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 INFECTIOUS DISEASE BACTERIOLOGY IMMUNOLOGY MYCOLOGY PARASITOLOGY VIROLOGY

VIDEO LECTURE

 

PARASITOLOGY - CHAPTER   SEVEN  

PART ONE

ARTHROPODS  

Dr Abdul Ghaffar and Dr Richard Hunt

 

 
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Logo image © Jeffrey Nelson, Rush University, Chicago, Illinois  and The MicrobeLibrary

 

  

All life cycle diagrams in this section are courtesy of the DPDx Parasite Image Library 
Centers for Disease Control (CDC)

PART ONE
ARTHROPODS

PART TWO
TICKS

Numerous species of arthropods play a role in human disease. Most of these are as vectors of different pathogens and, in other sections of this text, we have dealt with such disease vectors. There are also a number of arthropods that cause harm due their venom but these are not parasites. Here, we shall deal with arthropods that are parasitic to and cause disease in man.  For example, myiasis (burial of larvae in tissue) is an obligatory step in the life cycle of some flies and incidental for others. Species that cause myiasis in the Americas are Cochliomyia (Screw worm fly), Calliphora, Oestrus, Sarcophaga, Gastrophilus, etc. Myiasis may be cutaneous, arterial, intestinal or urinary in normal tissue or in pre-existing wounds, some of which may result from other infections. Larvae can burrow through necrotic or healthy tissue using their mandibular hooks aided by proteolytic enzymes. They can cause mechanical damage and the affected area may be the site of a secondary infection. Cutaneous myiasis may require surgical removal of burrowed larvae. Eggs and maggots may be washed from hair, skin and wounds with soap and water. Urinary myiasis usually clears itself. Purgation with anti-helminths may be necessary for gastrointestinal myiasis.
 

LICE

Three types of sucking lice are important for human health: Pediculus humanus capitis (head louse), P. humanus humanus (body louse) and Pthirus pubis (crab louse). Lice spend all of their life on one very specific host and both male and female feed on blood and leave one host only to transfer to another (figure 1and 2).

Head Lice

In the developed world, 2 to 10% of children are infested with head lice. Light infestations cause moderate itching of the scalp due to sensitization to louse saliva. Heavy infestations may result in fever, aches and secondary infections. Diagnosis is based on finding live lice or empty egg shells (nits measuring 0.8 x 0.3 mm) attached to hair, often behind the ears. Topical application of soothing lotions relieves irritation. Hair should be washed with shampoo containing 1% benzene hexachloride (Kwell). Application of a mixture of pyrentins (0.2%) and pipronyl butoxide (2%) or copper oleate may be as effective and less toxic than benzene hexachloride.

 

WEB RESOURCES
Head lice - CDC

    


Lice-lc.gif (16968 bytes)  Figure 1 
Life Cycle of the Head Louse

The life cycle of the head louse has three stages: egg, nymph, and adult.
Eggs: Nits are head lice eggs.  They are hard to see and are often confused for dandruff or hair spray droplets.  Nits are laid by the adult female and are cemented at the base of the hair shaft nearest the scalp  .  They are 0.8 mm by 0.3 mm, oval and usually yellow to white.  Nits take about 1 week to hatch (range 6 to 9 days).  Viable eggs are usually located within 6 mm of the scalp.
Nymphs: The egg hatches to release a nymph  .  The nit shell then becomes a more visible dull yellow and remains attached to the hair shaft.  The nymph looks like an adult head louse, but is about the size of a pinhead.  Nymphs mature after three molts ( ,  ) and become adults about 7 days after hatching .
Adults: The adult louse is about the size of a sesame seed, has 6 legs (each with claws), and is tan to grayish-white  .  In persons with dark hair, the adult louse will appear darker.  Females are usually larger than males and can lay up to 8 nits per day.  Adult lice can live up to 30 days on a person’s head.  To live, adult lice need to feed on blood several times daily.  Without blood meals, the louse will die within 1 to 2 days off the host.

