Abstract
Avian influenza A (H5N1) viruses cause severe disease in humans1,2, but the basis for their virulence remains unclear. In vitro and animal studies indicate that high and disseminated viral replication is important for disease pathogenesis3,4,5. Laboratory experiments suggest that virus-induced cytokine dysregulation may contribute to disease severity6,7,8,9. To assess the relevance of these findings for human disease, we performed virological and immunological studies in 18 individuals with H5N1 and 8 individuals infected with human influenza virus subtypes. Influenza H5N1 infection in humans is characterized by high pharyngeal virus loads and frequent detection of viral RNA in rectum and blood. Viral RNA in blood was present only in fatal H5N1 cases and was associated with higher pharyngeal viral loads. We observed low peripheral blood T-lymphocyte counts and high chemokine and cytokine levels in H5N1-infected individuals, particularly in those who died, and these correlated with pharyngeal viral loads. Genetic characterization of H5N1 viruses revealed mutations in the viral polymerase complex associated with mammalian adaptation and virulence. Our observations indicate that high viral load, and the resulting intense inflammatory responses, are central to influenza H5N1 pathogenesis. The focus of clinical management should be on preventing this intense cytokine response, by early diagnosis and effective antiviral treatment.
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Acknowledgements
We thank the medical and nursing staff of the Hospital for Tropical Disease, the Pediatric Hospital Number One and the Pediatric Hospital Number Two in Ho Chi Minh City for their help in this study and their care of patients suspected or diagnosed with influenza H5N1. This work was funded by the Wellcome Trust.
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The study was designed and analyzed by M.D.d.J., C.P.S., T.T.H. and J.F.; laboratory studies were performed by T.T.T., V.M.H., T.N.B.C., D.M.H. and D.Q.H.; sequence analyses were done by G.J.D.S., Y.G. and J.S.M.P.; clinical data collection was done by N.V.V.C., T.H.K., V.C.D., P.T.Q., B.V.C. and N.T.C.; M.D.d.J., C.P.S., J.S.M.P., T.T.H. and J.F. contributed to the writing of the paper.
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Supplementary information
Supplementary Fig. 1
Pharyngeal virus load in relation to onset of illness during H5N1 influenza and human influenza. (PDF 425 kb)
Supplementary Fig. 2
Nasal, pharyngeal and tracheal virus load during the course of illness in two H5N1 patients (PDF 650 kb)
Supplementary Table 1
Peripheral blood lymphocyte counts and CD4+/CD8+ ratios. (PDF 59 kb)
Supplementary Table 2
GenBank accession numbers of H5N1 viral sequences and clinical outcome of patients. (PDF 64 kb)
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de Jong, M., Simmons, C., Thanh, T. et al. Fatal outcome of human influenza A (H5N1) is associated with high viral load and hypercytokinemia. Nat Med 12, 1203–1207 (2006). https://doi.org/10.1038/nm1477
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DOI: https://doi.org/10.1038/nm1477
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