Cattle have so far not been associated with being infected in large-scale, and as a result, cattle have not been well studied as domestic hosts for influenza A virus species. In contrast to the notion that cattle are considered to be almost resistant to infection with influenza A virus, H5N1 virus, which was first detected in dairy cattle in Texas in late March, has rapidly spread to 37 herds in nine States in the U.S. as of May 7.
On April 24, the U.S. FDA said that in a nationally representative commercial milk sampling study of pasteurised milk, about one in five of the retail samples tested positive for bird flu viral fragments. A greater proportion of positive results were in milk from areas with infected herds. An NIH-funded study had found an absence of infectious virus in milk samples. The April 23 report of the FAO noted that H5N1 virus was detected in “high concentrations in milk from infected dairy cattle and at levels greater than that seen in respiratory samples”. That the concentration was less in the respiratory samples of the infected cows compared with the milk samples strongly suggest that the pathogenesis of H5N1 virus in cattle differs from other mammals, says a study posted as a preprint; preprints are yet to be peer-reviewed.
One reason for dairy cattle milk containing high concentrations of H5N1 virus fragments could be the propensity of the virus to infect the mammary glands of cows as a previous study had found. On evaluating the expression of H5N1 receptors in the mammary gland, respiratory tract and cerebrum of cattle, the authors found both the human and the duck receptors to be highly expressed in the mammary glands. In the mammary gland, the human receptors and the duck receptors were found to be widely distributed in the alveoli but not in the ducts. Chicken-type influenza receptors were common in the cow respiratory tract. The high concentration of H5N1 virus fragments in milk from H5N1-infected cows could be due to local viral replication in the mammary glands of cows as H5N1 has high affinity for the receptor, the authors say.
