INFECTIOUS DISEASE

VIDEO LECTURE

VIROLOGY - CHAPTER  SIXTEEN

PARAINFLUENZA, RESPIRATORY SYNCYTIAL AND ADENO VIRUSES

Dr N. Narayan and Dr Joseph Horvath

Parainfluenza viruses are important viral pathogens causing upper and lower respiratory infections in adults and children. They are second to respiratory syncytial virus cause of lower respiratory tract disease in young children.

Family Paramyxoviridae

Genus Members

Paramyxovirus - Parainfluenza [PIV types 1,2,3,4]

Rubulavirus - Mumps virus

Newcastle Disease Virus [birds]

Sendai virus [mice]

Morbillivirus - Measles virus

Canine Distemper Virus

Pneumovirus - Respiratory Syncytial Virus (RSV)

Metapneumovirus

WEB RESOURCES

Big Picture Book of Viruses
Paramyxoviruses

CDC Parainfluenza information

Figure 1. Structure of a paramyxovirus

Figure 2.  Paramyxovirus © Dr Linda Stannard, University of Cape Town, South Africa (used with permission) 

Parainfluenza viruses are relatively large viruses of about 150-300 nm in diameter. They have a spherical or pleomorphic shape (figure 1 and 2). The RNA is negative sense, unsegmented and single stranded (ss). The nucleocapsid core is filamentous or herringbone-like, has helical RNA tightly associated with Nucleoprotein (NP) Phosphoprotein (P) and Large protein (L)

These are enveloped viruses with a host-derived lipid bilayer associated with two virus-specific glycoproteins:

Hemagglutinin-Neuraminidase (HN). This is a viral attachment protein, that also causes hemadsorption and hemagglutination.

Fusion protein (F). The F protein forms spikes out from the envelope. It promotes the fusion of host and viral cell membranes which is an initial step in infection. It is synthesized as a biologically inactive form (F₀), which is activated by proteolytic cleavage to an active form that has 2 subunits, F₁ and F₂, linked by a disulfide bond.

Matrix (M) protein, located just within the envelope, is hydrophobic

Cell lines such as primary Rhesus monkey kidney epithelial Cells (PRMK), LLC-MK-2, and human embryonic kidney cells are used. Cytopathic effects occur such as rounding, bridging, cell lysis, and syncytium formation.

Hemadsorption (due to the interaction of viral hemagglutinin with specific erythrocyte receptors on guinea pig red cells) can be observed at 4° C. This may be seen even before the appearance of cytopathic effects and has been used for early diagnosis (especially PIV-1 and PIV-3).

The first step in the infection cycle involves attachment of the virus to host cell sialic acid receptors. This is mediated by viral attachment protein, a function served by the HN glycoprotein.

Next, the F protein catalyzes fusion of the viral envelope and host cell membrane, resulting in uncoating and release of the nucleocapsid structure into the host cell cytoplasm.

For transcription and protein synthesis to occur, first mRNA is formed with the help of RNA-dependent RNA polymerase which must be supplied by the virus. The polymerase function is carried out by the P and L proteins, and possibly also the NP. The genome is replicated by formation of a full-length positive sense RNA template onto which a negative sense RNA is then transcribed.

Assembly of the nucleocapsid occurs and M proteins are then associated with the viral glycoprotein modified cell membranes. Mature virions are released from host cell membranes by budding.

The virus is ubiquitous and most people have had a primary infection by the age of five; infections occur as epidemics as well as sporadically. There can be repeated infections throughout life.

Parainfluenza viruses are sensitive to detergents and heat but can remain viable on surfaces for up to 10 hours.

Transmission occurs via the following routes:

Large droplets - person to person through close contact

Aerosols of respiratory secretions

Fomites (virus survives on surfaces)

Figure . Weekly reports of parainfluenza type 1 in the US. Seasonal variation. CDC

Figure . Weekly reports of parainfluenza type 2 in the US. Seasonal variation. CDC

Figure . Weekly reports of parainfluenza type 3 in the US. Seasonal variation. CDC

Primary infections and re-infections occur but most infections are asymptomatic, especially in older children and adults. The incubation period is 2 to 6 days. Reinfections are clinically less severe, most commonly involve the upper respiratory tract and occur throughout life.

Fever and a spectrum of respiratory infections are caused by PIVs:

• Rhinorrhea/rhinitis, pharyngitis, cough, croup (laryngotracheobronchitis), bronchiolitis, and pneumonia.
   
• Croup - the subglottic region becomes narrower and results in difficulty with breathing, a seal bark-like cough and hoarseness.

PIV types 1 and 2 most often cause outbreaks of croup in autumn/early winter, with an alternate year pattern. PIV-1 tends to attack children ages 2-6 years.

