The Increasing Risk of Highly Pathogenic Avian Influenza
The last few years has seen a dramatic increase in the number of outbreaks of highly pathogenic avian influenza (HPAI). Besides the problems currently being experienced in Asia with H5N1 strains of HPAI, there have also been outbreaks due to different strains of HPAI in Italy, Holland & Belgium. A case of HPAI in the USA involving an H5N2 strain in Texas has been confirmed in the last few days.
The H5N1 strain in Asia (and to a lesser extent, the H7N7 strain in Holland) has shown an increased capability of infecting other species including humans, with a number of deaths being attributed to infection with this virus in people who were in contact with affected poultry. This phenomenon has raised concerns over the possibility of new mutants arising in humans co-infected with influenza strains of mixed origin (human as well as avian). This problem is largely related to the polyculture systems prevalent in Asia, where fish, pigs, waterfowl and poultry are reared in close association with one another. Pigs and waterfowl are recognized “mixing vessels” for AI viruses, allowing genetic re-assortment between different strains of AI.
Historically, control of HPAI has been achieved by a slaughter-out policy. Such a policy can only be effective where the systems are in place to ensure effective surveillance of the poultry population, rapid confirmation of the diagnosis and immediate depopulation of affected (or potentially affected) flocks. A good example of how effective this system can be involves the Belgian outbreaks. It is hoped that the case in Texas has been handled in a similar fashion.
As a result of the increasing frequency of new outbreaks of HPAI worldwide, there has been a move to attempt to control this disease through vaccination, so as to avoid the financial implications of a slaughter-out policy. This was first attempted on a large scale in Italy and has apparently also been applied in China. Vaccination in the face of AI, whether it be HPAI or low-pathogenicity AI (LPAI) is extremely controversial. This is because the vaccines currently used (haemagglutinin-specific oil emulsions) induce a strain-specific immunity. As the AI virus is extremely unstable, strain-specific vaccination induces further selection pressure to mutate. This is thought to have occurred both in Italy as well as in China, assisting in the emergence of “new” strains of AI (such as the current H5N1 strains in Asia).
We cannot regard the South African poultry industry as being isolated from what has being happening in the rest of the world. The ease with which the H6 LPAI strains infected supposedly biosecure broiler breeder flocks in South Africa over the last 2-3 years demonstrates how poorly our industry is prepared! The fact that the LPAI outbreaks occurred over a wide area before being acknowledged and that the industry had to resort to vaccination to control LPAI is also concerning. Compensation for slaughtered-out flocks is not in place in South Africa mitigating against prompt reporting of suspect flocks. Any delay in the confirmation of HPAI would seriously impair attempts to control and eradicate a new outbreak.
Fortunately, with regard to the current HPAI outbreaks in the rest of the world, we are geographically removed from them and AI viruses are relatively labile. In addition, we are not on the major migration routes of feral waterfowl and other birds from the affected areas. We hope the Government will successfully stop imports of birds and product from the affected countries. Harbours and airports remain areas of high risk, which need to be monitored closely – do we have the manpower in place doing the job effectively?
Date published: 2004-02-01
Dr David Allwright