PARASITOLOGY - CHAPTER   TWO  

BLOOD AND TISSUE PROTOZOA  

Dr Abdul Ghaffar 

Reading:
Medical Microbiology
Murray et al. (6th ed.), Chapter 82

Images on this page credited to CDC come from the DPDx Parasite Image Library
 

Blood protozoa of major clinical significance include members of genera Trypanosoma (T. brucei and T. cruzi); Leishmania (L. donovani, L. tropica and L. braziliensis); Plasmodium (P. falciparum, P. ovale, P. malariae and P. vivax); Toxoplasma gondii;  and Babesia (B. microti).

TRYPANOSOMIASIS

African trypanosomiasis (Sleeping sickness)

Etiology
There are two clinical forms of African trypanosomiasis: 1) a slowly developing disease caused by Trypanosoma brucei gambiense and 2) a rapidly progressing disease caused by T. brucei rhodesiense.

Epidemiology
T. b. gambiense is predominant in the western and central regions of Africa, whereas T. b. rhodesiense is restricted to the eastern third of the continent (figure 2E). 6,000 to 10,000 human cases are documented annually. 35 million people and 25 million cattle are at risk. Regional epidemics of the disease are cause of major health and economic disasters.

Morphology
T. b. gambiense and T. b. rhodesiense are similar in appearance: The organism measures 10 - 30 micrometers x 1-3 micrometers. It has a single central nucleus and a single flagellum originating at the kinetoplast and joined to the body by an undulating membrane (Figure 2A-D). The outer surface of the organism is densely coated with a layer of glycoprotein, the variable surface glycoprotein (VSG).

TEACHING OBJECTIVES

Epidemiology, morbidity and mortality
Morphology of the organism
Life cycle, hosts and vectors
Disease, symptoms, pathogenesis and site
Diagnosis
Prevention and control

Figure 1A  
During a blood meal on the mammalian host, an infected tsetse fly (genus Glossina) injects metacyclic trypomastigotes into skin tissue.  The parasites enter the lymphatic system and pass into the bloodstream .  Inside the host, they transform into bloodstream trypomastigotes , are carried to other sites throughout the body, reach other blood fluids (e.g., lymph, spinal fluid), and continue the replication by binary fission .  The entire life cycle of African Trypanosomes is represented by extracellular stages.  The tsetse fly becomes infected with bloodstream trypomastigotes when taking a blood meal on an infected mammalian host (, ).  In the fly’s midgut, the parasites transform into procyclic trypomastigotes, multiply by binary fission , leave the midgut, and transform into epimastigotes .  The epimastigotes reach the fly’s salivary glands and continue multiplication by binary fission .  The cycle in the fly takes approximately 3 weeks.  Humans are the main reservoir for Trypanosoma brucei gambiense, but this species can also be found in animals.  Wild game animals are the main reservoir of T. b. rhodesiense. 

  Figure 1B  Forms of  Trypansoma brucei obsreved in the tstese fly and in the human blood stream
T. brucei is transmitted by tsetse flies of the genus Glossina. Parasites are ingested by the fly when it takes a blood meal on an infected mammal. The parasites multiply in the fly, going through several developmental stages in the insect gut and salivary glands (procyclic trypanosomes, epimastigotes, metacyclic trypanosomes). The cycle in the fly takes approximately 3 weeks.  When the fly bites another mammal, metacyclic trypanosomes are inoculated, and multiply in the host's blood and extracellular fluids such as spinal fluid. Humans are the main reservoir for T. b. gambiense, but this species can also be found in animals. Wild game animals are the main reservoir of T. b. rhodesiense. 

 Figure 2B
Blood smear from a patient (a U.S. traveler) with Trypanosoma brucei rhodesiense. A dividing parasite is seen at the right. Dividing forms are seen in African trypanosomiasis, but not in American trypanosomiasis (Chagas' disease) CDC DPDx Parasite Image Library

  Figure 2E 
Distribution of West African or Gambian Sleeping Sickness and East African or Rhodesian Sleeping Sickness 

  Figure 2F  Reported number of cases of African trypanosomiasis
in Uganda, 1939-1998  WHO

Between 1962 and 1975, no cases were reported. Increased reporting during 1977 to 1983 reflected an epidemic of rhodesiense sleeping sickness in Busuga (south-eastern Uganda). However the increases shown between 1986 and 1992 corresponded to both the resumption of systematic population screening for gambiense sleeping sickness in the western part of the country and to a resurgence of rhodesiense sleeping sickness in Busuga.

Life cycle
The infective, metacyclic form of the trypanosome is injected into the primary host during a bite by the vector, the tsetse fly (figure 3). The organism transforms into a dividing trypanosomal (trypomastigote) blood form (figure 1B) as it enters the draining lymphatic and blood stream. The trypanosomal form enters the vector during the blood meal and travels through the alimentary canal to the salivary gland where it proliferates as the crithidial form (epimastigote) and matures to infectious metacyclic forms (Figure 1B). Trypomastigotes can traverse the walls of blood and lymph capillaries into the connective tissues and, at a later stage, cross the choroid plexus into the brain and cerebrospinal fluid. The organism can be transmitted through blood transfusion.

Symptoms
The clinical features of Gambian and Rhodesian disease are the same, however they vary in severity and duration. Rhodesian disease progresses more rapidly and the symptoms are often more pronounced. The symptoms of the two diseases are also more pronounced in Caucasians than in the local African population. Classically, the progression of African trypanosomiasis can be divided into three stages: the bite reaction (chancre), parasitemia (blood and lymphoid tissues), and CNS stage.

Bite reaction: A non-pustular, painful, itchy chancre (Figure 4 A and B) forms 1-3 weeks after the bite and lasts 1-2 weeks. It leaves no scar.

Parasitemia: Parasitemia and lymph node invasion is marked by attacks of fever which starts 2-3 weeks after the bite and is accompanied by malaise, lassitude, insomnia headache and lymphadenopathy and edema (figure 4E). Painful sensitivity of palms and ulnar region to pressure (Kerandel's sign) may develop in some Caucasians. Very characteristic of Gambian disease is visible enlargement of the glands of the posterior cervical region (Winterbottom's sign) (Figure 4C). Febrile episodes may last few months as in Rhodesian disease or several years as in Gambian disease. Parasitemia is more prominent during the acute stage than during the recurrence episodes.

CNS Stage: The late or CNS stage is marked by changes in character and personality. They include lack of interest and disinclination to work, avoidance of acquaintances, morose and melancholic attitude alternating with exaltation, mental retardation and lethargy, low and tremulous speech, tremors of tongue and limbs, slow and shuffling gait, altered reflexes, etc. Males become impotent. There is a slow progressive involvement of cardiac tissue. The later stages are characterized by drowsiness and uncontrollable urge to sleep. The terminal stage is marked by wasting and emaciation. Death results from coma, intercurrent infection or cardiac failure (figure 5).

