Soon after the discovery of highly pathogenic avian influenza (HPAI) strain H5N1 in dairy cattle, scientists learned that milk was a primary vector in spreading the disease from cow to cow.
But what happens when waste milk from the sick cows is fed to other animals? Early in the disease outbreak, domestic cats consuming raw milk from infected cows began dying. This prompted researchers at the USDA National Animal Disease Center in Ames, Iowa to conduct a study on the potential of passing H5N1 to preweaned calves via unpasteurized waste milk.
Four Holstein calves ranging from 7-11 weeks of age were fed approximately 1 quart of unpasteurized waste milk from experimentally inoculated cows twice a day for 4 days. One additional calf served as a control, and was fed milk from HPAI-free cows in the same experiment period.
Calves fed infected milk showed signs of illness that included nasal discharge, mild fever, mild lethargy, loose stools, and slightly increased respiratory effort for 5-6 days. The researchers noted that all symptoms were mild and may not be readily recognized in an on-farm setting with other environmental or health stressors.
Transmission of the H5N1 virus to the 4 calves fed infected milk was confirmed via nasal swabs collected 2-4 days after the feedings. Within 2 weeks after the experimental feeding trial, all calves in the study were humanely euthanized. Presence of the virus was further confirmed via tissue samples from lung lesions, lymph nodes, and tonsils from the 4 calves receiving infected milk.
The USDA researchers said the study underscored the importance of pasteurizing waste milk fed to calves, as pasteurization has been proven to kill the virus. This is especially crucial because research on infected herds has shown that cows begin shedding the virus in their milk up to 2 weeks before they start to show clinical signs of illness.
Meanwhile, researchers at the University of California-Davis have investigated a practical alternative to pasteurization of waste milk: acidification. In a study recently published in the Journal of Dairy Science, they noted that fewer than 50% of large dairy farms routinely pasteurize waste milk fed to calves.
They found that in a laboratory setting, acidification of waste milk with citric acid to a pH of 4.1-4.2 effectively deactivated the H5N1 virus. The acidification process takes only 6 hours and does not require refrigeration.
The UC-Davis researchers posed that acidification can be a practical, affordable, accessible method of preventing the spread of H5N1 to calves in operations that do not have pasteurization systems for waste milk.
Calves will be a critical control point in navigating H5N1 in the U.S. dairy industry. The USDA researchers noted that 1 out of every 10 dairy operations in the country raise their heifers off site, and most veal and beef-cross calves are transported to another facility for raising.
Protecting those calves from H5N1 will be a crucial element in preventing spread of the disease.
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