The respiratory viruses are recognized as the most frequent lower respiratory tract pathogens for infants and young children in developed countries but less is known for developing populations. The authors conducted a prospective study to evaluate the occurrence, clinical patterns, and seasonal trends of viral infections among hospitalized children with lower respiratory tract disease (Group A). The presence of respiratory viruses in children's nasopharyngeal was assessed at admission in a pediatric ward. Cell cultures and immunofluorescence assays were used for viral identification. Complementary tests included blood and pleural cultures conducted for bacterial investigation. Clinical data and radiological exams were recorded at admission and throughout the hospitalization period. To better evaluate the results, a non- respiratory group of patients (Group B) was also constituted for comparison. Starting in February 1995, during a period of 18 months, 414 children were included- 239 in Group A and 175 in Group B. In Group A, 111 children (46.4%) had 114 viruses detected while only 5 children (2.9%) presented viruses in Group B. Respiratory Syncytial Virus was detected in 100 children from Group A (41.8%), Adenovirus in 11 (4.6%), Influenza A virus in 2 (0.8%), and Parainfluenza virus in one child (0.4%). In Group A, aerobic bacteria were found in 14 cases (5.8%). Respiratory Syncytial Virus was associated to other viruses and/or bacteria in six cases. There were two seasonal trends for Respiratory Syncytial Virus cases, which peaked in May and June. All children affected by the virus were younger than 3 years of age, mostly less than one year old. Episodic diffuse bronchial commitment and/or focal alveolar condensation were the clinical patterns more often associated to Respiratory Syncytial Virus cases. All children from Group A survived. In conclusion, it was observed that Respiratory Syncytial Virus was the most frequent pathogen found in hospitalized children admitted for severe respiratory diseases. Affected children were predominantly infants and boys presenting bronchiolitis and focal pneumonias. Similarly to what occurs in other subtropical regions, the virus outbreaks peak in the fall and their occurrence extends to the winter, which parallels an increase in hospital admissions due to respiratory diseases.

In a study of acute respiratory disease, two collections of nasopharyngeal aspirates (NPA) were obtained from children hospitalized at the Pediatric Clinic of the University Hospital, São Paulo, in 1995 and 2000. Adenovirus was detected in 33 (8.2%) of 401 children followed. These viruses were isolated in HEp-2, HEK-293, or NCI-H292 cells and serotyped by neutralization. The genome types were determined after restriction analyses of the genomic DNA extracted from infected cells. Nineteen isolates were characterized as Human adenovirus B, genome types HAdV-3a, HAdV-7h, and HAdV-7h1; 11 as Human adenovirus C, genome types HAdV-1D10, HAdV-2D25, HAdV-5D2, and HAdV-6D3. Our findings show that species C adenoviruses present an endemic infection pattern, with co-circulation of different serotypes and genome types; no new genomic variant was observed. Species B adenoviruses showed epidemic infection patterns, with shifts in the predominant genome type. The isolates from 1995 belong to genome type 7h, or the variant 7h1, with a clear substitution of the type 7b, prevalent in São Paulo for more then 10 years. In 2000, the variant 7h1 predominated and the emergence of the type 3a was observed. Almost 10 years passed between the identification of HAdV-7h in Argentina and its detection in São Paulo. The geographic isolation of these two countries was reduced by the increase in population mobility due to growing commercial relationships.

We have evaluated the cellular and humoral immune response to primary respiratory syncytial virus (RSV) infection in young infants. Serum specimens from 65 patients £12 months of age (39 males and 26 females, 28 cases <3 months and 37 cases ³3 months; median 3 ± 3.9 months) were tested for anti-RSV IgG and IgG subclass antibodies by EIA. Flow cytometry was used to characterize cell surface markers expressed on peripheral blood mononuclear cells (PBMC) from 29 RSV-infected children. There was a low rate of seroconversion in children <3 months of age, whose acute-phase PBMC were mostly T lymphocytes (63.0 ± 9.0%). In contrast, a higher rate of seroconversion was observed in children >3 months of age, with predominance of B lymphocytes (71.0 ± 17.7%). Stimulation of PBMC with RSV (2 x 10 5 TCID 50 ) for 48 h did not induce a detectable increase in intracellular cytokines and only a few showed a detectable increase in RSVspecific secreted cytokines. These data suggest that age is an important factor affecting the infants' ability to develop an immune response to RSV.

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