(Bloomberg) -- A Michigan farmworker tested positive with bird flu, the second person to contract the potentially lethal virus that has run rampant in US cattle.

The farmworker experienced mild symptoms in the eye after contact with an infected cow and has since recovered, the US Centers for Disease Control and Prevention said Wednesday. Michigan officials said Monday that three additional cattle herds had tested positive for bird flu.

Health officials are increasingly on guard for cases of H5N1, a bird virus that’s known to jump between species and can sometimes cause severe cases in people. The country’s first human case of bird flu from cattle was announced in a Texas farm worker in late March. That patient had an eye infection with the virus, called H5N1, and was treated with an antiviral. Health officials maintain that the risk to the public remains low, as there’s been no human-to-human transmission. 

As of late Wednesday, the US Department of Agriculture has reported 52 herds in nine states have tested positive for bird flu. Health officials said they have antivirals and vaccine candidates on hand should they be needed. 

The US is also in talks with Moderna Inc. and Pfizer Inc. about the development of messenger RNA avian flu vaccines, Assistant Secretary for Preparedness and Response Dawn O’Connell said on a call with reporters. Moderna’s vaccine is in early-stage trials, the company said in a statement, and results are expected soon. The shares closed up 14% in New York trading. 

Pfizer didn’t immediately respond to a request for comment. The company’s shares gained 3.6% in New York. 

Given the high levels of the virus in raw, unpasteurized milk and the extent of spread through dairy cows, the CDC said, more cases may be identified in people.

Michigan Farmworker

The human infection in Michigan “was not unexpected,” Nirav Shah, principal deputy director of the CDC, said on the call. “We found this case because we were looking for this case.” 

After Michigan identified an H5N1-infected cattle herd, the farm worker was enrolled in an active surveillance program, which is currently monitoring 170 people across the state. The program sends daily texts to assess workers’ symptoms, Shah said. 

When the worker in question reported mild eye symptoms, Shah said, health officials obtained swabs of the worker’s eyes and nose. 

Those samples arrived at the CDC on Tuesday. While the nasal swab tested negative for an H5 subtype of influenza A virus, the eye swab tested positive. Shah said it’s “reassuring” that the nasal swab tested negative, because it reduces the likelihood of respiratory spread. 

The CDC said eye infections, which have been present in both cases of cattle-to-human transmission, may be a result of a “splash of contaminated fluid” or touching the eyes with something contaminated with the virus, such as the hand.

Given the high levels of the virus in raw, unpasteurized milk and the extent of spread through dairy cows, the CDC said, more cases may be identified in people. The case underscores the importance of taking precautions, such as using personal protective equipment if exposed to sick or dead livestock, the agency said. Shah added that he didn’t know whether the farmworker was wearing protection, and Michigan said it wasn’t revealing more details about the farm or farmworker to protect privacy.

The agency notified Michigan on Tuesday after receiving the positive result and will make results of the viral sequence available within days. It will also conduct a genetic analysis to see if the virus has mutated in a way that could make it more dangerous, as well as how it interacts with existing antivirals and vaccine candidates, Shah said. 

Michigan said that state officials have been tracking the situation closely since H5N1 was identified in cattle in the state on March 29. Three USDA management teams have been monitoring poultry farms in Michigan and one USDA epidemiological team has been deployed to monitor dairy herds.

--With assistance from Damian Garde and Gerry Smith.

(Updates with company’s and health official’s comments.)

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