The control of H5 or H7 mildly pathogenic avian influenza - a role for inactivated vaccine - Biosecurity is the first line of defence in the prevention and control of mildly pathogenic avian influenza (MPAI). Its use has been highly successful in keeping avian influenza (AI) out of commercial poultry world-wide.

Abstract

However, sometimes AI gets introduced into poultry populations, and when that occurs biosecurity again is the primary means of controlling the disease. There is agreement that routine serological monitoring, disease reporting, isolation or quarantine of affected flocks, application of strict measures to prevent the contamination of and movement of people and equipment, and changing flock schedules are necessities for controlling AI.

There is disagreement as to the disposition of MPAI infected flocks: some advocate their destruction and others advocate controlled marketing. Sometimes biosecurity is not enough to stop the spread of MPAI. In general, influenza virus requires a dense population of susceptible hosts to maintain itself. When there is a large population of susceptible poultry in an area, use of an inactivated AI vaccine can contribute to AI control by reducing the susceptibility of the population.

Does use of inactivated vaccine assist, complicate or interfere with AI control and eradication? Yes. It assists MPAI control (which may reduce the risk of highly pathogenic AI [HPAI]), but unless steps are taken to prevent it, vaccination may interfere with sero-epidemiology in case of an HPAI outbreak. Does lack of vaccine assist, complicate or interfere with AI control and eradication? Yes.

It assists in identification of sero-positive (convalescent) flocks in a HPAI eradication program, but interferes with MPAI control (which in turn may increase the risk of emergence of HPAI). A number of hypothetical concerns have been raised about the use of inactivated AI vaccines. Infection of vaccinated flocks, serology complications and spreading of virus by vaccine crews, are some of the hypothetical concerns.

The discussion of these concerns should take place in a scientific framework and should recognize that control of MPAI reduces the risk of HPAI. That inactivated vaccines have reduced a flock's susceptibility to AI infection, have reduced the quantity of virus shed post challenge, have reduced transmission and have markedly reduced disease losses, are scientific facts. The current regulations preventing vaccination against H5 or H7 MPAI have had the effect of promoting circulation of MPAI virus in commercial poultry and live poultry markets. In the absence of highly pathogenic avian influenza, there is no justification for forbidding the use of inactivated vaccine.

Conclusion

In spite of hypothetical concerns, inactivated AI vaccines have contributed successfully to preventing morbidity, mortality and egg production loss, reducing economic loss and controlling the spread of disease. Contrary to the prevailing attitude among some regulatory officials that vaccines are a last resort, there is rationale and evidence to support their immediate use in helping to stop a H5 or H7 MPAI outbreak.

Controlled vaccination against MPAI H5 and H7 should be available as part of a science-based influenza control strategy that includes tight biosecurity and:

• monitoring all flocks at risk,

• using controlled vaccination for flocks deemed to be at risk by industry veterinarians (where regulatory veterinarians are informed of all vaccine use),

• monitoring sentinel birds left in the vaccinated flocks, or other appropriate monitoring methods,

• isolating or quarantining convalescent and vaccinated flocks, and

• controlled marketing of convalescent and vaccinated flocks.

It is time for the regulatory veterinarians to take a leadership role in this issue of vaccination for MPAI. The current regulations preventing vaccination against H5 or H7 MPAI have had the effect of promoting circulation of MPAI virus in commercial poultry and live poultry markets. Uncontrolled H5 and H7 MPAI has already been shown to mutate to HPAI.

Current international regulations, as interpreted, prevent veterinarians from adequately fighting mildly pathogenic avian influenza. It is time to work on an international basis to change the interpretation or to correct the regulations. Acknowledgements The disease control strategy outlined here is the result of years of exchange with scores of industry veterinarians and avian influenza researchers whose input is greatly appreciated.

Source: David A. Halvorson, Department of Veterinary Pathobiology, College of Veterinary Medicine, University of Minnesota - Taken from site November 2005