What we know (and don't) about bird flu in cows

How the virus presents in cattle

Generally speaking, H5N1 has been found in poultry and other birds, as well as sea lions, farmed mink, cats, and other mammals. Now that the virus is appearing in cattle, scientists are working to figure out exactly how cows are transmitting H5N1.

USDA said some tests from the respiratory tracts of infected cows have come back positive for H5N1, but found there wasn't much virus present, suggesting that transmission via the respiratory tract is possible but unlikely to be the main source of transmission.

Instead, much of the spread of the virus appears to be happening in milking parlors, where cows are strapped to milking machines. H5N1 appears to primarily infect the mammary glands of dairy cows, and the amount of virus present in the udders of infected cows is extremely high.

"You can imagine that if such a cow is milked in a milking stall, even a few drops of milk remaining on the teat cup which is used to milk will then subsequently contaminate the teat of the next cow," said Thijs Kuiken, a pathologist in the department of viroscience at Erasmus Medical Center in the Netherlands. "So you can imagine very easily how, in the course of a few days, the virus could spread very fast among the cows being milked on that farm."

Not much is known about when infected cows are most contagious with H5N1. Research on outbreaks in Kansas and Texas found that incidence of the virus peaked four to six days after the first animals began displaying symptoms. Spread started to taper off within 10 to 14 days.

Researchers have yet to systematically and repeatedly collect samples from infected cattle to understand how much virus is present in their noses or shedding into their milk during the course of an infection.

Cow-to-human transmission

Jared Taylor, a professor of veterinary pathobiology at Oklahoma State University, expressed concern about how H5N1 might evolve after living in cows. "The concern is if it becomes effective as a respiratory pathogen in cattle, it's more likely to become effective as a respiratory pathogen in humans," he said.

As of now, there has only been one reported infection of H5N1 in a human, which was a Texas dairy farm worker who developed conjunctivitis. However, there have been anecdotal reports of other farm workers experiencing conjunctivitis and other mild respiratory symptoms.

According to Nirav Shah, principal deputy director at CDC, many farmers haven't been willing to test their cows or workers. It's also possible that workers presenting mild signs of illness, like conjunctivitis, could make cases more difficult to spot.

According to Kuiken, milking parlors' setup makes workers very susceptible to coming in contact with viruses that are shed in milk. Handling the milk and milking equipment and cleaning the milking parlor presents "all different ways in which these milk workers can easily become infected," he said.

How regulators are responding to the outbreak

To stem the spread of H5N1 in cattle, USDA last week said it would start requiring that dairy cows moving across state lines be tested for the virus.

USDA said that lactating cows have to test negative for influenza A viruses, which includes bird flu, before they're transported. Owners of herds with positive tests will have to provide data on the movements of their cattle to help investigators trace the disease.

This testing will help protect the livestock industry, limit the spread of H5N1, and "better understand this disease," according to Mike Watson, a senior USDA official.

Research is also still being done to figure out if all pasteurization techniques eliminate H5N1, and results have so far been reassuring.

FDA has begun a national survey of milk samples, analyzing 297 samples of retail dairy products from 38 states. The agency has said that early polymerase chain reaction tests have found that roughly one in five retail milk samples contained genetic fragments of H5N1, however, FDA's egg inoculation tests found no sign of live virus in its first batch of samples.

"The answer at this point seems pretty conclusive that pasteurized milk is safe," said Samuel Scarpino, a professor of the practice in health sciences at Northeastern University. "The fact that it's coming back negative is really strong evidence that, at least in the samples they tested, there's no live virus."

Some health experts have criticized the slow response from federal agencies, citing a lack of transparency from USDA.

Following mounting pressure, USDA earlier this month released 239 genetic sequences of H5N1 from poultry, wild birds, and dairy cows. Researchers hope this will allow them to look for new clues regarding the spread of the virus. However, the data was released without any additional information regarding where and when each animal was infected.

"It creates unnecessary delays and further is likely not a complete set of all the sequence data USDA has collected so far," said Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan.

"It's important to figure out how this got into the cattle and the mechanism by which it's spreading in them, but the incomplete metadata is ultimately an obstacle to that goal," she added.

Michael Osterholm, director of the University of Minnesota's Center for Infectious Disease Research and Policy, said that USDA "is primarily focused on protecting the animal production industry, and I think that's a short-term goal, but a long-term mistake." Osterholm added that it's unclear what investigations USDA has carried out, making it unclear how the cows became infected in the first place.

Meanwhile, Eric Topol, director of the Scripps Research Translation Institute, said in a tweet, "I'm not worried about H5N1 transmission to humans (yet) but the [USDA's] pathetic lack of transparency, how long it took for the genomes to be released, lack of testing asymptomatic cattle … all detracts from 'confidence.'" (Branswell/Molteni, STAT, 4/30; Stone, Forbes, 4/27; Weiland et. al., New York Times, 4/24; Weiland/Mueller, New York Times, 4/26; Cullinan/Newey, The Telegraph, 4/23)

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