While the findings are not entirely unexpected, the strikingly higher risk of infection coupled with the fact that pigs can be infected by swine viruses, bird (avian) viruses as well as human flu viruses--thereby acting as a virtual virus "mixing bowl," especially on farms where pigs, chickens and people coexist--is a potential public health concern, the study authors assert. The paper appears online this week in Clinical Infectious Diseases.
"Pigs play a role in transmitting influenza virus to humans," says NIAID Director Anthony S. Fauci, M.D. "The worry is that if a pig were to become simultaneously infected with both a human and an avian influenza virus, genes from these viruses could reassemble into a new virus that could be transmitted to and cause disease in people."
The study results strongly suggest that occupational exposure to pigs significantly increases the risk of developing swine influenza infection. Agricultural workers should, therefore, be considered in developing flu pandemic surveillance plans and antiviral and immunization strategies, according to the study's co-investigator, Gregory C. Gray, M.D., director of the University of Iowa Center for Emerging Infectious Diseases.
"If migratory birds introduce the H5N1 bird flu virus into swine or poultry populations in this country, agricultural workers may be at a much greater risk of developing a variant H5N1 and passing it along to non-agricultural workers," Gray says. "Not protecting agricultural workers could amplify influenza transmission among humans and domestic animals during a pandemic and cause considerable damage to the swine and p oultry industries, as well as the U.S. economy." While swine in other countries have been infected by the H5N1 virus, to date, the virus has not become readily transmissible between swine.
Swine influenza infections generally produce mild or no symptoms in both pigs and humans. However, exposure to swine flu virus at a 1988 Wisconsin county fair resulted in serious illness for 50 swine exhibitors and three of their family members; one previously healthy woman who became infected died.
The U.S. swine industry, which employs about 575,000 people, has shifted during the past 60 years from primarily small herds located on family farms to large herds maintained in expansive but confined agricultural facilities. Crowded conditions coupled with the constant introduction of young pigs to existing herds have made swine flu infections among pigs a year-round occurrence rather than the seasonal event they once were. As a result, there is a constant opportunity for people who are occupationally exposed to pigs to become infected with influenza viruses and, conversely, a continual opportunity for human flu viruses to mix with swine or bird flu viruses.
To determine the prevalence of swine influenza infection among swine-exposed employees, the researchers, led by Dr. Gray and graduate student Kendall P. Myers, examined serum samples taken from four adult populations in Iowa between 2002 and 2004. Three populations were occupational groups exposed to pigs: 111 farmers, 97 meat processing workers and 65 veterinarians. The fourth control group included 79 volunteers from the University of Iowa with no occupational pig exposure.
The researchers tested the serum samples for antibodies to several then-current swine and human influenza A viruses. The results showed that all three occupational study groups had markedly elevated antibodies to swine flu viruses compared with the control group. Farmers had the strongest indication of exposure to swine flu vi ruses, as much as 35 times higher than the control group. Similarly, comparable values were as much as 18 times higher for veterinarians and as much as 7 times higher for meat processors than the control group. In contrast, exposure to human flu virus in the occupational groups was not significantly different than that of the control group.
To date, the H5N1 avian virus has not appeared in the United States in any animal population or in humans.