Despite the dire projections of public health authorities in 2003 warning of an imminent avian influenza pandemic, infection of humans with H5N1 strain HPAI has been limited to a relatively small number of rural inhabitants of countries in SE Asia, Eurasia and recently in Egypt. This is no reason for complacency with regard to suppression and prevention of AI in both subsistence and commercial poultry in nations where the disease has become endemic. The International Conference on Options for the Control of Influenza which took place in Toronto during the week of June 18th stressed the need for action to limit circulation and dissemination of infection which represents a potential danger to the World’s population.
Recurring infections of poultry in Thailand and Viet Nam, in addition to de facto endemicity in Indonesia and areas of China represent a danger of spread to as yet unaffected nations. Possible routes include smuggling, legal commerce and by migratory species. Most countries with large populations of subsistence poultry lack the resources to diagnose and control infection.
Vaccination appears to be losing effectiveness in some areas. This may be due to declining potency of commercial vaccines, inconsistent administration with admixture of susceptible and immune flocks, and extensive movement of poultry for sale in live bird markets. There is evidence that field isolates of H5N2 are becoming more virulent. Genetic drift is creating greater susceptibility in flocks receiving conventional vaccines according to scientists at the Southeast Poultry Research Laboratory. Problems which have been identified include a dearth of new vaccines and the close association between chickens and ducks in SE Asia resulting in immunologic pressure in vaccinated but incompletely immunized chickens.
Both the WHO and the FAO recognize the major problems of practical control of the infection, especially in Africa where veterinary and public health authorities lack training, resources and commitment. A concurrent concern is that ineffective attempts at eradication in non-industrialized countries might lead to infection of susceptible and unprotected workers. Reports of mild influenza in personnel involved in handling infected flocks in Canada, Holland and the UK suggest the potential for interspecies transmission of both H5 and H7 avian strains. The attack rate for rural inhabitants in SE Asia in contact with flocks infected with H5N1 is admittedly low and has remained so for at least four years. The 200 or so documented cases of H5N1 infection should be viewed against the billions of person-days of potential exposure. This suggests that at the present time humans are refractory to infection based on the incompatibility of avian virus with influenza receptors on the cells lining the upper respiratory tract.
The principal lesson to be derived from the Conference is that vaccination is an effective short term response following exposure of poultry if campaigns are implemented rapidly and diligently using a homologous vaccine. Surveillance, repeated vaccination of flocks, imposing quarantines over affected areas and strict biosecurity measures to prevent introduction of infection into commercial flocks are all necessary and complementary components of control. This presumes availability of resources, prior planning with enabling legislation and a rational approach to suppression by all levels of government and industry in affected countries.
Persistence of HPAI in poultry on two Continents does represent a risk of potential transmission to humans. Accordingly the health authorities in both industrialized and developing nations need to establish contingency plans for a pandemic which may not occur as a result of the present infection of poultry in SE Asia but could emerge following the epidemiologic pattern of previous episodes in the 20th Century.
