Dr Robert Owen of Alpharma, formerly affiliated with the University of Pennsylvania, was the keynote speaker at the Southern Conference of Avian Diseases and Annual Meeting of the Southern Poultry Science Society. His presentation was on two outbreaks of low pathogenic avian influenza (LPAI) in turkeys in the USA last year.

Dr Owen prefaced his remarks with the cautionary comment, "This will happen again and we'll have to deal with it again." He reviewed the current approach to detection and response to LPAI in commercial flocks. The two cases involved separate and unrelated sporadic incidents in turkeys in West Virginia (H5N2 LPAI) and in Virginia (H5N1 LPAI).

The source of the infective agents is unknown but it is presumed that free-living waterfowl served as reservoirs with extension to the commercial flocks as a result of defective biosecurity.

Neither flock showed clinical signs and detection occurred as a consequence of an on-going comprehensive surveillance programme applying agar gel immuno-diffusion (AGID) serology.

Since cases of H5N2 LPAI have occurred in previous years as outbreaks on individual farms operated by contractors on the Delmarva Peninsula, a federal programme was developed in co-operation with individual states and the poultry industry to respond to infection. The imposition of trade restrictions and the new guidelines from the World Organisation for Animal Health (OIE), which require immediate reporting of H5 and H7 isolates, have serious economic and logistic implications. In terms of the current programmes implemented by state departments of agriculture and approved by the federal government, compensation can be paid to owners of depleted flocks. This is predicated on a state programme based on surveillance, approved detection procedures, prompt reporting and a plan for euthanasia and disposal coupled with biosecurity and preventive measures.

Two outbreaks of LPAI

In his presentation, Dr Owen described each of the two outbreaks and contrasted the responses by state officials and company representatives. The outbreak in West Virginia involved 25,000 turkey toms of approximately 20kg. Routine pre-slaughter AGID assay confirmed the presence of positive reactors. An H5 avian influenza virus was isolated from the tracheas of sacrificed birds, notwithstanding the clinically unaffected status of the flock. Diagnostic procedures were completed on 30 March 2007 and depopulation commenced on 1 April, with completion on 3 April. Composting of the approximately 500 tonnes of dead birds was initiated immediately after depopulation and the process was completed by 5 April, demonstrating the benefits of prior contingency planning. The integrator responsible for the flock completed the depopulation using foam and the entire programme was completed successfully without a defined command structure.

The outbreak in Virginia involved 24,000 toms of approximately 20kg. A flock of 30,000 3-week-old turkeys also present on the farm had to be depleted. The presence of antibodies by AGID in the older turkeys was detected on 6 July 2007. A polymerase chain reaction (PCR) assay was conducted by a state diagnostic laboratory. It yielded a suspicious-positive result. Initially, tracheal swabs were negative on viral isolation but subsequent attempts yielded an H5 avian flu virus. Virginia did not have a mass depopulation plan and depletion of the flock was delayed by approximately five days to 11 July but was completed by 14 July with composting of carcasses on 17 July.

Lessons learned

Dr Owen commented favourably on the co-operation among integrators operating in both areas, which have a high density of turkeys. The use of foam to de-populate heavy birds proved to be effective and humane although it was necessary to make certain modifications for the large birds. In the Virginia outbreak, a commercial contractor was responsible for depletion, transport and composting. Despite contingency planning, there was a problem acquiring sufficient cellulosic material for composting. Establishing responsibility for command and control of quarantine, euthanasia and disposal was delayed in one case.

Two issues should result in modifications to response programmes. With LPAI in asymptomatic flocks, diagnosis of infection should be expedited and the laboratory criteria must be clearly defined. Guidelines issued by OIE should be followed but specific new protocols will be required following consultation among federal, state and industry specialists. The second problem relates to appraisal of the value of flocks. In view of the high cost of heavy turkeys US$3 million in these two cases or in the event of extensive infection, both US Department of Agriculture (USDA) and state officials are currently required to jointly agree on the value of flocks.

Previous outbreaks of LPAI caused by viral strains other than H5 and H7 in turkeys in Minnesota during the past two decades have not resulted in depletion. Affected flocks were quarantined followed by controlled processing and, if clinically normal, with distribution of product from birds that show no lesions at the time of slaughter. A precedent was established in Connecticut where over two million hens infected with H7 AI in a large complex were quarantined and vaccinated but were not depleted. Questions therefore arise as to whether the quarantine and controlled processing approach could be used in future outbreaks to avoid needless euthanasia with associated welfare and cost implications. Given the current perception of AI following the ongoing sequence of highly pathogenic H5N1 outbreaks in south-east Asia, and the more recent isolated episode in United Kingdom turkeys, it is felt that consumer rejection would occur following controlled processing and distribution of turkey products from asymptomatic flocks infected with LPAI. The fact that both outbreaks were associated with an H5 strain was also a consideration in relation to the export market, notwithstanding the OIE principle of regionalisation. Although West Virginia is not a significant exporter of poultry products, Virginia with the adjacent states of Maryland and Delaware are responsible for over 12% of total US broiler production, which approaches 170 million birds per week. Integrators in these states rely heavily on export of leg quarters from processed broilers averaging over 3kg liveweight.

These two outbreaks occurred in areas with a high density of flocks, requiring strict enforcement of quarantine. The successful implementation of control measures is evidenced by the fact that no secondary outbreaks were recorded despite intensive surveillance in the two concentric zones established in accordance with OIE recommendations.

These episodes denote the high degree of readiness of the US poultry industry to respond to outbreaks of diseases that have trade or catastrophic significance. Transparency in reporting and review of detection and control also offer a learning experience to poultry industries in other countries.