Avian Influenza (Influenza A Virus, Bird Flu)

Michigan Surveillance and Response Plan for Avian influenza in free-ranging wildlife

Avian influenza (AI), or bird flu, is an influenza type A virus that occurs naturally in wild water birds (ducks, geese, swans, and shorebirds), and domestic poultry. However, other birds and mammal species can also become infected. AI viruses have two characteristic proteins: hemagglutinin (H) and neuraminidase (N), of which 16 H's and 9 N's have been identified. Thus, there are 144 different subtypes of AI known. All of these subtypes fall under two levels of severity: low pathogenic avian influenza (LPAI), which are the most common strains, and highly pathogenic avian influenza (HPAI). The pathogenicity of the virus is based on its ability to produce disease in domestic birds. LPAI causes mild signs in domestic poultry, rarely any clinical signs in wild birds, and does not pose a significant threat to human health. HPAI can cause severe symptoms resulting in high mortality in domestic birds and wild birds. Some evidence suggests that while wild birds may act as reservoirs of LPAI viruses for domestic poultry, those viruses likely mutate to HPAI forms within domestic poultry flocks under intensive husbandry. Infections in wild birds with HPAI were considered rare until 2022, when HPAI subtype H5N1 caused widespread mortality in wild birds and mammals across North America. Both LPAI and HPAI are easily spread through domestic poultry populations once the virus has been introduced.

Distribution     

LPAI is found in wild bird populations worldwide. H5 and H7 subtypes of LPAI are more likely to mutate into highly pathogenic strains and, as a result, are more closely monitored. Outbreaks of different strains of HPAI have occurred worldwide. In the United States, HPAI detections in domestic and wild birds have occurred since 2014.

As part of a National Surveillance Program led by the United States Department of Agriculture (USDA), Animal and Plant Health Inspection Service (APHIS), National Wildlife Disease Program (NWDP), the Michigan Department of Natural Resources (DNR) Wildlife Division has assisted with HPAI surveillance of wild bird populations since 2006.

HPAI was detected in wild birds in Michigan for the first time in June of 2015, when twelve Canada geese from Macomb County tested positive for the HPAI H5N2. In March of 2022, numerous wild birds and wild mammals tested positive for a different strain, HPAI H5N1.. The positive birds and mammals in 2015 and in 2022 were clinically ill or found dead, being adversely affected by the virus, not just asymptomatic carriers. Since 2022, HPAI H5N1 is considered to be widespread and endemic in Michigan and North American. The DNR continues to submit samples for testing from birds displaying symptoms consistent with HPAI before death on a case by case basis.

Transmission and Development

Infected birds shed the virus in saliva, nasal secretions, and feces. Susceptible birds can become infected when they come in contact with the virus shed by infected birds. Birds can also become infected when they have contact with environments contaminated with the virus. Fecal-oral transmission seems to be most common. Once the virus has infected a domestic flock, it spreads quickly, and symptoms, if any, appear rapidly. HPAI infection can cause exceptionally high mortality (90-100%) in some domestic species such as chickens. Spillover can occur from HPAI infected domestic birds to wild birds. Although wild birds can be asymptomatic when carrying the virus, they can still spread it to other birds and can potentially disseminate it over long distances during migration. The survivability of the virus outside of its host depends on the material it is in (feces, surfaces, soil, etc.) and the temperature and humidity of that environment. It has been shown to survive from days to over a month depending on those factors. There is an inverse relationship between virus persistence in the environment and temperature. At >80 F the virus can last a few days at most.

Clinical Signs and Pathology

Although wild birds infected with AI virus generally do not exhibit symptoms, wild birds and mammals can have clinical signs similar to those observed in domestic birds which may include lack of energy, reduced egg production, ruffled appearance, coughing and sneezing, diarrhea, stumbling, blindness, seizures, and other neurological disorders. Death can also occur in birds without observable symptoms. The severity of symptoms depends on the subtype of the virus and the pathogenicity. Pathological findings, if any, are restricted to the lungs with congestion and edema being present.

Diagnosis

Waterfowl are reported and collected by DNR Wildlife Field Offices if die-off events occur involving six or more birds or birds are observed displaying neurological symptoms (e.g. head tremors, seizures, swimming in circles) or labored breathing. Species susceptible to illness and death from HPAI (e.g. bald eagles, hawks, owls, juvenile wild mammals) are tested for HPAI on a cases by case basis. USDA APHIS conducts surveillance for HPAI of hunter harvested waterfowl from designated regions of the state. Oral and cloacal swabs are collected from suspect or surveillance cases and are submitted to Michigan State University's Veterinary Diagnostic Laboratory for initial screening. A real time reverse-transcript polymerase chain reaction (rRT-PCR) assay that targets the "matrix gene" for type A influenza virus is used for initial screening. If influenza virus is detected, a second round of testing is performed using subtyping rRT-PCR assays specific for H5 and H7. A presumptive H5 or H7 positive sample is forwarded to the USDA National Veterinary Services Laboratory (NVSL) in Ames, IA for confirmatory testing. NVSL will confirm the detection of the virus by rRT-PCR testing and virus isolation and will determine the strain type.

Treatment and Control

There are no feasible treatments for AI in wild bird populations. Culls are nearly impossible for free-ranging birds and are ineffective because environmental contamination is typically widespread, and birds are not limited to a geographic location. Surveillance and monitoring for HPAI occurs through testing of symptomatic wildlife, large die-offs of waterfowl, and sampling of live-caught or hunter-harvested birds.

When an HPAI outbreak occurs in domestic birds, the flock is depopulated. The effectiveness of depopulation depends on the time between detection and action, as the virus spreads rapidly. Quarantine and testing of nearby flocks is frequently necessary to determine the extent of the outbreak, followed by depopulation if tests are positive. To control and prevent outbreaks in domestic flocks and spillover to wild birds, proper biosecurity is key. Typically, transmission of HPAI between poultry flocks is caused by inadequate decontamination of people or equipment moving from one flock to another.

Significance

The impact of AI on wild populations was previously thought to be minimal; however, with the large number of wild birds and mammals affected by HPAI H5N1 since 2022, the population level impacts on vulnerable populations such as bald eagles, black vultures, and California condors is currently unknown. LPAI causes few clinical signs or widespread mortality, but the potential exists for these viruses to mutate and become highly pathogenic The economic impact of HPAI in domestic populations can be severe Outbreaks in the U.S. have resulted in the loss of millions of domestic birds (turkeys, chickens) which have resulted in billions of dollars lost by the poultry industry.

Humans are only occasionally infected by AI. Most cases of humans being infected by these viruses occurred from direct contact with infected poultry. Human to human transmission has been extremely rare. Symptoms can range from typical flu-like symptoms to severe life-threatening complications. H5N1 and H7N9 are the subtypes of most concern as they have caused the greatest number of human infections and have the greatest potential for a worldwide outbreak (i.e. pandemic). A vast majority of human cases were caused by AI viruses of Asian lineage and occurred overseas. There has been on case of human infection with HPAI H5N1 in North America in Colorado in a poultry facility worker involved in culling of infected poultry in 2022. The North American lineage AI viruses rarely infect people and pose little threat to human health.

For more information on avian influenza and how it affects human health, you can visit the Centers for Disease Control and Prevention (CDC) website at https://www.cdc.gov/bird-flu/virus-transmission/avian-in-humans.html.