Epidemiological studies of highly pathogenic avian influenza in Vietnam : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University

This thesis utilises data on highly pathogenic avian influenza (HPAI) subtype H5N1 from the Vietnamese national surveillance system and purpose-designed field studies to enhance the understanding of the epidemiological features of HPAI H5N1 in Vietnam. The findings obtained from the first study show that the presence of a HPAI H5N1 human case was associated with an increase in the likelihood of disease being detected in poultry one and four weeks later, indicating that the occurrence of clinical disease in poultry is not a useful predictor of subsequent human cases in the same locality. The analyses from the second study demonstrate that the epidemiology of HPAI H5N1 in poultry in Vietnam has changed over time and the infection transmission occurs by a combination of local and long-distance spread. The findings from a cross-sectional survey of management practices of itinerant grazing ducks suggest that surveillance strategies for this type of duck management should focus on both layer and larger flocks as they are more likely to be moved outside of their home district, facilitating long-distance disease spread. The results from a matched case-control study in poultry identify factors associated with the presence of HPAI H5N1 and provide evidence that disease control strategies should emphasise the reduction of household-level, rather than village-level, risks for disease. In the last study, spatio-temporal interaction of disease risk in poultry was observed within a distance of 10 kilometres and 12 days following the detected onset of clinical signs. Household-tohousehold infection rate within a commune was approximately 50 times greater than the household-to-household infection rate between communes. These findings show that the predominant mechanism of HPAI H5N1 infection transfer was local spread. The lessons learnt from the series of studies presented here should assist Vietnamese animal health authorities to implement the necessary systems and infrastructure that will allow novel and emerging disease syndromes to be investigated promptly and efficiently