Orthomyxoviruses /ParamyxovirusesGeneral characteristics:

Similarities:

Both orthomyxo and paramyxo viruses are RNA enveloped viruses.

Both have helical capsid symmetry.The term Myxo refers to the ability of these viruses to interact

with mucins , that are glycoproteins on the cell surface.Both have SS, linear and negative polarity RNA’s.

Differences:

The orthomyxoviruses have a segmented RNA genome (usually 8 pieces). Paramyxoviruses have their genome as a single piece.

Orthomyxoviruses are smaller than paramyxoviruses.

OrthomyxovirusesInfluenza viruses are the only members of this

family.These are important human pathogens as they

cause both outbreaks and pandemics (infrequently) that kill thousands of people.

Three kinds of Influenza viruses known:Influenza A virus causes worldwide epidemics.Influenza B virus causes major outbreaks of

Influenza.Influenza C virus cause mild respiratory tract

infections and no outbreaks.

Influenza virus genome is a segmented, SS, negative polarity RNA with a helical nucleocapsid and an outer lipoprotein envelope.

The virion contains its own RNA polymerase. The envelope has 2 different spikes, a hemagglutinin

(HA) and a neuraminidase (NA).The function of the HA is to bind to the cell surface

receptor usually a neuraminic acid or sialic acid; to infect the cell. The HA is highly antigenic and a target of the neutralising antibody.

The hemagglutinin functions at the beginning of infection.

The hemagglutinin agglutinates red blood cells, and this is the basis of diagnostic Hemagglutination inhibition test.

The neuraminidase cleaves neuraminic acid to release the progeny virus from the infected cell.

Neuraminidase functions at the end.Neuraminidase also degrades the protective

mucosal layer in the respiratory tract and enables the viruses to gain entry to the respiratory epithelial cells.

Changes in the antigenicity of hemagglutinin and neuraminidase confers on the Influenza A virus the ability to cause pandemics.

Two types of antigenic changes are known:

1) Antigenic shift that involves a major change based on the reassortment of segments of the genome RNA.

2) Antigenic drift signifies a minor change based on mutations in the genome RNA.

Thus, Influenza viruses have many serotypes.

Influenza A virus have 2 matrix proteins M1 and M2.M1 is located between the internal nucleoprotein

and the envelope and provides structural integrity.M2 protein forms an ion channel between the

interior and the exterior of the virus. It transports protons into the virion causing the disruption of the envelope. This leads to the uncoating of the virus and frees the nucleocapsid containing the RNA genome and allows it to migrate to the nucleus.

A non structural protein called NS-1, by its ability to inhibit the production of interferon mRNA reduces the host innate defence and is an important determinant of virulence of the virus.

Many species of animals have their own Influenza A viruses (aquatic birds, chickens, swine, horse etc…). These animal viruses are the source of the RNA segments that encode the antigenic shift variants and cause epidemics.

There are 16 types of HA (H1 to H16) and 9 types of NA (N1 to N9) found in water fowl.

3 types of HA (H1 to H3) and 2 types of NA (N1 and N2) are found in human viruses.

When two influenza A viruses from 2 different species infect the same cell, there can be an exchange and reassortment of the coding RNA segments leading to a new variant of the virus.

The H1N1 and H3N2 strains of Influenza A virus are the most common at this time and they are included in the current vaccine.

H2N2 caused a pandemic in 1957. H5N1 influenza A virus (avian influenza, bird flu) caused an outbreak in 1997. H1N1 influenza A virus (of swine origin, swine flu, Swine origininfluenza virus / S-oiv) ) caused an outbreak in 2009.

The spread from person to person is a major concern as the reassortment with human adapted strains would increase the spread dramatically.

Influenza B virus is only a human virus with no animal sources. Therefore does not undergo enough antigenic shifts to produce pandemics, but enough to change the strain. Hence, a new version of Influenza vaccine needs to be produced each year.

Influenza viruses have both Group-specific and Type-specific antigens.

The group-specific antigen includes the internal Ribonucleoprotein that distinguishes Influenza A,B and C viruses. Antibodies against group specific antigens do not offer protection against the infection.

The Type-specific antigens include the hemagglutinin and neuraminidase present on the cell surface. Antibodies against type-specific antigens offer protection. Antibodies against neuraminidase only decreases the amount of viral progeny released from the infected cell and thus reducing the sread.

Replicative Cycle:

After viral hemagglutinins interact with the surface receptors, the virus enters the cell in vesicles and uncoats mediated by the M2 proteins and is facilitated by the low pH within the endosome/vesicle.

The viral nucleocapsid enters the cytoplasm and migrates to the nucleus where the genome RNA (8 segments) gets transcribed into mRNA by the viral RNA polymerase (transcriptase).

