Bird flu is widening its reach. Maps show US counties affected by outbreak.

Four Colorado poultry farm employees are the latest confirmed cases of bird flu – also known as Avian influenza H5N1 . The U.S. total stands at nine, although another worker's test results could soon push it to 10.

The outbreak worries researchers because the virus can infect other animals, such as cattle and poultry. A recent study shows that the virus can jump back and forth between birds and cows, which may help it spread over large geographic areas.

Bird flu virus outbreak in backyard and commercial poultry

Of the nine human cases of bird flu in the U.S. since 2022, five followed exposure to poultry. The other cases followed exposure to dairy cows. While no one needed to be admitted to the hospital, each had a mild respiratory condition and conjunctivitis.

As of July 15, nearly 100 million poultry birds in the U.S. have been affected by the virus since 2022, according to the Centers for Disease Control and Prevention. The outbreak has reached 526 counties in 48 states.

Unable to view our graphics? Click here to see them.

More: Colorado workers exposed to bird flu while killing infected chickens, feds say

Bird flu virus outbreak in dairy cows

The multistate outbreak, the first seen in cattle, probably began late last year .

The CDC and the U.S. Department of Agriculture say milk and milk products are still safe although more than 160 dairy herds (as of July 18) have been affected in 13 states.

Health officials say they are analyzing raw meat samples from markets to ensure it's safe for consumption in places where cows have contracted bird flu.

Tracking the spread of bird flu

Health officials worry these outbreaks could expose people who work with animals more frequently to the virus. In turn, the virus might mutate and become transmissible among people – potentially leading to an outbreak or even a pandemic.

Reports say the virus has been transmitted from wild bird to cow, cow to cow, cow to poultry, cow to human and bird to human. There is no evidence so far of human-to-human transmission.

What are the symptoms of bird flu in humans?

It's uncommon for bird flu viruses to jump to humans, but it does occasionally occur. Humans can contract the disease by mouth, eyes, or nose, or by inhaling the virus after coming into contact with an infected animal.

According to David Boucher, director of the agency's infectious disease preparedness and response, the Department of Health and Human Services' Administration for Strategic Preparedness and Response is continuing to produce millions of doses of the bird flu vaccine through July as part of a pre-pandemic stockpile.

There is no recommendation yet for people to receive the vaccine, which has not been fully tested or approved for use.

CONTRIBUTING: Eduardo Cuevas and Jim Sergent

SOURCE Reuters, Centers for Disease Control and Prevention, U.S. Department of Agriculture, Nature.com and Harvard Medical School, American Veterinary Medical Association and USA TODAY research

This article originally appeared on USA TODAY: Bird flu is widening its reach. Maps show US counties affected by outbreak.