Pathogenesis of Parainfluenza Viruses Infection
C. Pathogenesis
Parainfluenza viruses enter the host through the inhalation of infected droplet nuclei. Virus multiplication occurs throughout the tracheobronchial tree, inducing the production of mucus. The vocal cords of the larynx become grossly swollen, causing obstruction to the inflow of air, which is manifested by inspiratory stridor. In adults, the virus is usually limited to causing inflammation in the upper parts of the respiratory tract. In infants and young children, the bronchi, bronchioles and lungs are occasionally involved, which may reflect on the small size of the airways and the relative immunological immaturity. Viraemia is neither an essential nor a common phase of infection.
D. Clinical Features
Croup is the commonest clinical manifestation of parainfluenza virus infection in children. Typically, the children exhibit a croupy cough, inspiratory stridor, hoarse voice or cry and respiratory difficulty on inspiration. Patients are usually afebrile. About 80% of patients exhibit a cough and runny nose 1 to 3 days before the onset of the cough. Respiratory rhonchi are heard frequently throughout the lung fields. Radiological examination is usually normal. Occasionally the epiglottitis is grossly swollen and reddened due to infection by parainfluenza viruses or haemophillus influenzae b. Severe airway obstruction may ensue, necessitating an emergency tracheotomy.
Hospitalized patients are usually admitted within 3 - 24 hours after onset of croup. They should be placed promptly into plastic tents with cooled humidified oxygen. Usually, respiratory symptoms subside within 1 or 2 days. Children aged less than 3 years may experience recurrent attacks with the same serotype of parainfluenza virus due to a combination of narrow diameters throughout the larynx, trachea and bronchi, plus ineffective production of antibody.
Parainfluenza viruses are found uncommonly in association with other respiratory tract infections in children such as tracheobronchitis, bronchiolitis and bronchopneumonia. However, these syndromes arise most frequently from RSV infection. Conversely, other viruses can induce croup, such as influenza viruses, RSV, measles and chickenpox. The majority of parainfluenza virus infections result in a non specific upper respiratory tract infection, even in children. Parainfluenza virus infections in adults are relatively uncommon, and symptoms are usually less severe in adults than children.
E. Diagnosis
Croup is a well-defined, easily recognized clinical entity. Although 80% of the cases are caused by parainfluenza viruses, other agents such as influenza and RSV should be excluded. Conversely, although bronchiolitis in children is usually caused by RSV, a few cases may be caused by parainfluenza viruses. Virological diagnosis depends on 3 categories of tests, (1) rapid diagnosis tests, (2) virus isolation, and (3) serology. As in the case of RSV, the best specimens are nasopharyngeal secretions collected by aspiration (NPA) for children under 3 years of age. For older children and adults, a mouth gargle sample with 0.15 M saline is more suitable. Throat swabs provide less suitable specimens for virological diagnosis, and they should be collected only when it is not feasible to obtain NPA or mouth gargle specimens.
1. Rapid Diagnosis Methods ;- when an accurate virological diagnosis is achieved by same-day non-cultural techniques, it may be possible to discharge the patient one or more days earlier than those without demonstrable viral aetiology, resulting in an appreciable saving in cost.
However, immunofluorescence and EM cannot be relied on solely as they have poor sensitivity. A study carried out in Arizona between 1983 and 1985 show that immunofluorescence detected parainfluenza 1 in only 63% of throat specimens from which this serotype was isolated in tissue cultures. Parainfluenza 3 in only 31% of virus-culture positive throat secretions.
2. Virus Isolation ;- primary monkey kidney and continuous diploid human fibroblasts are usually used. Alternatively, LLC-MK2 with trypsin added to the media can be used. It has been shown that proteases are necessary for parainfluenza virus replication, and it is hypothesized that proteases present in primary cell cultures are absent in continuous cell lines. HEK cells are also sensitive to parainfluenza viruses. The majority of parainfluenza viruses do not produce CPE in cell culture. When CPE is seen in MK cells, it consists of syncytia formation. The customary screening procedure is to detect virus by haemadsorption. Haemadsorption is performed after 3, 7, and 10 days of incubation (7 and 14 days at the RVL) with guinea-pig and avian erythrocytes. When haemadsorption is present, the haemagglutinin titre of the virus can be determined by serial 2 fold dilutions of 25ul volumes in 96 well microtitre plates. The virus can be serotyped by haemadsorption inhibition tests or by immunofluorescence. The prolonged incubation time (3 - 10 days) required for virus isolation severely restrict the usefulness of virus isolation in the acute management of patients.
3. Serology ;- a wide variety of serological tests are available for parainfluenza viruses.
F. Treatment
Most parainfluenza virus infections of infants and children manifest clinically as acute laryngotracheobronchitis (croup) although some patients may develop bronchitis, bronchiolitis or pneumonia. Where the symptoms are severe, the infants should be admitted to hospital and nursed in plastic tents supplied with cool, moistened oxygen (croupette), where they are held for a day or two until their respiration becomes unlaboured. Severe respiratory obstruction may require endotracheal intubation followed by a tracheotomy. Antibiotics are not normally prescribed in croup unless there is evidence of concomitant bacterial infection.
G. Prevention
Nosocomial spread of parainfluenza viruses is well documented. The prevention of spread of parainfluenza virus infections in a hospital can be attempted along the following lines. Paediatric patients who are admitted to hospital with croup should be nursed in wards, preferably with cubicles or barriers, which are reserved for the syndrome alone. Patients with other acute respiratory infections such as bronchiolitis should also be nursed in separately designated areas. During an epidemic period, elective admissions to the wards of patients with conditions such as cystic fibrosis and immunodeficiency.
