How does bird flu spread in cows? Experiment yields ‘good news’.

Ever since scientists discovered that flu was infecting American cows earlier this year, they have wondered how the flu spreads from one animal to another. An experiment conducted in Kansas and Germany has shed some light on the mystery.

Scientists could find no evidence that the virus can spread as a respiratory infection. Juergen Richt, a virologist at Kansas State University who helped lead the study, said the results suggested the virus is primarily contagious through contaminated milking machines.

In an interview, Dr. Target said the results offered hope that the outbreak could be stopped before the virus evolved into a form that could easily spread between people.

“I think this is good news, that we can probably control it more easily than people thought,” said Dr. Dir. “Hopefully now we can kick this thing in the ass and take it out.”

The findings have not yet been published online or in a peer-reviewed scientific journal.

Seema Lakdawala, a virologist at Emory University who studies the virus on dairy farms and was not involved in the new research, warned that breaking the chain of transmission would require serious changes in the way farmers milk their cows.

“It’s really great that these results are coming out,” she said. “But this is a real logistical problem.”

In January, veterinarians began noticing individual cows suffering from mysterious drops in milk production. They sent samples to the Department of Agriculture for testing. In March, the department announced that milk from cows in Kansas, New Mexico and Texas contained a deadly strain of influenza that is widespread among birds. They also found the virus in swabs taken from the mouth of a Texas cow.

Since, 132 herds in 12 states have tested positive for the virus. The cows suffer a drop in milk production and usually recover afterwards, although some cows do have died or were slaughtered because they did not recover.

Researchers have known for a long time that some strains of influenza viruses can infect the mammary cells in the udders and be excreted in the milk. But they had never seen such an epidemic of bird flu among cows as this year.

So far, state or federal officials have reported that only three people in the United States have been infected by the cows. Two of the infected farmworkers suffered from conjunctivitis, also known as pink eye. The third victim also suffered from coughing and other respiratory symptoms.

The rapid spread of the virus among cows confused scientists. One possible explanation for the transmission of the virus was that it took advantage of the way cows are milked on large farms. Workers clean a cow’s nipples, squeeze them by hand to produce a few squirts, and then attach four tubes, known as a claw. When the claw finishes sucking the cow’s milk, the worker removes it and places it on the next cow. A claw is typically used on hundreds of cows before being cleaned.

In another study published on Wednesday, Dr. Lakdawala and her colleagues found that the influenza virus could remain viable on a claw for several hours.

Scientists are also concerned that the cows could potentially spread the virus as a respiratory disease. A cow with the virus in its respiratory tract would emit droplets while breathing or coughing. Other cows can inhale the droplets or pick them up through physical contact.

If that were the case, the virus could potentially attack cows that are raised for meat rather than milk. It could also make it easier for the virus to spread between people.

In May, Dr. Richt and his colleagues in Kansas joined forces with German researchers to conduct experiments in which they deliberately infect cows. The two teams operate state-of-the-art biosecurity facilities that can house animals as large as cows.

Martin Beer and his colleagues at the Friedrich-Loeffler-Institut in Greifswald, Germany, injected the virus into the teats of three lactating cows. Within two days, the animals developed clinical signs of infection similar to what has been observed on farms: They developed fever, lost their appetite and produced much less milk.

The milk they produced was thick. “It’s like yogurt coming out of the udder,” Dr. Beer said.

To see if the flu strain in the cows was significantly different from other strains that infected birds, Dr. Beer and his colleagues also tested the cows with a different strain of the H5N1 bird flu virus. The cows showed the same clinical signs of infection.

“So this can happen anywhere this virus exists in the environment,” said Dr. Dir.

Dr. Richt also injected the cattle flu into three non-lactating female cows, as well as three male cows. Instead of injecting the virus into the udders, his team injected the viruses into the animals’ mouths and noses.

The cows developed mild infections and shed the virus through their nose and mouth for eight days.

Two days after the infection, three healthy cows, which were not infected with the virus, were placed in the same room as the sick one. Over the course of 19 days, the scientists checked whether the uninfected animals also developed the flu, by coming into contact with the sick cows or by inhaling droplets they exhaled.

None of the healthy cows became ill. “We have not seen any transmission,” said Dr. Dir. “The virus does not behave like a typical respiratory flu virus.”

He cautioned that the results from the two experiments involved a small number of cows. The scientists also studied an early strain of the virus. The virus mutated as it moved from animal to animal, and the researchers can’t say whether a more recent strain would behave more like a respiratory disease.

Dr. Lakdawala said the new findings from researchers in Kansas and Germany, which were consistent with epidemiological studies, added more urgency to stopping the spread of the virus among dairy cows.

But that may be easier said than done. Disinfecting the milking claws between each cow would slow down milk production on farms. The chemicals used to clean the claws could also end up in the milk supply. “We don’t want bleach in milk,” said Dr. Lakdawala.

In addition to stopping the spread from cow to cow, she said it is also critical to protect people from the virus. “We don’t want these dairy workers to get infected,” she said.

In a typical milking parlor, cows stand on a platform so that their udders are at eye level with workers. When milk splashes on the platform, it can turn into droplets that can fly into workers’ eyes or be inhaled. Personal protective equipment such as safety glasses and face shields can help block this route of infection.

Stopping the spread to dairy workers not only protects their health, it may also prevent the virus from having another chance to evolve in a human host and better adapt to our species.

“You never know what will happen in the future with this virus,” said Dr. Dir.