A third human case of H5 bird flu tied to the ongoing U.S. outbreak in cattle has been detected in a farm worker in Michigan, state health authorities confirmed on Thursday.
The unnamed individual worked on a dairy farm and was in close contact with infected cows, the state health department said in a statement. The farm involved is different from the one where an earlier human case was detected last week.
“With this case, respiratory symptoms occurred after direct exposure to an infected cow,” Natasha Bagdasarian, chief medical executive of the Michigan Department of Health and Human Services, said in a statement. The individual was not wearing protective equipment, she said.
In a separate statement, the Centers for Disease Control and Prevention said the individual had a cough without a fever and “eye discomfort” with watery discharge.
The worker was given flu antiviral drugs and is recovering, both statements said. The CDC said the individual was instructed to isolate, and household contacts were also offered antiviral drugs. The statement does not indicate whether they agreed to take them.
“No other workers at the same farm have reported symptoms, and all staff are being monitored. There is no indication of person-to-person spread of A(H5N1) viruses at this time,” the CDC said.
Of the four known cases of H5 infection that have been reported in the United States — three since the outbreak in cattle began — this is the first in which an infected individual reported respiratory symptoms. The two human cases reported earlier this year experienced only conjunctivitis — known as pink eye. Those cases occurred in Texas and in Michigan, and were both in farm workers who had contact with infected cows.
The first U.S. case of H5N1 occurred in April 2022 and involved a man in Colorado who was involved in culling chickens on a poultry farm where the virus had broken out; though he tested positive for the virus, he reported only feeling fatigue.
The significance of respiratory symptoms relates to the possibility of onward spread. In people, influenza transmits via the respiratory route. A person with flu virus in their airways could be more likely to spread the virus — if the virus has the capacity to transmit between humans — than a person with an infection in their eye. To date, H5 viruses have not been seen to spread easily among people, and Michigan officials said the risk to the general public remains low.
The Michigan statement described the virus solely as an H5 virus. The CDC, which does confirmatory testing for H5N1 cases, said it is attempting to generate a full genetic sequence of the virus and if the efforts are successful, it will release further information in a day or two.
Prior to this outbreak, it was not believed that cattle could be infected with this virus — an assumption that probably contributed to the slow realization that a mystery illness that was affecting milk production in some dairy herds located in the Texas panhandle starting in February was actually avian influenza.
Since the late March announcement that H5N1 was the reason for the drop in milk production, the U.S. Department of Agriculture has confirmed 67 infected livestock herds in nine states. One of the herds was alpacas, the rest have been dairy cattle.
Infectious disease experts suspect the outbreak is more widespread than the number of confirmed states would suggest, a belief that is supported by a survey of commercially purchased milk conducted by the Food and Drug Administration. Of nearly 300 pasteurized milk products bought in 38 states, one in five were positive by polymerase chain reaction testing for the H5N1 virus. Efforts to grow viable virus from milk that tested positive failed, supporting the FDA’s position that pasteurization kills the virus.
There are concerns, though, that unpasteurized milk — generally referred to as raw milk — could subject consumers to dangerous levels of the virus, if the herd that produced the milk was infected with H5N1.
A study published last week in the New England Journal of Medicine reported that mice fed raw milk known to have come from infected cows made the mice sick enough that they had to be euthanized. There have been multiple reports of deaths of cats on farms with infected herds, and one of the infected alpacas displayed neurological symptoms before it died, Sydney Kennedy, a public information officer for the Idaho State Department of Agriculture, told STAT in an email.
While the virus is deadly in poultry, and has been seen to kill multiple mammal species — from polar bears to foxes, from zoo tigers to sea lions — it does not, in general, seriously sicken cows. As a result, there’s been little upside for farmers to agree to have their animals tested. While the case count continues to grow in most of the states with confirmed herds, there has not been a new state to acknowledge having infected animals since Colorado joined the list on April 25.
A federal order that went into effect on April 29 requires farmers to test some of the lactating cows they plan to transport over state lines, though they are required to test a maximum of 30 animals per shipment, and the farmer can choose which animals are tested.
The USDA announced Wednesday that nearly 2,500 pre-movement tests have been conducted. It did not say how many of those tests produced positive results, and the agency has not acknowledged or responded to a STAT request for that figure.
Sarah Owermohle and Rachel Cohrs Zhang contributed reporting.