The US farmworker who caught bird flu from cows may be the first case of mammal-to-human transmission. His case also shows how difficult it is to track the infection.

CNN
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A US farmworker who caught bird flu after working with dairy cattle in Texas appears to be the first known case of mammal-to-human transmission of the virus, a new study shows.

The dairy worker sought care in late March after developing painful red, swollen, weeping eyes with burst blood vessels.  He had no fever, however, and his lungs were clear, according to a new letter about the case published in the New England Journal of Medicine on Friday.

He reported no contact with sick or dead birds or other animals, but he did have repeated, direct, close contact with dairy cows in the same part of the state with other infected herds.

Even though the man didn’t become seriously ill, his case is important because it confirms that humans can be infected with H5N1 after contact with cows.  At the same time, it also leaves critical questions unanswered and illustrates how hard it will be to track the infection in this vulnerable population of workers, where testing positive for an infectious disease might mean losing days of work and pay.

“For farmworkers specifically, certainly these are folks that are that are living in a state of economic desperation, and what they’re not going to do, is they’re not going to test for something if they don’t have paid sick leave, because they cannot afford to be sent home and told to stay home and not work,” said Elizabeth Strater, who is director of strategic campaigns for United Farm Workers.

Strater says UFW, like other groups, has heard rumors that there are dairy workers who are sick, but don’t want to be tested, but she said it’s nothing that they’re able to confirm.

Health officials in Texas said that they did test other sick dairy workers, including some with red eyes, but they turned out to have other illnesses, not bird flu.

“The people tested volunteered to be tested,” said Lara Anton, senior press officer with the Texas Department of State Health Services.

“It’s likely there were other people with symptoms who did not want to be tested so we cannot say with absolute certainty that no one else contracted H5N1. We can say for sure some of the people on dairy farms tested positive for other respiratory viruses that are commonly circulating in the human population,” Anton said.

In the case of the man who did test positive for bird flu, he and his and his close family members were given antiviral medications and they recovered without any lasting problems, the letter says.

Swabs of the patient’s eyes and lungs revealed something interesting, too. While his eyes were teeming with the H5N1 virus, there was hardly any virus in his lungs. That could mean the worker was infected through his eyes—either rubbing them with contaminated hands, or through splashes of contaminated milk—rather than through his lungs, and the virus never migrated there, or that the virus couldn’t get a foothold in his lungs because it was adapted primarily to infect birds, but not cells in the human airway.

The letter on the case was written by researchers at the US Centers for Disease Control and Prevention along with doctors at the Texas Department of State Health Services and researchers at the Texas Tech Bioterrorism Response Laboratory.

Health officials said they couldn’t do further investigation of how the man was infected because “epidemiological investigations were not able to be conducted at the farm” where he worked.  They were also unable to test other workers at the same farm.

That kind of testing is critical to answer questions about how the worker became infected, whether others were being infected and if so, for how long they were infected, what kind of symptoms they had, if they had any at all.

The CDC is currently looking for farms that will allow them to conduct such a detailed study.

“Understanding the current avian flu outbreak among dairy cattle is a vital priority to help protect human health,” the CDC said in a statement to CNN. “Discussions are under way with farms in multiple jurisdictions to participate in CDC-led epidemiological studies. In the meantime, states continue to test symptomatic farm workers and monitor those who have been exposed to infected animals. CDC also continues to closely monitor a robust, nationwide flu surveillance system. To date, it has not detected any unusual flu activity.”