The partially healed chancre on the arm of a female patient in a ward of a rural clinic.  WHO/TDR/Crump

Figure 4B
The leg of a teenage girl who has sleeping sickness, showing the chancre at the site of the tsetse fly bite WHO/TDR/Kuzoe 

  Figure 4C
Winterbottoms sign CDC  DPDx Parasite Image Library 

  Figure 4D
Neurological complications can occur as a result of infection and, as seen here, patients may be immobilised for their own safety. WHO/TDR/Kuzoe 

  Figure 4E
A male sleeping sickness patient with myxoedema. WHO/TDR/Kuzoe

  Figure 5A
The damaged brain of a patient who had died from African trypanosomiasis (or sleeping sickness). WHO/TDR/Kuzoe

  Figure 5B
A young boy with advanced African trypanosomiasis (or sleeping sickness) exhibiting marked wasting and skin damage caused as a result of the intense itching which can accompany late-stage disease. WHO/TDR/Kuzoe

Figure 5C
Neuropathology of Human African Trypanosomiasis: Acute haemorrhagic leucoencephalopathy (AHL): This slide shows very delicate fibrinoid necrosis in the wall of a small artery in the thalamus. Produced by the Dept. of Neuropathology, Southern General Hospital, Glasgow). 

  Figure 5D
Neuropathology of Human African Trypanosomiasis: Acute haemorrhagic leucoencephalopathy: This slide shows the foci of haemorrhage around small blood vessels. Produced by the Dept. of Neuropathology, Southern General Hospital, Glasgow). 

The clinical features of Rhodesian disease are similar but briefer and more acute. The acuteness and severity of disease do not allow typical sleeping sickness. Death is due to cardiac failure within 6-9 months.

Pathology and Immunology
An exact pathogenesis of sleeping sickness is not known, although immune complexes and inflammation have been suspected to be the mechanism of damage to tissues. The immune response against the organism does help to eliminate the parasite but it is not protective, since the parasite has a unique ability of altering its antigens, the VSG (see the chapter on Molecular Biology of Trypanosomes).  Consequently, there is a cyclic fluctuation in the number of parasites in blood and lymphatic fluids and each wave of parasite represents a different antigenic variant. The parasite causes polyclonal expansion of B lymphocytes and plasma cells and an increase in total IgM concentration. It stimulates the reticuloendothelial function. It also causes severe depression of cell mediated and humoral immunity to other antigens.

Diagnosis
Detection of parasite in the bloodstream, lymph secretions and enlarged lymph node aspirate provides a definitive diagnosis in early (acute) stages. The parasite in blood can be concentrated by centrifugation or by the use of anionic support media. Cerebrospinal fluid must always be examined for organisms. Immuno-serology (enzyme-linked immune assay, immunofluorescence) may be indicative but does not provide definite diagnosis.

Treatment and Control
The blood stage of African trypanosomiasis can be treated with reasonable success with Pentamidine isethionate or Suramin. These drugs have been reported also to be effective in prophylaxis although they may mask early infection and thus increase the risk of CNS disease. Cases with CNS involvement should be treated with Melarsoprol, an organic arsenic compound.

The most effective means of prevention is to avoid contact with tsetse flies. Vector eradication is impractical due to the vast area involved. Immunization has not been effective due to antigenic variation.

American trypanosomiasis (Chagas' disease)

Etiology
Chagas' disease is caused by the protozoan hemoflagellate, Trypanosoma cruzi.

Epidemiology
American trypanosomiasis, also known as Chagas' disease, is scattered irregularly in Central and South America, stretching from parts of Mexico to Argentina (figure 6). Rare cases have been reported in Texas, California and Maryland. It is estimated that 16-18 million people are infected by the parasite and 50 million are at risk. About 50,000 people die each year from the disease.

Morphology
Depending on its host environment, the organism occurs in three different forms (Figure 7 and 9B). The trypanosomal (trypomastigote) form (figure 7A), found in mammalian blood, is 15 to 20 microns long and morphologically similar to African trypanosomes. The crithidial (epimastigote) form (figure 7B) is found in the insect intestine. The leishmanial (amastigote) form (figure 7C), found intracellularly or in pseudocysts in mammalian viscera (particularly in myocardium and brain), is round or oval in shape, measures 2-4 microns and lacks a prominent flagellum.

Life cycle
The organism is transmitted to mammalian host by many species of kissing (riduvid) bug (figure 8), most prominently by Triatoma infestans, T. sordida, Panstrongylus megistus and Rhodnius prolixus. Transmission takes place during the feeding of the bug which normally bites in the facial area (hence the name, kissing bug) and has the habit of defecating during feeding. The metacyclic trypamastigotes, contained in the fecal material, gain access to the mammalian tissue through the wound which is often rubbed by the individual that is bitten. Subsequently, they enter various cells, including macrophages, where they  differentiate into amastigotes and multiply by binary fission. The amastigotes differentiate into non-replicating trypomastigotes and the cells rupture to release them into the bloodstream. Additional host cells, of a variety of types, can become infected and the trypomastigotes once again form amastigotes inside these cells. Uninfected insect vectors acquire the organism when they feed on infected animals or people containing trypomastigotes circulating in their blood. Inside the alimentary tract of the insect vector, the trypomastigotes differentiate to form epimastigotes and divide longitudinally in the mid and hindgut of the insect where they develop into infective metacyclic trypomastigotes (figure 9C). Transmission may also occur from man to man by blood transfusion and by the transplacental route.

 
Figure 7C. Trypanosoma cruzi. Leishmanial form CDC

Figure 8 Riduvid bug, the vector of American trypanosomiasis

Figure 9A  Ramana's sign: unilateral conjunctivitis and orbital edema 

  Figure 9B Megacolon in Chaga's disease

More than one hundred mammalian species of wild and domestic animals including cattle, pigs, cats, dogs, rats, armadillo, raccoon and opossum are naturally infected by T. cruzi and serve as a reservoir.

Symptoms
Chagas' disease can be divided into three stages: the primary lesion, the acute stage, and the chronic stage. The primary lesion, chagoma, appearing at the site of infection, within a few hours of a bite, consists of a slightly raised, flat non-purulent erythematous plaque surrounded by a variable area of hard edema. It is usually found on the face, eyelids, cheek, lips or the conjunctiva, but may occur on the abdomen or limbs. When the primary chagoma is on the face, there is an enlargement of the pre- and post- auricular and the submaxillary glands on the side of the bite. Infection in the eyelid, resulting in a unilateral conjunctivitis and orbital edema (Ramana's sign) (figure 9A), is the commonest finding.