Most RNA’s move to cytoplasm, some remain in the nucleus to serve as a template for the synthesis of negative polarity strand RNA genomes for the progeny, by a different subunit of viral RNA polymerase (replicase).

Newly synthesized NP protein and Matrix proteins bind to the progeny RNA genome in the nucleus and the entire complex is transported to the cytoplasm.

The helical NP protein (ribonucleoprotein)assembles, matrix protein mediates the interaction of the capsid with the envelope, and the virion progeny is released by budding off from the outer cell membrane at the site where the HA and NA are located. The viral progeny release is aided by the NA.

Influenza and retro viruses are the RNA viruses have their replication in the nucleus.

Transmission:Virus is transmitted by airborne respiratory droplets.

Pathogenesis:On inhalation of the virus, the NA degrades the

protective mucus layer and allows the virus to enter the cells of the upper and lower respiratory tract.

This is because the protease required to cleave the hemagglutinin into a modified hemagglutinin (that actually mediates attachment to the cell surface)is located in the respiratory tract. Infections are generally localized.

Systemic symptoms like severe myalgias are more due to circulating cytokines in the blood rather than the viremia as it rarely occurs.

Immunity depends on the secretory IgA in the respiratory tract and cytotoxic T-cells.

Clinical findings:

Include an incubation period of 24-48 hours.Symptoms include fever, myalgias, headache, sore

throat, and cough.Secondary bacterial pneumonia (usually due to

S.aureus) can lead to complications.Reye’s syndrome ( encephalopathy and liver

degeneration ) is a life threatening but rare complication (Aspirin given to reduce fever is generally implicated in the pathogenesis of Reye’s syndrome.)

Lab diagnosis:

Clinical diagnosis is sufficient.Virus detected in nasal/throat swabs/sputum using

direct fluorescent antibody tests, PCR or culture based tests

Rapid tests to detect viral antigens using monoclonal; antibodies against viral antigens or to detect neuraminidase, using a substrate for the enzyme that changes color when cleaved. Purpose of rapid tests is for effective treatment with neuraminidase inhibitors has to start within 48 hours.

Treatment:Oseltamivir (Tamiflu) and zanamivir both for

the treatment and prevention of Influenza. They are neuraminidase inhibitors and limits the release and thereby the spread of the infection.

Oseltamivir (Tamiflu) is given orally, but zanamivir is delivered by an inhalant directly into the respiratory tract.

Prevention:Both killed and live vaccine which contains

typically 2 strains of Influenza A virus (H1N1 and H3N2) and one strain of Influenza B virus are available.

The vaccine is reformulated every year to contain current antigenic strains.

Organisms grown in chick embryo, so individuals having severe hypersensitivity to eggs should not receive the vaccine.

The live vaccine contains temperature sensitive mutants of Influenza A and B viruses.

These mutants can grow only in cooler areas (33 degrees C) like the nasal mucosa producing the protective secretory IgA, and not in warmer places like the lower respiratory tract (37 degrees C). Vaccine sprayed in the nose and only immunizes without causing any disease.

Killed vaccines are poor immunogens, hence live vaccines preferred. Booster dose required every year.

People with chronic diseases, particularly respiratory and cardiovascular conditions, health care personnel, should receive the vaccine.

Paramyxoviruses

The Paramyxovirus family contains4 important human pathogens : measles virus, mumps virus, respiratory syncytial virus (RSV) and parainfluenza virus.

As the genome RNA is of negative polarity, they are non-infectious and carry their own RNA polymerase.

The envelope is covered with spikes,which contains a HA, NA or a fusion protein that causes cell fusion and sometimes hemolysis as well.

Measles virus.

The disease is characterized by a maculopapular rash and occurs primarily in childhood.

The nucleocapsid and genome RNA typical of paramyxo viruses, but their envelope spike are with HA activity and other, the cell-fusion type with hemolytic activity.

The replicative cycle is normal like other viruses.Transmission is through respiratory droplets

produced by coughing or sneezing.

The virus has the potential to cause epidemic, especially in populations not introduced or immunized to the measles virus.

Supplementation of Vitamin A greatly reduces the severity of the disease.

Pathogenesis:

Enters the blood and spreads to the skin after infecting the cells lining the upper respiratory tract and infecting the reticuloendothelial cells, where it replicates again.

The rash is primarily due to cytotoxic T-cells attacking the virus infected vascular endothelial cells in the skin.

Antibody mediated vasculitis may also be responsible for the rash.

Shortly after the rash appears (3-4 days) the virus can no longer be recovered and the patient cannot spread the virus.

Life long immunity occurs in infected individuals. Immunity primarily through cell-mediated immunity.

The virus binds to its receptor (called CD46) on the human macrophages and suppresses the production of IL-12 that is necessary for cell-mediated immunity. Thus, infection with measles virus transiently reduces cell-mediated immunity.

