Analysis of Cow, Cat H5N1 Avian Flu Samples Raises Concerns About Spread to Other Animals

May 01, 2024

By Lisa Schnirring

Microbiological examination of cow, milk, and cat samples early in the investigation of H5N1 avian flu in some of the first affected states found that the cats died shortly after they were fed raw colostrum from sick cows, highlighting the risk of spread from cows to other animals through contaminated milk.

A research team based at Iowa State University reported its findings on some of the earliest samples from cows and cats today in Emerging Infectious Diseases.

For the study, the scientists examined samples—milk, serum, and tissues—from cows on early affected farms in Texas and Kansas. They also analyzed samples from cats that died on the farms. The initial H5N1 highly pathogenic avian influenza (HPAI) findings prompted the initial announcement from the US Department of Agriculture (USDA) on March 25. 

H5N1 in mammary gland cells

The tissue samples in cows came from a three that were euthanized and three that died naturally. The researchers also performed postmortem exams on two adult cats from one of the farms, which had about 24 cats. The cats started showing symptoms a day after clinical disease was noted in the cows, and about half of the cats died.

Microscopic examination revealed that the H5N1 virus infects epithelial cells of mammary alveoli where milk is produced, prompting acute inflammation in the udder, which could explain the drop in milk production and high virus levels that have been turning up in milk.

The authors said the case series shows that H5N1 infection has more dramatic symptoms in cows than reflected in earlier reports of influenza A viruses in the animals. 

Cats showed symptoms soon after exposure

Meanwhile, they wrote that the clinical disease in cats, especially neurologic symptoms, is consistent with earlier reports of H5N1 in cats and tracks with reports of cats eating infected wild birds and poultry products.

Though wild-bird consumption can't be ruled out in the cats from the dairy farms, known consumption of unpasteurized milk and colostrum from infected cows, a fluid that contained a high viral load, makes it a likely exposure route, the team wrote. 

"Therefore, our findings suggest cross-species mammal-to-mammal transmission of HPAI H5N1 virus and raise new concerns regarding the potential for virus spread within mammal populations," they said.

Ingestion of feed contaminated with feces from wild birds is the most likely initial source of infection on dairy farms, according to the authors, who said more studies are needed to clarify transmission routes and pathogenesis within infected cattle.

FDA, CDC, USDA add updates

Over the past few days, federal agencies involved in the nationwide H5N1 investigation and response updated their findings and provided policy clarifications.

Egg inoculation tests were negative for live infectious virus in retail milk samples that were positive for H5N1 fragments in earlier polymerase chain reaction (PCR) tests, the Food and Drug Administration (FDA) said in an April 26 update. It noted that the test results are preliminary but affirm its assessment that the commercial milk supply is safe.

The FDA also said it tested several samples of retail powered infant formula and powdered milk products marketed as toddler formula, and all were negative for H5N1 remnants on PCR testing.

Work is still under way on 297 retail samples from 38 states. The agency said that samples that are positive on PCR testing will go through egg inoculation testing.

Meanwhile, the Centers for Disease Control and Prevention (CDC) in an update on the same day says its ongoing susceptibility testing on commercially available antiviral drugs has found that the current H5N1 strain is susceptible to baloxavir marboxil (Xofluza).

The CDC said its earlier studies showed that the H5N1 virus from the human case in Texas was susceptible to other antivirals, including oseltamivir. However, it said one of the 200 publicly posted H5N1 samples from dairy cows found a marker with a known link to reduced susceptibility to neuraminidase inhibitors.

"The detection of this marker in one of more than 200 specimens is not surprising or concerning at this time in terms of the clinical usefulness of these drugs, but it does underscore why this kind of constant monitoring is important," the agency wrote.

Finally, the US Department of Agriculture (USDA) on April 27 clarified its recent federal order barring interstate movement of lactating dairy cows without tests to ensure that the animals aren't infected with HPAI H5N1.

It said the clarification notes that the order doesn't apply to the interstate movement of dairy cows going to sale barns, which are sometimes used to consolidate and move cattle to slaughter out of state. "We are announcing this clarification over the weekend to ensure small farms have the guidance necessary to continue to move cull cattle and limit animal welfare issues," the USDA said.

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