Paramyxoviruses

GENERAL CONCEPTS:

The Paramyxoviruses are an important cause of respiratory disease in children. Illnesses include croup, bronchiolitis and pneumonitis.
This group of viruses share similar features; they possess a double layered envelope with spikes, have a helical symmetry and contain a negative stranded RNA genome. An RNA-dependent RNA polymerase (RDRP) is contained within the virus particle.
Paramyxoviruses replicate in the cytoplasm and are released by budding.
Virus-specific antigens include those associated with the envelope and those contained within the nucleocapsid.
The host range for the Paramyxoviruses includes humans and monkeys.
These viruses produce multinucleated giant cells (syncytia) by production of a cell fusing factor, and then cause host cell lysis.

DISTINCTIVE PROPERTIES:

Parainfluenza: About 30-40% of acute respiratory infections in infants and children can be attributed to the Parainfluenza virus. Disease ranges from mild and cold-like to life-threatening (croup, bronchiolitis and pneumonia). Parainfluenza viruses are the most common cause of croup and there are 4 serotypes, numbered 1-4.
Mumps: A common acute disease of children, the mumps virus produces inflammation of salivary glands leading to obvious enlargement. Some severe manifestations can result from infection but there is only one serotype.
Measles: Another acute childhood disease, measles virus commonly causes a fever with a rash, occasionally producing conjunctivitis and, sometimes, pneumonia. More serious complications include encephalitis and subacute sclerosing panencephalitis (SSPE). Only one serotype exists and disease is limited to humans and monkeys.
RSV: Respiratory Syncytial virus is a major cause of bronchiolitis and pneumonia in infants under 1 year. Reinfection in adults usually involves the upper respiratory tract. The viruses produce a characteristic syncytia formation, hence the name. There is only one serotype that affects humans.

PATHOGENESIS:

Parainfluenza: These viruses generally produce local infections in the upper and lower respiratory tract. The viruses implant in ciliated epithelia of respiratory tract (nose and throat). The virus can be shed over 3-16 days and the main pathologic response is inflammation. The most important (i.e. serious) diseases are croup, bronchiolitis and pneumonia. The severe diseases occur most often with types 1 and 2.
Mumps: Mumps is a systemic infection spread by viremia. The major targets include glandular and nervous tissue. The virus enters via the pharynx or conjunctiva, there is local multiplication followed by viremia. A secondary viremia disseminates the virus to salivary glands, testes, ovaries, pancreas and the brain. The incubation period is 18-21 days and disease is asymptomatic in about 35% of those infected. The most characteristic response is painful enlargement of the parotid glands. Severe cases may progress to include epididymoorchitis in prepubescent males, which can cause atrophy of the testes, but rarely sterility. Mumps can also produce a transient high frequency deafness.
Measles: Measles is also a systemic infection spread by viremia. Acute disease affects the lymphatic and respiratory systems and occasionally the brain. Persistence of the virus can produce SSPE. Measles virus generally enters via the oropharynx. Local multiplication precedes the viremia and spread to the reticuloendothelial system. Extensive multiplication produces a secondary viremia 5-7 days later with spread to the mucosa of the respiratory, urogenital or gastrointestinal tracts or the central nervous system. Clinically, there are respiratory symptoms during the prodromal stage (malaise, fever, cough) and a rash during the eruptive phase (Koplik's spots, rash on head then body). Complications are mainly bacterial superinfection but other rare complications (e.g. SSPE) can occur.
RSV: The RS virus initiates a local infection in the upper or lower respiratory tract but illness varies with age and previous experience. The virus infects ciliated epithelia of the nose, eye and mouth and remains generally confined. Virus spreads extracellularly and by fusion. Severe disease may present as bronchiolitis, pneumonia or croup, particularly in infants. Some evidence suggests that there are possible immunopathologic mechanisms involved.

HOST DEFENSES:

Parainfluenza: Interferon and neutralizing IgA are important but IgA is often short lived.
Mumps: Interferon limits viral spread while IgM and IgG are protective. Long lasting IgG affords life-long immunity.
Measles: Interferon affects viral spread but the cell-mediated response is associated with recovery (disease is more severe in persons with a T-cell immunodeficiency). Life long persistence of IgG affords protection.
RSV: Interferon, age, immune competence and neutralizing IgA (short lived) all contribute to host defense. Breast milk may contain neutralizing factors as well.

EPIDEMIOLOGY:

Parainfluenza: Found worldwide, Parainfluenza affects mostly children. The virus is endemic in some areas and epidemic at times. The source for infections is the respiratory tract of humans. Disease is contagious for 3-16 days, transmission is person to person or by droplets. At 5 years of age, about 90-100% have antibodies against type 3, about 74% for type 1 and about 58% for type 2.
Mumps: Also found worldwide, mumps is endemic with peaks of acute disease appearing from January through May. Epidemics occur every 2-7 years and humans are the only host. Transmission is via salivary secretions and disease is contagious just before and after the symptoms. It is less contagious than Parainfluenza so intimate contact is usually required. In unvaccinated populations, about 45% of people are infected by age 5 and about 95% by 15 years of age. About 35% of those infected are subclinical, however.
Measles: Endemic worldwide, epidemics occur every 2-4 years in developed countries when the population susceptibility reaches about 40%. The source of the virus is human (respiratory) secretions. Disease generally affects those in the 4-7 year age group. The incidence of SSPE is about 6-20 per million.
RSV: Also endemic worldwide, epidemics occur yearly, usually between January and March. Reinfection is common. The source for infection is the human respiratory tract and disease is usually contagious about 4-5 days after the symptoms. Most infants experience upper respiratory infections but between 15% and 50% experience lower (more serious) respiratory infections.