Clinical findings include incubation period of 10-14 days.

Characterized by fever, conjunctivitis that causes photophobia, running nose and cough.

Red lesions with a white, central dot seen on the buccal mucosa are known as Koplik’s spots and are diagnostic.

The maculopapular rash spreads through out the body in a few days and later on develops a brownish hue.

Encephalitis is a feared complication seen in 1:1000 cases with a permanent sequeale of deafness and mental retardation in 40% of such cases.

Measles in pregnant women leads to still birth rather than congenital abnormalities, as it causes a more severe disease in the fetus than rubella.

Lab diagnosis: cell culture, fluoroscent antibody staining, Antibody rise of 4-fold in subsequent test done after 10 days is diagnostic, PCR etc…

Clinical diagnosis is sufficient.

Prevention:Live attenuated vaccine given sub-cutaneously at 15

months of age, usually in combination with rubella and mumps vaccine.

As contains live virus, not for pregnant women and immunocompromised.

Immune globulin to unimmunized immunocompromised individuals early in the incubation period amy be necessary.

Mumps virus.

Mumps is characterized by the swelling of Parotid glands. It occurs primarily in childhood.

Typical of paramyxo, but has envelope spikes with both HA and Na activity aqnd other with cell-fusing and hemolytic activity.

Internal nucleocapsid protein present is the soluble S-antigen detected in the CFT.

Humans are the natural host.

Transmitted through respiratory droplets.Viruses infect the upper respiratory tract and

spreads through the blood to infect the parotid glands and spread to other organs such as testes, ovaries , pancreas etc…

Life long immunity after infection and the belief that unilateral mumps will be followed subsequently again on the other side is unfounded.

Clinical findings include incubation period of 18-21 days.Fever, malaise, and anorexia followed by tender swelling of

the parotid glands, either unilateral or bilateral.Complications such as Orchitis in post pubertal males, if

bilateral will lead to sterility.

Lab diagnosis:

Clinical diagnosis sufficient, Lab diagnosis for confirmation.Cell culture from specimen such as saliva, spinal fluid or

urine. 4-fold rise in subsequent titer and hemaagglutination

inhibition or the CFT that assays both the S-antigen and V(viral antigens).

S-antigen indiates current infection; V-antibody indicates, infection in the past.

Prevention:Live attenuated vaccine.Given at 15 months of age and a booster after

4-6 years.Not for immunocompromised and pregnant

women.

Respiratory Syncytial virus (RSV).

Most important cause of pneumonia and bronchiolitis in infants.

Important cause of otitis media in children and pneumonia in the elderly and patients with chronic cardio-pulmonary diseases.

Typically paramyxo, the envelope surface spikes are Fusion proteins alone. No HA and no NA present.

The fusion proteins cause the cells to fuse forming multinucleated giant cells (syncytia), hence the name.

Humans are natural host and 2 serotypes known.Transmission through respiratory droplets.RSV outbreaks occur every year in winter, unlike

other viruses causing cold.Pathogenesis is more severe in infants involving

the lower respiratory tract, than in older children and adults. The cause may be immunopathogenic due to the maternal antibody passed on by the mother which may react with the virus, form immune complexes and damage the respiratory tract cells.

Immunity is incomplete and the individual can have multiple infections.

IgA antibodies of the respiratory cells may have a protective effect as the person ages.

Clinical findings include lower respiratory tract diseases such as bronchiolitis and pneumonia in children, immunocompromised, elderly and cardio-pulmonary patients.

Lab diagnosis includes:

Rapid antigen test which is a enzyme immunoassay detecting the presence of RSV antigens in respiratory secretions.

Immunofluorescence on smears of the respiratory epithelium or by isolation in culture for further identification.

CPE of multinucleated giant cells in cell culture is a characteristic finding.

Treatment:Combination of aerosolized Ribavirin and

immuneglobulins more effective than ribavirin alone.

Parainfluenza virus:

Cause croup (acute laryngotracheobronchitis), laryngitis, bronchiolitis and pneumonia in children.

Typically paramyxo. Surface spikes have HA, NA (fused together on a single spike) and fusion proteins (separate).

Fusion proteins mediate formation of multinucleated giant cells.

Transmission, CPE, etc… same like other paramyxo. Antibody to the HA and Fusion protein neutralizes infectivity.

Pathogenicity:

Cause upper and lower respiratory tract diseases without viremia.

Parainfluenza 1 and 2 are major cause of Croup in children below 5 years of age, and characterized by harsh cough and hoarseness. Parainfluenza 3 in children and 4 causing mild cold like symptoms are also known.

Diagnosis mostly clinical. Cell culture, PCR for confirmation.

No antiviral therapy or vaccine is available.