 

Figure 2  

    

lice2.jpg (31572 bytes)  
A. Head Lice: Nit
Nits are head lice eggs. They are hard to see and are often confused for dandruff or hair spray droplets. Nits are laid by the adult female and are cemented at the base of the hair shaft nearest the scalp. They are 0.8 mm by 0.3 mm, oval and usually yellow to white. Nits take about 1 week to hatch (range 6 to 9 days). Viable eggs are usually located within 6 mm of the scalp. CDC

 

lice3.jpg (25863 bytes)  
B.  Head Lice: Nymph hatching through the operculum of an egg. The egg hatches releasing a nymph. The nit shell then becomes a more visible dull yellow and remains attached to the hair shaft.  The nymph looks like an adult head louse, but is about the size of a pinhead. Nymphs mature after three molts and become adults about 7 days after hatching. CDC

lice4.jpg (76813 bytes)  
C. Head Lice: Adult louse and two nymphs. The adult louse is about the size of a sesame seed, has 6 legs (each with claws), and is tan to grayish-white. In persons with dark hair, the adult louse will appear darker. Females lay up to 8 nits per day. Females are usually larger than males. CDC

pcapiaeg.jpg (45445 bytes) 
D. Pediculis humanus var. capitis adult and egg (nit) on hair shaft  © PeterDarben, used with permission
 

  lice5.jpg (63750 bytes)  
E. Female head louse, Pediculus humanus var capitis  CDC/Dr. Dennis D. Juranek ddj1@cdc.gov 

   
F. Head Louse on Human Hair - Pediculus humanus capitis  (SEM x55)
© Dennis Kunkel Microscopy, Inc.  Used with permission

   
G. Head Louse on Human Hair - Pediculus humanus capitis  (SEM x55) 
© Dennis Kunkel Microscopy, Inc.  Used with permission

   
H. Head Louse Egg Case on a Human Hair, Pediculus humanus capitis. (SEM x130) 
© Dennis Kunkel Microscopy, Inc.  Used with permission

 


Body Lice

The body louse is similar to the head louse except that it is found on the body and clothes. Diagnosis is based on finding eggs or nits (eggs) in seams of clothing. Symptoms and treatment are the same as those for head lice.

 

 

Pubic Lice

Pubic lice (figure 3), also known as crab lice, infest widely-spaced coarse hair in the pubic area in adults or eye lashes in children. Transmission in adults is usually by sexual contact. Diagnosis is based on finding lice or nits in the infested area; crab lice may be difficult to see at the base of the hair. The pubic area is treated in the same manner as the infested head. Nits and lice may be removed from eye lashes with forceps. Ointments with physostigmine (0.25%) or yellow mercury oxide are effective.

 

Figure 3

 
A. This patient presented with an infestation of Phthirus pubis, or crab lice (arrows). A Phthirus pubis infestation has caused the erythematous lesions seen in the pubic region of this patient in response to the bites of the crab lice arthropods
CDC/Joe Miller

  B. Pubic Louse - Phthirus pubis  (SEM x110)  © Dennis Kunkel Microscopy, Inc.  Used with permission

    C. Pubic Louse, Pthirus pubis  (SEM x130)  © Dennis Kunkel Microscopy, Inc.  Used with permission

 ppubiadt.jpg (22072 bytes)  D. Pubic Louse, Phthirus pubis adult © PeterDarben, used with permission

   F. Phthiriasis pubis (Pubic lice) on eyelids  © Bristol Biomedical Image Archive. Used with permission

    

 


FLEAS

Most fleas (figure 4) are of clinical significance to man because they are vectors for other parasites. However, the jigger flea or chigoe (Tunga penetrans) is a serious pest in the tropical and subtropical regions of the Americas and Africa. Diagnosis of tungiasis is rare in North America. An epidemiological study in a traditional fishing village in Ceará State, north-eastern Brazil found about 51% of the population was infested

Both sexes feed on blood. The female flea, after insemination, burrows itself in the skin of the toes and the sole of the foot. The female swells to the size of a pea, produces eggs and dies in the tissue. There is local reaction to the bite and the eggs and dead flea produce reaction. The infested tissue can get infected and gangrenous; auto-amputation is not uncommon. Treatments are symptomatic: infestation may be physically removed; secondary infections are treated appropriately. Shoes should be worn in infested areas.

