Avian Influenza (Influenza A Virus, Bird Flu)

Waterfowl In FlightDescription

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. 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 are rare with few, if any, symptoms, and are most frequently found in rare high mortality events. 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 across 23 states (Alaska, Arkansas, California, Idaho, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Missouri, Montana, Nebraska, New Mexico, Nevada, North Dakota, Oregon, South Dakota, Tennessee, Utah, Washington, Wisconsin, and Wyoming) between the years 2014 and 2017.

As part of a National Surveillance Program led by the United States Department of Agriculture (USDA), Animal and Plant Health Inspection Service (APHIS), the Michigan Department of Natural Resources (DNR) Wildlife Division conducted surveillance of wild bird populations from 2006-2017.

HPAI was detected in wild birds in Michigan in June of 2015, when three goslings in Macomb County tested positive for the highly pathogenic subtype H5N2. A total of 12 wild Canada geese tested positive for H5N2 that year, all from Macomb County. This was the only time HPAI has been detected in Michigan. HPAI has not been detected in domestic poultry in Michigan. In recent years there have been no HPAI detections nationwide and routine surveillance for AI has ceased. The MDNR does continue to submit samples for testing from birds displaying symptoms consistent with HPAI before death.

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 are generally asymptomatic, symptomatic birds and rapid death have been reported, as was the case in 2015 in Michigan. However, when domestic birds contract the virus, clinical signs prior to death may include lack of energy, reduced egg production, ruffled appearance, coughing and sneezing, diarrhea, stumbling, and other neurological disorders. Death can also occur peracutely without any symptoms. Large die-offs in domestic flocks should be investigated for AI because death without any symptoms can indicate an AI outbreak.  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 or if birds are observed displaying neurological symptoms (i.e. head tremors, seizures, swimming in circles) or have labored breathing before death. 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.

Serological testing can be used to determine if AI antibodies are present in a bird. This can indicate if the bird was exposed to the virus at one time, but not whether the bird was ever infectious.   

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. Regular surveillance is used to monitor for AI in wild birds. Surveillance through sampling of live-caught or hunter-harvested birds normally only occurs when there is heightened concern of a potential outbreak. Monitoring for AI continues through testing waterfowl that are symptomatic or when large die-offs occur.

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 is minimal, as LPAI causes few clinical signs, and rapid die-offs in HPAI-affected wild birds limit transmission. However, the economic impact on domestic populations can be severe if the virus mutates and becomes highly pathogenic. Outbreaks in the U.S. between 2014-2017 resulted in the loss of millions of domestic birds (turkeys, chickens) which 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 have been no human infections with H5N1 or H7N9 in the United States. 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/flu/avianflu/avian-in-humans.htm.