Avian Influenza Risks to Humans and Poultry

 The H5N1 strain of avian influenza, or bird flu, was first identified in Hong Kong in 1997. Since then the virus has spread among poultry flocks in a number of south-east Asian countries. It has also spread westwards into eastern Turkey and some European countries along the migration paths of wild birds, particularly waterfowl. Transmission from birds to humans has occurred. More than 140 people have become sick and of those approximately half have died. WHO keeps an up-to-date count of human infections and deaths caused by H5N1. Compared to most strains of influenza that infect humans the mortality rate is very high. However, despite the severity of the human disease, the close contact that people have with poultry in much of Asia, and widespread H5N1 infection in poultry, infection of humans by the H5N1 strain is relatively rare. It is even more difficult for the virus to be transmitted from human to human, although may have happened on a few occasions within households. The virus has not, since its initial appearance, developed the capacity to transmit from human to human readily enough to sustain an epidemic. It has the potential to do so, but this has not happened.

Risk to humans

What is the risk of the introduction of the H5N1 avian influenza virus into the United States? The H5N1 virus can get here in two ways – as a disease already in humans, or as an animal infection. If it has already made the transition to a human to human infection capable of producing a large epidemic, it is most likely to do so in Asia, where human-bird contact is close. In that case the most likely scenario is for it to enter the US in an infected traveler from overseas, and have the potential to produce a human epidemic of H5N1 influenza in this country. If the virus has retained its ability to cause a severe disease in humans, and if it has additionally gained the capability to spread very easily among people, then there is a danger of a very large and very serious North American epidemic.

Risk to domestic poultry

If it comes to the US as a bird disease, how would that happen, and what would be the consequences? It is most likely to enter as an infection in migratory water birds, and the most likely path would be through Alaska. In this region the East-Asian Flyway connects with at least one of the North American flyways used by migratory birds traveling south. As with previous outbreaks of avian influenza in the US, domestic poultry would be infected by contact with wild birds. The diagnosis of avian influenza would activate disease control plans by federal and state veterinary authorities, and by the commercial poultry industry, which has contingency plans for dealing with such an outbreak. Previous outbreaks of highly pathogenic avian influenza (HPAI) caused by strains other than H5N1 occurred in the US in 1924, 1983 and 2004, and were eradicated. Low pathogenicity avian influenza (LPAI) occurs in US birds on a regular basis.

The size of an H5N1 influenza epidemic in poultry would depend on the time lag between the first appearance of the infection and the time the diagnosis was made, whether infected live birds were marketed, and the effectiveness of existing flock biosecurity measures. In the 1983 outbreak 17 million birds were destroyed to contain the disease, while the 2004 outbreak was confined to one flock. In neither outbreak were humans affected, but it is possible that a few people in close contact with H5N1 infected poultry could be made sick. Because human contact with live domestic birds is infrequent in the general population it is unlikely that H5N1 would convert to a human pandemic form in the US.

Dr. Simon Kenyon, Extension Veterinarian, Purdue University