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One Health Resource Center
ByStephen I. Feller
Fact checked byCarol L. DiBerardino, MLA, ELS

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November 01, 2024
4 min read
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Q&A: First case of bird flu in a US pig raises concern

ByStephen I. Feller
Fact checked byCarol L. DiBerardino, MLA, ELS

Key takeaways:

  • A pig at a noncommercial farm in Oregon tested positive for H5N1 avian influenza.
  • There is no threat to the commercial pork supply and the threat of H5N1 to humans remains low, officials said.

The United States Department of Agriculture this week confirmed the first case of highly pathogenic H5N1 avian influenza in a U.S. pig, raising concerns about the potential health risk to humans.

After poultry at a noncommercial farm in Crook County, Oregon, tested positive for H5N1, the Oregon Department of Agriculture tested other animals on the farm because livestock and poultry on the farm share water sources, housing and equipment, the USDA said. Of five pigs on the farm, one tested positive for H5N1, two tested negative, and tests for two others are pending, according to the department.

AdobeStock_Pigs_1200x630
The first case of H5N1 bird flu in a U.S. pig was detected in Oregon. Image: Adobe Stock

The ongoing bird flu outbreak in the U.S. has affected more than 104 million poultry and 403 dairy herds in 14 states, according to the CDC. There also have been 39 cases of H5N1 reported in humans, including nine in Washington this week — three of which were detected in Oregon when they traveled during their infection.

The CDC continues to say that the public health risk from H5N1 is low. The agency is using its influenza surveillance systems to help monitor potential H5 activity in people, which includes wastewater surveillance.

We asked Benjamin Anderson, PhD, MPH, CPH, an assistant professor of environment and global health at the University of Florida, about H5N1’s jump to pigs and what it could mean for people.

Healio: How worrisome is it that H5N1 has been confirmed in a pig?

Anderson: This seems to be an isolated case for the moment. However it still raises some concern. Swine are a known “mixing vessel” for influenza A viruses because they are able to be infected by avian and human influenza virus strains.

If H5N1, an avian influenza virus, were to regularly transmit among pigs, the greatest concern is that the genetic material from other influenza virus strains circulating in pigs may recombine with it to make a new progeny virus that is more transmissible to humans. This is likely how the 2009 influenza pandemic began.

Furthermore, the information that all five of the pigs at the farm were not clinically ill is concerning for surveillance purposes. Most of our testing for H5N1 in farm settings to date has occurred only due to clinical outbreaks. If the virus causes subclinical — mild illness — or asymptomatic infections in other livestock, then we may not catch it without regular ongoing testing.

Healio: Does the jump to pigs make it more likely that this could start a pandemic?

Anderson: This one case does not indicate a larger spread of the virus is guaranteed to happen. However, if the virus does establish itself in swine then, yes, the probability of recombination with other circulating influenza virus strains and gaining human transmissibility is higher.

Healio: The number of states detecting new avian influenza cases in livestock has declined in the last 30 days, but California has confirmed H5N1 in 193 dairies and reported 16 human cases and Colorado is testing cow’s milk for influenza. What do you think about the current situation?

Anderson: Many epidemiologists, myself included, see California as an encouraging example of how active surveillance can identify more cases of a virus both in farms and in humans who may have had an exposure. Other states that have also implemented strong active surveillance efforts are Colorado and Michigan, which has resulted in a higher number of positive farms identified.

There is certainly an epidemiological curve that is being seen in states where outbreaks have occurred, represented by a spike in cases and then a drop off as the virus either burns out or response efforts mitigate its spread. However, without uniform and robust surveillance across all states pre- and post-outbreak, it is difficult to make comparisons about the degree of circulation because it is likely cases of H5N1 are being missed in places where active surveillance is not being carried out.

Healio: Does anything worry you about how the United States has handled bird flu so far?

Anderson: The response to date, in my opinion, has been very fragmented because of the way that outbreak investigations are handled on a state-by-state basis. Some might be critical of the USDA and CDC, but it is the states that are largely responsible for how surveillance is implemented and outbreak investigations are conducted.

Much of the reluctance we have seen to carry out more extensive testing in farms is primarily related to the economic concerns of identifying the virus, and on the farm level there is uncertainty regarding regulations for how state and federal agencies will respond to those detections. These concerns are legitimate, and I do not believe the public health response to date has fully taken those into account.

References:

Sources/Disclosures

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Disclosures: Anderson reports no relevant financial disclosures.

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h5n1 avian influenza
 

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