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H5N1 HPAI in Different Species

3.7 Conclusions

The evidence presented above strongly suggests that domestic ducks – birds that in most cases are reared extensively – have played a key role in the genesis of new genotypes and spread of Asian-lineage H5N1 HPAI viruses.

It is not clear exactly how infected ducks infect other poultry. However, there is considerable sharing of environments by ducks and other poultry, especially at village level and in some live-poultry markets. Water contaminated with viruses from ducks could also transmit the virus to other avian species.

As long as ducks are reared under management systems with poor biosecurity, including free grazing and channel and pond rearing, they represent a risk that H5N1 HPAI will be spread and maintained. In areas where infection is present, methods will be required to protect these birds from infection and to stop them becoming a source of infection for other poultry and possibly wild birds.

Thailand has done this by restricting the grazing of ducks and culling known infected flocks. The effect of these measures cannot be isolated from those due to other measures, but overall there was a marked fall in the number of reported cases in 2006 compared with 2004. It is highly likely that measures related to ducks were a key factor in controlling HPAI there. However, care needs to be taken in changing management practices for ducks to ensure that the ecological balance is not changed given the role that ducks are likely to play in pest management and fertilization of rice paddies.

Other countries such as Viet Nam are promoting a shift away from grazing ducks to intensive duck production but this requires substantial investment and increased inputs for feeding and maintenance of duck flocks. Given the size of the duck population in Viet Nam, it seems unlikely that more than a small percentage of these will be reared indoors under intensive conditions for some years to come. Nevertheless, if reared under biosecure conditions, experiences from Thailand suggest that they can be kept free from infection.

Vaccination is the other method available for reducing the virus load in ducks and is being used in China, Indonesia and Viet Nam. Use of vaccine in ducks will likely be required for some time given that there are few other ways of significantly enhancing biosecurity for ducks reared outdoors on rice fields, ponds or channels.

Wild birds have almost certainly played some role in the spread of H5N1 HPAI viruses but as will be seen in Chapter 4 their relative contribution is likely to be lower than that associated with human activities involving rearing and marketing of poultry, especially in those countries where infection persists in poultry.

The main conclusion that can be drawn so far from human cases is that the majority of these occur in rural settings where there is close association with poultry. Humans have acted as sentinels for infection in poultry indicating that there is considerable under-reporting of this disease in poultry in rural areas. Cases where disease has not been closely associated with sick poultry have occurred in places where the virus is known or suspected to be present in live-poultry markets, or sufficient samples have not been tested to rule out this possibility. Spillover infections have occurred in other mammalian species.

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