The past couple of decades have yielded repeated - and lethal - reminders of how animals can make people sick. Think apes and AIDS, mosquitoes and West Nile virus.
The latest example: pigs and MRSA, the bacterium that in recent weeks has infected schoolchildren and caused custodians to scour emptied classrooms, dousing any trace of the germ.
Canadian researchers who studied farmhands and the pigs they tend found that a quarter of the swine and a fifth of the humans harbored Methicillin-resistant staphylococcus aureus, a bacterium that's especially feared because first-line antibiotics are useless in treating it. And it wasn't just any type of MRSA, according to the study from the University of Guelph in Ontario.
"In every situation where we found MRSA in a human, we found the same strain in pigs on their farm," said Dr. Scott Weese, an author of the study in the journal Veterinary Microbiology.
That's potentially powerful evidence that community cases of MRSA, which until recent years had been limited to hospital patients, may have originated in animals. Already, human cases in Europe have been linked to swine.
But there's also evidence that people can transmit the germ to pigs.
"Everybody assumes it goes just one way, but it really goes both ways," said Dr. George Saperstein, chairman of the Department of Environmental and Population Health at the Tufts University Cummings School of Veterinary Medicine. "The question of the day is, which way did it go? Did the pigs give it to the people? Did the people give it to the pigs?"
Both the pigs and the people on the Ontario farms tended to harbor one of two strains.
The first is the same type of MRSA that has been infecting swine in Europe and spreading to humans. It's called ST398. The second predominant strain is called USA100, and it has been most often associated with human cases of MRSA, suggesting the pigs caught it from the people.
Research has shown that three of every four newly emerging diseases in the world have their roots in the animal kingdom. A prime culprit is the pig. Swine have long been recognized as remarkably efficient incubators for germs - and for the ability to take parts of germs from different species and reassemble them into dangerous, novel viruses and bacteria.
And then it doesn't take much for a germ from a pig to make the leap into a human.
In part, that's because pigs turn out to bear a striking anatomical resemblance to people. "That's one of the reasons why they are commonly used as animal models in research," Saperstein said.
MRSA-colonized pigs were first found in the Netherlands, where they have been blamed for human outbreaks of the disease. A series of research papers published in the past two years has documented the germ in swine in at least three other nations: Denmark, France, and Singapore.
But, until the Canadian researchers went hunting for it, nobody else had searched for MRSA in pigs in North America, according to the study currently posted online. The researchers were looking solely at living pigs, because cooking typically kills bacteria. American agricultural authorities said that while they have not tested swine for MRSA in the past, they are reviewing that policy. An industry group, the National Pork Board, intends to start testing for the germ in coming weeks.
Testing needs to be made routine, said researchers with science-based advocacy groups.
"A lot of these bacteria are going to be washed off into the waterways, into the environment, and we just don't know what it really means," said Margaret Mellon, director of the Food and Environment Program at the Union of Concerned Scientists. "I want people to understand whether or not a lot of MRSA is being generated on the farm and whether it's moving back to humans."
Groups such as Mellon's are also interested in the issue because of their long-standing concern about overuse of antibiotics in livestock. They suggest MRSA in swine could be evidence that misuse of antibacterial drugs in animals has allowed drug-resistant strains of germs to flourish.
Weese acknowledged those concerns. "It's logical to assume that there could be a role there, but that's currently unproven."
There was one piece of good news from the Canadian study: Neither the pigs nor the people got sick from the MRSA. That's to be expected with the pigs, who make for good carriers precisely because the germ usually doesn't make them ill.
People, though, aren't always so luckyThe Boston Globe
