Since scientists discovered this year that the flu was infecting American cows, they have wondered how it spreads from one animal to another. An experiment carried out in Kansas and Germany resolved the mystery.
Scientists have not found 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 findings suggest the virus is primarily infectious 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 humans.
“I think it’s good news because we can probably control this virus more easily than people thought,” Dr. Richt said. “Hopefully now we can kick him in the behind and knock 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 studies the virus on dairy farms and was not involved in the new study, warned that breaking the chain of transmission would require serious changes in how farmers milk their cows.
“It’s really great that these results are being published,” she said. “But it poses a real logistical challenge.”
In January, veterinarians began noticing that some cows were suffering from a mysterious drop in milk production. They sent samples to the Ministry of Agriculture for analysis. In March, the department announced that milk from cows in Kansas, New Mexico and Texas contained a deadly strain of flu, which is widespread among birds. They also found the virus in swabs taken from the mouth of a Texas cow.
Since then, 132 herds across 12 states have tested positive for the virus. Cows experience a drop in milk production and then usually recover, although some cows have died or been put down because they were not recovering.
Researchers have long known that some strains of influenza viruses can infect mammary cells in udders and be excreted in milk. But they have never seen an outbreak of bird flu spread among cows like this year.
So far, federal and state authorities have reported only three people in the United States have been infected by cows. Two of the infected farmworkers suffered from conjunctivitis, also known as pink eye. The third victim also had a cough and other respiratory symptoms.
The rapid spread of the virus among cows has intrigued scientists. One possible explanation for the transmission of the virus is that it took advantage of the way cows are milked on large farms. Workers clean a cow’s teats, squeeze them by hand to produce a few streams, then attach four tubes, called claws. When the claw has finished drawing milk from 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 his colleagues found that the flu virus could remain viable on a claw for several hours.
Scientists also fear that cows could spread the virus as a respiratory disease. A cow with the virus in its respiratory tract would expel droplets when it breathed or coughed. Other cows may inhale the droplets or pick them up through physical contact.
If this were the case, the virus could potentially 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 teamed up with German researchers to conduct experiments in which they deliberately infected cows. Both teams operate high-level 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 udders of three lactating cows. Within two days, the animals developed clinical signs of infection similar to those observed on farms: fever, loss of appetite and significantly lower milk production.
The milk they produced was thick. “It’s like yogurt coming out of the udder,” Dr. Beer said.
To see if the strain of flu 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 avian flu virus. The cows showed the same clinical signs of infection.
“So this can happen anywhere this virus is found in the environment,” Dr. Richt said.
Dr. Richt also injected bovine flu virus into three female cows that were not lactating, 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-grade infections and shed the virus through their noses and mouths for eight days.
Two days after infection, three healthy cows, not infected with the virus, were placed in the same room as the sick cows. Over 19 days, the scientists checked whether the uninfected animals had also developed the flu, either by coming into contact with the sick cows or by breathing in the droplets they exhaled.
None of the healthy cows got sick. “We didn’t see any transmission,” Dr. Richt said. “The virus doesn’t behave like a typical influenza respiratory virus.”
He stressed 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 has been transmitted from animal to animal, and researchers cannot say whether a newer 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, made the need to stop the spread of the virus in dairy cows even more urgent.
But that may be easier said than done. Disinfecting milking claws between cows would slow 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.
In addition to stopping the spread of the virus from cow to cow, it is also essential to protect people from the virus, she added. “We don’t want these dairy farm 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 onto the platform, it can turn into droplets that can reach workers’ eyes or be inhaled. Personal protective equipment such as goggles and face shields could help block this route of infection.
Stopping the spread of the virus to dairy workers will not only protect their health. It could also prevent the virus from having another opportunity to evolve within a human host and better adapt to our species.
“You never know what will happen to this virus in the future,” Dr. Richt said.