The rash of H5N2 avian influenza outbreaks has spread farther across the Midwest, with a Wisconsin chicken farm joining the list of affected sites today, following two new turkey outbreaks in South Dakota and another in Minnesota over the weekend.
Poultry scientists say it's a mystery why the virus is popping up in many widely scattered locations in a short period, especially given that on most farms, just one of several barns is hit by the virus. In Minnesota, wildlife experts have not found the virus in any wild birds so far.
Wisconsin, South Dakota, Minnesota outbreaks
The outbreak in southeastern Wisconsin's Jefferson County is the first on a commercial chicken farm in the Midwest, all of the previous ones having involved turkey farms (though a backyard poultry flock in Kansas was hit in March). The virus struck a commercial flock of 200,000 layer chickens, the US Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) reported.
Increased deaths in the flock prompted officials to commission testing by the Missouri Department of Agriculture Veterinary Diagnostic Laboratory, with results confirmed by the APHIS National Veterinary Services Laboratories in Ames, Iowa, the USDA said.
The Wisconsin Department of Agriculture, in a statement today, put the size of the flock at 180,000. The USDA and state officials said they plan to mount the standard response to the outbreak, including quarantine of the farm, culling of all the chickens, and testing of poultry in the surrounding area.
The two new outbreaks in South Dakota, announced Apr 10, are the state's third and fourth. One involves a farm with 53,000 turkeys in McCook County, which is northwest of Sioux Falls, and the other involves a flock of 46,000 turkeys in McPherson County in the northeastern part of the state.
McPherson County is just across the border from Dickey County, North Dakota, where an H5 outbreak in turkeys came to light last week. Over the weekend (Apr 11), the USDA confirmed that the virus in North Dakota was the highly pathogenic H5N2 strain. The affected farm had 40,000 turkeys.
The USDA outlined the same response steps for the South Dakota and North Dakota outbreaks as in all the others announced recently.
South Dakota had two earlier H5N2 outbreaks, both on turkey farms in the east-central part of the state. The first, in Beadle County, was announced Apr 2, and the second was reported a week later in Kingsbury County, which adjoins Beadle on the east.
In Minnesota, the latest outbreak is in the west-central county of Kandiyohi, which already had two recent outbreaks. It is the state's 14th incident since early March, when the virus surfaced in Pope County, which borders Kandiyohi.
This time the virus hit a farm with 38,000 turkeys. USDA and state announcements did not specify how close the farm is to the other two outbreak sites. Minnesota is the nation's leading turkey-producing state, and Kandiyohi is the state's top turkey-producing county.
A mysterious pattern
The H5N2 virus has been found in a few wild birds, which can carry it without appearing sick. Officials believe wild waterfowl have been spreading it across the country. Infected birds shed the virus in their feces, leading to the suspicion that the recent outbreaks have been sparked by feces somehow carried into poultry barns by vehicles or people.
But in Minnesota, at least, investigators haven't been able to confirm or rule out that suspicion, nor have they found the virus in fresh fecal samples collected from wild ducks near affected farms.
Bethany Hahn, spokeswoman for the Minnesota Board of Animal Health (MBAH), said today that the investigation by a large group of USDA and state officials has not yet come up with any solid answers on how the virus is spreading.
"Some of the affected flocks are in close proximity to each other," she commented by e-mail. "But other than geographical, there are no apparent connections between the affected flocks. Once the epi[demiologic] investigations are finished and we have a more complete picture, we will share that information."
The state Department of Natural Resources (DNR) has been testing fecal samples from wild ducks near some of the affected turkey farms and has not yet found the H5N2 virus. Michelle Carstensen, PhD, the DNR's wildlife health program supervisor, reported today that 240 samples collected last week all tested negative.
She said 120 samples were from Stearns County, the site of four outbreaks, and 120 came from Lac Qui Parle County, which has had one incident. More samples are being collected this week in those two countries and three others, she added.
An odd lack of links
Carol Cardona, DVM, PhD, who holds the Ben Pomeroy Chair in Avian Health at the University of Minnesota, said in a recent interview that the lack of evidence of connections between the affected farms in Minnesota is very puzzling. She noted that she is not directly involved in the investigation of the current outbreak, but she and her colleagues have been studying virus introductions in turkeys for several years.
"This is really unprecedented," she said. "It really makes us scratch our heads, because it's not what we're brought up to think about. We're brought up to look for those connections, and when they're not there, its baffling."
When there was a large avian flu outbreak in Pennsylvania back in 1983-84, she observed, "It wasn't [a matter of] continuing multiple introductions over a wide area. The virus came out of a live-bird market and spread from farm to farm. All of our efforts are directed to stop the spread between turkey farms, but it's not even spreading to the barn next door, so how do we make sense of that mystery?"
She alluded to the fact that in most of the recent outbreaks, the virus struck in only one of several barns on a farm.
Cardona said she believes the outbreaks mark the first major foreign animal disease problem in Minnesota. "This is a huge deal not just for Minnesota, but for the US," she added.
She said the evidence that the virus somehow spreads from the environment to turkeys but does not easily spread from barn to barn has prompted some new biosecurity measures and recommendations. She commented that it's hard to change long-standing practices and attitudes, but given the high susceptibility of turkeys to the virus, it appears that some of the biosecurity efforts are paying off.
"Minnesota is a huge turkey state," she said. "There's a lot of turkey flocks that have had no infection and are testing negative. It's too early to say, but right now in some of the regions that have been hardest hit, not all turkey flocks have been infected, and they're trying hard and succeeding in many cases."
While it's unclear how the virus is getting into turkey barns, Cardona noted that a tiny amount of bird feces can carry a large dose of virus: "Our former poultry extension vet used to say that one milligram of duck feces carries about 10 turkey infectious doses, so that gives you an idea of how small were talking."
"We're putting a lot of effort into blocking that route of transmission," she said.
Chickens less susceptible
In other comments, Cardona said there is some experimental evidence that chickens are much less susceptible to the H5N2 virus than turkeys are, which would help explain why nearly all the outbreaks have involved turkey farms.
She said the USDA's Southeast Poultry Research Laboratory in Athens, Ga., ran some experiments shortly after the virus surfaced in Washington state in December. The scientists found that when uninfected turkeys were housed with an infected turkey, they easily became infected. They also found that chickens could be infected with the virus, but it did not readily spread to healthy chickens housed with infected ones.
Apr 13 USDA statement on H5N2 in Wisconsin chickens
Apr 13 Wisconsin Department of Agriculture statement
Apr 10 USDA statement on H5N2 in South Dakota
Apr 11 USDA statement on H5N2 in North Dakota
Apr 11 USDA statement on H5N2 in Minnesota
MBAH avian flu updates