23.06.2017

A combined outbreak of highly pathogenic avian influenza (HPAI) and low pathogenic avian influenza (LPAI) started in early March 2017. The outbreak was first suspected by an increase in mortality on a commercial breeder farm in Tennessee and was confirmed as H7N9 highly pathogenic avian influenza (HPAI) of North American wild bird lineage (AM H7N9 HPAI) via laboratory testing. After initial efforts to control the disease, a series of epidemiologic, genetic,and wildlife investigations were started. This is a report of the findings available to-date, and these analyses are intended to assist in understanding disease introduction and transmission pathways.
These studies were undertaken collaboratively with the poultry industry, State agriculture personnel, and the Animal and Plant Health Inspection Service.The outbreak area lies primarily in south central Tennessee and northern Alabama, but cases have occurred in southern Kentucky and western Georgia. This area is bounded by dense areas of poultry production, including broiler, table-egg layer, and turkey operations. Sampling and testing of wildlife was implemented near a commercial premises infected with AM H7N9 Influenza A virus (IAV). Initial results have not confirmed the presence of H7N9 IAV from wild bird samples; however, limited evidence suggests that one or more animals sampled may have been previously exposed to an IAV. Genetic analyses determined that all H7N9 viruses detected from this event are of North American wild bird lineage, and the HPAI and LPAI viruses are highly similar across all eight genes, excluding the multi-basic amino acid insertion at the cleavage site in the HPAI virus.Initial genetic and epidemiologic evidence suggest the possibility of more than a single introduction with limited lateral transmission. These viruses were most likely introduced inlate February based on available barn-level egg production, mortality, and testing information. H7 detections have occurred during summer, fall, and winter and throughout the Mississippi and Atlantic Flyways. Annually, H7 detections tend to peak in early winter. This pattern may have contributed to the timing of this outbreak in the southeastern United States. The six commercial AM H7N9 LPAI farms involved five different integrated poultry complexes suggesting unique sources of feed, pullets, males, egg transport trucks, and crews for most of the cases. Potential routes of lateral spread between farms, related to potential concerns such as the biosecurity of egg pickup, trucks, visitors, equipment and disposal activities, did not appear to be risk factors during this outbreak. However, factors such as the presence of rodents and other wild mammals and waterfowl near barns, the condition of the housing, and breaches in biosecurity protocols were identified as risk factors that could bring viruses into the barns from the environment.
The fact that backyard operations have also been impacted adds credence to the theory that this outbreak was governed in large part by exposure to environmental sources of virus.Broiler flocks located in the area were unaffected during this outbreak. Results from the expert elicitation highlighted differences in housing type, age of birds, and length of the production cycle, which could help explain why broiler flocks were not infected.