Fourth human case of bird flu connected to dairy cattle outbreak identified in the US

A fourth person in the United States has tested positive for H5 bird flu in connection to an ongoing outbreak among dairy cattle across the country.

The Colorado Department of Public Health and Environment announced Wednesday that it has identified the state’s first human case of H5 avian influenza associated with the multistate outbreak of the virus in dairy cattle. This case is the fourth identified nationally in connection to the outbreak; two other human cases have been reported in Michigan and one in Texas.

More than a quarter of dairy herds in Colorado have reported cases of bird flu, according to data from the state health department. As of Wednesday, federal data shows that seven states have confirmed cases in the past 30 days, and 40% of them are in Colorado — more than any other state.

The infected person in Colorado had mild symptoms, reporting only conjunctivitis, or pink eye, as a symptom, according to state health officials. He worked at a dairy farm in northeast Colorado and had direct exposure to dairy cattle infected with avian flu.

After testing positive, the man was treated immediately with the antiviral medication oseltamivir, and he has recovered, according to the state health department.

“The risk to most people remains low. Avian flu viruses are currently spreading among animals, but they are not adapted to spread from person to person. Right now, the most important thing to know is that people who have regular exposure to infected animals are at increased risk of infection and should take precautions when they have contact with sick animals,” Dr. Rachel Herlihy, state epidemiologist at the CDPHE, said in Wednesday’s announcement. The last human case of H5N1 in Colorado was in 2022 in a person who was exposed to infected poultry.

In the current outbreak connected to infected dairy cattle, the first two human cases identified in the United States also reported eye-related symptoms and eye infections or conjunctivitis. The third human case in the US reported more respiratory symptoms, such as cough, congestion, sore throat and watery eyes. None of those three people had contact with each other, but all worked with cattle, suggesting that they are instances of cow-to-human transmission of the virus.

On the national level, the US Centers for Disease Control and Prevention has said that it continues to monitor the situation carefully, using its flu surveillance systems to watch for H5N1 activity in people. The agency said Wednesday that the human case in Colorado “does not change CDC’s current H5N1 bird flu human health risk assessment for the U.S. general public, which the agency considers to be low.”

However, the case “underscores the importance of recommended precautions in people with exposure to infected animals,” according to the CDC. “People with close or prolonged, unprotected exposures to infected birds or other animals (including livestock), or to environments contaminated by infected birds or other animals, are at greater risk of infection.”

The CDC repeatedly has recommended that dairy workers wear personal protective equipment to reduce their risk. The US Department of Agriculture has offered financial support to farms with infected animals to provide workers with protection. Research is ongoing into how infections are transmitting between cows and from cows to people who work with them.

CDC officials said Tuesday that the United States has enough H5 bird flu tests to respond to the outbreak.

“We have an ample number of H5-specific tests in the public health system for the current outbreak. As of right now, there are roughly 750,000 H5-specific tests available today and another 1.2 million such tests coming online in the next two to three months,” Dr. Nirav Shah, principal deputy director of the CDC, said at a news conference Tuesday.

“This is on top of a robust supply of influenza A tests, the general tests that are available at doctor’s offices around the country,” Shah said. “The way that our system works is such that if someone tests positive for influenza A at the doctor’s office, their specimen is sent to a public health laboratory for what’s called subtyping. That tells us if the influenza A virus is a common seasonal one, or a rarer version like H5N1. That subtyping happens more commonly than you might think.”

CNN’s Deidre McPhilips contributed to this report.

For more CNN news and newsletters create an account at