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ACVP Avian Influenza Factsheet

Avian Influenza "Bird Flu" Factsheet

Definition
Avian Influenza (H5N1 HPAI), also known as “bird flu,” is a highly infectious, systemic disease of poultry caused by an Orthomyxovirus of the genus Influenzavirus A. Most AI viruses cause asymptomatic infections or mild diseases in poultry. However a few AI viruses of the H5 and H7 subtypes cause severe, systemic disease in chickens with nearly 100% mortality.

Significance
The bird flu has raced through Asia’s poultry farms and killed at least 27 people. Health officials are concerned the virus might mix with a regular human influenza strain to create a mutant form that could trigger a human flu pandemic.

History
The H5N1 HPAI virus was first identified in an infected goose in China during 1996. In the spring of 1997, this virus caused high mortality in chickens on three farms in Hong Kong. By the fall of 1997, the virus was causing high mortality in poultry sold in the live bird poultry markets in urban areas. By the end of 1997, 18 people were hospitalized and six people died from infection of the “bird flu” virus. Roughly 1.4 million chickens and other poultry from Hong Kong farms and retail markets were killed to prevent additional human cases of the disease. A 1998 CDC study indicated the transmission of H5N1 Hong Kong HPAI virus from 1997 was bird-to-human.

In 2001 and 2002, H5N1 HPAI re-emerged in poultry in Hong Kong. Roughly 2.5 million birds were killed during these two years to prevent the spread of the virus.

Also in 2001, the H5N1 HPAI virus was isolated from duck meat imported by China by the National Veterinary Research and Quarantine Bureau (South Korea). In a laboratory setting, this virus infected ducks but caused no clinical disease. However, in chickens this virus caused 100 percent mortality.
From December 2002 to February 2003, wild and captive birds infected with H5N1 HPAI virus were discovered dead in two parks in Hong Kong. In the lab, this virus caused high mortality in ducks, chickens, turkey and other gallinaceous poultry species.

In January/February 2003, a family from Hong Kong visiting relatives in China developed respiratory disease. Two died and three were hospitalized. H5N1 influenza A virus was isolated from two family members.

On December 15, 2003, H5N1 HPAI was diagnosed in South Korea in broiler breeder chickens. On January 8, 2004, H5N1 HPAI was diagnosed in Vietnam in broiler breeder chickens. Additional virus discoveries were made in Japan, Thailand, Cambodia and Indonesia in late January 2004. The disease was discovered for the first time in Malaysia in mid-August 2004.

Transmission
Human risk appears to result from exposure to live poultry rather than handling, cooking or eating poultry meat.

Avian influenza viruses are transmitted between poultry primarily through direct or indirect exposure to respiratory secretions or feces of infected birds. A primary method of AI virus spread between farms has been through the movement of contaminated equipment, shoes and clothing. Airborne transmission is theoretically possible, but has been demonstrated only over short distances, such as 50-100 meters.

Transmission from bird-to-human can be prevented to a large degree by limiting exposure to live poultry.

Animal Link
Migratory birds are involved as reservoirs of avian influenza viruses, but most of the viruses cause asymptomatic infections or mild diseases. Historically, migratory birds have not been involved as reservoirs of HPAI viruses and have played only minor roles in spreading the viruses. However, the appearance of H5N1 HPAI in wild and captive birds in Hong Kong in 2003 suggests a possible change in the ecology of the H5N1 virus and a need for examination of wild bird populations as reservoirs and disseminators of H5N1 HPAI virus.

The H5N1 HPAI virus is not present in poultry or other birds on European, African, Australian and North and South American continents.

Prevention and the search for a vaccine
Disinfectants and heat are effective in stopping the spread of avian influenza viruses. Some of the most effective compounds include sodium hypochlorite, phenolics, quaternary ammonia and alkali disinfectants. Normal cooking temperatures and times will kill any AI virus in food products.

Because there are 15 different subtypes of AI viruses, a broadly protective vaccine program is cost prohibitive. However, because three outbreaks of H5N1 HPAI have occurred in Hong Kong, some vaccinations are now required in that country for live poultry. Preventing exposure to AI viruses is the most widely used control strategy.

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