Influenza A and methicillin-resistant Staphylococcus aureus co-infection in rhesus macaques – A model of severe pneumonia

BACKGROUND: Influenza results in up to 500,000 deaths annually. Seasonal influenza vaccines have an estimated 60% effectiveness, but provide little or no protection against novel subtypes, and may be less protective in high-risk groups. Neuraminidase inhibitors are recommended for the treatment of severe influenza infection, but are not proven to reduce mortality in severe disease. Preclinical models of severe influenza infection that closely correlate to human disease are needed to assess efficacy of new vaccines and therapeutics. METHODS: We developed a nonhuman primate model of influenza and bacterial co-infection that re-capitulates severe pneumonia in humans. Animals were infected with influenza A virus via intra-bronchial or small-particle aerosol inoculation, methicillin-resistant Staphylococcus aureus, or co-infected with influenza and methicillin-resistant S. aureus combined. We assessed the severity of disease in animals over the course of our study using tools available to evaluate critically ill human patients including high-resolution computed tomography imaging of the lungs, arterial blood gas analyses, and bronchoalveolar lavage. RESULTS: Using an intra-bronchial route of inoculation we successfully induced severe pneumonia following influenza infection alone and following influenza and bacterial co-infection. Peak illness was observed at day 6 post-influenza infection, manifested by bilateral pulmonary infiltrates and hypoxemia. The timing of radiographic and physiologic manifestations of disease in our model closely match those observed in severe human influenza infection. DISCUSSION: This was the first nonhuman primate study of influenza and bacterial co-infection where high-resolution computed tomography scanning of the lungs was used to quantitatively assess pneumonia over the course of illness and where hypoxemia was correlated with pneumonia severity. With additional validation this model may serve as a pathway for regulatory approval of vaccines and therapeutics for the prevention and treatment of severe influenza pneumonia.