Other names for Tunga penetrans

 

 Figure 4  

flea1.jpg (24558 bytes)  
A. Scanning Electron Micrograph of a Flea  CDC/Janice Carr  

sifo13cc.jpg (60048 bytes) 
B. The impregnated female Tunga embeds itself in the skin under the toenails and fingernails of man - where the resultant sores may fill with pus and become infected .  ©
Marcelo de Campos Pereira, PhD, University of Sao Paulo. Used with permission

sifo7.jpg (104245 bytes) 
C. Tunga penetrans is known as the chigger, jigger, chigoe, bicho do pé or sand flea. The head is angular, it has no comb of spines, and the thoracic segments are narrow at the top.The female feeds by burrowing into the skin of its host. The abdomen becomes enormously enlarged between the second and third segments so that the flea forms a round sac with the shape and size of a pea  ©
Marcelo de Campos Pereira, PhD, University of Sao Paulo. Used with permission

tunga01.jpg (16591 bytes)
F. Tungiasis from a dog.  Tunga penetrans may also pose significant problems in dogs but infestation tends to be sporadic,geographically isolated,or related to special environmental circumstances. The impregnated Tunga female attaches firmly to the dog,usual areas being the interdigital spaces,under the pads, and the scrotum. The presence of a number of adult T.penetrans in the the paws can be crippling, and the damage to the skin can facilitate the entry of other pathogens leading to secondary infection and ulceration ©  Marcelo de Campos Pereira, PhD, University of Sao Paulo. Used with permission

    

 

 

 

MITES

Scabies mite (Sarcoptes scabei) (figure 5) is the cause of scabies and is distributed worldwide. Epidemics of the disease may occur for long periods but mites may be common at all times in very poor communities with inadequate washing facilities. The mite transmitted by contact burrows into the skin on the webbing side of fingers, later spreading to the wrists, elbows and the rest of the body (figure 6 and 7). The buttocks, women's breasts and external genitalia may be involved. The mite tunnels itself through the upper layer of the skin depositing eggs. Larvae escape the tunnel and wander on the skin and start new burrows and mature there to continue the cycle. Scabies itch is due to the sensitization of the patient to the mite and eggs and is characteristically nocturnal. Septic pustules may develop after scratching, if the hygiene is poor. Diagnosis is made by the characteristic rash and by smearing black ink on the skin and observing burrows when the ink is wiped away. Microscopic examination of a skin scraping shows the mites. Treatment involves swabbing of the whole body from the neck down with 1% malathion or benzene hexachloride (crotamiton for infants). Topical steroids must not be used. If possible, the whole family should be treated. Contact with an infested person should be avoided. Clothes should be washed in hot water.

 

Figure 5     

mite1.jpg (37383 bytes)  
A. Sarcoptes scabei mite. Females are 0.3 to 0.4 mm long and 0.25 to 0.35 mm wide. Males are slightly more than half that size. CDC   


scabies03.jpg (502537 bytes)  B. Microscopic view of Sarcoptes scabiei  © Bristol Biomedical Image Archive. Used with permission

sscabads.jpg (49297 bytes) C. Sarcoptes scabei adult, whole and in section of skin (H&E)    © PeterDarben, used with permission
 

Figure 6     

Scabies-lc.gif (37801 bytes)
Life cycle of  the Scabies mite (Sarcoptes scabei)
Sarcoptes scabei
undergoes four stages in its life cycle; egg, larva, nymph and adult.  Females deposit eggs at 2 to 3 day intervals as they burrow through the skin  .  Eggs are oval and 0.1 to 0.15 mm in length  and incubation time is 3 to 8 days.  After the eggs hatch, the larvae migrate to the skin surface and burrow into the intact stratum corneum to construct almost invisible, short burrows called molting pouches.  The larval stage, which emerges from the eggs, has only 3 pairs of legs  , and this form lasts 2 to 3 days.  After larvae molt, the resulting nymphs have 4 pairs of legs  .  This form molts into slightly larger nymphs before molting into adults.  Larvae and nymphs may often be found in molting pouches or in hair follicles and look similar to adults, only smaller.  Adults are round, sac-like eyeless mites.  Females are 0.3 to 0.4 mm long and 0.25 to 0.35 mm wide, and males are slightly more than half that size.  Mating occurs after the nomadic male penetrates the molting pouch of the adult female  .  Impregnated females extend their molting pouches into the characteristic serpentine burrows, laying eggs in the process.  The impregnated females burrow into the skin and spend the remaining 2 months of their lives in tunnels under the surface of the skin.  Males are rarely seen.  They make a temporary gallery in the skin before mating.
Transmission occurs by the transfer of ovigerous females during personal contact.  Mode of transmission is primarily person to person contact, but transmission may also occur via fomites (e.g., bedding or clothing).  Mites are found predominantly between the fingers and on the wrists.  The mites hold onto the skin using suckers attached to the two most anterior pairs of legs.