Acute Stage: The acute stage appears 7-14 days after infection. It is characterized by restlessness, sleeplessness, malaise, increasing exhaustion, chills, fever and bone and muscle pains. Other manifestations of the acute phase are cervical, axillary and iliac adenitis, hepatomegaly, erythematous rash and acute myocarditis. There is a general edematous reaction associated with lymphadenopathy. Diffuse myocarditis, sometimes accompanied by serious pericarditis and endocarditis, is very frequent during the initial stage of the disease. In children, Chagas' disease may cause meningo-encephalitis and coma. Death occurs in 5-10 percent of infants. Hematologic examination reveals lymphocytosis and parasitemia.

Chronic Stage: The acute stage is usually not recognized and often resolves with little or no immediate damage and the infected host remains an asymptomatic carrier. An unknown proportion (guessed at 10-20%) of victims develop a chronic disease. They alternate between asymptomatic remission periods and relapses characterized by symptoms seen in the acute phase. Cardiac arrhythmia is common. The chronic disease results in an abnormal function of the hollow organs, particularly the heart, esophagus and colon. The cardiac changes include myocardial insufficiency, cardiomegaly, disturbances of atrio-ventricular conduction and the Adams-Stoke syndrome. Disturbances of peristalsis lead to megaesophagus and megacolon (figure 9B).

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Pathology and Immunology
The pathological effects of acute phase Chagas' disease largely result from direct damage to infected cells. In later stages, the destruction of the autonomic nerve ganglions may be of significance. Immune mechanisms, both cell mediated and humoral, involving reaction to the organism and to autologous tissues have been implicated in pathogenesis.

T. cruzi stimulates both humoral and cell mediated immune responses. Antibody has been shown to lyze the organism, but rarely causes eradication of the organism, perhaps due to its intracellular localization. Cell mediated immunity may be of significant value. While normal macrophages are targeted by the organism for growth, activated macrophages can kill the organism. Unlike T. brucei, T. cruzi does not alter its antigenic coat. Antibodies directed against heart and muscle cells have also been detected in infected patients leading to the supposition that there is an element of autoimmune reaction in the pathogenesis of Chagas' disease. The infection causes severe depression of both cell mediated and humoral immune responses. Immunosuppression may be due to induction of suppressor T-cells and/or overstimulation of macrophages.

Diagnosis
Clinical diagnosis is usually easy among children in endemic areas. Cardiac dilation, megacolon and megaesophagus in individuals from endemic areas indicate present or former infection. Definitive diagnosis requires the demonstration of trypanosomes by microscopy or biological tests (in the insect or mice). Antibodies are often detectable by complement fixation or immunofluorescence and provide presumptive diagnosis.

Treatment and Control
There is no curative therapy available. Most drugs are either ineffective or highly toxic. Recently two experimental drugs, Benznidazol and Nifurtimox have been used with promising results in the acute stage of the disease, however their side effects limit their prolonged use in chronic cases.

Control measures are limited to those that reduce contact between the vectors and man. Attempts to develop a vaccine have not been very successful, although they may be feasible.

LEISHMANIASIS

Etiology
Several species of Leishmania are pathogenic for man: L. donovani causes visceral leishmaniasis (Kala-azar, black disease, dumdum fever); L. tropica (L. t. major, L. t. minor and L. ethiopica) cause cutaneous leishmaniasis (oriental sore, Delhi ulcer, Aleppo, Delhi or Baghdad boil); and L. braziliensis (also, L. mexicana and L. peruviana) are etiologic agents of mucocutaneous leishmaniasis (espundia, Uta, chiclero ulcer).

Epidemiology
Leishmaniasis is prevalent world wide: ranging from south east Asia, Indo-Pakistan, Mediterranean, north and central Africa, and south and central America.

Morphology
Amastigote (leishmanial form) is oval and measures 2-5 microns by 1 - 3 microns (figure 10A-D), whereas the leptomonad measures 14 - 20 microns by 1.5 - 4 microns, a similar size to trypanosomes (Figure 10E).

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A   B C Leishmania tropica amastigotes from a skin touch preparation. In A, a still intact macrophage is practically filled with amastigotes, several of which have clearly visible a nucleus and a kinetoplast (arrows); in B, amastigotes are being freed from a rupturing macrophage. Patient with history of travel to Egypt, Africa, and the Middle East. Culture in NNN medium followed by isoenzyme analysis identified the species as L. tropica minor. CDC

   Figure 10D
Leishmania mexicana mexicana in skin biopsy. Hematoxylin and eosin stain. The amastigotes are lining the wall of two vacuoles, a typical arrangement. The species identification was derived from culture followed by isoenzyme analysis. 26-year old man from Austin, Texas, with a lesion on his left arm. CDC

 Figure 10E 
Leishmania donovani, leptomonad forms.  CDC  DPDx Parasite Image Library

  Figure 10G
Bone marrow smear showing Leishmania donovani parasites in a bone marrow histiocyte from a dog (Giemsa stain).  CDC/Dr. Francis W. Chandler 

 Figure 10I
Leishmania donovani in bone marrow cell. Smear.  CDC/Dr. L.L. Moore, Jr.

 

 Figure 10H 
Erythrophagocytosis in the liver (H&E X 400) WHO/TDR/El-Hassan

 
Life cycle

The organism is transmitted by the bite of several species of blood-feeding sand flies (Phlebotomus) which carry the promastigote in the anterior gut and pharynx. The parasites gain access to mononuclear phagocytes where they transform into amastigotes and divide until the infected cell ruptures. The released organisms infect other cells. The sandfly acquires the organisms during the blood meal; the amastigotes transform into flagellate promastigotes and multiply in the gut until the anterior gut and pharynx are packed. Dogs and rodents are common reservoirs (figure 11F).

Symptoms

Visceral leishmaniasis (kala-azar, dumdum fever): L. donovani organisms in visceral leishmaniasis are rapidly eliminated from the site of infection, hence there is rarely a local lesion, although minute papules have been described in children. They are localized and multiply in the mononuclear phagocytic cells of spleen, liver, lymph nodes, bone marrow, intestinal mucosa and other organs. One to four months after infection, there is occurrence of fever, with a daily rise to 102-104 degrees F, accompanied by chills and sweating. The spleen and liver progressively become enlarged (figure 11B, C and E). With progression of the diseases, skin develops hyperpigmented granulomatous areas (kala-azar means black disease). Chronic disease renders patients susceptible to other infections. Untreated disease results in death.

