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Texas Agriculture Archive

March 19, 2004

Bird virus a moving target

With no birds infected with avian influenza found in a 10-mile quarantine radius of the first south Texas farm infection, there's a good chance the outbreak has been contained, according to the director of the Texas Veterinary Medical Diagnostic Laboratory.

"The concern is that there's possibly foci of this strain somewhere else, and finding this foci is of high priority," said Dr. Lelve Gayle, speaking at the opening of the newly upgraded TVMDL Poultry lab in Center.

However, though all signs point to the outbreak being contained, the nature of AI makes predicting the outcome a "moving target," Gayle said. The virus can live for weeks outside the host if the conditions are favorable and as yet, the foci of the original infection has not been positively identified.

The infected poultry were not found in Shelby County but at a farm nearly 300 miles to the southwest in Gonzales County. The incident caused enough concern to put a damper on the dedication ceremonies of the new 3,200-square foot facility. Local producers, veterinarians and those from other poultry-producing regions in Texas were asked to stay away to lessen the chance of infecting flocks in the county where poultry constitutes an $160 million annual industry. About 50 attended the ceremony, mostly local elected officials, poultry lab personnel and Texas A&M administrators.

The outbreak occurred about a week after a Gonzales poultry producer visited a Houston live bird market. He sold some birds at the market but brought some back to the farm.

AI is highly infectious and can be transmitted by air or by feces. The virus can also survive for some time on the hands of those examining birds, making live bird markets a prime suspect when there's an outbreak. Because the time delay between the producer's Houston trip and the onset of symptoms corresponds to the incubation period of the type of avian influenza, it's suspected the Gonzales birds may have been infected at the market, Gayle said.

Because the disease is highly infectious to poultry and potentially of devastating to the industry, TVMDL went into high gear at the first signs of the disease. The first bird death on the Gonzales farm was on Feb. 15, a Sunday. Birds were examined post mortem at the TVMDL Poultry Lab in Gonzales on the following day. The lab suspected avian influenza and shipped samples to the TVMDL lab in College Station. Before noon, Feb. 17, the College Station lab had confirmed the Gonzales diagnosis and determined it was the H5 strain of avian influenza. Samples were sent overnight to the National Veterinary Services Laboratory in Ames, Iowa, which confirmed the College Station laboratory finding and did further testing, identifying the strain as H5 subtype N2, or just H5N2 for short.

"By genotype characterization alone, the USDA calls H5N2 a high-path strain," Gayle said.

Avian influenza comes in several flavors. There are 15 H types with nine subtypes. Of the H types, H5 and H7 are considered the most virulent, but the subtype is an indicator of just how highly pathogenic—"high path"—the strain may be. But AI can mutate once it's in a flock, changing from a lowly or mildly pathogenic disease—types slow to infect and with a relatively low mortality rate—to a highly pathogenic disease that spreads like wildfire and kills most if not all of a flock. This mutation may not occur overnight, but occurs a few days, a week or longer after the first bird deaths.

Once AI is diagnosed, regulatory agencies take over. At the state level, it's the Texas Animal Health Commission; at the federal, the U.S. Department of Agriculture. Within hours of the confirmation of a high-path strain, the USDA and the TAHC imposed a 5-mile radius quarantine zone around the initial site of infection.

The initially infected flock–termed the "index flock"–was killed and buried. To be on the safe side, the 5-mile radius was expanded to a 10-mile radius. The initial round of blood and tissue sample was followed by a second eight to 10 days later.

"Because no further bird mortalities or positive testing samples turned...more than a week after the initial infection, we are hopeful that the outbreak has been contained," Gayle said.

The question remains, however, concerning the origin of the infection. The disease just didn't spontaneously generate. Somewhere, there is a reservoir of the virus.

The most likely reservoir is a wild avian species or habitat. A virus that is highly pathogenic for domestic poultry can emerge from the pool of viruses in wild birds at any time. Given the fact that ducks, geese and other waterfowl are often present at many live bird markets—and assuming the Houston live bird market was where the Gonzales infection was contracted—then the Gonzales outbreak may be associated with waterfowl at the live bird market.