Mexican veterinary authorities are confirming this week there has been an outbreak of avian influenza near Guadalajara that has caused the death of nearly a quarter million chickens since early June and so far has forced a quarantine zone around three poultry processing facilities in the Mexican state of Jalisco.

In a follow-up report submitted to the World Organization for Animal Health (OIE), Mexican animal health officials said intravenous pathogenicity tests revealed a highly pathogenic H7N3 subtype that is the cause of the current outbreak. Mexican veterinary authorities are intensifying avian influenza control efforts in the region, which houses several large commercial farms.

The event represents the first highly pathogenic avian influenza outbreaks in Mexican flocks since the country battled H5N2 in the mid 1990s.

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 nearly 220,000 of them.

Problem in U.S.

Outbreaks of avian influenza A occur among U.S. poultry flocks from time to time. Since early February 2004, avian influenza outbreaks have been reported in several locations in the United States, most recently in Texas in 2004 when a highly pathogenic H5N2 strain was discovered at a poultry facility in Gonzales County.

Dr. Marty Ficken, resident director of the Texas Veterinary Medical Diagnostic Laboratory in Gonzales, says avian influenza viruses typically do not infect humans; however, several instances of human infections and outbreaks of avian influenza have been reported since 1997. It is believed that most cases of avian influenza infection in humans have resulted from contact with infected poultry or contaminated surfaces. Other means of transmission also are possible, such as the virus becoming aerosolized and landing on exposed surfaces of the mouth, nose, or eyes, or being inhaled into the lungs.

“There is the potential for these types of viruses to spread to the human population but they usually do not cause a disease of consequence. Unlike many viruses, human infections of avian A type are generally limited to mild flu-like symptoms,” he said.

He says the threat of this outbreak reaching Texas and New Mexico is very low, citing USDA import requirements and border inspections as adequate defense. But he suggests transmission could be possible by the illegal import of live poultry or egg shells contaminated with the pathogen.

All poultry imported from all countries except Canada must be quarantined for at least 30 days at a USDA Animal Import Center and be accompanied by import permits and veterinary health certificates. Canadian poultry entering the United States must be accompanied by a veterinary health certificate issued within 30 days of import date.

Egg shell risk

“Vertical transmission is lacking, but infected material on an egg shell could spread the virus under the right conditions. But a comprehensive program of quarantine that regulates movement will also include destroying both poultry and eggs and sanitizing areas within the quarantine zone,” he added.

Ficken says the greatest threat is to the poultry industry because highly pathogenic strains of influenza A can spread rapidly and destroy a great number of birds. But he says safeguards at Texas poultry facilities are ever diligent as routine testing and observation is ongoing. In Texas, substantial poultry facilities are located in Gonzales, Waco, Amarillo, and scattered through other areas of the state.  

According to the latest update from Mexico’s animal health officials, 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 according to Mexican health officials.

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. According to the latest OIE update, they are also assessing biosecurity practices and overseeing depopulation efforts at the affected farms.

Officials at OIE said that in some parts of Mexico, large populations of backyard poultry, live poultry markets, and commercial farms exist within close proximity, making inspection and control more difficult during times of disease outbreak.

Producers cautious

Ficken said U.S. poultry producers, especially those in Texas, are always cautious about the potential for disease introduction from indirect contact with Mexican poultry.

“The level of security goes up when new virus outbreaks surface,” Ficken said.

John Glisson, DVM, PhD, director of research programs for the U.S. Poultry and Egg Association, said in a recent statement that "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."

Nearly 20 million birds were destroyed in Canada in 2004 when highly pathogenic H7N3 outbreaks in British Columbia’s Fraser Valley occurred, leading to the culling of nearly 20 million birds. In addition, two related human infections were confirmed when poultry workers, both men, had been exposed to infected poultry on the farms. They were the first known H7N3 infections in humans. Both had conjunctivitis with mild flu-like symptoms and recovered without major incident.