 Figure 11C 
A 12-year-old boy suffering from visceral leishmaniasis. The boy exhibits splenomegaly and severe muscle wasting. WHO/TDR/El-Hassan

  Figure 11E   
Enlarged spleen and liver in an autopsy of an infant dying of visceral leishmaniasis. WHO/TDR/El-Hassan

Leishmaniasis is transmitted by the bite of female phlebotomine sandflies.  The sandflies inject the infective stage, promastigotes, during blood meals .  Promastigotes that reach the puncture wound are phagocytized by macrophages and transform into amastigotes .  Amastigotes multiply in infected cells and affect different tissues, depending in part on the Leishmania species .  This originates the clinical manifestations of leishmaniasis.  Sandflies become infected during blood meals on an infected host when they ingest macrophages infected with amastigotes (, ).  In the sandfly's midgut, the parasites differentiate into promastigotes , which multiply and migrate to the proboscis .   CDC

Cutaneous leishmaniasis (Oriental sore, Delhi ulcer, Baghdad boil): In cutaneous leishmaniasis, the organism (L. tropica) multiplies locally, producing of a papule, 1-2 weeks (or as long as 1-2 months) after the bite. The papule gradually grows to form a relatively painless ulcer. The center of the ulcer encrusts while satellite papules develop at the periphery. The ulcer heals in 2-10 months, even if untreated but leaves a disfiguring scar (figure 12). The disease may disseminate in the case of depressed immune function.

Mucocutaneous leishmaniasis (espundia, Uta, chiclero): The initial symptoms of mucocutaneous leishmaniasis are the same as those of cutaneous leishmaniasis, except that in this disease the organism can metastasize and the lesions spread to mucoid (oral, pharyngeal and nasal) tissues and lead to their destruction and hence sever deformity (figure 12E). The organisms responsible are L. braziliensis, L. mexicana and L. peruviana.

Pathology
Pathogenesis of leishmaniasis is due to an immune reaction to the organism, particularly cell mediated immunity. Laboratory examination reveals a marked leukopenia with relative monocytosis and lymphocytosis, anemia and thrombocytopenia. IgM and IgG levels are extremely elevated due to both specific antibodies and polyclonal activation.

Diagnosis
Diagnosis is based on a history of exposure to sandfies, symptoms and isolation of the organisms from the lesion aspirate or biopsy, by direct examination or culture. A skin test (delayed hypersensitivity: Montenegro test) and detection of anti-leishmanial antibodies by immuno-fluorescence are indicative of exposure.

Treatment and Control
Sodium stibogluconate (Pentostam) is the drug of choice. Pentamidine isethionate is used as an alternative. Control measures involve vector control and avoidance. Immunization has not been effective.
 

Figure 11F    

  Figure 12C 
Scar on skin of upper leg representing healed lesion of leishmaniasis CDC 

 Figure 12E 
Girl with diffuse muco- cutaneous leishmaniasis of the face which is responding to treatment  WHO/TDR/El-Hassan

MALARIA

Etiology
Four Plasmodium species are responsible for human malaria These are P. falciparum, P. vivax, P. ovale and P. malariae.

Epidemiology
There are an estimated 200 million global cases of malaria leading a mortality of more than one million people per year. P. falciparum (malignant tertian malaria) and P. malariae (quartan malaria) are the most common species of malarial parasite and are found in Asia and Africa. P. vivax (benign tertian malaria) predominates in Latin America, India and Pakistan, whereas, P. ovale (ovale tertian malaria) is almost exclusively found in Africa (figure 12G).

Morphology
Malarial parasite trophozoites are generally ring shaped, 1-2 microns in size, although other forms (ameboid and band) may also exist. The sexual forms of the parasite (gametocytes) are much larger and 7-14 microns in size. P. falciparum is the largest and is banana shaped while others are smaller and round. P. vivax causes stippling of infected red cells (figure 13-17).

Plasmodium falciparum: Blood Stage Parasites: Thin Blood Smears  Fig. 1: Normal red cell; Figs. 2-18: Trophozoites (among these, Figs. 2-10 correspond to ring-stage trophozoites); Figs. 19-26: Schizonts (Fig. 26 is a ruptured schizont); Figs. 27, 28: Mature macrogametocytes (female); Figs. 29, 30: Mature microgametocytes (male)  CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971	Plasmodium falciparum: Blood Stage Parasites: Thick Blood Smears Illustrations from: Wilcox A. Manual for the Microscopical Diagnosis of Malaria in Man. U.S. Department of Health, Education and Welfare, Washington, 1960.  CDC
Plasmodium malariae: Blood Stage Parasites: Thin Blood Smears Fig. 1: Normal red cell; Figs. 2-5: Young trophozoites (rings); Figs. 6-13: Trophozoites; Figs. 14-22: Schizonts; Fig. 23: Developing gametocyte; Fig. 24: Macrogametocyte (female); Fig. 25: Microgametocyte (male)   CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971	Plasmodium malariae: Blood Stage Parasites: Thick Blood Smears Illustrations from: Wilcox A. Manual for the Microscopical Diagnosis of Malaria in Man. U.S. Department of Health, Education and Welfare, Washington, 1960.  CDC
Plasmodium ovale: Blood Stage Parasites: Thin Blood Smears Fig. 1: Normal red cell; Figs. 2-5: Young trophozoites (Rings); Figs. 6-15: Trophozoites; Figs. 16-23: Schizonts; Fig. 24:  Macrogametocytes (female); Fig. 25: Microgametocyte (male)   CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971	Plasmodium vivax: Blood Stage Parasites: Thin Blood Smears  Fig. 1: Normal red cell; Figs. 2-6: Young trophozoites (ring stage parasites); Figs. 7-18: Trophozoites; Figs. 19-27: Schizonts; Figs. 28 and 29: Macrogametocytes (female); Fig. 30: Microgametocyte (male)   CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971
Figure  13  Trophozoites: Blood stages malarial parasites  DPDx Parasite Image Library

