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The CDC is preparing for the possibility of H5N1 posing an increased risk to human health, even though that current risk remains low, the agency said on Friday.

"It is possible that influenza A(H5N1) viruses could change in ways that allow them to easily infect people and to efficiently spread between people, potentially causing a pandemic," the agency stated in an email to media.

As of May 22, about 350 farm workers are being monitored for illness after exposure to infected cows or infected raw cow's milk, according to researchers led by Shikha Garg, MD, of the CDC's National Center for Immunization and Respiratory Diseases, writing in the .

"The number of persons monitored continues to increase," they added.

In April, the CDC announced that a dairy farm worker in Texas developed conjunctivitis associated with H5N1 infection, the first human case linked to an ongoing multistate outbreak in dairy cows. Then earlier this week, a second case in Michigan was reported of a person who also developed conjunctivitis from infection with H5N1. This most recent case was identified through daily monitoring of farm workers, the report said.

Of note, a in the New England Journal of Medicine showed that when researchers fed mice raw milk from dairy cows infected with H5N1, the mice rapidly developed signs of illness. At 4 days after exposure, high levels of virus were found in the animals' nasal passages, trachea, and lungs and moderate-to-low virus levels were identified in other organs, a finding that is consistent with H5N1 infections found in other mammals.

The FDA has confirmed that pasteurization inactivates H5N1 viruses and that the commercial milk supply is safe, however "all persons should avoid consuming raw milk or products produced from raw milk," noted Garg and colleagues.

Clinicians should consider the possibility of H5N1 when evaluating patients who present with conjunctivitis or respiratory illness following a relevant exposure, the CDC authors emphasized. People who have job-related or recreational exposure to infected birds, poultry, dairy cattle, or other infected animals or contaminated materials are at highest risk for infection and should take precautions, including using personal protective equipment, self-monitoring for symptoms of illness, and seeking prompt medical care if they develop symptoms.

While reiterating that the current risk to the U.S. public from H5N1 viruses is low, if a novel influenza A virus acquired the ability to infect and be transmitted easily in humans in a sustained manner, "an influenza pandemic could occur," the CDC authors wrote.

"Comprehensive worldwide surveillance and investigation of every novel influenza A virus case in humans is essential to prepare for any developments that increase the risk to human health," Garg and coauthors emphasized.

The CDC is currently monitoring trends in influenza activity by looking for unusual changes in the percentage of positive influenza tests and through the CDC's National Wastewater Surveillance System. As of May 18, no indicators of H5N1 or other unusual human influenza activity has been detected.

The CDC is also over the coming months by working with commercial laboratories to increase submission of influenza-positive test specimens to public health laboratories for virus subtyping. Also, the CDC is collaborating with manufacturers of commercial diagnostic tests to potentially develop an H5N1 test that could be made widely available if needed.

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