Highly pathogenic H7N3 outbreak in Mexico- largest in nearly 20 years: millions of chickens could be culled

MEXICO – Mexican veterinary authorities are intensifying avian influenza control efforts in a region that houses several large commercial farms after further tests determined that the strain responsible for more than 200,000 bird deaths at three farms is the highly pathogenic H7N3 subtype. The events represent the first highly pathogenic avian influenza outbreaks in Mexican flocks since the country battled H5N2 in the mid 1990s. In a follow-up report submitted today to the World Organization for Animal Health (OIE), Mexican animal health officials said intravenous pathogenicity tests revealed the highly pathogenic H7N3 subtype. The initial report on Jun 21 said preliminary tests suggested a low-pathogenic H7 subtype. The outbreaks began at three large commercial farms in Jalisco state on Jun 13, causing clinical signs in the layer flocks that included gasping, lethargy, fever, and death. The disease sickened 587,160 of more than 1 million susceptible birds, killing 211,424 of them. About 60,000 have been culled so far to curb the spread of the virus. Today’s update said that, based on the latest test results, authorities are sampling birds at about 60 poultry farms near the outbreak area, and quarantine measures are under way in the region, which has about 500 production units. Full gene sequencing and an epidemiologic investigation to determine the source of the virus are also in progress. Jalisco state, in western Mexico, is the country’s top egg producer. Officials have also limited poultry movements near the outbreak area and are testing birds at commercial farms, backyard flocks, and poultry markets. They are also assessing bio-security practices and overseeing depopulation efforts at the affected farms, according to the OIE report. David A. Halvorson, DVM, an avian health expert at the University of Minnesota in St. Paul, told CIDRAP News that Mexico’s last high pathogenic avian influenza outbreaks occurred in 1994 and 1995 and involved H5N2. He added that low-pathogenic H5N2 circulated in the country for several years. John Glisson, DVM, PhD, director of research programs for the US Poultry and Egg Association, said in an e-mail statement to CIDRAP News, “The US poultry industry would strongly agree with the idea that the disease should be dealt with quickly and that quarantine of these farms and elimination of infected flocks would be a prudent measure.” In 2004, highly pathogenic H7N3 outbreaks in British Columbia’s Fraser Valley led to the culling of 19 million birds, and two related human infections were confirmed. The patients, both men who had been exposed to infected poultry on the farms, were the first known H7N3 infections in humans. Both had conjunctivitis with mild flulike symptoms, according to a December 2004 report on the cases in Emerging Infectious Diseases. –CIDRAP

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