Anthropogenic factors and the risk of highly pathogenic avian influenza H5N1: prospects from a spatial-based model

Paul, Mathilde; Tavornpanich, Saraya; Abrial, David; Gasqui, Patrick; Charras-Garrido, Myriam; Thanapongtharm, Weerapong; Xiao, Xiangming; Gilbert, Marius; Roger, Francois; Ducrot, Christian

First, we generated maps of the relative risk of HPAI H5N1 for chicken and duck flocks, and showed that the spatial pattern for chickens and ducks was similar. This indicated that chickens and ducks either infected each other or shared the same spatial source of infection. The results of the multivariate analysis suggest that both explanations may be valid. When, for example, we considered chickens, we found that the relative risk in ducks and several other risk factors were significant when adjusted for each other in the final model. Furthermore, the model for chickens and the model for ducks indicated a common set of risk factors. Second, our results were consistent with previous studies on ecological risk factors of HPAI in Thailand [8, 9] . However, adding on to previous work, our analysis made it possible to identify classes of values associated with higher risk, which provides greater detail regarding the possible role of ecological risk factors. A high HPAI risk was associated with a high density of free-grazing ducks (> 10 ducks per km 2 ), more than one rice crop per year, and a short distance to a river ( 2 km). Altitude may be considered as an indicator of other unmeasured environmental variables related to HPAI risk. Subdistricts with a low average altitude ( 50 m) were associated with a high risk of HPAI. The mixture of wetlands, ponds, irrigation networks and agriculture in these areas combined with intensive land use [9] , may have constituted a favourable environment for the HPAI H5N1 virus. In contrast, subdistricts with a medium altitude (50.01-400 m) have higher slopes and a land cover dominated by forests and permanent vegetation [9] . Medium average altitude in subdistricts associated with low RR was found to constitute a kind of protective factor regarding HPAI risk. We also provide new insight into the role of factors related to poultry farming in the spread of HPAI. Like Tiensin et al. [24] , we found no indication that native chickens represent an increased HPAI risk despite the fact that these chickens are raised in low biosecurity systems and were affected massively by the disease (920 out of the 1 158 chicken flocks infected during the second wave). Conversely, a high density of native chickens (100-300 and > 300 native chickens/km 2 ) was associated with RR significantly below 1. In Thailand, wet markets have always been rare [2] and native chickens mainly are raised for family consumption using little input and involving little trading activity. This may have resulted in a protective effect against HPAI in subdistricts with a high density of native chickens. These subdistricts probably were less exposed to the virus because they were not connected to trading chains which potentially spread the disease. In addition, the pre-emptive culling, which focussed in the beginning on native chickens around an outbreak, may have contributed to containing the spread within these subdistricts. Fighting cocks are believed to have worsened the HPAI situation in other Asian countries [29] and in Thailand [22] . The association we found between high densities of fighting cocks and HPAI risk was significant but weak, as did Gilbert et al. [8] . In Thailand, fighting cocks were also targeted when control measures were implemented in 2004, with a prohibition on cockfighting, compulsory registration, and disease monitoring. Given their high monetary and cultural value, roosters receive very special attention from their owners,