Dept title


Current state of knowledge on Highly Pathogenic Avian Influenza

2.6 Conclusions

HPAI is not a new disease and occurred well before the industrialization of poultry production. The number of outbreaks of HPAI has increased dramatically in the last eight years, in part due to spread of Asian-lineage H5N1 HPAI, but also as a result of outbreaks of diseases due to other HPAI viruses.  Although a number of possible causes for this increase have been proposed, the reasons are not known and it is probably multifactorial.

Aquatic birds are the natural hosts of avian influenza viruses but are not normally infected with HPAI viruses. H5N1 HPAI has proved the exception to this rule by causing both clinical and sub-clinical infection in domestic ducks.

HPAI viruses are derived from LPAI viruses, often after multiplication in commercial poultry. Asian-lineage H5N1 HPAI viruses were first detected in 1996 and all viruses in this lineage isolated since then have been highly pathogenic. The circumstances that led to the emergence of this lineage of HPAI viruses are not known.

Asian-lineage H5N1 HPAI viruses have now spread across three continents. The mode of spread of these viruses is still not entirely clear. Trade in poultry and poultry products and movement of wild birds have played a role, although the relative contributions of these in the initial introduction into countries previously free from infection has not always been determined. Much of the evidence supporting these various modes of introduction is circumstantial and complicated in many cases by failure to detect the initial incursion of virus. Molecular epidemiology has assisted greatly in assessing links between various isolates.

Speed of detection has also played a role in determining the effectiveness of control and eradication programmes. Many factors influence the timeliness of reporting, which is key to determining the speed of responses. Producers in all production sectors have not reported disease, sometimes due to failure to recognize the disease but also because of fears regarding the consequences of reporting. Compensation has been proposed as an incentive to report, and the absence of a compensation system provides little incentive to report. However, field experiences suggest that availability of compensation (which does not cover consequential losses or even the full value of poultry) does not necessarily lead to reporting of disease.

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