A highly pathogenic avian influenza strain not seen in the United States in 20 years was confirmed Feb. 23 in a flock of some 6,600 broiler chickens in Texas by the U.S. Department of Agriculture.

Although in the United States there are no known humans infected with high- or low-pathogenic H5N2 viruses, public health officials were monitoring area farm workers as a precaution. The farm in Gonzales County, Texas, is under state quarantine, and the infected flock was depopulated Feb. 21.

The USDA and Texas Animal Health Commission are conducting an epidemiologic investigation that includes determining the source of the infection and surveillance testing within a 10-mile radius of the infected flock.

The highly pathogenic H5N2 strain was last seen in the United States in 1983-1984 in commercial poultry in Pennsylvania and Virginia.

In recent weeks, two strains of low-pathogenic avian influenza have been detected in the United States. The H7N2 strain was found in two flocks in Delaware and four of the 35 live chicken markets in northern New Jersey. The low-pathogenic H2N2 strain was detected in a Pennsylvania flock. In Delaware, chickens on more than 226 farms have been tested, with no additional infection detected.

The virus currently affecting several Asian countries is a highly pathogenic H5N1 strain, according to Dr. Ron DeHaven, the USDA's chief veterinary officer.

News of the outbreak in Texas prompted the European Commission to ban all imports of U.S. eggs and live birds into the European Union.

The virus was discovered Feb. 17 when routine surveillance samples sent to the Texas Veterinary Medical Diagnostic Laboratory received a preliminary positive result for an H5 avian influenza virus.

Further testing at the USDA National Veterinary Services Laboratories in Ames, Iowa, determined the strain to be the highly pathogenic virus.

For more information on avian influenza, visit the USDA's Animal and Plant Health Inspection Service Web site at www.aphis.usda.gov or the national Centers for Disease Control and Prevention Web site at www.cdc.gov.