Robert Roos  News Editor

Feb 23, 2004 (CIDRAP News) – Genetic analysis has shown that an avian influenza outbreak at a poultry farm in Texas involves a highly pathogenic H5N2 virus, but it poses little risk to humans, the US Department of Agriculture (USDA) announced today.

H5N2 viruses—unlike the H5N1 virus currently widespread in Asia—have not been known to infect humans, but the Centers for Disease Control and Prevention (CDC) is carefully watching the situation, a CDC official said at a USDA news briefing.

Ron DeHaven, the USDA"e;s chief veterinary officer, said test results early this morning showed that the outbreak at a farm 50 miles east of San Antonio involved a highly pathogenic form of H5N2 virus. The flock of 6,600 broiler chickens has been sacrificed, and poultry flocks within 10 miles are being tested, he said.

H5 and H7 viruses can have either low or high pathogenicity, and "moderate" mortality in the Texas flock had led to a belief that the virus was a low pathogenic form, DeHaven said. "We did genetic sequencing, and it"e;s that genetic sequencing that"e;s causing us to call this a high-pathogenic avian influenza," he said.

DeHaven said chickens from the farm were shipped to two live-bird markets in Houston, and preliminary tests indicated that some of those birds were infected. One of the markets had chicken products but no live chickens when it was inspected, and the other had a few live ducks, which were being destroyed, he said.

He said the last highly pathogenic avian flu outbreak in the United States occurred in 1983-84 in Pennsylvania and Virginia and also involved an H5N2 virus.

Nancy Cox of the CDC said there were no known human cases in the 1983-84 outbreak or at any other time. "Past experience with H5N2 viruses has indicated that there is a low threat to human health," she said. "But in this situation we must keep an open mind and really monitor the situation as we go." She said the CDC is working with state and local public health agencies to do that and would test the outbreak virus for susceptibility to antiviral drugs.

DeHaven said there was "no known epidemiologic link whatsoever at this point between any of the avian influenza viruses in the United States with the situation in Asia." Eight Asian countries have had H5N1 outbreaks in poultry, and the World Health Organization has reported 32 human cases, with 22 deaths.

In recent weeks, low-pathogenic H7N2 outbreaks have occurred in two chicken flocks in Delaware and in four live bird markets in New Jersey. Last week an H2N2 outbreak believed to be low-pathogenic was reported at one farm in Pennsylvania.

DeHaven said the Texas virus was analyzed at the USDA"e;s national veterinary lab in Ames, Iowa. He said further tests are under way in which healthy chickens have been inoculated with the virus and are being monitored.