 

Figure 7     

mite.jpg (399398 bytes)  A. Child's arm exhibiting papular rash due to Sarcoptes scabiei © Bristol Biomedical Imafe Archive. Used with permission

 

 

mite2.jpg (89211 bytes)  B. Scabies lesions are caused by Sarcoptes scabei burrowing under the skin. A typical location is on the hands, particularly the webbing between the fingers, as shown in this image. CDC

mite3.jpg (91270 bytes)  C. Cross sectional view of the burrows created in the epithelium by Sarcoptes scabei. CDC

 scarc-darb.jpg (49297 bytes) D. Sarcoptes scabei adult, whole and in section of skin (H&E) © Dr Peter Darben, Queensland University of Technology clinical parasitology collection. Used with permission

Figure 8A
24hrs after a chigger bite, genus Trombicula, this patient presented with a red welt or bleb at the bite site.
These chiggers  can cause intense itching and reddish welts on skin, but are not known to transmit infectious diseases in the U.S. However, in some Asian countries, they are vectors of scrub typhus. CDC/M. A. Parsons

Figure 8B
Chigger (adult) actual size 1/128 inch. Clemson University, South Carolina
Figure 8C
Chigger mite © Dennis Kunkel Microscopy

 


CHIGGERS

Chiggers (the larvae of red mites or harvest mites - figure 8) (Trombiculidae) are an important group of ectoparasites affecting humans. They attach to the skin in the ankles, waistline, armpits and perianal area after the host walks through a grassy environment. Contrary to popular belief, these mite larvae do not feed on blood and do not burrow into the skin. They pierce the skin near a hair follicle and feed on partially digested skin cells using enzymes in the chigger's saliva. They then drop off the host. The host reacts to the mouth parts and saliva of the mite, however, and after a few hours an erythematous papule appears (figure 8) that is highly pruritic. The intensity of the eruption depends on the sensitivity of the host and may be followed by fever. 

Treatment with a local anesthetic is useful. Insect repellents (DEET) may be effective in avoiding chigger bites.

In south Asia, chiggers are the vectors for scrub typhus (Orientia tsutsugamushi), a rickettsial disease that can (rarely) be life-threatening.

See also Bacteriology Chapter 21: Rickettsia

Figure 9A.
First instar larva of Cuterebra, The north American botfly which causes warbles in a variety of mammals but does not usually infect humans. CDC/Dr. George Healy

Figure 9B
Mosquito bite with open lesion as a result of a bot fly infection

Figure 9C
The bot fly larva can be seen in the lesion at the top left

Figure 9D
The bot fly removed after application of vaseline for 24 hours

BOT FLY AND TUMBU FLY -  Myiasis

Myiasis is the parasitism of a vertebrate host by the larva of a dipteran fly. It frequently occurs in domestic and wild animals and therefore is important in veterinary medicine. Human cases are rare but occur especially in tropical countries. In the United States, the most common agents of myiasis are Dermatobia hominis (bot fly, berne) and Cordylobia anthropophaga (tumbu fly). The former occurs in central (particularly Mexico and Belize) and south America and some Caribbean islands such as Trinidad, while the latter occurs in tropical Africa. The larvae of both of these flies are obligate parasites of mammals.

