An overview of the 2002 outbreak of low-pathogenic H7N2`avian influenza in Virginia, West Virginia and North Carolina

Abstract

During the spring and summer of 2002, an outbreak of low-pathogenic H7N2 avian influenza virus (AIV) infected 210 flocks of chickens and turkeys in Virginia, West Virginia and North Carolina, and caused the destruction of more than 4.7 million birds. Although no epidemiologic link was established, the virus was related to the H7N2 virus circulating in the live-bird market system (LBMs) since 1994. An avianinfluenza Task Force (TF), comprised of industry, state and federal personnel, was utilized in the control programme. The use of good safety and biosecurity practices was emphasized by TF commanders. Carcass-disposal options, which included burial in sanitary landfills, incineration and composting, proved to be problematic and caused delays in depopulation of infected premises. Surveillance activities focused on once-a-week testing of dead birds from all premises, biweekly testing of all breeder flocks and pre-movement testing. Additional surveillance carried out in backyard flocks and local waterfowl did not detect the H7 virus or specific antibodies to the virus. The outbreak emphasized the need to establish effective biosecurity barriers between the LBMs and commercial poultry. Avian influenza (AI) is a viral disease that can affect many species of wild and domestic birds, including poultry. The AI virus (AIV) is comprised of 15 subtypes based on differences in antigenic nature of the surface haemagglutinin (HA) protein and is classified, based on pathogenicity, into low-pathogenic (LPAI) and highly pathogenic (HPAI) viruses (Swayne and Halvorson 2003). The natural reservoirs of avian influenza virus (AIV) are migratory waterfowl and shorebirds (Kawaoka et al. 1988; Slemons et al. 1974). However, the live-bird market system (LBMs) has been recognized as a significant man-made reservoir of poultry-adapted AIV and has been implicated in several outbreaks of AIV in commercial poultry in the United States (Committee on Transmissible Diseases of Poultry and Other Avian Species 2002; Davison et al. 2003). The highly pathogenic form of AI is extremely contagious and lethal, causing sudden death in poultry, often without any warning signs of infection. Mortality in flocks infected with HPAI can often reach 100%. It has been documented that PAI can evolve through the mutation of LPAI H5 or H7 precursor viruses after circulating for extended periods in unnatural hosts such as domestic poultry (Capua and Marangon 2000; Horimoto et al. 1995; Kawaoka, Naeve and Webster 1984; Webster 1998). Low-pathogenic strains of AI can also be highly contagious often resulting in subclinical infections, allowing the virus to spread undetected for a period of time. In March 2002, a LPAI H7N2 virus similar to a strain of H7N2 virus known to be present in the LBMs in Northeast United States was found to be present in commercial poultry in Virginia, West Virginia and North Carolina. To reduce the possibility of the H7 virus mutating to HPAI, a control programme was implemented to eradicate the H7N2 virus from commercial poultry in the region. Agriculture authorities in Virginia initially took steps to control the H7N2 LPAI through diagnostic testing, quarantines, surveillance, and depopulation and disposal of infected poultry. However, the rapid increase in the number of positive cases quickly overwhelmed the State’s capacity to manage the outbreak. Consequently, the Commonwealth of Virginia asked the USDA for assistance in controlling the outbreak. This paper will provide an overview of the outbreak and methods used to control the outbreak.