PIV-3 can cause croup, though less commonly than PIV-1 and 2 and is sporadic. It often occurs in the spring and summer. Primary infection with PIV 3 in young infants and children of less than two years of age is a common cause of bronchiolitis (although RSV is a more common cause).

PIV-4 is associated with mild upper respiratory infections. The upper airway is more often involved than the lower airway. There are two types of PIV4: A and B

Otitis media, parotitis, aseptic meningitis occur although they are rare.

Particularly severe and persistent infections are known to occur in immunocompromized children and adults in whom prolonged viral shedding is seen.

Antigen detection

Radio-immunoasay, enzyme immunoassay, fluoro-immunoassay, and immunofluoresence methods are used for antigen detection. Nasopharyngeal secretions are collected, from swabs or washings, and transported in viral transport medium and on ice. These techniques are 70-90% sensitive.

Shell vial assay is useful in detecting growth in 4-7 days. Hemadsorption can be noted before cytopathic effects. Immunofluoresence is confirmatory.

Antibody Detection

Serology uses hemagglutinin inhibition to demonstrate a difference between acute and convalescent levels. A 4-fold increase in antibody titers is considered positive. However, serologic diagnosis is of limited value because of the presence of nonspecific inhibitors and the antibody being heterotypic  (antibody that is common to different PIV types as well as the mumps virus)

There is no specific treatment. Supportive treatment for croup includes humidification of air and racemic epinephrine (Racemic epinephrine is a 1:1 mixture of the dextrorotatory and levorotatory isomers of epinephrine. The L form is the active component) . Corticosteroids may be used in moderate to severe cases.

Immunity following infection is short lived. The role of antibody is not clear since reinfection has been seen even with high levels of antibody. Cell-mediated immunity is probably more important for limiting infection.

Infection control

Asymptomatic shedding is common, making it difficult to contain spread of infection. Hand washing and preventing contamination of surfaces with respiratory secretions are important for limiting nosocomial spread.

Classification and structure

Family Paramyxoviridae, genus Pneumovirus.

Infection of cells by RSV often results in syncytium formation, hence the name. The virus was first discovered in chimpanzees (Chimpanzee Coryza Virus) and accidental infection in humans led to its recognition as a human pathogen.

These are spherical or pleomorphic enveloped viruses (100-350 nm) with single-stranded, negative sense linear RNA. There are two non-structural and eight structural proteins.

The envelope has 2 glycoproteins: F protein, the fusion protein, is important for fusion of viral particles to target cells and fusing infected cells to neighboring cells to form syncytia. G protein, which is highly glycosylated, is important for viral attachment to host cells. Antigenic variations in the type of G protein determine the subgroup (A or B). RSV lacks H/N proteins, unlike other members of the family Paramyxoviridae.

These viruses survive on surfaces for up to 6 hours, on gloves for  less than 2 hours. They rapidly lose viability with freeze-thaw cycles, in acidic conditions and with disinfectants.

Pathology and Pathogenesis

RSV attaches (via the G protein) to cells of nasal mucosa and upper respiratory tract. The F protein allows fusion of the viral envelope with the host cell plasma membrane. The virus can also infect the eye. Infected cells may undergo necrosis and syncytia form through cell-cell fusion (which is often cell with cultured cells). Cell to cell transfer of virus leads to spread from upper to lower respiratory tract.

Mucosa edema occurs and there is increased mucin secretion. There is also cell necrosis that leads to sloughing of debris. Smaller airways (bronchioles) become plugged with debris and mucin; bronchoconstriction also occurs. Peribronchial lymphocytes may infiltrate the tissue.

The host immune response also induces some of the pathological changes. IgE response in some individuals is linked to airway hyper-reactivity. Cell-mediated immunity and humoral response limits the severity of the infection.

RSV has a worldwide distribution and and is an important cause of lower respiratory tract disease in young infants. Most children have had an RSV infection by age 4 years. RSV is the most frequent cause of bronchiolitis but is an infrequent cause of croup.

Out breaks are seasonal occurring from late fall through spring (November to May), the virus being  transmitted via large droplets, through fomites and via the hands. RSV can survive on surfaces for up to six hours. Viral shedding continues for  less than 1 to 3 weeks but longer in immuno-compromised hosts.

75,000 to 125,000 infants are hospitalized each year in the US because of RSV infections. These account for 50 to 90% of hospitalizations for bronchiolitis.

Nosocomial spread is common. Viral shedding can last for up to three weeks and infants can show a high titer of shed virus, especially initially (10⁷ viral particles per ml). Asymptomatic viral shedding is also observed. There is prolonged shedding in immunocompromized individuals.

