A new study of an ongoing avian flu outbreak on dairy farms provides an unprecedented look into how efficiently the virus spreads among cows and from them to other mammals, including cats and raccoons.

This suggests the virus is developing new capabilities that have bird flu experts concerned.

Bird flu is a concern in the United States due to various poultry populations. Over the past few years, scientists have feared the virus could become more transmissible from person to person and spark the next pandemic.

To the best of researchers' knowledge, there has not yet been any human-to-human transmission, but the number of cases of person-to-person transmission is continuing to increase, Colorado health officials confirmed. 3 new cases Fourteen new cases of bird flu were confirmed on Thursday, bringing the total in the country to 14.

All of the human cases have been farm workers who became infected after coming into contact with sick animals, and all but one was diagnosed within the past four months. Little is still known about the three most recent infections, but the remaining infections are mild.

New research shows that the virus is spreading from one mammal species to another, a relatively new phenomenon that could make it difficult for authorities to control.

The longer the virus goes uncontrolled, the greater the chance it will evolve and adapt, becoming a greater threat to people, say the authors of the new study. The study was published Tuesday in the journal Nature.

“This virus is not very effective at infecting or transmitting to humans, but that could change if the virus continues to circulate among dairy cows and then from there to other mammals,” said study author Diego Diehl, head of the virology lab at Cornell University's Animal Health Diagnostic Center. “This is cause for concern.”

The new study assessed the first outbreaks of avian influenza among cows on nine farms in Texas, New Mexico, Kansas and Ohio, taking animal samples and comparing the genetic similarities of the viruses between them.

Researchers found that the virus, a specific strain of avian flu known as H5N1, spread rapidly between farms. When infected cows were moved from Texas to another farm in Ohio, the virus was soon found in the Ohio cows. Genetic sequences also suggest that cats and raccoons have died from the virus, possibly after drinking raw milk.

Studies have shown that sick cows eat less feed, chew their cud less, produce less milk, and the color of their milk changes. On some infected farms, cows died at twice the normal rate.

Andrew Bowman, a professor of veterinary preventive medicine at Ohio State University who was not involved in the study, said the research reflects the experience of veterinarians on many U.S. farms.

“It's consistent with the clinical picture,” Bowman said. “This is exactly what we've been seeing on dairy farms. This is really just the first documentation that's been published.”

The study adds to the growing evidence that handling and drinking unpasteurized milk is dangerous.

Diehl said infected cows shed incredible amounts of the virus from their mammary glands, at concentrations higher than could easily be cultured in a lab.

“Drinking raw milk is extremely dangerous,” Diehl said.

Various studies have detected live H5N1 virus in raw milk. Pasteurization has been shown to inactivate virusesCommercially produced milk will be safe to drink.

Farmers must not send potentially contaminated milk to production.

“Milk from sick cows should not be used in the milk supply,” Bowman said.

As scientists continue to study the avian flu outbreak, Bowman said he's interested to know whether cows can shed the virus before they start showing signs of illness and whether the virus can continue to spread from mammals infected by cows.

This is only the third time a virus has spread fairly quickly between groups of mammals, said Anise Loewen, a virologist and professor at the Emory University School of Medicine, who was also not involved in the new study. Spreading on mink farms and Between sea lions and fur seals last summer.

Some mammals have severe cases, such as sea lions. sealamong other species.

Lowen said health officials should also consider the risk that people could be infected with H5N1 and seasonal influenza at the same time this winter: When people are infected with more than one influenza virus, the viruses can exchange genetic information, a process called “reassortment.”

This process could give the virus an evolutionary shortcut and change how the virus spreads.

“The viruses currently infecting cattle pose a relatively low risk to humans — that's what the CDC says, and they're right,” Loewen said. “But the risk I see is viral evolution. Viruses change as they adapt to new hosts. They change as they reassort and swap genes.”

The Centers for Disease Control and Prevention declined to comment on the new study.