Plasmodium falciparum: Gametocytes Figs. 27, 28: Mature macrogametocytes (female); Fig. 29, 30: Mature microgametocytes (male) CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971		Plasmodium malariae: Gametocytes Fig. 23: Developing gametocyte; Fig. 24: Macrogametocyte (female); Fig. 25: Microgametocyte (male) CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971
Plasmodium falciparum: Gametocytes: An asplenic, 41 y.o. woman, immigrant from Haiti, who returned to the US 2 days ago; high P. falciparum parasitemia; the presence of such young gametocytes in the peripheral blood is exceptional (specimen contributed by Florida SHD) CDC	Plasmodium falciparum: Gametocytes: A patient from Haiti; mature gametocytes (specimen contributed by Florida SHD) CDC	Plasmodium malariae: Gametocytes: Smear from patient:  56 y.o. man who had traveled to Kenya (specimen contributed by Wisconsin SHD) CDC	Plasmodium malariae: Gametocytes: Smear from patient:  56 y.o. man who had traveled to Kenya (specimen contributed by Wisconsin SHD) CDC
Plasmodium ovale: Gametocytes Fig. 24: Macrogametocyte (female); Fig. 25: Microgametocyte (male). CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971		Plasmodium vivax: Gametocytes Fig. 28 and 29: Nearly mature and mature macrogametocyte (female); Fig. 30: Microgametocyte (male) CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971
A B Plasmodium ovale: Gametocytes Smears from patients: Note the Schüffner's dots in A, and the fimbriation of the erythrocyte in B. The erythrocytes in P. ovale infections are less enlarged than with P. vivax, and are not as deformed. A, B: Male patient born in Nigeria, who came to the US 5 days ago (specimen contributed by Michigan SHD) CDC		A  B C Plasmodium vivax: Gametocytes Smears from patients: Note the variability in Schüffner's dots. A: A pregnant woman who visited India 6 months ago (specimen contributed by New Jersey SHD) B,C: 50 y.o. woman 3 months ago from a 1-month visit to India (specimen contributed by Indiana SHD) CDC
Figure 14  Gametocytes DPDx Parasite Image Library

Plasmodium falciparum: Ring Stage Parasites. Fig. 1: Normal red cell; Figs. 2-10: Increasingly mature ring stage parasites. CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971	Plasmodium malariae: Ring Stage Parasites Fig. 1: Normal red cell; Figs. 2-5: Rings CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971
Appliqué form   Ring with double chromatin dot   Older ring stage parasite   Doubly infected erythrocyte   Multiple infections, 6 rings in 2 erythrocytes An asplenic, 41 y.o. woman, immigrant from Haiti, who returned to the US 2 days ago; high P. falciparum parasitemia (specimen contributed by Florida SHD). CDC	Plasmodium malariae: Ring Stage Parasites  Smears from patients:  56 y.o. man who had traveled to Kenya (specimen contributed by Wisconsin SHD) CDC
Plasmodium ovale: Ring Stage Parasites Fig. 1: Normal red cell; Figs. 2-5: Ring stage parasites CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971	Plasmodium vivax: Ring Stage Parasites Fig. 1: Normal red cell; Figs. 2-6: Ring stage parasites (young trophozoites) CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971
A  B C   Plasmodium ovale: Ring Stage Parasites Smears from patients: Note the relatively large chromatin dots. A, C: 54 y.o. man who returned the previous month from a visit to Kenya and Malawi. P. ovale, confirmed by PCR (specimen contributed by New Mexico SHD). B: 20 y.o. man who returned 10 months ago from a visit to Mozambique, Zimbabwe and Swaziland; this attack is thus a relapse (specimen contributed by New York SHD). CDC	Plasmodium vivax: Ring Stage Parasites Smears from patients: A: Rings in 2 slightly enlarged RBCs; 17 y.o. man with a relapse due to P. vivax (PCR confirmed), 6 months after returning from a visit to Papua New Guinea (specimen contributed by Virginia SHD) B: Double infection with rings, RBC enlarged and deformed, Schüffner's dots beginning to become visible; 69 y.o. woman born in India who was symptomatic on the day of arrival to the US (specimen contributed by Pennsylvania SHD)  C: Late ring in a RBC with Schüffner's dots; 60 y.o. man who returned 2 months ago from a 3 month trip to Laos and North Korea (specimen contributed by Hawaii SHD) CDC
Figure 15  Ring stage parasites DPDx Parasite Image Library

Plasmodium falciparum: Schizonts Figs. 19-25: Increasingly mature schizonts; Fig. 26: Ruptured schizont CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971	Plasmodium malariae: Schizonts.  Increasingly mature schizonts CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971
A B Plasmodium falciparum: Schizonts. Smears from patients: Schizonts are seen only rarely in P. falciparum malaria. An asplenic, 41 y.o. woman, immigrant from Haiti, who returned to the US 2 days ago; high P. falciparum parasitemia A: Young schizont with 10 nuclei; B: Mature schizont with 24 nuclei, ready to rupture (“segmenter”)   (specimen contributed by Florida SHD) CDC	A B C   D Plasmodium malariae: Schizonts.Smears from patients: The parasites are compact and the infected erythrocytes are not enlarged. In C and D, the merozoites are arranged in a rosette pattern.  A, B, C, D: 56 y.o. man who had traveled to Kenya (specimen contributed by Wisconsin SHD) CDC
Plasmodium ovale: Schizonts  Increasingly mature schizonts CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971	Plasmodium vivax: Schizonts Figs. 19-27: Increasingly mature schizonts  CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971
A B Plasmodium ovale: Schizonts Smears from patients: A, B: 54 y.o. man who returned the previous month from a visit to Kenya and Malawi. Infection with P. ovale, confirmed by PCR   Note the fimbriation of the erythrocyte in A. (specimen contributed by New Mexico SHD). CDC	A B C D E Plasmodium vivax: Schizonts Smears from patients: Note that in these patients, the Schüffner's dots are not conspicuous. (This happens in many of the smears received at CDC; it is probably related to variability in staining.)  A, C, D, E: A pregnant woman who visited India 6 months ago (specimen contributed by New Jersey SHD) B: 17 y.o. man with a relapse due to P. vivax (PCR confirmed) (specimen contributed by Virginia SHD) CDC
Figure 16  Schizonts DPDx Parasite Image Library

