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Flu scare could spur zoonotic research

Producers remain biggest casualty of outbreak, DVMs say
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Jun 01, 2009

NATIONAL REPORT — Investigators may never confirm the source of the H1N1 outbreak of swine influenza, but the outbreak could serve as a wake-up call to veterinarians and legislators to be more vigilant about emerging zoonotic diseases.

Veterinary experts came out in full force after news broke of the H1N1 outbreak of swine influenza — a new strain combining swine, avian and human influenza viruses. They outbreak established veterinarians' role in keeping watch on animal diseases that jump species and become human problems, but Dr. Rodney "Butch" Baker, senior clinician in the Food Supply Veterinary Medicine Unit at Iowa State University's College of Veterinary Medicine and current president of the American Association of Swine Veterinarians (AASV), says the attention to the importance of veterinarians in battling zoonotic diseases isn't enough without financial backing for better surveillance and more food-supply veterinarians.

"We really don't know where this virus came from. There's been no federal dollars for a surveillance effort," Baker says. "What little money has been coming out of the national coffers has primarily gone to avian surveillance."

No official surveillance program exists for swine, but Baker says research centers and universities have been informally studying swine influenza samples for some time, though. They started going back and testing and re-testing old samples that had fallen by the wayside once Mexico started reporting the new H1N1 strain, Baker says.

Researchers at Iowa State believe the currently circulating swine H1 viruses are descendents from a common ancestor of one of the two viruses that reassorted to create the novel H1N1 strain which is now circulating in humans. They have not found the human virus in pigs, and the ones that are in pigs are only partially related, Baker says.

"We went back and looked for the genetic code of the new novel H1N1 and discovered that this human virus isn't present in our samples and the hemagglutinin gene is 10 percent different from those triple reassortment viruses circulating today," Baker explains. "The viruses observed in the field and sequenced here at Iowa State change at a rate of about 1 percent per year. Since there is on average 10 percent change in the code, it appears that one of the ancestors of this new virus has been out of U.S. pigs for about five to 10 years. We suspect it's been on another continent.

"You can guess that our triple assortment virus, somewhere maybe five to 10 years ago, reassorted with a virus of Asian or Eurasian origin and created this virus," he continues. "We'll probably never know how this virus was created, but we are almost certain it wasn't created in North America because we don't import pigs from Asia. But Asia imports pigs from North America."

Dr. Christopher Olsen, associate dean for academic affairs and public health professor at the University of Wisconsin-Madison School of Veterinary Medicine, specilializes in influenza studies and says that, while it's too early to know the virus' origin, labs worldwide are working to crack the genetic code. The current form of the virus could have been mixed within a pig, but also in humans or another species, he explains.

Pigs have for some time carried forms of the influenza virus, he explains. Four forms now are common in many herds, including the "classic" H1N1 virus, which pigs caught from people around 1919 following the Spanish influenza outbreak. No influenza was detected in pigs before that time, Baker adds.

Other strains present in today's swine include two triple reassortment H1N1 viruses, which Baker says are "distant cousins" of the new H1N1 strain. Finally, there is a human-like virus that pigs probably originally caught from humans that quickly adapted to swine. Baker says to this point, that particular virus has yet to be transferred from person to person and human cases are rare.

"These virus species adapt very quickly, but once they do, they are slow to jump back to another species," he says. "We would expect this current virus, if it does get into pigs, will adapt to the species very quickly. If you're a doomsday person, you'll think these viruses will all get together and some 'supervirus' will come from that. But historically, that's not the case."

One of the "silver linings" of this outbreak is that the virus may prove to be a valuable research tool, says Olsen, explaining it could help researchers identify the genetic markers that allow viruses to be transmitted from pigs to people and vice versa.

Another risk, however, is if the current H1N1 virus strain circulates among humans this fall and worsens, it will be more likely to enter swine herds. But by then, vaccines may have been created for both people and pigs.

Recent cases in one Canadian swine herd showed only 20 percent morbidity, meaning 80 percent of the herd either showed no clinical signs of infection with the virus or didn't get infected in the first place. No deaths were reported in that herd, Baker says. Additionally, the National Animal Disease Center in Ames, Iowa, recently tested the virus on pigs with no other forms of influenza and found that it presented no worse that any other influenza strain in pigs.