Calves that drank unpasteurized milk from cows experimentally infected with highly pathogenic H5N1 bird flu contracted the virus and developed mild illness, according to a peer-reviewed study published in Nature Communications on August 21. The findings provide direct evidence that contaminated milk can transmit the virus to young cattle and reinforce warnings against feeding raw waste milk from infected or suspect cows to calves.
Researchers from the US Department of Agriculture's National Animal Disease Center studied nine Holstein calves aged about seven to 11 weeks. One control animal received milk from uninfected cows. Four calves received 0.95 liters twice daily for four days from infected cows early in infection, before those cows had developed neutralizing antibodies. Four others received a mixture that included infected milk containing antibodies from cows later in infection.
All four calves given infected milk without antibodies showed nasal discharge, mild fever, lethargy, loose stool and slightly increased breathing effort for five to six days. Viral RNA appeared consistently in their nasal swabs from two to four days after exposure, and investigators confirmed infection through viral material or antigen in lung lesions, lymph nodes and tonsil tissue, together with the later development of antibodies.
The second exposed group had no clear clinical illness, and researchers detected viral RNA in nasal swabs from only one of four calves. The authors concluded that neutralizing antibodies in the milk offered partial protection, but the result did not make contaminated raw milk safe. The single control calf remained negative, supporting the link between infected milk and transmission under the experimental conditions.
The study used a small, controlled sample and tested the B3.13 genotype, so it does not establish how often calves become infected on working farms or whether they transmit the virus between premises. The researchers nevertheless warned that mild signs could resemble common calf illnesses and escape notice. Because calves are frequently moved, their possible role in farm-to-farm spread now requires evaluation.
USDA guidance says infected cows can shed high concentrations of H5N1 in milk and recommends pasteurizing or heat-treating all milk fed to calves and other mammals. The agency also says commercial pasteurization inactivates the virus. The new results identify a practical route that biosecurity programs can block while scientists continue investigating transmission, viral adaptation in cattle and possible vaccination strategies.
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