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A new paper challenged two ideas about how the H5N1 bird flu behaves in cattle: that it always produces mild illness, and that asymptomatic animals don't spread disease.

Instead, the paper showed that cattle mortality was twice as high during outbreaks on two of the nine farms assessed, and almost a third of nasal swabs in asymptomatic animals were positive for the virus, as were half of urine samples.

The findings underscore "the need for robust measures to prevent and control the infection and further spread of HPAI [highly pathogenic avian influenza] H5N1 in dairy cattle," Kiril Dimitrov, DVM, PhD, of Texas A&M Veterinary Medical Diagnostic Laboratory in College Station, and co-authors wrote in . "This would reduce the risk of the virus adapting in this new mammalian host species, thereby decreasing the pandemic risk to humans."

Dimitrov and colleagues studied nine farms -- five in Texas, two in New Mexico, one in Kansas, and one in Ohio -- that had outbreaks from February 11 to March 19.

The New York Times that 99 cows died during a 3-week outbreak on the Ohio farm, which is twice the normal mortality rate. The other increased mortality event occurred on a Texas farm, according to the paper.

The researchers found evidence of subclinical infection in some cows, with viral RNA detected in six of 19 nasal swabs and four of eight urine samples. Interestingly, animals with clinical illness shed virus at a lower frequency in nasal swabs and urine, they noted.

That has implications for the risk of spreading the virus between farms, as the researchers said epidemiologic and genomic data showed cow-to-cow transmission after apparently healthy cows from a Texas farm were transported to a farm in Ohio.

It's "possible that the virus infects through respiratory and/or oral routes replicating at low levels in the upper respiratory tract, from where it could disseminate to other organs via a short and low-level viremia," Dimitrov and colleagues wrote, notably to the mammary glands.

Indeed, the paper confirmed that H5N1 has high tropism for mammary gland tissue in cows, which is "consistent with high expression of sialic acid receptors with an alpha 2,3 (avian-like receptor) and alpha 2,6 (human-like receptor) galactose linkage in these cells."

It also confirmed prior reports about key symptoms of the disease in cattle, including "decreased feed intake, decreased rumination time, mild respiratory signs (clear nasal discharge, increased respiratory rate, and labored breathing), lethargy, dehydration, dry/tacky feces or diarrhea, and milk with abnormal yellowish colostrum-like color, thick and sometimes curdled consistency."

Cattle often had an abrupt drop in milk production that could last for about a month, the researchers pointed out, but they generally recovered from their illness in 5 to 14 days, returning to their pre-outbreak health status.

The virus spread widely among other animals on or near the farm, including to cats, raccoons, and wild birds, which was confirmed by genomic analysis. "These observations highlight complex pathways underlying the introduction and spread of HPAI H5N1 in dairy farms, underscoring the need for efficient biosecurity practices and enhanced surveillance efforts in affected and non-affected farms," Dimitrov and co-authors concluded.

"The spillover of HPAI H5N1 into dairy cattle and evidence for efficient and sustained mammal-to-mammal transmission are unprecedented," they wrote. "This efficient transmission is concerning as it can lead to the adaptation of the virus, potentially enhancing its infectivity and transmissibility in other species, including humans."

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