Plasmodium falciparum: Trophozoites Figs. 11-18: Increasingly mature trophozoites CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971	Plasmodium malariae: Trophozoites Figs. 6-13: Increasingly mature trophozoites; Fig. 13 is a "band form". CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971
A B Thin smears from two patients with high parasitemias: A: An asplenic, 41 y.o. woman, immigrant from Haiti, who returned to the US 2 days ago; high P. falciparum parasitemia (specimen contributed by Florida SHD) CDC B: A patient who acquired malaria by blood transfusion and died with extremely high parasitemia; PCR confirmed P. falciparum; one of the 2 RBCs contains 3 young trophozoites, which have begun to accumulate pigment (specimen contributed by Missouri SHD); CDC	A B C Plasmodium malariae: Trophozoites  Smears from patients:  The infected erythrocytes are not enlarged (sometime they even appear smaller than non-infected ones). C is a "band form" trophozoite. A, B, C: 56 y.o. man who had traveled to Kenya (specimen contributed by Wisconsin SHD) CDC
Plasmodium ovale: Trophozoites Increasingly mature trophozoites. Note the fimbriated red cells (Figs. 8, 13) CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971	Plasmodium vivax: Trophozoites Figs. 8-18: Increasingly mature trophozoites of P. vivax CDC  Illustrations from: Coatney GR, Collins WE, Warren M, Contacos PG. The Primate Malarias. U.S. Department of Health, Education and Welfare, Bethesda, 1971
Plasmodium ovale: Trophozoites   Smears from patients: Note the lack of ameboidicity in the older trophozoites (B,C) and the fimbriation of the erythrocyte in C. The erythrocytes in P. ovale infections are less enlarged than with P. vivax, and are not as deformed. The Schüffner's dots are visible in A, but not B and C.  A: 20 y.o. man who returned 10 months ago from a visit to Mozambique, Zimbabwe and Swaziland (specimen contributed by New York SHD). CDC B, C: 23 y.o. man who arrived to the US 5 months ago after having been in Liberia and Ivory Coast (specimen contributed by Kentucky SHD) CDC	A B C   D  E Plasmodium vivax: Trophozoites Smears from patients: Increasingly mature trophozoites. The RBCs are enlarged and deformed, the parasites are ameboid, and the Schüffner's dots vary in intensity. A, B: 26 y.o. woman who spent 2 weeks in Papua New Guinea 5 months ago (specimen contributed by Pennsylvania SHD)  CDC C, E: 60 y.o. man who returned 2 months ago from a 3-month visit to Laos and North Korea (specimen contributed by Hawaii SHD) D: 28 y.o. woman who returned 3 months ago from a 2 weeks visit to Kenya (specimen contributed by Texas SHD) CDC
Figure  17  Trophozoites DPDx Parasite Image Library

Life cycle
Malarial parasites are transmitted by the infected female anopheline mosquito which injects sporozoites present in the saliva of the insect (Figure 18). Sporozoites infect the liver parenchymal cells where they may remain dormant (hypnozoites) or undergo stages of schizogony to produce schizonts and merogony to produce merozoites (meronts). When parenchymal cells rupture, thousands of meronts are released into blood and infect the red cells. P. ovale and P. vivax infect immature red blood cells whereas P. malariae infects mature red cells. P. falciparum infects both. In red cells, the parasites mature into trophozoites. These trophozoites undergo schizogony and merogony in red cells which ultimately burst and release daughter merozoites. Some of the merozoites transform into male and female gametocytes (figure 19) while others enter red cells to continue the erythrocytic cycle. The gametocytes are ingested by the female mosquito, the female gametocyte transforms into ookinete, is fertilized, and forms an oocyst (figure 20) in the gut. The oocyte produces sporozoites (sporogony) (figure 20) which migrate to the salivary gland and are ready to infect another host. The liver (extraerythrocytic) cycle takes 5-15 days whereas the erythrocytic cycle takes 48 hours or 72 hours (P. malariae). Malaria can be transmitted by transfusion and transplacentally.

 

Stage II (central) and stage III (bottom right) immature gametocytes (blood film, wet mount, x1000 magnification under oil immersion) Image courtesy of Dr Andrew Taylor-Robinson, University of Leeds, UK  © Dr Andrew Taylor-Robinson	Stage IV immature gametocyte, located centrally (blood film, wet mount, x400 magnification) Image courtesy of Dr Andrew Taylor-Robinson, University of Leeds, UK  © Dr Andrew Taylor-Robinson
Stage V mature gametocyte, showing characteristic sausage-shaped morphology, located centrally (blood film, wet mount, x1000 magnification under oil immersion) Image courtesy of Dr Andrew Taylor-Robinson, University of Leeds, UK  © Dr Andrew Taylor-Robinson	Male (micro)gametocyte exflagellation - extrusion of motile, flagella-like microgametes with vigorous movement (blood film, wet mount, x1000 magnification under oil immersion) (an unusually clear picture of this metabolically dynamic and visually striking event)  Image courtesy of Dr Andrew Taylor-Robinson, University of Leeds, UK  © Dr Andrew Taylor-Robinson
Figure 19  Sexual stages of the malaria parasite Plasmodium falciparum

Two oocysts, dissected from the outer wall of the Anopheles stephensi midgut, 10 days post infection of the mosquito (wet mount, x400 magnification) Image courtesy of Dr Andrew Taylor-Robinson, University of Leeds, UK  © Dr Andrew Taylor-Robinson	Single oocyst, dissected from the outer wall of the Anopheles stephensi midgut, 10 days post infection of the mosquito (wet mount, x400 magnification) Image courtesy of Dr Andrew Taylor-Robinson, University of Leeds, UK  © Dr Andrew Taylor-Robinson
Single oocyst, dissected from the outer wall of the Anopheles stephensi midgut, 10 days post infection of the mosquito (wet mount, x1000 magnification under oil immersion) Image courtesy of Dr Andrew Taylor-Robinson, University of Leeds, UK  © Dr Andrew Taylor-Robinson	Isolated bow-shaped sporozoite, dissected from the salivary glands of Anopheles stephensi, 17 days post infection of the mosquito (wet mount, x1000 magnification under oil immersion)   Image courtesy of Dr Andrew Taylor-Robinson, University of Leeds, UK  © Dr Andrew Taylor-Robinson
Figure 20  Developmental stages of Plasmodium falciparum in the Anopheles mosquito vector

FLASH MOVIE

Life cycle of Plasmodium

WHO/Welcome Trust

Symptoms
The symptomatology of malaria depends on the parasitemia, the presence of the organism in different organs and the parasite burden. The incubation period varies generally between 10-30 days. As the parasite load becomes significant, the patient develops headache, lassitude, vague pains in the bones and joints, chilly sensations and fever. As the disease progresses, the chills and fever become more prominent. The chill and fever follow a cyclic pattern (paroxysm) with the symptomatic period lasting 8-12 hours. In between the symptomatic periods, there is a period of relative normalcy, the duration of which depends upon the species of the infecting parasite. This interval is about 34-36 hours in the case of P. vivax and P. ovale (tertian malaria), and 58-60 hours in the case of P. malariae (quartan malaria). Classical tertian paroxysm is rarely seen in P. falciparum and persistent spiking or a daily paroxysm is more usual.

