Understanding the Implications of H5N1 Bird Flu in Dairy Cattle

A recent paper has challenged some prevailing ideas about how the H5N1 bird flu behaves in cattle. Traditionally, it was believed that this virus produces only mild illness in cattle and that asymptomatic animals do not spread the disease. However, the study showed that the mortality rate among cattle during outbreaks was actually twice as high on some farms, indicating that the virus can have serious consequences. Additionally, a significant number of nasal swabs from asymptomatic animals tested positive for the virus, as did half of urine samples. These findings highlight the urgent need for robust measures to prevent and control the infection and spread of HPAI H5N1 in dairy cattle.

Researchers, led by Kiril Dimitrov, DVM, PhD, from Texas A&M Veterinary Medical Diagnostic Laboratory, conducted a study on nine farms to assess the impact of H5N1 bird flu outbreaks in cattle. The results revealed that not only did cattle mortality increase during outbreaks, but there was also evidence of subclinical infection in some cows. This means that animals were carrying the virus without showing any outward signs of illness. Interestingly, animals with clinical symptoms shed the virus at a lower frequency compared to asymptomatic animals, raising concerns about the risk of virus transmission between farms.

According to the researchers, epidemiologic and genomic data indicated that the virus could spread between cows after apparently healthy cows from one farm were transported to another. The virus likely enters the body through respiratory or oral routes and replicates at low levels in the upper respiratory tract before disseminating to other organs. The high tropism of H5N1 for mammary gland tissue in cows suggests that the virus may infect through the mammary glands, leading to further spread and transmission among animals on the farm.

The paper confirmed that key symptoms of H5N1 bird flu in cattle include decreased feed intake, respiratory signs, lethargy, and abnormal milk production. Cattle experienced an abrupt drop in milk production that lasted for about a month, but they generally recovered within 5 to 14 days. The virus also spread to other animals on or near the farm, such as cats, raccoons, and wild birds, highlighting the complex pathways of virus introduction and spread among different species.

The study concluded that the spillover of HPAI H5N1 into dairy cattle and the evidence of efficient mammal-to-mammal transmission are unprecedented. Therefore, efficient biosecurity practices and enhanced surveillance efforts are crucial in both affected and non-affected farms to prevent further outbreaks. The findings of this study shed light on the importance of understanding the implications of H5N1 bird flu in dairy cattle and the potential risks it poses to both animal and human health.