Ever since scientists discovered that influenza was infecting American cows earlier this year, they have been wondering how it is transmitted from one animal to another. An experiment in Kansas and Germany has shed some light on the mystery.
Scientists have failed to find evidence that the virus can spread as a respiratory infection. Juergen Richt, a virologist at Kansas State University who helped lead the research, said the results suggested the virus is infectious primarily through contaminated milking machines.
In an interview, Dr. Richt said the results offered hope that the outbreak could be stopped before the virus evolved into a form that could easily spread between humans.
“I think it’s good news that we can control it more easily than people thought,” Dr. Richt said. “Hopefully now we can kick him in the butt and take him out.”
The results have not yet been published online or in a peer-reviewed scientific journal.
Seema Lakdawala, a virologist at Emory University who is researching the virus on dairy farms and was not involved in the new study, cautioned that breaking the chain of transmission would require serious changes in the way farmers milk their cows.
“It’s really fantastic that these results are coming to light,” he said. “But this is a real logistical problem.”
In January, veterinarians began noticing that some cows were suffering from a mysterious drop 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 samples taken from the mouth of a cow in Texas.
Since then, 132 herds in 12 states have tested positive for the virus. Cows suffer a drop in milk production and then usually recover, although some cows have died or been slaughtered because they were not recovering.
Researchers have long known that some strains of influenza viruses can infect mammary cells in the udder and can be shed in milk. But they have never seen a bird flu epidemic circulating among cows like this year.
So far, only three people in the United States have been reported by state or federal officials to have been infected by cows. Two of the infected farmworkers suffered conjunctivitis, also known as conjunctivitis. The third victim also experienced cough and other respiratory symptoms.
The rapid spread of the virus among cows puzzled scientists. One possible explanation for the virus’s transmission was that it took advantage of how cows are 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 claws. When the claw is finished extracting the cow’s milk, the worker removes it and places it on the next cow. Typically, one claw is used on hundreds of cows before it is cleaned.
In another study published Wednesday, Dr. Lakdawala and her colleagues found that the influenza virus could remain viable in a claw for several hours.
Scientists are also concerned that cows could transmit the virus as a respiratory disease. A cow with the virus in her respiratory tract would expel droplets when breathing or coughing. Other cows could inhale the droplets or collect them through physical contact.
If that were the case, the virus could have the potential to attack cows raised for meat rather than milk. It could also allow the virus to spread more easily between humans.
In May, Dr. Richt and his colleagues in Kansas joined forces with German researchers to conduct experiments in which they deliberately infected cows. The two teams have high-security biosecurity facilities that can house animals as large as cows.
Martin Beer and his colleagues at the Friedrich-Loeffler Institute in Greifswald, Germany, injected the virus into the teats of three lactating cows. Within two days, the animals developed clinical signs of infection very similar to those seen on farms: they had a 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 test whether the flu strain present in the cows was significantly different from other strains infecting birds, Dr. Beer and his colleagues also injected the cows with a different strain of the H5N1 bird flu virus. The cows suffered the same clinical symptoms of infection.
“This can happen anywhere the virus is present in the environment,” Dr. Richt said.
Dr. Richt also injected the bovine flu virus into three non-lactating female cows and also into 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 shed the virus through 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. For 19 days, the scientists checked whether uninfected animals also contracted the flu, either by contact with the sick cows or by inhaling the droplets they exhaled.
None of the healthy cows became sick. “We didn’t see transmission,” Dr. Richt said. “The virus doesn’t behave like a typical respiratory flu virus.”
He cautioned that the results of the two experiments involved a small number of cows. Scientists also studied an early strain of the virus. The virus has mutated as it passed from animal to animal, and researchers can’t say whether a newer strain would behave more like a respiratory illness.
Dr. Lakdawala said the new findings from researchers in Kansas and Germany, which were consistent with epidemiological studies, added more urgency to the task of stopping the spread of the virus in dairy cows.
But that may be easier said than done. Disinfecting milking claws between each cow would reduce 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 the milk,” Dr. Lakdawala said.
As well as stopping the spread from cow to cow, he also said it was vital to protect people from the virus. “We don’t want these dairy workers to get infected,” she said.
In a typical milking parlor, cows are placed standing on a platform so that their udders are at eye level for workers. When milk splashes onto the platform, it can turn into droplets that can fly into workers’ eyes or be inhaled. Personal protective equipment, such as goggles and face shields, could help block that route of infection.
Stopping the spread among dairy workers will not only protect their health. It may also prevent the virus from having a new opportunity to evolve within a human host and better adapt to our species.
“You never know what will happen with this virus in the future,” Dr. Richt said.