The malarial paroxysm is most dramatic and frightening. It begins with a chilly sensation that progresses to teeth chattering, overtly shaking chill and peripheral vasoconstriction resulting in cyanotic lips and nails (cold stage). This lasts for about an hour. At the end of this period, the body temperature begins to climb and reaches 103-106 degrees F (39- 41degrees C). Fever is associated with severe headache, nausea (vomiting) and convulsions. The patient experiences euphoria, and profuse perspiration and the temperature begins to drop. Within a few hours the patient feels exhausted but symptom-less and remains symptomatic until the next paroxysm. Each paroxysm is due to the rupture of infected erythrocytes and release of parasites.

Without treatment, all species of human malaria may ultimately result in spontaneous cure except with P. falciparum which becomes more severe progressively and results in death. This organism causes sequestration of capillary vasculature in the brain, gastrointestinal and renal tissues. Chronic malaria results in splenomegaly, hepatomegaly and nephritic syndromes.

Pathology and immunology
Symptoms of malaria are due to the release of massive number of merozoites into the circulation. Infection results in the production of antibodies which are effective in containing the parasite load. These antibodies are against merozoites and schizonts. The infection also results in the activation of the reticuloendothelial system (phagocytes). The activated macrophages help in the destruction of infected (modified) erythrocytes and antibody-coated merozoites. Cell mediated immunity also may develop and help in the elimination of infected erythrocytes. Malarial infection is associated with immunosuppression.

Diagnosis
Diagnosis is based on symptoms and detection of parasite in Giemsa stained blood smears. There are also antibody tests (Figure 20B).

Treatment and Control
Treatment is effective with various quinine derivatives (quinine sulphate, chloroquine, meflaquine and primaquine, etc.). Drug resistance, particularly in P. falciparum and to some extent in P. vivax is a major problem. Control measures are eradication of infected anopheline mosquitos. Vaccines are being developed and tried but none is available yet for routine use.

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BABESIOSIS

Etiology
Babesia microti is the only member of the genus that infects man.

Morphology
The trophozoite is very similar to the ring form of the Plasmodium species (figure 21A and B).

WEB RESOURCES
Babesiosis - CDC

 
Life cycle
The organism (sporozoite) is transmitted by a tick and enters the red cell where it undergoes mitosis and the organisms (merozoite) are released to infect other red cells. Ticks acquire the organism during feeding on an infected individual. In the tick, the organism divides sexually in the gut and migrates into the salivary gland (figure 21C).

Symptoms
Babesiosis is associated with hemolytic anemia, jaundice, fever and hepatomegaly, usually 1-2 weeks after infection.

Diagnosis
Diagnosis is based on symptoms, patient history and detection of intraerythrocytic parasite in the patient or transfer of blood in normal hamsters which can be heavily parasitized.

Treatment and Control
Drugs of choice are clindamycin combined with quinine. The patient may recover spontaneously. One should avoid tick exposure and, if bitten, remove the tick from the skin immediately.

 

   Figure 21B  
Infection with Babesia. Giemsa-stained thin smears. Note  the tetrad (left side of the image), a dividing form pathognomonic for Babesia.  A 6 year old girl, status post splenectomy for hereditary spherocytosis, infection acquired in the US. CDC

  Figure 21C

The Babesia microti life cycle involves two hosts, which includes a rodent, primarily the white-footed mouse, Peromyscus leucopus.  During a blood meal, a Babesia-infected tick introduces sporozoites into the mouse host .  Sporozoites enter erythrocytes and undergo asexual reproduction (budding) .  In the blood, some parasites differentiate into male and female gametes although these cannot be distinguished at the light microscope level .  The definitive host is a tick, in this case the deer tick, Ixodes dammini (I. scapularis).  Once ingested by an appropriate tick , gametes unite and undergo a sporogonic cycle resulting in sporozoites .  Transovarial transmission (also known as vertical, or hereditary, transmission) has been documented for “large” Babesia spp. but not for the “small” babesiae, such as B. microti .  Humans enter the cycle when bitten by infected ticks.  During a blood meal, a Babesia-infected tick introduces sporozoites into the human host .  Sporozoites enter erythrocytes and undergo asexual replication (budding) .  Multiplication of the blood stage parasites is responsible for the clinical manifestations of the disease.  Humans are, for all practical purposes, dead-end hosts and there is probably little, if any, subsequent transmission that occurs from ticks feeding on infected persons.  However, human to human transmission is well recognized to occur through blood transfusions .

  Figure 21D
Thin blood film of B. microti ring forms with a typical Maltese Cross (four rings in cross formation). © MicrobeLibrary and Lynne Garcia, LSG & Associates

TOXOPLASMOSIS

Etiology
Toxoplasma gondii is the organism responsible for toxoplasmosis

Epidemiology
Toxoplasma has worldwide distribution and 20%-75% of the population is seropositive without any symptomatic episode. However, the infection poses a serious threat in immunosuppressed individuals and pregnant females.

Morphology
The intracellular parasites (tachyzoite) are 3x6 microns, pear-shaped organisms that are enclosed in a parasite membrane to form a cyst measuring 10-100 microns in size. Cysts in cat feces (oocysts) are 10-13 microns in diameter (figure 22).

Life cycle
The natural life cycle of T. gondii occurs in cats and small rodents, although the parasite can grow in the organs (brain, eye, skeletal muscle, etc.) of any mammal or birds (Figure 22). Cats gets infected by ingestion of cysts in flesh. Decystation occurs in the small intestine, and the organisms penetrate the submucosal epithelial cells where they undergo several generations of mitosis, finally resulting in the development of micro- (male) and macro- (female) gametocytes. Fertilized macro-gametocytes develop into oocysts that are discharged into the gut lumen and excreted. Oocysts sporulate in the warm environment and are infectious to a variety of animals including rodents and man. Sporozoites released from the oocyst in the small intestine penetrate the intestinal mucosa and find their way into macrophages where they divide very rapidly (hence the name tachyzoites) (figure 23) and form a cyst which may occupy the whole cell. The infected cells ultimately burst and release the tachyzoites to enter other cells, including muscle and nerve cells, where they are protected from the host immune system and multiply slowly (bradyzoites). These cysts are infectious to carnivores (including man). Unless man is eaten by a cat, it is a dead-end host.

Symptoms
Although Toxoplasma infection is common, it rarely produces symptoms in normal individuals. Its serious consequences are limited to pregnant women and immunodeficient hosts. Congenital infections occur in about 1-5 per 1000 pregnancies of which 5-10% result in miscarriage and 8-10% result in serious brain and eye damage to the fetus. 10-13% of the babies will have visual handicaps. Although 58-70% of infected women will give birth to a normal offspring, a small proportion of babies will develop active retino-chorditis or mental retardation in childhood or young adulthood. In immunocompetent adults, toxoplasmosis, may produce flu-like symptoms, sometimes associated with lymphadenopathy. In immunocompromised individuals, infection results in generalized parasitemia involvement of brain, liver lung and other organs, and often death.

