APPLICATION OF BIOLOGICAL AND MOLECULAR TECHNIQUES TO THE DIAGNOSIS AND CONTROL OF AVIAN INFLUENZA AND OTHER EMERGING POULTRY PATHOGENS
Location: Exotic and Emerging Avian Viral Diseases Research Unit
Title: Epidemiology of avian influenza in agricultural and other man-made systems
Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: August 6, 2007
Publication Date: March 1, 2008
Citation: Swayne, D.E. 2008. Epidemiology of avian influenza in agricultural and other man-made systems. In: Swayne, D.E., editor. Avian Influenza. Ames, Iowa: Blackwell Publishing. p. 59-85.
Mankind has changed the natural ecosystems of birds through captivity, domestication, agriculture, and commerce which began thousands of years ago and continues through today. This has profoundly changed the existence of LPAI viruses from being a diverse group of viruses circulating asymptomatically in certain free-living aquatic birds to also becoming a less diverse group of influenza A viruses, which have arisen from reassortment or adaptation of whole viruses, causing endemic respiratory disease in horses, pigs, man, and some domestic poultry. In addition, the diverse LPAI viruses of free-living aquatic birds have caused sporadic infections in a variety of wild and domestic mammals and poultry as these viruses attempt to establish new niches. The man-made systems are very diverse and include hobby, village, and rural poultry; fighting cocks; captive wild birds; outdoor-reared non-commercial and commercial poultry; and industrial indoor-reared poultry. The definition of a commercial farm has changed dramatically since the last 1800s, and the development of indoor commercial production has accelerated since the 1950s.
HPAI viruses are not maintained in a wild bird reservoir like LPAI viruses. HPAI viruses have arisen by mutation of H5 and H7 LPAI viruses following uncontrolled circulation of the viruses in susceptible gallinaceous poultry. Historically, HPAI viruses have not been very infectious for domestic or free-living waterfowl (geese and ducks), but over the past 2 decades, the H5N1 HPAI virus that originated in Southern China has adapted to both wild and domestic waterfowl with production of infection, disease, and in some situations death. Transfer of LPAI viruses from free-living aquatic birds requires a complex, multi-step process which includes exposure and adaptation of the viruses to a new host species. Such transmissibility implies exposure, host adaptation, and efficient virus replication in the host species with easy transfer between individuals.
At high risk for introduction of AI viruses from free-living aquatic birds are outdoor reared domestic poultry, especially domestic waterfowl (ducks, geese, and swans). Additional risk factors include intermixing of poultry species on the same premise, and lack of biosecurity and movement controls. There are five means or ways of introducing an AI viruses onto a premise: 1) direct contact with infected wild or domestic birds; 2) exposure to contaminated fomites such as agricultural equipment, vehicles, and materials); 3) through human movement with AI virus contained on shoes, clothing, hair, hands, and skin; 4) contaminated water; and 5) airborne contaminated dust or water droplets. The importance for each of these ways depends upon the individual farm circumstances.