  Figure .
Transmission electron micrograph of respiratory syncytial virus. Long
filamentous form. CDC/Dr. Erskine Palmer 

Morphologic traits of the Respiratory Syncytial Virus. The virion is variable in shape, and size (average diameter of between 120-300nm)
 

WEB RESOURCES

CDC RSV information

RSV in a child-care situation (CDC)

Figure . Weekly reports of RSV isolation in the US

Figure . Section of lung: acute pneumonia, epithelial syncytia formation in alveoli, respiratory syncytial virus
infection, calf pneumonia © Bristol Biomedical Image Archive. Used with permission

The incubation period is 4 to 6 days (range: 2 to 8 days). First, there is an upper respiratory infection (‘bad cold’) in older children and adults with clinical features of fever, rhinitis, pharyngitis. Lower respiratory infection (bronchiolitis and/or pneumonia) may occur after  the upper respiratory infection and results in the clinical features of cough, tachypnea, respiratory distress, hypoxemia, cyanosis. The cough can persist for 3 weeks.

In young infants one may observe  apnea, lethargy, irritability, poor feeding, otitis media and croup.

Radiological examination may show atelectasis, streaking, hyperinflation and perihilar infiltrates, especially in the right middle and upper lobes.

Severe infections occur in pre-term infants (especially less than 35 weeks gestation and those with chronic lung disease), children with cyanotic congenital heart disease, and immunocompromized hosts. There are up to 3000 deaths per year in the United States.
 

Nasal washings, nasal aspirates or swabs should be transported on ice and processed immediately. Rapid diagnosis is carried out using  DFA, IFA, ELISA.

Viral culture is carried out in cell lines such as HeLa, Hep-2, Monkey Kidney cells. Cytopathic effects are usually seen in 2-5 days. Shell vial technique with immunofluorescence is useful. 

Treatment

Treatment is usually supportive by the provision of fluids, oxygen, humidification of air, respiratory support. Steroids and bronchodilators have not proved useful.

Ribavirin (Virazole) (see chemotherapy section) , a guanosine analogue (aerosol) has been used with some efficacy but is used  only in persons at high risk for severe disease (premature and immunocompromized infants).


Immunity

Humoral immunity 

Neutralizing antibody to F and G proteins, IgA is also produced. The level of neutralizing antibody does not correspond to neutralizing activity. Immunity is short lived, therefore reinfections are common.

Newborns may have some innate immunity

IgE response occurs in some individuals and may be a marker for future airway hyper-reactivity.

Cell mediated immunity

 This is carried out by T cells. Cytokine production also contributes to illness.

Hand washing is important as is isolation and cohort nursing. Health care providers should wear protective gear, i.e.  gowns, gloves, masks and goggles

Immunization

Active immunization
The inactivated vaccine is no longer used because it was associated with an increase in severity of disease. Other subunit vaccine candidates are in trial phases.

Passive immunoprophylaxis
There have been some encouraging results from trials using pooled hyperimmune globulin (RespiGam) as monthly injections to susceptible infants during the RSV season. Now, a monoclonal antibody against F protein has been synthesized (Palivizumab - marketed as Synagis). It is used to prevent disease in children who are at risk for severe RSV infection. Neither of these approaches is yet proven, however. Synagis is very costly but can reduce hospitalizations by 50%; however, there is no difference in mortality.

 

WEB RESOURCES

Synagis information

Synagis package insert (pdf file)

Synagis Product monograph 
(pdf file)

RespiGam package insert (pdf file)

Metapneumovirus is ubiquitous and, by the age of five, most people are seropositive and have thus been infected by the virus. Many infections are asymptomatic but the virus can cause both upper and lower respiratory tract infections with symptoms of a cold, otitis media, pneumonia or bronchitis. There can also be pneumonia in marrow recipients that can possibly be fatal.

There are distinct epidemics in the winter months. There are two main HMPV types (A and B), each with 2 subtypes (A1, A2; B1, B2) and allfour of these circulate in the population with the dominant strain varying.

Metapneumovirus can be detected by PCR but there is no commercial available testing.

READING

Medical Microbiology, 3^rd Ed.-1998; Mosby (Murray et al)

Manual Of Clinical Microbiology, 6^th Ed.-1995; ASM Press (Murray, Baron, Pfaller)

2000 Red Book; American Academy of Pediatrics

Textbook of Pediatric Infectious Diseases, 4^th ed. 1998 (R.D. Feigin and J. D. Cherry)

They belong to family Adenoviridae, genus Mastadenovirus.

Adenoviruses are further classified into 6 subgroups (A through F), based on hemagglutinating properties and DNA homology.

About 47 serotypes have been isolated from humans.

Types 40, 41 belong to subgroup F and are enteric pathogens.

Common serotypes are 1 -  8, 11, 21, 35, 37, and 40.

Figure Structure of adenovirus

Adenovirus ©t Dr Stephen Fuller, 1998 

These are non-enveloped viruses with a diameter of 70-90nm.

The genome is made of linear double-stranded (ds) DNA with 2 major proteins.

