Nation enters third year of historic HPAI epizootic

Deadly bird flu persists as USDA continues vaccine trials

The largest epizootic of highly pathogenic avian influenza (HPAI) in the nation’s history is entering its third year, but there is hope. The U.S. Department of Agriculture (USDA) continues to evaluate several vaccines shown in initial studies to protect chickens against the deadly disease. A vaccine for turkeys is also underway. And researchers are looking into why the most recently circulating strain is one of the most virulent to date.

Since January 2022, multiple HPAI viruses have infected tens of millions of wild birds, commercial poultry flocks, and more than two dozen species of terrestrial and marine mammals worldwide. This bird flu panzootic has also spread across Africa, Asia, the Americas, Australia, Europe and even into Antarctica. Subsequent epidemiologic and genetic investigations determined that the Eurasian (EA) H5 and H5N1 subtypes of the HPAI A virus are the cause of most infections.

Species affected

A 2023 report by the Food and Agriculture Organization (FAO) of the United Nations described the HPAI panzootic as unprecedented in terms of its extensive spread and the number of wild birds and mammals affected. “The devastation caused by this virus underscores the need to prevent new strains from establishing in wild birds and causing further waves of infection,” the report states.

Northern Shoveler ducks fly over water
Northern Shovelers fly over water. Waterfowl like these have been among the wild species mainly involved in the viral cycle of avian influenza but even mountain lions, striped skunks, red foxes, and harbor seals have contracted the virus as have several outdoor cats.

According to World Animal Health Organisation (WOAH) data collected since 2005, HPAI appears to be seasonal, with spread being lowest in September, beginning to rise in October, and peaking in February.

The main wild species involved in the viral cycle of avian influenza are waterfowl, gulls, and shorebirds; however, this clade of the virus seems to pass easily between different bird species, WOAH information states. Direct exposure of farmed birds to wild birds is a likely transmission route. Consequently, limiting farmed poultry exposure to wild birds is critical to lessening the risk of introduction of avian influenza into flocks. Farmers might experience a high level of mortality in their flocks, with rates often around 50%.

In the United States, HPAI infections have been detected in wild birds in every state save Hawaii. As of December 11, 2023, the virus was confirmed in just over one thousand commercial and backyard poultry flocks in 47 states, affecting over 72 million birds, according to the USDA Animal and Plant Health Inspection Service (APHIS). Mountain lions, striped skunks, red foxes, and harbor seals have contracted the virus as have several outdoor cats, the agency said.

Canadian government officials in April reported that a domestic dog in Oshawa, Ontario, had tested positive for HPAI. The dog became sick and soon died after chewing on a wild goose carcass.

“The number of documented cases of avian influenza H5N1 in non-avian species, such as cats and dogs, is low, despite the fact that this virus has caused large avian outbreaks globally over the last few years,” the Canadian Food Inspection Agency and the Public Health Agency of Canada stated.

Studying the spread

Like the FAO, Dr. David Swayne, who ran the USDA’s Southeast Poultry Research Laboratory in Athens, Georgia, for two decades, said the current HPAI outbreak is unlike anything seen before. “We have no historical evidence that anything like this has existed previous to this particular outbreak,” said Dr. Swayne, now a consultant on global HPAI control.

He explained that the Eurasian H5 HPAI strain was responsible for the 2014-15 HPAI outbreak, which circulated for several months among poultry flocks before killing its avian hosts. Not so with the latest EA H5 and H5N1 subtypes. Migrating birds from Europe, likely arriving in Canada during the spring of 2021, carried a highly infectious EA virus already lethal to a wide range of bird species.

The first harbinger that the EA HPAI outbreak had reached North America appeared in December 2021 when the virus was identified in a wild great black-backed gull in Newfoundland, Canada. By January 2022, the virus had spread hundreds of miles to South Carolina, where a wild American wigeon tested positive for the EA H5 virus. Then in February 2022, the USDA announced a commercial turkey flock in Dubois, Indiana, had been infected.

Studies as to why the current HPAI virus is infecting far more bird species than previous strains are in the early stages, Dr. Swayne said, and he believes something within the influenza virus’s genetic makeup changed to make it so virulent.

“As far as pinpointing what's changed about the virus, allowing it to infect so many more diverse species of wild birds than the previous viruses, that’s the big question,” he said.

