Cattle testing for H5N1 bird flu will be more limited than USDA initially announced

New federal rules aimed at limiting the spread of the H5N1 bird flu virus among dairy cattle go into effect Monday, but detailed guidance documents released Friday by the U.S. Department of Agriculture reveal its mandatory testing order is less stringent than initially described.

While that is easing concerns from farmers and veterinarians about the economic and logistical burden of testing, it leaves questions about how effective the testing program will be at containing additional outbreaks.

“More testing is better,” said Jennifer Nuzzo, an epidemiologist and director of Brown University’s Pandemic Center. “But in many ways this policy is very leaky in terms of how much virus it will allow to move. And because we still don’t know what’s driving transmission between cows, we should not pin our hopes on this policy making a major dent in the infections we’re seeing.”

On Wednesday, the USDA issued a federal order requiring farms to ensure lactating dairy cows test negative before being moved across state lines. Laboratories and state veterinarians also must report to the USDA any animals that have tested positive for H5N1 or any other influenza A virus. The guidance issued Friday narrowed the scope of that order.

It says farmers only have to test up to 30 animals in a given group. The guidance does not say how farmers should determine which 30 animals to test in larger groups that are being readied to be moved. The USDA did not respond to STAT’s questions about the rationale for the 30-animal cap.

Jamie Jonker, chief scientific officer for the National Milk Producers Federation, said the group supports the testing program as an important step in response to the outbreak, one that dairy farmers are ready to take “as part of their responsibility to ensure the safety of their animals and the milk supply.”

While pragmatic, researchers who spoke to STAT were split on whether the policy will be effective. Anice Lowen, an influenza researcher at Emory University School of Medicine, told STAT via email that the approach is likely sufficient to detect an H5N1-positive herd. “I think this approach is reasonable,” she said.

Nuzzo had concerns, however, that in very large herds, like those around 500 or more, infected animals could be missed. In herds where outbreaks have occurred, only somewhere between 5% to 15% of cows have presented with clinical symptoms, Terry Lehenbauer, a bovine disease epidemiologist and director of the Veterinary Medicine Teaching and Research Center at UC Davis, told STAT. “My general experience would suggest that there are not a lot of lactating cattle that get shipped interstate on a regular basis, so we’re probably looking at fairly small numbers of animals that will be required,” he said.

The federal order is in recognition of epidemiological evidence that the virus is spreading between cows in affected herds and between herds as cattle are moved. As of April 26, H5N1 outbreaks have been confirmed in 34 dairy herds in nine states, with the first outbreak in Colorado reported Friday.

But analysis of viral genomes from cows infected with H5N1, combined with evidence that genetic traces of the virus have been found broadly in milk in grocery stores, indicate that the outbreak is much more widespread.

The risk of infection from ingesting milk is believed to be very low because pasteurization should kill the virus. Academic researchers did not find any live virus in a small study of commercial milk products. The U.S. Food and Drug Administration is conducting its own, much larger study on the viability of virus in milk, results from which are expected in the coming days. On Friday, the FDA issued an update saying that tests of several samples of retail powdered infant and toddler formula were negative, indicating no presence of H5N1 viral fragments or whole virus. It provided no details on the quantity tested.

Because farmers are required to divert milk from sick animals out of the national food supply, the traces of H5N1 in grocery store products indicates that asymptomatic animals may also be shedding the virus. In an FAQ posted online Thursday, the USDA confirmed that cows without signs of illness can still test positive for virus, acknowledging it had found H5N1 in the lungs of an asymptomatic cow in an affected herd.

Under the new rules, cows that are to be moved between states must have samples collected and tested no more than a week prior to transport. A licensed or accredited veterinarian has to collect the samples — between 3 and 10 milliliters of milk per animal taken from each of the four teats. That’s very important, the USDA noted, because there have been reports of infected animals having virus in only one teat.

A strange feature of H5N1’s jump from birds into cows is that the virus seems to have developed an affinity for mammary tissue. Samples from sick cows show the highest levels of virus not in their noses but in their milk, suggesting that udders seem to be where H5N1 migrates to or infects.

The USDA order does not apply to beef cattle or non-lactating dairy cattle, including calves, due to their lower risk profile, according to the guidance. But influenza researchers told STAT that not enough yet is known about the risks to non-lactating animals to leave them out. “Testing such cattle destined to move between states would not only guard against interstate spread of the virus, it would give important insight into the susceptibility of non-lactating animals,” Lowen said.

Thijs Kuiken, a professor of comparative pathology in the Department of Viroscience at Erasmus Medical Center in Rotterdam, is particularly concerned about the potential for milk from infected cows to harm calves. The U.S. Food and Drug Administration has encouraged farmers to discard milk from H5N1-positive cows, but if that’s not possible and farmers intend to feed calves with that milk, they should first heat it to kill any viruses and bacteria.

Newborn calves need to consume colostrum, the antibody-rich milk cows produce in the first few days after birth, in order to start building their immune systems to ward off all the microbial threats that exist on a farm. Without it, calves often quickly succumb to infection.

If a farmer doesn’t know that a cow has H5N1 because it’s not showing symptoms, calves could inadvertently be consuming the virus. The reason that has Kuiken worried is because of a cluster of fatal H5N1 cases in baby goats, reported in Minnesota in March. Genomic analyses showed they likely contracted the virus from a backyard poultry flock that had been depopulated due to H5N1 days before the goats were born. The animals had shared the same enclosure, including a water source that was likely contaminated. Ten of the kids died after showing neurological symptoms; necropsies on five of them showed virus in the brains and other organs.

“Because we don’t know the extent of this virus in dairy herds in North America,” Kuiken said, “I would expect there will be neurologically affected calves turning up sometime. My prediction is that if it has not already happened, that young dairy calves on affected farms will be found with severe highly pathogenic avian influenza H5N1 infection.”

At this time, there have been no reported cases of H5N1-positive dairy cattle exhibiting any signs of neurological disease in the U.S.

Correction: An earlier version of this article misspelled Thijs Kuiken’s surname.

Helen Branswell contributed reporting.