Avian influenza is a major health and economic risk throughout the world. For example, the most notorious influenza epidemic, the Spanish Flu of 1918, caused the death of 20 to 50 million people worldwide. The avian influenza virus (bird flu) has been detected in six western states in the U.S., British Columbia, Asia, and Europe since the beginning of December. This disease is caused by certain strains of the influenza A virus, a type of highly infectious, negative sense, single-stranded RNA virus. Bird flu virus can be intermittently detected in chicken and turkey flocks in many parts of the world. These viruses can sometimes infect humans in food production sites and rapidly mutate to produce strains capable of causing global pandemics.
The World Health Organization (WHO) states that the primary risk factor for human transmission is exposure to live or dead poultry or contaminated environments in farms and animal markets. Additionally, the U.S. FDA says transmission is possible by exposure to environments contaminated by infected birds. It is transmitted by saliva, feces, and nasal secretions of infected birds. While the virus is generally located in the gastrointestinal and respiratory tracts of infected animals, it can also be found in the meat of infected birds. Avian influenza virus can also be located outside surface shell or within the interior of chicken eggs. According to the FDA, avian influenza is not transmissible by eating poultry or eggs that have been prepared properly and the chance of infected poultry or eggs entering the food chain is extremely low because of the rapid onset of symptoms as well as the safeguards in place. However, because of the devastating economic consequences of avian flu to poultry producers and the potential zoonotic hazards presented by exposure to infected birds, avian influenza is a major disease concern to poultry producers and human health officials alike. A great deal of time, effort, and expense is expended each year in curtailing the spread of avian influenza in chicken and turkey farms worldwide.
Individual strains of avian influenza viruses are classified according to the specific structures of two antigens, hemagglutinin (H) and neuraminidase (N), on the viral surface. These antigens control virus-host cell binding interactions during infection. Different virus strains possess different pathogenicity and zoonotic characteristics and can be classified into two different types: weakly pathogenic strains and highly pathogenic strains.
While weakly pathogenic strains of avian influenza can adversely affect poultry (chicken and turkey flocks), the more deadly highly pathogenic strains can cause tremendous economic damage to poultry farms and, more importantly, pose a direct threat to human health by occasionally crossing over the “species boundary” to infect and sicken humans. Highly pathogenic strains tend to be of type H5 and H7 and can rapidly infect an entire flock, killing 100 percent of the birds in less than 48 hours, according to WHO; infection can easily destroy an entire flock and threaten neighboring farms in the region. Some highly pathogenic strains, called zoonotic strains, can be transmitted to humans; these strains also usually possess the H5 or H7 subtype of hemagglutinin antigen. It is important to note however that some H5 and H7 avian influenza strains can possess low pathogenicity at first and gradually mutate to high pathogenicity forms. A minority of avian flu strains can be transmitted from birds to humans. These strains are most problematic since zoonotic strains, such as H5N1 and H7N9, can put farm workers’ health at risk and trigger global human influenza pandemics.
Measures to Detect and Control
The serious disease risks posed by poultry farming have created a need to prevent, detect, and control the spread of avian influenza throughout poultry farming regions and into the human food supply chain. The rapid mutation rates of the influenza virus make effective implementation of these control measures especially challenging.