Using protective gear during milking is important to reduce the chance of transmitting pathogens from cows to humans, maintaining worker safety and the quality of dairy products.

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Recent studies published by the CDC in Emerging Infectious Diseases have suggested a persistence of highly pathogenic avian influenza A (H5N1) virus on milking equipment surfaces, as well as its presence in dairy cattle mammary glands. These findings, in total, underscore a need for enhanced biosecurity measures and surveillance to mitigate the risks posed by avian influenza viruses in dairy farming, particularly concerning the handling of unpasteurized milk and the distribution of virus receptors in the mammary glands of dairy cattle.

Risks in Milk Handling

Studying the survival of highly pathogenic H5N1 from cattle and human influenza A(H1N1)pdm09 pandemic viruses in unpasteurized milk revealed that both can remain infectious on milking equipment surfaces for several hours. These findings underscore potential risks associated with unpasteurized milk handling and the virus’s ability to exploit specific receptors, emphasizing the importance of surveillance and biosecurity in dairy farming practices.

“We observed that the H5N1 cattle virus remained infectious in unpasteurized milk on stainless steel and rubber inflation lining after 1 hour, whereas infectious virus in PBS fell to below the limit of detection after 1 hour. That finding indicates that unpasteurized milk containing H5N1 virus remains infectious on materials within the milking unit,” the researchers wrote.¹

The milking process involves the automated collection of milk using clusters attached to cow teats but starts with manual forestripping to stimulate milk release and check quality. This step can create milk splatter. Workers manually clean teats before attaching clusters, which regulate milk flow with rubber liners. During milking, workers are exposed to potentially infectious milk due to their lower positioning. Research focuses on influenza virus persistence in unpasteurized milk on milking surfaces, highlighting health risks to workers.

HPAI H5N1 Virus in US Dairy Cattle

In March 2024, the US Department of Agriculture’s Animal and Plant Health Inspection Service confirmed the presence of highly pathogenic avian influenza (HPAI) A (H5N1) virus in dairy cattle in the US, marking its first detection in this species. One key determinant of susceptibility to HPAI H5N1 is the presence of specific virus receptors on host cells.

Notably, HPAI H5N1 demonstrated epitheliotropism within the mammary gland, with minimal detection in macrophages as observed through rare immunolabeling. These observations underscore the potential for HPAI H5N1 to exploit sialic acid (SA) receptors in mammary glands, potentially influencing its transmission dynamics and pathogenesis in dairy cattle.

“Sporadic human cases of H5N1 virus infection have occurred when humans are in close and prolonged contact with animals. Humans have some α2,3-linked SA receptors deep within their lungs, and prolonged close contact with infected birds is postulated to cause infection attributable to inhaling large amounts of virus from those birds with introduction into the deeper recesses of the lungs,” according to the investigators led by Eric R. Burrough, DVM, PhD, of the Iowa State University Veterinary Diagnostic Laboratory.²

Researchers compared the distribution of sialic acid (SA) receptors in the respiratory tract and mammary glands of dairy cattle naturally infected with HPAI H5N1. The study found that both respiratory and mammary gland tissues of HPAI H5N1-infected dairy cattle exhibit a high concentration of SA receptors, particularly SA α2,3-gal, which is specific to avian influenza viruses. Immunostaining of mammary gland tissues revealed co-localization of sialic acids and influenza A virus nucleoprotein, indicating that HPAI H5N1 has a strong affinity for SA α2,3-gal receptors within these tissues.

“The presence of HPAI H5N1 virus in dairy cattle, more so in the mammary gland and milk, once again highlights the importance of IAV adaptability to other nontraditional species and cross-species transmission. This finding reiterates the need for active IAV [influenza A virus] surveillance efforts in animal species. Like coronaviruses, IAVs have a broad host range involving avian and mammal species,” according to the second group of investigators, led by Rahul Kumar Nelli, PhD, MVM, also from the Iowa State University Veterinary Diagnostic Laboratory.²

The Department of Agriculture, in collaboration with federal agencies, has taken proactive measures to safeguard the nation’s meat supply. Since late March, outbreaks of the H5N1 virus have been confirmed in across many US states, along with transmission to humans. In response, federal officials have launched a campaign to collect ground beef samples from retail stores in affected states for rigorous testing. While authorities express confidence in the safety of the meat supply, they are committed to ensuring there is no viral contamination present.³

Furthermore, the USDA has mandated testing for lactating dairy cows before interstate movement as part of containment efforts. However, cows destined for immediate slaughter are exempt from this requirement, needing only documentation of veterinary inspection.³

References

1. CDC. Highly Pathogenic Avian Influenza A(H5N1) Clade 2.3.4.4b Virus Infection in Domestic Dairy Cattle and Cats, United States, 2024. Emerging Infectious Diseases. Volume 30, Number 7—July 2024. Accessed July 2, 2024. https://wwwnc.cdc.gov/eid/article/30/7/24-0508_article 

2. CDC. Sialic Acid Receptor Specificity in Mammary Gland of Dairy Cattle Infected with Highly Pathogenic Avian Influenza A(H5N1) Virus. Emerging Infectious Diseases. Volume 30, Number 7—July 2024. Accessed July 2, 2024. https://wwwnc.cdc.gov/eid/article/30/7/24-0689_article

3. Abene, S. US Government’s Efforts to Ensure Meat Supply Amid Bird Flu Concerns. Contagion. Accessed May 6, 2024. Published July 2, 2024. https://www.contagionlive.com/view/us-government-s-efforts-to-ensure-meat-supply-amid-bird-flu-concerns