Influenza viruses can be classified as type A, B, or C. Influenza A viruses, the type responsible for most influenza outbreaks, are further classified by subtypes based on their surface proteins, hemagglutinin (H) and neuraminidase (N). There are 16 types of hemagglutinin and 9 types of neuraminidase.
Most influenza viruses are highly adapted to a single species or type of animal. Avian and swine influenza viruses, however, commonly affect other species, including humans. To date, all highly pathogenic avian strains have been H5 and H7, whereas human epidemics have been caused by H1, H2, H3, H5, N1, and tN2.
All avian species are susceptible to avian influenza (AI), but domestic poultry tend to be highly sensitive to it. The low pathogenic form of avian influenza (LPAI) is common worldwide; affected birds may develop mild disease including slight respiratory signs, ruffled feathers, and reduced egg production. Many infected birds never develop clinical signs of disease.
Free-ranging waterfowl may be carriers of LPAI and shed the virus in their excrement. Domestic poultry in backyard flocks or markets exposed to LPAI are highly susceptible to the virus, and LPAI circulating in these flocks can mutate into highly pathogenic avian influenza (HPAI). HPAI has the capacity for a greater number of protein cleavage sites, which can infect more tissue types and increase morbidity in the host animal.
HPAI was first identified in 1878 in Italy. Affected birds show a sudden onset of severe disease including respiratory compromise and massive internal hemorrhage, with virus isolated from multiple organs and tissues. HPAI spreads rapidly and has a very high mortality rate in birds, sometimes reaching 100 percent within 24 hours of onset of clinical signs. AI is transmitted between birds via respiratory droplets and by fecal-oral route. Vector transmission is another important mode of transmission. The virus can survive up to 35 days in fecal material at low temperatures and up to 6 days at warmer temperatures.
Current HPAI Outbreak and Surveillance
In 2014 a mortality event involving turkeys in British Columbia heralded an outbreak of a highly pathogenic strain of H5N2 avian influenza. Since then, more than 48 million chickens and turkeys have died or been slaughtered as government officials attempt to contain this virus.
Cases of HPAI H5N2 in poultry have been confirmed in Washington, Oregon, California, Idaho, Montana, North Dakota, South Dakota, Nebraska, Kansas, Minnesota, Iowa, Missouri, Arkansas, Wisconsin, and Indiana.
At this time, the source of the virus is unknown. Routes of transmission may include wild bird carriers, human vectored transmission, or airborne spread. Free-ranging waterfowl are carriers of all subtypes of influenza A and are predominantly asymptomatic while shedding the virus into the environment.
National wild bird surveillance programs have detected HPAI H5N8 and H5N2 in Canada geese, wild ducks, and recently chickadees. Affected birds displayed neurologic signs, including head tremors, persistent head tilt, and seizures. Some raptors appear to be highly susceptible to HPAI virus infection, but most cases involved captive raptors being fed meat from infected, wild-caught waterfowl.
Veterinarians on Alert
The incidence of HPAI in wild waterfowl is likely to increase with the onset of cooler weather, the comingling of birds in staging areas prior to migration, and the introduction of naïve young birds into established flocks.
Consequently, it is important for veterinarians and avian caregivers to be alert for signs of HPAI in wild and domestic birds. If suspicious cases are detected or if a mortality event occurs, veterinarians are asked to contact the state veterinarian immediately for further instructions. While the current health risk to humans from H5N2 appears to be low, implementation of strict biosecurity protocols when working with infectious disease suspects is prudent.
—Julia Whittington, DVM