Case report: Increased viral receptor expression associated with high viral load and severe pneumonia in a young patient infected with 2009 H1N1 influenza a with no pre-existing conditions.

A case of unusually high severity of influenza pneumonia leading to acute respiratory distress syndrome and death was investigated. This was a previously a healthy 28-year-old man with no underlying conditions, admitted to a hospital during the first wave of influenza pandemic in Thailand in July 2009. He had experienced high fever and influenza-like illness for 5 days before coming to the hospital. He developed acute respiratory distress syndrome and expired on day 7 after admission. In comparison to three other cases of influenza pneumonia in the same outbreak with known risk factors for severe influenza, such as pregnancy and diabetes mellitus, a much higher viral load was detected in the lungs of this patient despite antiviral treatment. In agreement with the high viral load, the lung specimens from this patient, but not the other three patients, showed a high expression of α-2,6-linked sialic acid by lectin staining. The gene responsible for the synthesis of this sialic acid was also found to be upregulated. The data indicated overexpression of the viral receptor as a potential mechanism for severe disease in some patients.