Pigs Often Infect Farmers, Meatpackers
Pigs have their own versions of influenza, and studies of farmers and meatpackers suggest that the animals fairly regularly infect people.
Pigs play an important role in the origin of pandemics because they can be infected by both bird and human strains of flu virus. Consequently, they can serve as "mixing vessels" in which different viruses can combine to form new and potentially dangerous strains.
In most cases, the transmission involves common pig flu that does not go beyond the person who had contact with the animal. But in a few, there is evidence of secondary human-to-human transmission -- which appears to be happening in California and Mexico.
"I suspect that there has been a lot of transmission of swine influenza, causing mild disease in the United States, for a number of years," said Gregory C. Gray, a physician who heads the Center for Emerging Infectious Diseases at the University of Iowa.
In August 2007, about 25 people and 160 pigs developed flu at a county fair in Ohio. Analysis showed they were infected with the same strain -- an H1N1 type containing genes of human, bird and swine origin.
A 2004 study found that in Iowa, 20 percent of swine veterinarians and 3 percent of meatpackers, but no university workers, had antibodies in their blood indicating they had been infected with swine flu.
Another study, of 804 rural Iowans, found that pig farmers were 50 times more likely, and their spouses about 30 times more likely, than university workers to carry swine flu antibodies. The researchers think that in many cases the spouses were infected by their partners.
-- David Brown