4th person catches bird flu from cows, this time in Colorado

A fourth dairy worker in the U.S. has tested positive for bird flu following exposure to infected cows, the Centers for Disease Control and Prevention (CDC) reported Wednesday (July 3).

The case is the first human illness detected in Colorado in connection with an ongoing outbreak in cattle. It follows one human case in Texas and two in Michigan. The second case in Michigan caused mild respiratory symptoms, such as coughing, but the other three cases — including the most recent in Colorado — have only included eye symptoms, such as redness.

The culprit behind these infections is a bird flu virus called iNFLuenza A(H5N1). This subtype of flu was first spotted among U.S. dairy cows in March. Since then, dozens of herds across the country have tested positive for the virus.

Recent research suggests that, when it infects cows, the virus can end up in the animals' milk and remain infectious in unpasteurized milk for at least an hour after harvest. This poses a significant risk to dairy workers, who may come into contact with contaminated milk through contact with cows or with milking equipment. The pasteurized milk supply, on the other hand, is safe because the pasteurization process inactivates any viruses in the milk. 

Related: 'Playing Russian roulette with your Health': Officials warn that social media trend of consuming raw milk will not protect you from bird flu

Because of the risk posed by raw milk, the CDC recommends that dairy workers wear personal protective equipment.

In the recent Colorado case, the individual was being monitored because they'd been exposed to H5N1-infected cattle at work. Clinical samples from the person were sent to the CDC, who confirmed an infection with an iNFLuenza A(H5) virus. The agency is continuing to analyze the sample to confirm that the virus is in fact H5N1.