Scientists examine pig farming for answers on H1N1

The animals don’t know the feel of grass, mud, or sunshine, and hardly the touch of human hands in their 6 months of life. But they are also free of many of the infections that slow the growth and occasionally end the lives of their outdoor cousins.

“We’re producing the most efficient animal, one that is healthy every day,’’ said Devon Schott, 34, the farmer who owns the building. To do that, he said, “biosecurity is of utmost importance.’’

Despite the efforts of farmers such as Schott, many scientists think pig farming presents a serious and overlooked risk to public health. Proof of that assertion - indirect but indisputable, in the opinion of virologists - is the 2009 H1N1 pandemic influenza.

Little is known about the origin of the novel H1N1. But one thing is virtually certain: The bug now infecting the people of more than 190 countries began in a pig. Detecting and preventing such cross-species transfers quickly is an urgent priority in a field that has spent most of its energy in recent years worrying about the emergence of flu from birds in Asia. A major concern now is what might happen if the pandemic H1N1 virus spreads widely in pigs, and then out again into the human population.

“We really need to know more about what is happening in the pig population in the United States,’’ said Robert Webster, a leading avian influenza virologist.

Scientists at the University of Minnesota and the University of Iowa revealed last week that they had identified the H1N1 strain in seven pigs at the Minnesota State Fair in late summer as part of a study of virus exchange between swine and people.

Some of those animals might have caught the virus from the hordes of visitors at the 12-day event. But not all: One infected animal was swabbed while being unloaded and almost certainly arrived with the virus, said Gregory Gray, a physician and epidemiologist at the University of Iowa who helped run the study.

What worries virologists is the mixing of human and swine flu strains - or, worse, human, swine, and bird strains. That can lead to “reassortment,’’ in which strands of genetic material are exchanged to yield a new virus, often with behavior not seen in its parents. Those features can include higher contagiousness, rapid growth, the ability to infect the lungs and, most important, an unfamiliar appearance to the immune system.

Reassortment is rare, and it’s even rarer when the product is a strain that can spread like wildfire. That’s one reason influenza pandemics occur only a few times a century. (The last one was in 1968.)

A major goal of public health is to make such events even more rare. One way is to keep pigs and humans away from each other’s flu viruses. It’s been clear for a while, however, that there’s a small but steady traffic of virus between America’s 110 million pigs and the 120,000 people who care for them.

In 2006, a team of researchers at the University of Iowa examined blood samples from 111 farmers, 65 veterinarians, and 97 meat-processing workers, and compared them with 79 university employees and students who had no contact with pigs. The scientists looked for antibodies to two common swine influenza viruses. They found that 17 percent to 20 percent of farmers and 11 percent to 19 percent of veterinarians had evidence of previous infection by the two strains. None of the meatpackers or students did.

Another study by the same research team found that the wives of half of infected pig farmers had the antibodies - suggesting that person-to-person transmission of the viruses was possible.

Influenza is transmitted from pig to human the same way it is transmitted from human to human - in respiratory droplets and in hand-to-mouth contact. Most cross-species infections end on the farm because swine flu strains, even if occasionally acquired by animal handlers, are almost never well-adapted to human hosts.

But there have been some close calls. One occurred in 2006, when the US Department of Agriculture lab in Ames, Iowa, got samples from two farms in Missouri where pigs were ill.

The virus was found to be well adapted to humans, but the transmission did not occur.