Studying the source
Scientists study pig farming for answers on swine flu
Sunday, October 25, 2009
TIPTON, IOWA -- It may be crowded and carpeted in manure, but the long, white building beside State Route 38 is one of the most pathogen-free homes a pig could have.
The animals never know the feel of grass, mud or sunshine, and hardly the touch of man, in their six 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, the 34-year-old farmer who owns the building. To do that, he said, "biosecurity is of utmost importance."
Despite the buttoned-up methods of farmers such as Schott, many experts 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 such cross-species transfers quickly -- or, better yet, preventing them -- 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 G. Webster, a leading avian influenza virologist. Scientists at the University of Minnesota and the University of Iowa revealed last week 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 may have caught the bug 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 C. 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 is even rarer when the product is a strain that can spread like wildfire. That is 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 has been clear for a while, however, that there is 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 to 20 percent of farmers and 11 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 U.S. Department of Agriculture lab in Ames, Iowa, got samples from two farms in Missouri where pigs were ill.
"We were in for a big surprise," recalled Juergen A. Richt, a veterinary microbiologist at Kansas State University who was working in the Ames lab at the time.
The virus was a reassortment between a swine flu strain and avian flu strains found in mallard ducks and blue-winged teal. (Both farms got water for the pigs from outdoor ponds, which scientists theorize contained flu virus from wild ducks.) More important, the H protein on its surface, which determines its transmissibility, was perfectly adapted to humans and nearly identical to the one found in the 1957 "Asian flu" pandemic virus.
"We had good luck that this did not take ahold, and did not jump into humans," Richt said.
The farm as laboratory
It is to prevent such encounters that operations like Devon Schott's are a kind of down-on-the-farm version of "Hot Zone" laboratories.
Anyone entering his $525,000 metal building must shower and get dressed in coveralls and boots. That clothing and the towels -- like the pigs -- never go outside. Most of the time, Schott and his helpers wear rubber gloves and masks.
All 2,400 pigs come from the same source, arriving as seven-pound weanlings and departing five months later as 250-pound market hogs. They have about eight square feet of space apiece and can eat and drink as much as they like.
Trucks deliver 23 tons of feed -- ground corn, distiller's grain, fat -- to three conical silos outside the barn. The food enters the barn on an automatic conveyer. Twice a day, a man comes through to make sure everything is running smoothly.
The purpose of such "confined animal feeding operations," or CAFOs, is to maximize fast and healthy growth. Gone are many of the things that once stood in the way -- hog lice, mange, roundworms, hookworms and whipworms.
"We have eliminated or minimized so many diseases that used to be standard and common in the swine industry," said Mike Male, 57, a veterinarian who provides the medical care to Schott's animals. Influenza, however, isn't one of them.
A survey done in 2006 found that 58 percent of pig farms had at least one animal with antibodies to influenza. Some farmers seek to protect their herds with vaccines against swine flu strains. Some also offer seasonal flu vaccine (and, this year, the H1N1 vaccine) to their workers, although how many take it is unknown. A sow farm where Male consults offered flu shots last year to its 18 workers; about one-third said yes.
"It was a kind of underwhelming success," he said.
CAFOs such as Schott's are inherently safer than backyard pig farms, where the animals mingle with people and birds fly overhead. But if multiple flu viruses were to get into a CAFO, the crowding of the animals would make widespread transmission, and the chance of reassortment, likely. Mathematical modeling suggests CAFOs can function as "amplifiers" of pandemic strains.
Limiting the virus
The importance of keeping a human pandemic flu strain out of pigs, or limiting its spread, is underappreciated, according to many experts.
"The thing we're concerned about is if this [novel H1N1] virus gets into pigs and then comes back out of pigs into people," said Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. "The question is what may happen to the genetics during the time it's in pigs."
Concerned about just that possibility, Gregory Gray, of the University of Iowa, campaigned via editorials in three medical journals to have swine workers be made a "priority group" in any pandemic vaccine program. He was not successful.
A soft-voiced former Navy physician, Gray has led the effort to document the flow of flu virus between pigs and their keepers. It has been difficult.
Swine farmers have a long-standing suspicion of strangers on their farms. They fear attacks by animal rights and environmental activists; they don't want outsiders bringing bugs to their biosecure herds; and they are wary of scientific projects that may link, even indirectly, human illness and the animal that provides "the other white meat."
Most American pig farmers -- who have been losing money since the fourth quarter of 2007 -- don't want to know whether the new strain is in their herds. Twenty-seven countries banned imports of U.S. pork last spring after the new virus was discovered in people -- even though it wasn't in pigs and, in any case, flu can't be contracted from pork. Later, when several hogs on a swine operation in Alberta, Canada, were found to have H1N1, the farmer couldn't find a packinghouse for them; he eventually euthanized the whole herd.
"People are really scared if their farm is the first one to find an outbreak of pandemic H1N1," said Richt, the Kansas State microbiologist. "They are afraid they will lose their livelihood."
The search for influenza in pigs has actually decreased in the six months since the H1N1 strain was discovered in California and Mexico in April. Diagnostic labs in Minnesota, Kansas and Iowa report a decline in samples submitted by veterinarians; the lab at Iowa State University recently eliminated three positions because of "decreases in overall case revenue."
Gray, Richt and others are putting their hope on a program launched last spring in which the Agriculture Department, not farmers, pays for testing sick pigs and sampling herds where flu is suspected.
In many experts' minds, the program is long overdue.
Most of the flu viruses already in American pigs contain genes derived from human, pig and bird strains. Such mongrelized viruses, scientists believe, are more likely than others to reassort again, setting the stage for another pandemic.