The capsid is icosahedral, comprised of 252 capsomeres. 240 are hexons; at the vertices are 12 pentons, from which a fiber with a terminal knob projects. This complex is toxic to cells - causing rounding and death of cells through inhibition of protein synthesis. The fiber proteins determine target cell specificity.

10 structural proteins are known.

Adenovirus © Dr Linda M Stannard, University of Cape Town, South Africa, 1995 (used with permission).

Figure . Transmission electron micrograph of adenovirus  
CDC/Dr. G. William Gary, Jr.

Virus primarily attacks mucoepithelial cells of the conjunctiva, respiratory tract, gastrointestinal and genitourinary tracts. Attachment to host cell receptor occurs via the fiber protein. The virus replicates in the cytoplasm of host cells, but viral DNA replicates within the host cell nucleus. Early and late phases of replication occur, followed by assembly and release of virions.

Three types of infections occur in target cells:

Lytic - cell death occurs as a result of virus infection (mucoepithelial cells)

Latent / persistent / occult - virus remains in the host cell, which is not killed (lymphoid tissues such as tonsils, adenoids, Peyers patches)

Oncogenic transformation - cell growth and replication continue without cell death. This is seen in hamsters, most often with group A viruses (see oncogenic virus section).

Adenovirus also replicates in associated lymphoid tissues, and subsequent viremia can cause secondary infection in visceral organs.

Inefficient (error-prone) replication of the virus results in many excess antigenic components. These are liberated into the culture fluid in vitro as soluble antigens and lead to formation of basophilic staining intra-nuclear inclusion bodies in cells.

Adenoviruses are stable in the environment and to low pH, bile, and proteolytic enzymes - These properties make it possible for them to replicate to high titers in the GI tract.

WEB RESOURCES

CDC Adenovirus information

Almost half of adenoviral infections are subclinical

Most infections are self-limited and induce type-specific immunity

Incubation period is 2-14 days; for gastroenteritis usually 3-10 days

Different clinical syndromes have been described:

Eye

Epidemic Keratoconjunctivitis (EKC), acute follicular conjunctivitis, pharyngoconjunctival fever

Respiratory system

Common cold (rhinitis), pharyngitis (with or without fever), tonsillitis, bronchitis, pharyngoconjunctival fever, acute respiratory disease (LRI), pertussis-like syndrome, pneumonia- sometimes with sequelae

Genitourinary

Acute hemorrhagic cystitis, orchitis, nephritis, oculogenital syndrome

Gastrointestinal

Gastroenteritis, mesenteric adenitis, intussusception, hepatitis, appendicitis. Diarrhea tends to last longer than with other viral gastroenteritides

Rare results of adenovirus infections include- Meningitis, encephalitis, arthritis, skin rash, myocarditis, pericarditis, hepatitis. Fatal disease may occur in immunocompromised patients, as a result of a new infection or reactivation of latent virus

Endemic, epidemic and sporadic infections occur. Outbreaks have been noted in military recruits, swimming pool users, residential institutions, hospitals, day care centers etc.

Transmission: Droplets, fecal-oral route (direct and through poorly chlorinated water), fomites

Many infections are subclinical

Infections are most communicable in the first few days of illness, however infective period continues since clinical infection may be followed by intermittent and prolonged rectal shedding

Secondary attack rate within families: up to 50%;

Adenovrius outbreaks are seasonal: Respiratory disease mainly occurs in late winter through early summer. Pharyngoconjunctival and EKC infections occur in the summer months while GI disease does not seem to be seasonal

Clinical specimens, such as swabs (nasopharyngeal, conjuncticval, rectal, or other) and washings, corneal scrapings, stool, urine or biopsy and autopsy materials etc. should be transported in viral transport medium.

Viral Isolation in cell cultures is carried out in HeLa, human embryonic kidney (HEK) and human fetal diploid cells (HDFL). A549 cells lines are used for types 1-39.

Subgroup F (serotypes 40, 41) do not grow well in these cell lines, but do grow in Graham-293 (a modified HEK cell line).

Shell vial culture technique aids in faster detection.

Cytopathic effects include swelling and rounding of cells. Cells may become refractile and clustered into irregular clumps.

Isolation of virus from a pharyngeal specimen is more suggestive of a current clinical infection than from fecal specimen.

Rapid detection of enteric types (serotypes 40, 41) is by ELISA or immunofluorescnece antibody. Immune EM (aggregation with sera) may also be used

Other detection methods in current use include electron microscopy, polymerase chain reaction and nucleic acid probes.

Serology is mainly used for epidemiologic studies

Handwashing

Contact precautions, respiratory precautions in health care settings

Adequate chlorination of swimming pools

Sterilization / disinfection of ophthalmologic equipment and use of single dose vials of ophthalmic medications

Vaccine:  live, enteric coated, oral vaccine (types 4, 7, 21)

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