Chapter 4: Avian Influenza and Highly Pathogenic Avian Influenza 2 CE Hours
Expiration Date : May 4, 2027 Learning objectives
After completing this course, the learner will be able to: Describe avian influenza viruses, including their genetic classification, wildlife reservoirs, geographic distribution, and modes of transmission. Discuss the effects of avian influenza on domestic poultry flocks, both clinically and economically.
List potential effects of avian influenza on wild bird species, mammalian species, and humans. Outline the efforts underway by U.S. government agencies to monitor and limit the spread of avian influenza.
INTRODUCTION
Avian influenza is a highly contagious viral respiratory illness of birds, which tends to circulate primarily within the avian population but can occasionally affect domestic animals. Periodically, strains of avian influenza arise that are termed “highly pathogenic,” because they can significantly impact domestic birds, especially chickens and turkeys. In the spring of 2015, a new strain of highly pathogenic avian influenza (HPAI) was introduced in the United States. This strain was identified as HPAI H5N8 and, like many strains, this virus originated in Asia. This virus is now established within the U.S. wild bird population and is expected to continue having ongoing, significant impacts on the nation’s domestic poultry. Highly pathogenic avian influenza viruses, including H5N8, cause significant illness and HPAI: What's the issue? The current strain of HPAI, the H5N8 strain, was initially documented in 2014 in China and South Korea. The virus spread to Russian waterfowl by September 2014 and reached the Pacific Northwestern United States by December 2014 1,2 . Genetic sequencing suggests that the spread across continents was caused by contact between Asian and Alaskan migratory birds at their summer breeding grounds; these areas are close to each other and provide a route for avian influenza viruses to enter the United States 2 . The current H5N8 strain of HPAI appears to be well adapted to waterfowl hosts, causing little or no clinical disease and thus allowing rapid distribution of the virus along the flyways of migrating birds 1 . By late 2015, HPAI had been detected in domestic or wild birds in 21 states across the western and central United States 2 and the virus is expected to spread across the remainder of the United States. Unfortunately, domestic birds (such as chickens and turkeys) do not have the same tolerance for HPAI demonstrated by waterfowl. Therefore, this disease can have tremendous agricultural and economic significance. In the most recent U.S. outbreak of HPAI H5N8, in the spring and summer of 2015, 211 commercial and 21 backyard poultry operations were affected. This outbreak resulted in the depopulation of 7.5 million turkeys and 42.1 million chickens, costing federal taxpayers over $950 million in compensated losses and control efforts 2 . Furthermore,
mortality rates in domestic poultry and may also impact wild bird populations. While HPAI H5N8 has not yet been observed to cause illness in humans or other mammals, related strains of avian influenza have had zoonotic effects; therefore, the situation warrants close monitoring. Many U.S. agencies, including the U.S. Department of Agriculture (USDA), National Wildlife Health Center (NWHC), and Centers for Disease Control and Prevention (CDC) are involved in the monitoring and control of HPAI and are now working together to lessen avian influenza’s potential reach in the United States. Due to the possible economic significance of this disease, it is important for all veterinarians to have a basic understanding of this virus so they can provide appropriate guidance to their clients and the community. HPAI outbreaks can lead to trade restrictions between affected and unaffected areas, further contributing to substantial economic losses 1 . Highly-pathogenic H5N8 virus is now found in waterfowl and backyard poultry flocks across much of the United States. This virus has reassorted with low-pathogenicity avian influenza (LPAI) to form new HPAI strains 1 , further increasing the virus’s potential reach. Because this virus is well adapted to waterfowl while causing high pathogenicity in domestic poultry, it is likely that HPAI will remain a long-term presence in the United States 1 , circulating continually in waterfowl with periodic spillover events into domestic poultry. Risks are especially high for poultry farms that do not practice adequate biosafety practices (e.g., backyard flocks) and those that provide outdoor access for organic poultry. While commercial poultry operations may employ a dedicated poultry veterinarian, owners of small backyard chicken flocks often do not have a designated veterinarian for their birds. Therefore, these backyard bird owners may seek guidance from their small animal or equine veterinarian if their chickens fall ill. For this reason, it is important that all veterinarians have at least a basic understanding of avian influenza. This knowledge can help them recognize possible avian influenza outbreaks in their community and direct clients to the appropriate resources.
ETIOLOGY OF AVIAN INFLUENZA
Avian influenza viruses are members of the family Orthomyxoviridae, a virus family that contains Influenza A, Influenza B, Influenza C, and two other types of viruses. Avian influenza viruses are members of the genus influenzavirus A; this genus is the cause of most human disease and all global pandemics. Influenza B viruses are another cause of human seasonal influenza, while Influenza C viruses are typically associated only with mild respiratory illness in humans. Influenza A viruses are then further divided into subtypes, based on two proteins that coat the surface of the viral particle. These
two proteins are hemagglutinin (HA) and neuraminidase (NA) 3,4 . There are 16 recognized HA antigens (H1 to H16) and nine recognized NA antigens (N1 to N9) in avian influenza, while one additional HA and one additional NA have been isolated in influenza from bats. These surface proteins, especially HA, are the primary targets of the body’s immune response. Variation in these HA or NA types results in the immune system recognizing the virus; therefore, there is little cross-protection between different influenza strains.
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