[Respiratory allergy and the viruses]

Virus respiratorio sincitial, metapneumovirus y virus parainfluenza humanos: cuadro clínico y fisiopatología

El VRSh causa la mayor parte de las bronquiolitis del lactante, infecciones respiratorias bajas que provocan trastornos respiratorios obstructivos y sibilancias. Los factores relacionados con las características del paciente (prematuridad, enfermedades asociadas, predisposición genética), con el virus (VRShA, genotipo, etc.) o el ambiente (contaminación, tabaquismo, etc.) son determinantes en la génesis de las bronquiolitis y determinan su gravedad. La mortalidad de la bronquiolitis es del 0,005-0,2% dependiendo de las características del paciente. No se conoce la fisiopatología de la bronquiolitis por VRSh. El papel directo del virus sobre el epitelio, la inmadurez del pulmón del lactante y, como se ha demostrado recientemente, el papel de la reacción inflamatoria, son factores destacados. También es probable que haya mecanismos inmunopatológicos (anticuerpos IgE, respuesta CD4+ Th2, etc.) que intervengan en ciertas bronquiolitis. La inmunidad anti-VRSh nunca llega a ser completa y las reinfecciones son frecuentes en los niños pequeños y las personas de edad avanzada, en quienes la infección por el VRSh representa aproximadamente el 5% de las afecciones respiratorias bajas. Con frecuencia se presentan con un cuadro de neumopatía infecciosa atípica con signos de bronquiolitis. Las infecciones debidas al VPIh3 suelen manifestarse por bronquiolitis, y las infecciones secundarias al VPIh1 y al 2 por laringotraqueítis. No se conoce bien la expresión clínica de las infecciones por VPIh4. El MPVh causa bronquiolitis comparables en todos sus aspectos a las que origina el VRSh.

[Viral infection and asthma: immunologic mechanisms]

The role of viral respiratory infections in lactating infants and other children continues to generate controversy. The debate concerns the difference, or the apparent differences, in the natural history of wheezing. Viral infections frequently provoke wheezing episodes in non-asthmatic small children but in the majority of these the wheezing disappears without the child subsequently developing asthma. In some cases, however, the wheezing persists and in others the child has asthma. Both the role of viral infection and the mechanisms by which wheezing can be produced in a previously healthy child or exacerbated in asthmatic children are unknown. Several hypotheses have been put forward to explain the relationship between viral infections and persistent wheezing and asthma: 1. Altered immune response to various allergens, whether producing sensitization to these allergens or inhibiting tolerance response to airborne allergens. The number of such patients is increasing, among them those with bronchiolitis, asthma, positive skin tests and specific IgE antibodies. Although there is no unanimity on the matter, these patients also present elevated IL-4 levels and reduced IFN-gamma levels. 2. Induction of inflammation typical of allergic asthma. This occurs when the virus interacts with T lymphocytes; (the natural response to viral infection is Th0 and Th1 lymphocyte differentiation and release of IFN-gamma, which has antiviral properties. In children infected with respiratory syncytial virus Th2 lymphocyte differentiation is produced, which is characteristic of allergic reactions, to the detriment of Th1); epithelial cells (in these cells active viral infection activates nuclear transcription kappa-beta and nuclear IL-6 factor, producing the release of numerous pro-inflammatory cytokines and chemokines as well as expression of adhesion molecules); eosinophils (inducing variable eosinophilia which, to a certain degree, has predictive value for the persistence of wheezing) and other inflammatory cells such as neutrophils and macrophages. In the same context, during viral respiratory infection, the presence of mediators (leukotrienes, especially LTC4, histamine, prostaglandins and tryptase) are observed in respiratory secretions and a correlation between levels of specific IgE mediators can be observed. 3. Increased allergic inflammation--producing bronchial hyperreactivity, mediator release by the various inflammatory cells and neuropeptides from C-sensitive fibers, and even interfering with nitric oxide bronchodilators. In spite of all of the above, it seems that recurrent wheezing after childhood bronchiolitis is not exclusively the result of viral infection and that other factors also play a role in this disease.