A new study of ongoing bird flu outbreaks on dairy farms describes in unprecedented detail how effectively the virus spreads between cows and from cows to other mammals, including cats and raccoons.
This is an indication that the virus has developed new abilities that worry bird flu experts.
Bird flu has infected more than 100 million wild waterfowl, commercial flocks and backyard poultry in the U.S. over the past few years, and its spread has scientists on alert for the possibility that the virus could one day spread from person to person, sparking the next pandemic.
As far as researchers know, there has not yet been human-to-human transmission, but the total number of cases among humans continues to rise: Colorado state health officials confirmed three new human cases of bird flu on Thursday, bringing the national total to 14.
All of the human cases were farm workers who became infected after exposure to sick animals, and all but one were diagnosed in the last four months. The rest were mild, although little is yet known about the three most recent infections.
The new study suggests the virus is spreading from one mammal species to another, a relatively new phenomenon that could make it harder for authorities to contain it.
The longer the virus remains uncontrolled, the greater its potential to evolve and pose a greater threat to humans, according to the authors of a new study published Tuesday in the journal Nature.
“The virus is not very effective at infecting humans and being transmitted between humans, but if the virus continues to spread in dairy cows and from dairy cows to other mammalian species, that could change,” said Diego Diel, one of the study’s authors and director of the Virology Laboratory at Cornell University’s Animal Health Diagnostic Center. “It’s concerning.”
The new research evaluated the first outbreaks of bird flu in cows on nine farms in Texas, New Mexico, Kansas and Ohio, taking samples from the animals and then comparing the genetic similarity of the virus to the cows.
Researchers found that a particular strain of the bird flu virus, known as H5N1, was spreading rapidly between farms. When infected cows were moved from Texas to a different farm in Ohio, the virus was soon discovered in Ohio cows. And genetic sequencing suggests that cats and a raccoon contracted the virus and likely died after drinking raw milk.
According to the study, sick cows ate less feed, chewed less cud, had reduced milk production and their milk became discoloured. On some affected 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 new research, said the research reflects the experience of veterinarians on many farms in the United States.
“It matches the clinical picture,” Bowman said. “This is 100% of what we see on dairy farms. This is just the first real published documentation.”
The study supports evidence that handling or drinking unpasteurized milk is dangerous.
Infected cows shed incredible amounts of virus through their mammary glands when they are infected, at concentrations higher than can be easily cultured in a laboratory setting, Diel said.
“Drinking raw milk is extremely risky,” Diel said.
Several studies have detected live H5N1 virus in raw milk, while others have shown that pasteurization inactivates the virus, making commercially produced milk safe for consumption.
Farmers should not send any milk into production that could be contaminated.
“Milk from sick cows should not be added to the milk supply,” Bowman said.
As scientists continue to study the bird flu outbreak, Bowman said he will be interested in learning whether cows can shed the virus before they start looking sick and whether the virus continues to spread from mammals that get infected from cows.
This is the third episode where the virus has spread so rapidly among a group of mammals, said Anice Lowen, a virologist and professor at Emory University School of Medicine who was not involved in the new study. The virus previously spread last summer on mink farms and among sea lions and fur seals.
While cases were severe in some mammals, the virus caused mass deaths in many species, especially sea lions and seals.
Lowen said health officials also need to consider the risk of people catching H5N1 and seasonal flu at the same time this winter. When someone is infected with more than one influenza virus, those viruses can undergo a process called reassortment, in which they exchange genetic information.
This process can provide viruses with a shortcut to evolve and change how they spread.
“The virus that’s in cows right now poses a relatively low risk to humans. The CDC has said that, and they’re right,” Lowen said. “But where the risk lies in my mind is viral evolution. They change as they adapt to new hosts. They change as they go through a process of reassortment and gene exchange.”
The Centers for Disease Control and Prevention declined to comment on the new research.
This article was originally published on NBCNews.com