Report on H5N1 in cattle and cats increases concern about repercussion event

Cows infected with H5N1 bird flu had a mild illness characterized by a drop in milk production, but cats fed raw milk suffered a severe systemic illness with high mortality, researchers found.

The findings, published in the CDC’s Emerging Infectious Diseases, come from an analysis of samples from farms in Kansas and Texas collected in mid-March. They suggest cow-to-cow transmission of H5N1 and raise concerns about raw milk consumption as a potential transmission mechanism, reported Eric Burrough, DVM, PhD, of the Iowa State University College of Veterinary Medicine in Ames, Iowa, and colleagues.

“The recurrent nature of global outbreaks of the H5N1 virus and the detection of rebound events across a wide range of hosts are concerning and suggest increasing adaptation of the virus in mammals,” the researchers wrote. “Surveillance of [highly pathogenic avian influenza] HPAI viruses in domestic farm animals, including cattle, are needed to elucidate the evolution and ecology of influenza viruses and prevent interspecies transmission.”

Veterinarians in the US first sounded the alarm about a strange disease affecting dairy cattle in the Panhandle region of North Texas in February 2024. These cows experienced a precipitous drop in milk production, and what they produced was thickened. and creamy yellow in appearance, similar to colostrum, the milk produced in the first days after birth. The cows had reduced appetites, but most recovered within 10 to 14 days, the researchers reported.

In early March, similar cases were quickly reported in dairy cattle in southwestern Kansas and northeastern New Mexico, and at least one dairy farm in Texas reported deaths among cats that drank raw milk from sick cows.

In mid-March, the Iowa State University Veterinary Diagnostic Laboratory received samples from dairy cattle in Texas and Kansas, and two dead cats from a dairy farm in Texas. The samples tested positive for H5N1, prompting the US Department of Agriculture to issue a press release shortly afterwards.

Disease in cows

Burrough and colleagues reported that milk and mammary gland samples had very low cycle threshold (Ct) values ​​for H5N1, indicating a high viral load.

Some, but not all, animals had firm mammary glands, typical of mastitis, and the main microscopic lesion observed in the affected cows was acute multifocal neutrophilic mastitis, they reported.

Although the mode of transmission is not known, cow-to-cow transmission was suggested because cattle herds in Michigan, Idaho and Ohio that received infected animals also tested positive for H5N1, according to the study.

Burrough and colleagues wrote that feed contaminated with feces from infected wild birds is the most likely source of infection on dairy farms.

“Although the exact origin of the virus is unknown, migratory birds (Anseriformes and Charadriiformes) are likely sources because the Texas Panhandle region lies on the Central Flyway, and these birds are the main natural reservoir of avian influenza viruses,” wrote.

They noted that influenza A-related declines in milk production have occurred among dairy cattle in the past: in Japan in 1949 and in Europe in 1997-1998 and 2005-2006. “The sporadic occurrence of clinical disease in dairy cattle worldwide may be the result of changes in subclinical infection rates and the presence or absence of sufficient baseline [influenza A virus] IAV antibodies in cattle to prevent infection,” the researchers wrote.

Disease in cats

Among about 24 domestic cats that were fed milk from sick cows on a North Texas dairy farm, more than half fell ill and died by mid-March.

They showed signs of systemic influenza infection, including depressed mental status, rigid body movements, ataxia, blindness, circling movements and copious oculonasal discharge, the researchers reported. Neurological examination found no threat reflexes and pupillary responses to light with weak blink response.

Two animals were available for postmortem analysis. Brain and lung samples showed low Ct values, again indicating a high viral load in these animals, they reported.

They also found mild hemorrhages in the subcutaneous tissues over the dorsal part of the skull, along with multifocal meningeal hemorrhages in the brain. Microscopic lesions were consistent with severe systemic viral infection, including evidence of meningoencephalitis, pneumonia, myocarditis, and chorioretinitis.

Burrough and colleagues called the findings of blindness and chorioretinitis “unique,” ​​suggesting that “further investigation into possible ocular manifestations of HPAI H5N1 virus infection in cats may be warranted.”

Although consumption of dead wild birds cannot be ruled out as a source of transmission to cats, “the known consumption of unpasteurized milk and colostrum from infected cows and the high amount of virus nucleic acid in milk make consumption of milk and colostrum a probable route”. of exposure,” the researchers wrote.

“Therefore, our findings suggest mammal-to-mammalian interspecies transmission of the HPAI H5N1 virus and raise new concerns regarding the potential for spread of the virus in mammalian populations,” they concluded.

Burrough and co-authors added that hemagglutinin and neuraminidase sequences from cows and cats on different Texas farms had “a remarkable degree of similarity,” strongly suggesting a “shared origin.”

“Further research, case series investigations, and surveillance data are needed to better understand and inform measures to reduce the clinical effects, dissemination, and spread of HPAI viruses among mammals,” they wrote.

The H5N1 clade involved in this outbreak is 2.3.4.4b, which was first reported in the US in late 2021, the researchers noted. It has been involved in other recent reports of mammal spread in the US, including seals in New England and a number of land animals, including 50 red foxes, six opossums, four raccoons, two bobcats, two opossums, a coyote, a fisherman, and a gray fox. An outbreak of H5N1 among farmed mink in Spain also raised concerns in 2022.