[US] Animals Share Bugs With Us
The Washington Post - Thursday, May 7, 2009
New Virus, Old Tale: Animals Share Bugs With Us
By Joel Achenbach, Washington Post Staff Writer
Somewhere out there, somewhere along the way, a single creature got all this
started. A pig, presumably. Pig Zero.
Scientists suspect that two influenza viruses common in swine, one rooted in
Eurasia and the other in North America, came together in a single cell
within a pig. The two viruses exchanged their genes like a couple of kids
swapping school clothes. The result was a novel strain of virus, with,
according to scientists, two genes from the Eurasian virus and six genes
from the North American virus.
The new strain then jumped to humans. Where is unknown. Mexico is a
possibility, but so far the virus hasn't been found in any Mexican swine.
All of this is the latest iteration of a phenomenon dating to the dawn of
mankind: zoonosis. A zoonotic disease is one that spreads from animals to
humans, or vice versa. Bubonic plague came from a bacterium that infects
rats and can spread via fleas to humans. HIV is a virus that passed into
people from a monkey. Malaria, tuberculosis, rabies, yellow fever and
typhoid fever are zoonotic.
And it's a two-way street, as seen recently when a Canadian farmworker
infected with the new H1N1 swine flu apparently passed the disease to a herd
of pigs. When it comes to influenza, the thoroughfare between Homo sapiens
and Sus scrofa -- domesticated pigs -- is something of a superhighway.
From the perspective of an influenza virus, the receptors on the lungs of a
human being -- the places where the little spiky knobs on the virus can
attach themselves -- look very much like the receptors in a pig. A pig's
anatomy is so similar in certain respects to a human being's that pig heart
valves are routinely transplanted into human heart patients.
"Zoonotic agents don't care whether it's a human or an animal ," said
Juergen Richt, a professor of veterinary medicine at Kansas State
Thanks to vaccines and antibiotics, the war against infectious diseases
seemed to be nearly won by the second half of the 20th century, but the
pathogens have shown themselves to be resilient and adaptive. Meanwhile, the
human population has grown to more than 6 billion, sustained by billions of
farm animals, many raised in close quarters on factory farms, said JoLynn
Montgomery, an epidemiologist at the University of Michigan.
"There's more crowding in animals, and more crowding in people, and the
crowding is merging," she said. "People are getting diseases from animals
more frequently. I'm not sure the diseases themselves are getting worse."
Public health measures -- careful surveillance of zoonotic diseases -- can
counterattack the problem, she said.
Zoonotic diseases can also come from wild animals, and new pathogens can
emerge as human beings penetrate remote, isolated regions of the planet,
said Thomas J. Inzana, a bacteriologist at Virginia Tech. Some exotic
pathogens are so "hot" that they can't spread as easily as viruses that are
less lethal, he noted: "It doesn't do the pathogen any good to kill its
Which is why flu is such a problem: It has essentially co-evolved with
people, pigs, birds and other animals. And it's malleable. Influenza is what
is known as an RNA virus. Such viruses, mere snippets of genetic material,
replicate inexactly, like photocopy machines on the fritz. That sloppiness
enables them to evolve rapidly and find new hosts, and makes them a moving
target for vaccine makers.
The specific origin of the new flu strain remains a matter of intense
investigation. Even the presumption of a Pig Zero is just educated
guesswork. The new virus conceivably could have spliced itself together
inside a human being or some kind of bird. A pig is the most likely source
simply because two ancestral viruses had clear genetic markers of
swine-related flu, and a pig is the most likely place for two swine flus to
converge, said Andrew Pekosz, a virologist at the Johns Hopkins Bloomberg
School of Public Health.
That said, a veterinarian and spokeswoman for the National Pork Producers
Council, Jen Greiner, emphatically rejects any presumption that the new
strain emerged in a pig: "It has not been found in any pigs in Mexico; it
has not been found in any pigs in the U.S.," she says.
Influenza is hardly just a human-and-pig tango. Birds can also get flu. So
can horses. But although humans can, in rare cases, catch bird flu, it
doesn't then spread from person to person. Humans can't catch horse flu.
Dogs, however, can catch horse flu. Why these viruses do what they do and
jump where they jump is not well understood, Richt says. He says that's all
the more reason for veterinary health research to be on equal funding
footing with human medical research. "You need to understand what's
happening in the cattle, the pig, the raccoon, to protect the human
population," Richt said.
There's a lot a human would recognize in a pig with flu. "The pig feels hot,
and it doesn't eat as much," Richt said. If the condition of pigs worsens,
"they develop nasal and ocular secretions. And if they go further, they
start to have respiratory distress, so they breathe faster, and they can go
to coughing, sneezing."
Although factory farming has been a target of much criticism, it has its
defenders. Marie Gramer, a veterinarian at the University of Minnesota, said
enclosed farm buildings offer "biosecurity" from pathogens carried by wild
"The health now of the collective animal agriculture is better than it was
20, 40, 60 years ago, " Gramer said.
Although some of the worst fears about the current swine flu outbreak have
subsided -- the virus doesn't appear to be as virulent as first thought --
the very nature of influenza makes the future of this strain impossible to
predict. It will surely evolve further, Pekosz said.
"This is a brand-new virus and a brand-new host," Pekosz said. The process
of natural selection will tug the virus in new directions, he said. His
scientific prediction -- "That gene constellation is probably going to
optimize itself to replicate" -- strongly suggests that human beings haven't
heard the last of this new swine flu.
© 2009 The Washington Post Company
Join us at Animal Rights 2009 National Conference
July 16-20th, Los Angeles; www.ARConference.org