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Crossing the Road: Modeling Avian Influenza Outbreaks

Wednesday, June 3, 2015

By Tim Boyer, PhD, Epidemiologist, National Center for Food Protection and Defense

The ongoing highly pathogenic avian influenza (HPAI) outbreak illustrates the vulnerability of the United States food and agriculture system to transboundary animal diseases. As of May 28, 2015, the disease has been confirmed in 197 poultry operations in 15 states. Approximately 45 million birds have been affected, comprising 10% of the country’s laying hens and 8% of turkeys.

Turkey Farm

The Outbreak

Avian influenza viruses are classified as low pathogenic or highly pathogenic. Low pathogenic strains generally cause mild illness in domestic poultry while highly pathogenic (HPAI) strains cause serious illness often resulting in 90-100% mortality in domestic poultry. Low pathogenic avian influenza strains continuously circulate among wild birds in the United States and sporadically affect domestic poultry. The current HPAI outbreak is only the fourth recorded occurrence of HPAI in the United States and has been the most severe in terms of numbers of affected premises and birds. The other outbreaks occurred in 1924, 1983, and 2004.

Evidence suggests that the outbreak was caused by a Eurasian H5N8 HPAI strain that was presumably carried to North America by waterfowl migrating via the Pacific flyway. Some of the earliest detected flocks in Canada and the western U.S. were infected with the Eurasian H5N8 strain; however this strain reassorted with North American low pathogenic avian influenza strains and one of these, H5N2, has been the predominant cause of disease in domestic poultry during this outbreak.

Minnesota and Iowa have been the two hardest hit states with Minnesota having the largest number of infected, mostly turkey, premises (92 premises with 8 million birds) and Iowa having the largest number of birds, mostly egg laying hens, on affected premises (64 premises with 29 million birds).

The Paths of Avian Influenza

Simulating the Outbreak

NCFPD has been working with the USDA APHIS Center for Epidemiology and Animal Health to apply epidemiologic simulation models to questions related to the spread of HPAI in domestic poultry to help inform preparedness activities. For transboundary animal diseases with which the United States has had limited experience, simulation models are one tool that can help answer “what if” questions to aid decision making when preparing for the possibility of an outbreak.

Past NCFPD work in this area has included simulations of HPAI spread in commercial and backyard poultry in order to estimate the effectiveness of various control strategies and how those control strategies might be impacted by labor and financial resource constraints during an outbreak response. The HPAI simulations were also used to estimate the numbers of flocks and birds that would need to be serologically sampled to prove disease freedom according to guidelines described in the Federal HPAI Response Plan.

Affecting the Nation

The outbreak has had a devastating economic impact on the farmers and communities affected. An economic analysis by University of Minnesota Extension estimated that, as of May 11, the outbreak cost over $300 million in Minnesota due to direct losses, lost wages, reduced business to business spending and other ripple effects. An additional economic impact that has not yet been quantified is trade loss due to bans on U.S. poultry products by other countries. The effects of the outbreak have yet to reach U.S. consumers but the wholesale cost of liquid eggs have tripled in the past month leading food companies to find ways to mitigate the effects of the shortage by switching ingredients or discontinuing certain product lines. Prices at grocery stores are likely to increase.

The industry and government have made a substantial effort to strengthen biosecurity on poultry operations and to prepare for an HPAI outbreak such as this one. That this strain has caused such a devastating impact in spite of these efforts highlights the challenges of preparing for such an event and the need to constantly seek ways to mitigate the impacts and improve the resilience of our food and agriculture system.



Graphic Created by Kendra McCormack for the National Center for Food Protection and Defense
Country Outline Attribution: Map of United States of America with States - Multicolor by
Information Source: United States Department of Agriculture, Animal and Plant Health Inspection Service - AI Findings