The human bot fly has a very interesting life cycle. The female fly captures a mosquito and lays its eggs on its abdomen. When the released mosquito takes a blood meal (usually on the face, scalp or extremities), the body heat of the mammal causes the eggs to hatch into a first instar larva, which drops onto the skin of its host. The larva either enters the skin via the puncture wound made by the mosquito or actively penetrates the skin where it takes up residence in sub-cutaneous tissue. Here, the larva matures through second and third instars over a period of one or two months. The larvae are difficult to remove because of backward facing barbs (figure 9). When they are mature, the larvae emerge and fall to the ground where they pupate into the adult fly and the life cycle repeats. The adult fly only lives a few days and does not feed. 

The initial symptom of the presence of a bot fly is a cutaneous nodule that contains one larva (although this is not initially visible). The nodule is often called a warble or a furuncle and myiasis is often referred to as a furunculous disease. Unlike a puncture wound of a mosquito, the infected nodule discharges blood or serum continually because the larva needs to keep the wound open in order to breath. There is often pruritus and sometimes intense periodic shooting pain that occurs when the larva moves or matures to another instar. As the larva grows, movement can often be seen below the surface of the serosanguineous effusion. There may also be local lymphadenopathy, and fever and malaise if a secondary bacterial infection develops. 

Treatment entails removal of the larva, although some patients prefer to allow the larva to develop and emerge naturally. In the case of Dermatobia hominis, the Achilles heel is the need for the larva to breath air though the open wound. Cutting off the parasite's air supply is most often done by applying a thick layer (at least 5mm) of Vaseline, although nail polish, adhesive tape and even bacon have been reported to be used. Because of the lack of oxygen, the larva will usually emerge and can be pulled the remaining way out with forceps. However, it often takes a day for the larva to come far enough out to be grabbed with forceps. Surgery is not usually necessary unless the larva dies in situ and cannot be removed. 

Tumbu fly (Cordylobia anthropophaga) is found in sub-Saharan Africa and causes a furuncular myiasis similar to bot fly (figure 10). The patient comes in contact with eggs on the ground or deposited on clothes. After hatching, the larva burrows into the skin. The eggs are killed by ironing clothes but for many people that may be impractical.

 

VIDEO
Removal of a bot fly larva from the scalp at the London Hospital for Tropic Medicine

Infectious Diseases Case of the Month

Figure 9 E  
Dermatobia hominis
  (Bot fly) Frontal view.
Bot fly is a bluebottle-like fly with yellow to orange head and legs. The thorax is dark blue with a greyish bloom; the abdomen is short and broad and has a brilliant blue color. Adults have atrophied mouthparts and do not feed, relying instead on food reserves accumulated during the larval stage.
 ©
Marcelo de Campos Pereira, PhD, University of Sao Paulo. Used with permission

Figure 9F  
Dermatobia hominis
  Female lateral view
© Marcelo de Campos Pereira, PhD, University of Sao Paulo. Used with permission

Figure 9G  
Eggs of Dermatobia hominis
Eggs are glued to the abdomen of a carrier fly.  When the Dermatobia female is ready to oviposit, she captures another insect - usually a fly - and glues her eggs to the captured insect's abdomen. © Marcelo de Campos Pereira, PhD, University of Sao Paulo. Used with permission

Figure 9H    
Dermatobia hominis
pupa
Most mature larvae drop during early morning hours. They burrow into upper soil or debris for about 20 min and form a hardened puparium in 2-3 days.
©
Marcelo de Campos Pereira, PhD, University of Sao Paulo. Used with permission

Figure 10A
Furuncular breast lesion with multiple sinuses containing Tumbu fly larvae.
© 2004 Adisa and Mbanaso; licensee BioMed Central Ltd. This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original URL: http://www.biomedcentral.com/1471-2482/4/5

Figure 10B
Tumbu fly larvae after removal from patient   © 2004 Adisa and Mbanaso; licensee BioMed Central Ltd. This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original URL: http://www.biomedcentral.com/1471-2482/4/5
 

CASE REPORT
Furuncular myiasis of the breast caused by the larvae of the Tumbu fly (Cordylobia anthropophaga)
 

VIDEO
Removal of tumbu fly larvae from infected breast

This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original URL: http://www.biomedcentral.com/1471-2482/4/5
 

 

 

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