Dairy cow

Photo by Marta Knight

The susceptibility of dairy cows to highly pathogenic avian influenza raises questions about bovine veterinarians' occupational risk of contracting the virus.

When Dr. David Hernke attended a conference for bovine practitioners in Ohio last year, he was asked to roll up his sleeve for science.

The U.S. Centers for Disease Control and Prevention was collecting samples of blood from veterinarians who work with cattle to determine the prevalence of a highly virulent form of avian influenza that's been infecting a wide variety of animals besides birds — including dairy cows, seals, sheep and cats.

Hernke was a good research candidate. An assistant clinical professor at Tufts University's veterinary school in Massachusetts, he regularly works around cattle as part of the school's field services team, which provides veterinary care to nearby farms. Plus, he understood how the CDC research could benefit fellow dairy veterinarians.

"It was helpful to give some real hard data," he said, "to help figure out how is it spreading, where is it spreading, what are the risk factors. So that's why I signed up — it made sense."

The study detected antibodies to the virus in three out of 150 bovine veterinarians tested. None had been sick or even aware that they'd been exposed, and one practiced only in a region with no known infections in cattle or people.

The results, published in February, suggest that monitoring only symptomatic exposed workers might underestimate human infection — a finding with important implications for a disease with a worrisome potential to become the next pandemic.

Even absent broader public health consequences, veterinarians' exposure raises questions about their individual safety risk, particularly those working around susceptible livestock like cattle and poultry, in which cases continue to arise.

In the two months since the results of the CDC study were published, there have been 51 new confirmed highly pathogenic avian influenza (HPAI) cases in dairy cattle, all in Western states. No new cases have been reported in people — although, as the study indicated, the number of detections may be lower than the true number of cases.

Less lethal to humans so far

The current avian influenza outbreak predominantly involves a new form of the HPAI subtype H5N1. First identified in Europe in 2020, it showed up in U.S. commercial poultry flocks in 2022, then in U.S. dairy cows in 2024.

According to the CDC, there have been 70 confirmed human cases of H5N1 in the country since April 2024. Of these, nearly all had direct exposure to dairy cattle or poultry. The severity of illness has ranged from mild to severe to, in one instance, fatal. The fatal case occurred in January in Louisiana. The patient who died was a woman over age 65 and reported to have underlying medical conditions, according to that state's health department.

Most human cases have been mild, typically presenting like a mild flu, with a low-grade fever, cough and runny nose. There is one symptom not usually seen in seasonal flus: conjunctivitis, an inflammation commonly called pink eye.

While H5N1 is highly lethal among some types of animals, the CDC judges its risk to people as currently "low." For context, other outbreaks of H5N1 have been more deadly to humans: According to the World Health Organization, of 261 cases of human infection by H5N1 in the Asia-Pacific region between 2003 and Dec. 12, 2024, 142 were fatal — a case fatality rate of 54%.

Silent infections detected

In the CDC study that Hernke participated in, 25 of the test subjects reported recent exposure to a dairy herd with a suspected or confirmed case of H5, but none of the positive results came from this group. (The researchers use the term H5 because the test they used targets the H5 portion of the virus.)

One of the antibody-positive veterinarians had worked with an infected poultry flock, presenting a possible alternate path to infection (other than from cattle), though a CDC spokesperson told the VIN News Service they couldn't confirm that path.

"Our study underscores the potential for missed H5 infections in both animals and people, but the data suggests these undetected infections are uncommon," the CDC said by email. "This could indicate there are undetected infections in cattle, which illustrates the importance of testing of cattle herds, bulk milk and people at increased risk of H5 exposure."

A similar study published last year documented antibodies in farmworkers. Eight farmworkers out of 115 tested positive for H5 antibodies. Four of them recalled being ill.

Hernke, for his part, isn't fretting about his safety because he lives in the Northeast, where no cases have been detected in dairy herds to date. He acknowledges that concern could be higher among colleagues or farmworkers in the region who are immunocompromised or don't have health insurance.

"I'm not worried for myself, to be quite honest," Hernke said.

Precautions to take

Dr. Dean Blumberg, a pediatric infectious disease expert at the University of California, Davis, agreed that the risk to people of getting seriously sick from the current form of H5N1 is pretty low, at least at this juncture.

"It usually doesn't result in severe illness, for example, requiring hospitalization," he said.

"I do think it's being underdiagnosed," he added. "Not everybody who gets sick is going to be tested, and then obviously, people who have asymptomatic infection, there's no reason that they would present for testing to their health care provider."

Even asymptomatic infection provides the virus with an opportunity to mutate and become more dangerous. If a person is infected with a seasonal flu virus and avian flu virus simultaneously, that could enable a process known as reassortment, in which viruses trade genetic material, potentially creating a version of avian flu that infects humans more easily.

While vaccination against seasonal flu won't protect a person from avian flu, it can reduce the likelihood of reassortment. As for a specific avian flu immunization, the U.S. Food and Drug Administration has approved an H5N1 vaccine that the government has stockpiled, but it's not commercially available.

David Hernke TVFS.jpg

Photo courtesy of David Hernke

Dr. David Hernke participated in a federal study that tested bovine veterinarians for antibodies to the highly pathogenic avian influenza H5 virus.

As a treatment or, in some cases, as a post-exposure preventive measure, the CDC recommends taking the antiviral medication oseltamivir (brand name Tamiflu).

"The strains that have been tested so far all remain susceptible to oseltamivir or Tamiflu," Blumberg said, adding that most patients tolerate it well, with a minority experiencing an upset stomach or vomiting. "It's reassuring that we do have a treatment in place."

Part of calculating the risk for veterinarians is understanding how H5 progresses through a dairy herd.

Michigan State University's Dr. Zelmar Rodriguez has worked with dairy cows throughout his career and now studies H5's spread and impacts within herds.

"We have been working on understanding influenza in cattle since very early on because Michigan actually was the third state affected," he said.

While avian flu progresses quickly in poultry, the presentation can be less apparent in cattle. Poultry fatalities associated with H5 are high, so much so that any flock in which it's detected is isolated and exterminated. However, dairy cows may completely recover from their illness.

Rodriguez believes it's entirely possible veterinarians may show up to a farm where H5 is present and never know it.

"Veterinarians are visiting farms on a daily basis, and they are exposed to cattle," Rodriguez said. "I think the most important thing is that they know if the virus is at the farm so they can take extra precautions."

Those precautions, per Blumberg's recommendation, may include copious personal protective equipment: gown, gloves, goggles, masks, hoods and booties.

In the CDC study, all three antibody-positive test subjects reported wearing clothing covers and gloves while interacting with dairy cows, but not eye protection or masks.

Even Hernke, who is in a region with no identified cases in cattle, says there's no harm in taking extra protective measures.

"A few of our more proactive producers have implemented ongoing biosecurity measures," he said. "One of my dairies is really quite into that, because they do tours and they have some public-facing portions of their dairy. So we've been talking a lot about biosecurity and signing in, signing out; things like that."

One of his client dairies has even begun using face shields as a precaution.

"There's other benefits to biosecurity than just protecting against influenza, right?" Hernke said. "But I don't think we need to go full Tyvek suits at this point and make everybody on edge."