TORONTO – Contrary to conventional thinking, a high respiratory syncytial viral load may be protective against progression of bronchiolitis.
Elevated respiratory syncytial viral (RSV) load was unexpectedly associated with less severe bronchiolitis disease in a convenience sample of 63 children less than 2 years of age who presented to an ED with clinical signs of infection.
Children with an elevated respiratory syncytial viral load were more likely to be discharged home, to require less than 24 hours of intravenous fluids, and to not require intubation, Dr. Berkeley L. Bennett and associates reported in a poster presentation at the annual meeting of the Pediatric Academic Societies.
While these trends did not reach statistical significance, there was a highly significant association between higher RSV load and the need for more than 24 hours of oxygen therapy.
RSV load had an independent and protective effect on duration of oxygen therapy in a stepwise multivariate regression analysis that included RSV load, age, duration of illness, interleukin-8 (IL-8), and interleukin-10 (IL-10). A 10-fold increase in RSV load decreased the need for more than 24 hours of oxygen therapy by 43%.
“We hypothesize that an adequate viral load is necessary to induce an optimal inflammatory response that is capable of controlling the disease,” Dr. Bennett of Cincinnati Children's Hospital Medical Center, said in an interview. “Alternatively, with a small amount of viral load, you get a weak inflammatory response, and the disease will smolder and go unchecked.”
Nasal-wash samples obtained from 101 children prospectively enrolled during the RSV season between November 2004 and February 2005 were analyzed for viral pathogens via cell culture, RSV quantification via plaque assay, and cytokine and chemokine concentrations via the Bio-Plex Suspension Array System.
Of these, 63 were RSV positive and included for analysis. Twenty-two were discharged home from the ED and 41 were admitted in this study that was published simultaneously in the May 15 issue of the Journal of Infectious Diseases (J. Infect. Dis. 2007;195: 1532-40).
The mean RSV concentration was 3.95 log10 copies/mL (range 1.0-6.85 log10 copies/mL). Rather than using a numeric definition for an “elevated” viral load, the mean viral concentration was compared between groups, said Dr. Bennett, who conducted the study while at Baylor College of Medicine, Houston.
RSV load was directly correlated with age and inversely correlated with duration of illness at presentation. The investigators analyzed the patients in four age categories, and found viral load increased with age.
RSV load was significantly directly correlated to IL-8 and IL-10 concentrations.
“If an adequate level of RSV concentration is truly necessary to induce an optimal inflammatory response, that may explain why previous antiviral and anti-inflammatory therapies have not been as beneficial as hoped,” Dr. Bennett said.
She acknowledged that the study was limited by its cross-sectional design and single nasal-wash sampling; trials are needed that will analyze different samples at different time points.
We hypothesize that an adequate viral load is needed to induce an optimal inflammatory response. DR. BENNETT