Highly pathogenic avian influenza (HPAI) virus continues to take a heavy toll on US poultry while raising concerns about transmission to other species, including humans. How serious are these concerns and what are the transmission routes?
“To understand the big picture with transmission, we must remember that influenza A as a family has a wide host range,” stated Erica Spackman, PhD, USDA senior research scientist, during a recent webinar presented by the Poultry Science Association.
“In poultry specifically, individual strains of avian influenza A will adapt to host species through a process of circulating in that host,” she added.
Spackman also explained that the current H5 viruses are much more infectious than in the past, creating new challenges.
She discussed the latest HPAI research on transmission pathways and factors impacting infection during the webinar.
From dabbling ducks to chickens, mammals
“Dabbling ducks are the natural host species from which all influenza A viruses probably originated,” Spackman said. “It is the group that carries the virus, then the virus spills over into everything else…occurring through an environmental source is likely rather than direct transmission.”
Despite ongoing HPAI outbreaks, gallinaceous poultry are not natural hosts for influenza, according to Spackman. Poultry generally need high doses of the virus for transmission to occur.
Researchers are studying how influenza is transmitted to mammals such as dairy cows and cats. “Compared to birds, mammals tend to have a cold-adapted influenza A virus that grows better in their bodies,” Spackman explained.
“Cats seem to be universal victims of influenza transmission and have occasional influenza transmission from people,” she added. “We’ve known for decades that cats tend to get seasonal influenza from their owners. So, the random one-off transmission of influenza to other species — cats, in this case — isn’t really that remarkable.”
‘A thousand times more infectious’
The virus can become well-adapted after long-term exposure at high levels in some host species. “We have these H9N2 low-pathogenic avian influenza viruses that are chicken-adapted and not seen in other species,” Spackman said. “They are endemic in poultry in Africa, the Middle East and Asia and are generally controlled by vaccination.
“But with H5N1 HPAI, it’s a different dynamic. The infection is much shorter because of high mortality,” she continued.
Unfortunately for US poultry, the current influenza viruses became more infectious after circulating in Asia, Europe and Africa and making it back to the Western Hemisphere.
“The viruses seem to have adapted much more to gallinaceous birds,” Spackman said. “The viruses we see now right out of ducks, which are index cases from wild birds, are a thousand times more infectious for chickens than the H5 HPAIV in the US in 2014-2015.”
Consequently, the virus is significantly harder to combat through biosecurity and biosafety. And it transmits more effectively than just a few years ago.
Although vaccination is used in other parts of the world, it doesn’t completely block an influenza infection, Spackman noted. Instead, current vaccines reduce how much virus is shed and how much virus is replicating at the site of exposure, therefore reducing or eliminating the spread within the host.
Transmission dynamics
“Regarding transmission, HPAI is not magical or different among other pathogens,” Spackman said. “Viruses have only a few ways they can spread among farms, and those transmission routes aren’t that different among viruses. How we treat them is universal for many pathogens.”
Research indicates that how an animal is exposed influences virus transmission. One study found that intranasal exposure required 3 to 4 logs less virus for infection than exposure via eating or drinking.
“Breathing in the virus occurs through aerosols, and fomites like dust and feathers that get coated with the virus can spread it,” she said. “However, a little higher humidity level will reduce transmission.”
In field tests with outside-air sampling, researchers also learned that viruses do not travel far from the houses. “Generally, when viruses are detected outside a house, quantities are very low, and viability testing demonstrates that the virus is not always infectious,” she added.
In another study, in-house airborne fomites appeared to be very important in viral spread. A high amount of the HPAI virus was found in feather pulp from chickens, turkeys and ducks.
“Basically, the growing cells inside the feather shaft are full of the virus, and the feather shaft seems to protect it,” Spackman said. “Feathers are made to float around and stick to everything. So, if there’s a good fomite, it’s feathers.”
Virus in the environment
Considerable research is underway looking at how the virus is transmitted in the environment. Here are several highlights:
- In studies evaluating soil types for transmission, sandy material did not transmit the virus well. Denser soils transmitted the virus more effectively, and birds became infected.
- Research on feed indicated no evidence of HPAI transmission, from an epidemic standpoint. The virus didn’t survive 24 hours in feed.
- Surface water is a possible source for HPAI transmission. Research showed the virus survived in field water for a month at 82˚ F (28˚ C) and for 270 days at 63˚ F (17˚ C). All samples where the live virus persisted were in cold environments like Alaska and Minnesota. The virus also survived in sediment.
- A study on manure and litter, including pine shavings, found very little transmission of the virus.
- Whether transmission occurs through small animals on poultry farms requires more research. “In studies on infected farms, we can find the virus and evidence of infection in mice and birds that live around the farms,” Spackman said. “But it’s unclear if the poultry were infected by the small animals or vice versa or a common environmental source.”
- Studies of the virus in house flies and blow flies demonstrated that it did not replicate in the insect guts and died off quickly within 24 to 28 hours.
Less farm-to-farm spread
“The good news is we really don’t have farm-to-farm spread now,” Spackman continued.
“But there is a lot the poultry industry is doing right, and we’ve learned a lot from our outbreaks in the past,” she concluded.
