Since scientists discovered the flu infecting American cows earlier this year, they have wondered how it spreads from one animal to another. An experiment carried out in Kansas and Germany has shed some light on the mystery.
Scientists have been unable to find evidence that the virus can spread as a respiratory infection. Juergen Richt, a virologist at the University of Kansas who helped with the research, said the results suggest the virus is primarily contagious through contaminated milking machines.
In an interview, Dr. Richt said the results gave hope that the outbreak could be stopped before the virus evolved into a form that could spread easily between people.
“I think that’s good news that we can probably control it more easily than people thought,” said Dr. Richt. “Hopefully we can now kick this thing in the rear and hit it.”
The findings have not yet been published online or published in a peer-reviewed scientific journal.
Seema Lakdawala, a virologist at Emory University who researches the virus on dairy farms and was not involved in the new study, warned that breaking the chain of transmission would require major changes in the way farmers milk their cows.
“It’s really great that these results are coming out,” he said. “But that’s 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 flu that is widespread in birds. They also found the virus in swabs taken from the mouth of a Texas cow.
Since then, 132 herds in 12 states have tested positive for the virus. Cows suffer a drop in milk production and then typically recover, although some cows have died or been culled because they did not recover.
Researchers have long known that some strains of influenza viruses can infect mammary cells in the breasts and can be shed in milk. But they had never seen an epidemic of bird flu circulating among cows like they did 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 farm workers suffered from conjunctivitis, also known as pink eye. The third victim also developed a cough and other respiratory symptoms.
The rapid spread of the virus among cows puzzled scientists. One possible explanation for the transmission of the virus was that it took advantage of the way cows were milked on large farms. Workers clean a cow’s teats, squeeze them by hand to produce a few squirts, then attach four tubes, known as a claw. When the claw finishes milking the cow, the worker removes it and places it on the next cow. A claw will typically be used on hundreds of cows before being cleaned.
In another study published Wednesday, Dr. Lakdawala and her colleagues found that the flu virus could remain viable in a fingernail for several hours.
Scientists have also worried that cows may be able to transmit the virus as a respiratory disease. A cow with the virus in its respiratory tract would expel droplets as it breathed or coughed. Other cows may inhale the droplets or pick them up by physical contact.
If this were the case, the virus might have the potential to attack cows raised for meat rather than milk. It may also allow the virus to spread more easily between people.
In May, Dr. Richt and his colleagues in Kansas joined forces with German researchers to run experiments in which they deliberately infected cows. The two groups operate high-level 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 those seen on farms: They felt feverish, lost their appetite and produced much less milk.
The milk they produced was thick. “It’s like yogurt coming out of the breast,” Dr. Burr said.
To see if the flu strain in the cows was significantly different from other strains that infect birds, Dr. Burr and his colleagues also injected the cows with a different strain of the H5N1 bird flu virus. The cows suffered the same clinical signs of infection.
“So this can happen wherever this virus is in the environment,” said Dr. Richt.
Dr. Richt also injected three non-lactating female cows with bovine influenza, as well as three males. Instead of injecting the virus into the udders, his team injected the viruses into the animals’ mouths and noses.
The cows developed low-level infections and expelled the virus from their noses and mouths for eight days.
Two days after infection, three healthy cows that had not been infected with the virus were placed in the same room as the sick ones. Over the course of 19 days, the scientists checked whether the uninfected animals also developed the flu, either by coming into contact with the sick cows or breathing in droplets they exhaled.
None of the healthy cows became ill. “We didn’t see transmission,” said Dr. Richt. “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 mutates as it moves from animal to animal, and 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 virus from spreading to dairy cows.
But that may be easier said than done. Disinfecting milking hooves between each cow would slow down milk production on farms. Chemicals used to clean nails could also end up in your milk supply. “We don’t want bleach in the milk,” said Dr. Lakdawala.
In addition to stopping cow-to-cow spread, he also said it was vital to protect people from the virus. “We don’t want these dairy workers to get infected,” he said.
In a typical milking parlor, cows stand on a platform so that their udders are at eye level with the workers. When milk splashes on the platform, it can turn into droplets that can get into workers’ eyes or be inhaled. Personal protective equipment such as goggles and face shields could help prevent this route of contamination.
Stopping the spread in dairy farmers won’t just protect their health. It may also prevent the virus from getting a new chance to evolve inside a human host and better adapt to our species.
“You never know what happens with this virus in the future,” Dr. Richt said.
