Primary infection of mice with high titer inoculum respiratory syncytial virus: characterization and response to antiviral therapy

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Abstract
Intranasal infection of BALB/c mice with respiratory syncytial virus (RSV)-A2 (0.5 × 108– 2.0 × 108plaque-forming units, PFU) produced disease characterized by weight loss (2–3 g) and mortality (60%–100%) with the mean day of death ranging from 6–7 d after infection. The extent of RSV disease was inoculum titer-dependent and required a replication competent virus. Lung titers of virus peaked at 0.5-1 × 106PFU/g wet weight. Bronchoalveolar lavage fluid (BALF) levels of IL-1β, TNF-α, INF-γ IL-12, IL-6, MIP-1α, RANTES, and protein were elevated, whereas IL-2, IL-4, IL-5, IL-13, and IL-10 were unchanged. Histological assessment of lungs revealed marked inflammatory pathology characterized by bronchiolitis, vasculitis, and interstitial pneumonia. Whole-body plethysmography revealed significant disease-associated deficits of respiratory function. Therapy with ribavirin administered either by the intranasal, subcutaneous, or oral route significantly reduced disease in a dose-dependent manner. Delaying the initiation of therapy resulted in a loss of activity for ribavirin. Synagis®administered either intramuscularly as a single dose in prophylaxis or intranasally in prophylaxis, followed by therapy, also significantly reduced disease in a dose-dependent manner. Infection of mice with a high titer inoculum of RSV-A2 resulted in severe and fatal pulmonary disease that was responsive to treatment. This model may be useful to characterize the in vivo activity of experimental therapies for RSV infection.Key words: respiratory syncytial virus, disease, mortality, antiviral therapy.

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