Epidemiology, production losses, and control measures associated with an outbreak of avian influenza subtype H7N2 in Pennsylvania (1996-98)

Abstract

An outbreak of H7N2 low-pathogenicity (LP) avian influenza (AI) occurred in a two-county area in Pennsylvania from December of 1996 through April of 1998. The outbreak resulted in infection of 2,623,116 commercial birds on 25 premises encompassing 47 flocks. Twenty-one (one premise with infection twice) of the twenty-five infected premises housed egg-laying chickens and one premise each had turkeys, layer pullets, quail, and a mixed backyard dealer flock. Despite dose proximity of infected flocks to commercial broiler flocks, no infected broilers were identified. Experimentally, when market age broilers were placed on an influenza-infected premise they seroconverted and developed oviduct lesions. The outbreak was believed to have originated from two separate introductions into commercial layer flocks from premises and by individuals dealing in sales of live fowl in the metropolitan New York and New Jersey live-bird markets. Source flocks for these markets are primarily in the northeast and mid-Atlantic areas, including Pennsylvania. Mixed fowl sold include ducks, geese, guinea hens, quail, chukar partridges, and a variety of chickens grown on perhaps hundreds of small farms. Infections with the H7N2 AI virus were associated with variable morbidity and temporary decreases in egg production ranging from 1.6% to 29.1% in commercial egg-laying chickens. Egg production losses averaged 4.0 weeks duration. Mortality ranged from 1.5 to 18.3 times normal (mean of 4.3 times normal). Duration of mortality ranged from 2 to 13 weeks (average of 3.9 weeks) in flocks not depopulated. Lesions observed were primarily oviducts filled with a mucous and white gelatinous exudates and atypical egg yolk peritonitis. Quarantine of premises and complete depopulation were the early measures employed in control of this outbreak. Epidemiological studies suggested that depopulation furthered the spread of influenza to nearby flocks. Thereafter, later control measures included quarantine, strict biosecurity, and controlled marketing of products.