Another, more positive difference between the 2014-15 and current HPAI outbreaks, is commercial poultry operations are more secure this time around, according to Dr. Swayne. In the years following the 2014-15 outbreak, poultry farms audited their operations to identify and remedy weaknesses in their biosecurity plans and procedures. In a June 2023 report, the USDA APHIS estimated “at least 83 percent of U.S. detections in domestic birds and poultry are consistent with independent point source (wild bird origin) introductions,” meaning the chances of accidental farm-to-farm transmission have been greatly reduced.

Vaccine development, implications

Because of the HPAI outbreak in the U.S., the industry’s ability to export both poultry products and breeding stock has been significantly restricted.

Detection of HPAI in any country or region ceases international trade from that location based on international agreements informed by the World Organization for Animal Health (WOAH) Terrestrial Code.

Ducks in a farmyard
France is the first European nation to introduce an HPAI vaccine in poultry. It is now mandatory for all ducks on farms that have more than 250 birds and whose products are meant to be sold in the form of meat or foie gras, according to its agriculture ministry.

In October, the French government approved an HPAI vaccine for farmed ducks, becoming the first European nation to do so.

While the effectiveness of the French vaccination programs has yet to be determined, Dr. Swayne hopes the USDA will soon make an HPAI vaccine approved for poultry available to U.S. producers.

A USDA spokesperson confirmed the department’s Agricultural Research Service (ARS) has completed initial testing of five HPAI vaccine candidates. ARS scientists evaluated one HPAI vaccine developed in-house by the USDA and four commercial HPAI vaccines. These studies, which are not yet publicly available, showed that the five vaccines reduced oral and cloacal virus shedding significantly and provided near 100% clinical protection in chickens, the spokesperson said.

“ARS scientists have also continued to work on vaccine testing. They will evaluate vaccine efficacy in turkeys and the duration of immunity in several avian species. They have developed and will continue to optimize and validate the diagnostic tests needed with vaccination as well,” the USDA spokesperson said. “These are all longer-term studies. We estimate the turkey efficacy and duration of immunity study results will be available during summer 2024.”

That said, there are potential drawbacks. France’s use of their vaccine triggered a ban on exports of poultry, duck meat, and foie gras. If countries vaccinate for HPAI, international agreements require distinguishing vaccinated flocks from those exposed to HPAI virus. U.S. poultry vaccination for HPAI could have dramatic and long-lasting impacts on the trade of poultry genetics and poultry products without the ability to make and accept that distinction.

In addition, current vaccines may be relatively efficient at protecting against disease but less effective at eliminating virus shedding, which is a WOAH requirement. More research is needed to identify vaccine candidates and strategies that satisfy international trade protocols.

Multiple solutions

The International Alliance for Biological Standardization (IABS) has recommended forming a consulting committee with broad stakeholder participation, developing pilot vaccination programs in a number of countries to develop monitoring and surveillance programs, sharing isolates, examining reduction of infection in vaccinated flocks, supporting translational research to expedite vaccine commercialization, and development of risk assessment templates.

The USDA also believes as many approaches as possible must be considered to protect against and respond to HPAI, including vaccination of vulnerable bird populations.

“The decision to proceed with vaccination is complex—from vaccine development to production timelines to dissemination to flocks—there are many factors that make implementing a vaccine strategy a challenge and it would take time to deliver an effective vaccine,” the spokesperson said.

For now, biosecurity is the best defense against HPAI, and USDA strongly encourages all bird owners to review the department’s resources on managing wildlife to prevent avian influenza, evaluate their biosecurity plans, and develop a strategy to prevent any exposure to wild birds or their droppings.

Assuming a vaccine is approved, Dr. Swayne said next comes buy-in from producers, some of whom are resistant to using an HPAI vaccine because they worry it will cause actual HPAI infections in inoculated flocks. Producers also fear the same fate as France: unable to differentiate between inoculated and infected flocks, countries will simply ban imports of U.S. poultry outright.  

“At the end of the day, I'm hopeful that the scientific minds and scientific data will prevail and show that a vaccine has great utility at preventing any of these solid infections,” Dr. Swayne said. “In general, the threat and the fear of widespread infections within vaccinated flocks is not supported by science, and much of that concern is based upon misinterpreted vaccine efficacy data.”

“Vaccination doesn't replace biosecurity,” he continued. “If you have good biosecurity, and you only have little bits of the virus getting through, the vaccine gives you a very good prevention of infection.”

A version of this story appears in the February 2024 print issue of JAVMA


The AVMA has information on avian influenza and its epidemiology in the United States as well as how to recognize, diagnose, and prevent the disease in various animal species and how to report sick birds and other animals.