Bird flu is affecting more and more dairy cattle. There’s no sign yet of human-to-human spread, but new dimensions of the outbreak continue to emerge.
- Avian flu, highly contagious and damaging to poultry flocks, was found in dairy cattle in March.
- It’s since spread to hundreds of herds. Several dozen workers exposed to the cattle have become ill, with mild symptoms. Traces of the virus have been found in raw milk.
- There’s no indication that the virus has mutated and become a risk for human-to-human contagion, but public health officials are taking this possibility seriously. Dairy farms are already being negatively impacted.
In March of this year, bird flu was detected in dairy cattle, the first time the virus had been seen in cows. By April, the Centers for Disease Control and Prevention (CDC) confirmed a bird flu case in a human exposed to dairy cows, the first instance of the infection traveling from a mammal to a human.
To date, the U.S. Department of Agriculture (USDA) has confirmed avian flu infections in more than 700herds of dairy cattle, the great majority in California (see map). There are nearly 60 confirmed cases in humans. Both numbers continue to grow.
All of those who have contracted the virus worked on farms with infected animals, with the exception of two caseswith unknown causes. The most common symptom among workers is eye irritation, most likely the result of milk contaminated with the virus splashing in the eye. So far, there’s no sign the virus is being passed human to human.
Several dozen cases don’t amount to a public health emergency, but scientists have long been concerned that avian flu might mutate and become contagious among humans. Highly pathogenic avian influenza has a mortality rate up to 100 percent in infected poultry. An extremely contagious respiratory disease, it has killed 100 million chickens in the U.S. since an outbreak that began in 2022.
In rare cases, persons who live in close contact with infected birds have contracted avian flu. About 900 such cases have been documented worldwide in the past two decades. According to the World Health Organization, more than half did not survive.
In a population of billions, these numbers are extremely small, but their severity makes the possible transfer from cows to workers in an industrial farming system a major concern. A paper published last week in the journal Science suggests that a single mutation of the virus now in cows could be enough to create a version that could attach to human cells. Understanding more about the outbreak and containing it are immediate priorities for the public health and agricultural sectors.
Unknowns and Cryptic Cases
Detection of possible avian flu virus in raw milk and cream from a Fresno, Calif., dairy early this month was another indicator that the virus was spreading. A few days later, the USDA announced plans to immediately begin nationwide testing at milk processing sites.
There’s no evidence that anyone has contracted avian flu from drinking raw milk. Even so, the California Department of Public Health has warned the public not to consume it, citing risks from drinking, splashing into eyes or touching the eyes, nose or mouth with unwashed hands after contact with the milk. Only 1 percent of the commercial milk supply is unpasteurized, or raw; studies conducted since dairy cow infections have confirmed that pasteurization kills bird flu virus in milk.
For Scott Harris, state health officer for Alabama, the biggest concern about this news is the unknowns. “We feel comfortable making the statement that the risk is very low, and yet there’s a lot that’s just not known,” he says. There’s still much data to gather and analyze, and there are some cases out there that are difficult to explain.
One of these involves a child in Alameda County, Calif., who tested positive for bird flu. The child developed mild upper respiratory symptoms. An investigation by the county health department was not able to identify the source of exposure, though it ruled out raw milk products. No other cases were found, says Austin Wingate, the county’s public information manager. (An equally unexplained H5N1 case in a Canadian teenager led to serious respiratory issues and hospitalization.)
Another disturbing anomaly is the country’s first-ever case of bird flu in a pig— a troubling development, says Bobbi S. Pritt, a pathology researcher at the Mayo Clinic, given that pigs are well known as “mixing bowls” for flu viruses that can mutate and make the jump to humans. “[Pigs] can be infected with avian influenza strains as well as human influenza strains,” Pritt says. If both viruses are in the same animal at the same time, they could swap genetic material and create a new virus that would be infectious in humans.
It’s not inevitable that this will happen, Pritt says. But it’s a risk, and it only has to happen in one pig to set a chain of contagion in motion.
The pig in question was on a non-commercial backyard farm in Crook County, Ore. The pig wasn’t destined for the food supply and was euthanized, as were four other pigs and several dozen birds on the farm that tested positive. USDA testing of the virus in these animals found no sign it had mutated to become more of a risk to humans.
Questions remain about all the ways the virus can spread in dairy herds. The virus is present in high levels in milk and can move from cow to cow during the milking process if equipment isn’t effectively disinfected before it’s transferred. Workers moving from farm to farm is another potential risk factor, as are contaminated trailers and shared equipment such as water troughs.
At least two companies are working as fast as they can on a vaccine for dairy cattle, says James Roth, director of the Center for Food Security and Public Health at Iowa State University. He’s optimistic that when this becomes available it could stop the spread of the current outbreak.
In the meantime, the routine tools of public health are the front line of defense.
Doing the Usual
The CDC and state and local health departments are all monitoring workers exposed to infected animals and conducting contact tracing. Face masks, gloves and goggles can protect workers; the USDA has a program to reimburse producers who supply personal protective equipment to employees.
The country is entering the flu season. Vaccinating poultry and swine workers reduces the potential for new virus strains. “It won’t protect against H5N1, but the danger is that an individual gets seasonal flu and H5N1 at the same time,” Roth says. “That’s when the viruses can mix and match their genes and a brand new one can emerge.” (This precautionary recommendation extends to the general public.)
Encouraging physicians to test farm workers who come to them with flu-like symptoms for H5N1 is another important step, as is voluntary testing of those who don’t show symptoms. Some cases in humans have been asymptomatic, says Lori Tremmel Freeman, CEO of the National Association of City and County Health Officials. An antiviral treatment, oseltamivir, can help those who are ill recover.
The CDC is sharing genetic analysis of H5N1 virus discovered in workers. Researchers will benefit if others do the same, says Pritt. Wastewater is being monitored for signs of avian flu in some states, including California.
Public health regularly works with the agricultural industry, says Harris, but the current situation is making it necessary to form new relationships in real time. “We’re trying to build trust and understanding, so we’re all on the same page about our priorities,” he says. “It’s going to take some time, but it’s moving in the right direction.”
Three-fourths of new infectious diseases in humans come from animals. Public health officials are constantly on the alert for new pathogens. “Things pop up on the horizon, we monitor and prepare for them, and wait,” Harris says. “Many times nothing ever happens and the public’s not aware of what’s going on.”
Detection and mitigation efforts have helped slow the spread of H5N1, Freeman says. But public health agencies haven’t done enough to make it clear that investing in such work brings economic and health benefits, she says. “I would like to have that message heard and understood by this new administration as they’re considering ways to make government more efficient.”
Farmers Are Hurting
The long-term impact of infections among cattle on humans is still an open question, but avian flu is already having a significant impact on dairy farming. Death rates in some infected herds in California, which produces about one-fifth of the nation’s milk supply, have been as high as 15 or 20 percent.
The virus doesn’t kill dairy cows at the same rate as it does poultry, but the infections it causes in their mammary glands are a big problem, says Roth. Most quit producing milk while they are ill, he says, and their milk production can be reduced when they recover. Any abnormal milk cannot be sold.
Most cows get past their infections (nationally, only about about 2 percent are lost) but losing even one dairy cow is another financial hit. The value of a single animal can exceed $2,000. “The dairy farmers have a terrible time [with avian flu], because they have a bunch of sick cows at the same time that need to be supported and treated to get through it,” Roth says.
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