Immunology
Both humoral and cell mediated immune responses are stimulated in normal individuals. Cell-mediated immunity is protective and humoral response is of diagnostic value.

Diagnosis
Suspected toxoplasmosis can be confirmed by isolation of the organism from tonsil or lymph gland biopsy.

Treatment
Acute infections benefit from pyrimethamine or sulphadiazine. Spiramycin is a successful alternative. Pregnant women are advised to avoid cat litter and to handle uncooked and undercooked meat carefully.

Figure 23    

  Figure 23A  
Toxoplasma gondii in the bronchoalveolar lavage (BAL) material from an HIV infected patient. Numerous trophozoites (tachyzoites) can be seen, which are typically crescent shaped with a prominent, centrally placed nucleus. Most of the tachyzoites are free, some are still associated with bronchopulmonary cells.  CDC

   Figure 23B 
Toxoplasma gondii in tissue from a cat. CDC  

PNEUMOCYSTIS PNEUMONIA

Pneumocystis jiroveci (formerly known as Pneumocystis carinii)

Pneumocystis jiroveci was formerly thought to be a protozoan but is now known to be a fungus. It is included here because pneumocystis pneumonia is often described as an opportunistic parasitic disease. 

Pneumocystis pneumonia is an infection of immunosuppressed individuals and is particularly seen in AIDS patients. The organism is pleomorphic, exhibiting, at various stages of its life cycle: 1-2 micron sporozoites, 4-5 micron trophozoites and 6-8 micron cysts. It spreads from person to person in cough droplets. Infection in immunosuppressed individuals results in interstitial pneumonia characterized by thickened alveolar septum infiltrated with lymphocytes and plasma cells. Pneumonia is associated with fever, tachypnea, hypoxia, cyanosis and asphyxia. Diagnosis is based on isolation of organisms from affected lungs. Trimethoprim-sulphamethoxazole is the treatment of choice (figure 24).

WEB RESOURCES
Pneumocystis pneumonia - CDC

     Figure 24 B and C
Pneumocystis jiroveci cysts
B. 3 cysts in bronchoalveolar material, Giemsa stain; the rounded cysts (size 4-7 µm) contain 6-8 intracystic bodies, whose nuclei are stained by Giemsa; the walls of the cysts are not stained; note the presence of several smaller, isolated trophozoites.
C. cysts in lung tissue, silver stain; the walls of the cysts are stained black; the intracystic bodies are not visible with this stain; baby who died with pneumonia in California. CDC

This is a generalized life cycle proposed by John J. Ruffolo, Ph.D. (Cushion, MT, 1988) for the various species of Pneumocystis.  These fungi are found in the lungs of mammals where they reside without causing overt infection until the host's immune system becomes debilitated.  Then, an oftentimes lethal pneumonia can result.  Asexual phase: trophic forms replicate by mitosis to .  Sexual phase: haploid trophic forms conjugate and produce a zygote or sporocyte (early cyst) .  The zygote undergoes meiosis and subsequent mitosis to produce eight haploid nuclei (late phase cyst) .  Spores exhibit different shapes (such as, spherical and elongated forms).   It is postulated that elongation of the spores precedes release from the spore case.  It is believed that the release occurs through a rent in the cell wall.  After release, the empty spore case usually collapses, but retains some residual cytoplasm .  A trophic stage, where the organisms probably multiply by binary fission is also recognized to exist.  The organism causes disease in immunosuppressed individuals.  

FACULTATIVE PARASITIC PROTOZOA

These are free-living amebae that occasionally cause serious human disease. They are of particular significance in immunocompromised hosts.

Negleria fowleri
This organism is a flagellate that may inhabit warm waters (spas, warm springs, heated swimming pools, etc.) and gain access via the nasal passage to the brain and cause encephalitis (figure 25)

  Figure 25B  
Naegleria fowleri trophozoite in spinal fluid. Trichrome stain. Note the typically large karyosome and the monopodial locomotion. Image contributed by Texas SHD. CDC

   Figure 25D   
Histopathology of Naegleria infection of brain. CDC 

Free-living amebae belonging to the genera Acanthamoeba, Balamuthia, and Naegleria are important causes of disease in humans and animals.  Naegleria fowleri produces an acute, and usually lethal, central nervous system (CNS) disease called primary amebic meingoencephalitis (PAM).  N. fowleri has three stages, cysts , trophozoites , and flagellated forms , in its life cycle.  The trophozoites replicate by promitosis (nuclear membrane remains intact) .  Naegleria fowleri is found in fresh water, soil, thermal discharges of power plants, heated swimming pools, hydrotherapy and medicinal pools, aquariums, and sewage.  Trophozoites can turn into temporary flagellated forms which usually revert back to the trophozoite stage.  Trophozoites infect humans or animals by entering the olfactory neuroepithelium and reaching the brain.  N. fowleri trophozoites are found in cerebrospinal fluid (CSF) and tissue, while flagellated forms are found in CSF.
Acanthamoeba spp. and Balamuthia mandrillaris are opportunistic free-living amebae capable of causing granulomatous amebic encephalitis (GAE) in individuals with compromised immune systems.  Acanthamoeba spp. have been found in soil; fresh, brackish, and sea water; sewage; swimming pools; contact lens equipment; medicinal pools; dental treatment units; dialysis machines; heating, ventilating, and air conditioning systems; mammalian cell cultures; vegetables; human nostrils and throats; and human and animal brain, skin, and lung tissues.  B. mandrillaris however, has not been isolated from the environment but has been isolated from autopsy specimens of infected humans and animals.  Unlike N. fowleri, Acanthamoeba and Balamuthia have only two stages, cysts and trophozoites , in their life cycle.  No flagellated stage exists as part of the life cycle.  The trophozoites replicate by mitosis (nuclear membrane does not remain intact) .  The trophozoites are the infective forms and are believed to gain entry into the body through the lower respiratory tract, ulcerated or broken skin and invade the central nervous system by hematogenous dissemination .  Acanthamoeba spp. and Balamuthia mandrillaris cysts and trophozoites are found in tissue.  

Acanthemeba
Several species of free-living Acanthemeba are pathogenic to man. They normally reside in soil and can infect children who swallow dirt while playing on the ground. In normal individuals, the infection may cause mild disease (pharyngitis) or remain asymptomatic, but in immunodeficient individuals, the organism may penetrate the esophageal mucosa and reach the brain where it causes granulomatous encephalitis (figure 26).

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