Birgit Schauer

Spatio-temporal epidemiology of highly pathogenic avian influenza outbreaks in the two deltas of Vietnam during 2003–2007

Outbreaks of highly pathogenic avian influenza A subtype H5N1 have occurred in Vietnam as a series of epidemic waves since December 2003. We describe the spatial and temporal patterns of the HPAI H5N1 epidemics in the Red River Delta in the north (785 outbreaks in 606 communes) and the Mekong River Delta in the south of Vietnam (1313 outbreaks in 837 communes), where the epidemics were concentrated. Throughout the study period the percentage of outbreaks affecting ducks increased steadily to a peak of 78% during the 2006/2007 epidemic in both deltas. Five of the seven epidemic waves occurred in the period of active poultry population buildup immediately prior to the Vietnamese New Year (Tet festival). Recorded outbreaks were clustered in space and time within both deltas, consistent with infection transmission occurring via a combination of local and long-distance spread. Our analyses demonstrate that the epidemiology of HPAI in Vietnam has changed over the 4-year study period, with outbreaks now occurring in the warmer months of the year and ducks featuring more prominently as affected species. To determine the relative importance of local and long-distance spread on infection transmission, precise details of outbreak location, date of onset of clinical signs, and size and composition of the poultry population at risk need to be recorded during future outbreak responses.

A description of the management of itinerant grazing ducks in the Mekong River Delta of Vietnam

A cross-sectional survey of management practices of itinerant grazing ducks (known as field running ducks) was conducted during March 2008 in four districts of Can Tho and Bac Lieu provinces in the Mekong River Delta, Vietnam. The objective was to characterize the demographic structure of the itinerant grazing duck population and the management practices which might be related to the continuing outbreaks of H5N1 avian influenza in this region. Broiler duck flocks were owned by 55% of survey respondents, and layer flocks by 53%. Greater than 80% of ducks within 93% of villages and 99% of duck flocks were reported to have been vaccinated against H5N1 avian influenza, whereas only 19% of villages and 60% of chicken flocks had greater than 80% of chickens vaccinated. Fifty-nine percent of duck owners moved their ducks outside their home communes, whilst 37% and 28% of owners moved their ducks outside their home district and province, respectively. Larger flocks were more likely to be run outside their home district compared with smaller flocks. After adjusting for the effect of flock production type and district, the odds of an out-of-district field running duck flock movement was increased by a factor of 7.24 (95% CI 2.89-19.24) for households with flocks of more than 800 ducks, compared with flocks of less than 250. Most households sold ducks to traders (72%) or to neighbours (33%), whereas less than 20% sold their birds through markets. The findings of this study suggest that surveillance strategies for field running duck flocks should focus on layer flocks as well as larger flocks as they are more likely to be moved outside of their home district, facilitating long-distance disease spread.

Spatio–temporal analyses of highly pathogenic avian influenza H5N1 outbreaks in the Mekong River Delta, Vietnam, 2009

This paper presents the results of spatio-temporal analyses and epidemic modelling of HPAI H5N1 outbreaks that occurred in four provinces of the Mekong River Delta, Vietnam between January and March 2009. Significant spatio-temporal interaction of disease risk was observed within a distance of 10 km and 12 days following the detected onset of clinical signs. We estimate that the household-to-household infection rate within a commune was approximately 50 times greater than the household-to-household infection rate between communes. Our findings show that the predominant mechanism of infection transfer was local spread. A comparison of disease control procedures and veterinary capacity in communes with relatively high and low infection rates should help to identify procedures essential for effective outbreak management in this area of Vietnam.

Association Between Human Cases and Poultry Outbreaks of Highly Pathogenic Avian Influenza in Vietnam From 2003 to 2007: A Nationwide Study

This study quantifies the spatio-temporal association between outbreaks of highly pathogenic avian influenza H5N1 in domestic poultry (n = 3050) and human cases (n = 99) in Vietnam during 2003-2007, using rare events logistic regression. After adjusting for the effect of known confounders, the odds of a human case being reported to authorities increased by a factor of 6.17 [95% confidence interval (CI) 3.33-11.38] and 2.48 (95% CI 1.20 - 5.13) if poultry outbreaks were reported in the same district 1 week and 4 weeks later respectively. When jointly considering poultry outbreaks in the same and neighbouring districts, occurrence of poultry outbreaks in the same week, 1-week later, and 4 weeks later increased the odds of a human case by a factor of 2.75 (95% CI 1.43-5.30), 2.56 (95% CI 1.31-5.00) and 2.70 (95% CI 1.56-4.66) respectively. Our study found evidence of different levels of association between human cases and poultry outbreaks in the North and the South of the country. When considering the 9-week interval extending from 4 weeks before to 4 weeks after the week of reporting a human case, in the South poultry outbreaks were recorded in 58% of cases in the same district and 83% of cases in either the same or neighbouring districts, whereas in the North the equivalent results were only 23% and 42%. The strength of the association between human and poultry cases declined over the study period. We conclude that owner reporting of clinical disease in poultry needs to be enhanced by targeted agent-specific surveillance integrated with preventive and other measures, if human exposure is to be minimized.

Measuring the Attenuation in a Subject-specific Random Effect with Paired Data

This paper is motivated by an investigation into the growth of pigs, which studied among other things the effect of short-term feed withdrawal on live weight. This treatment was thought to reduce the variability in the weights of the pigs. We represent this reduction as an attenuation in an animal-specific random effect. Given data on each pig before and after treatment, we consider the problems of testing for a treatment effect and measuring the strength of the effect, if significant. These problems are related to those of testing the homogeneity of correlated variances, and regression with errors in variables. We compare three different estimates of the attenuation factor using data on the live weights of pigs, and by simulation.