Introduction to HPAI A(H5N1) in Cattle

The emergence of highly pathogenic avian influenza in dairy cows represents a significant shift in the epidemiology of clade 2.3.4.4b. While initial reports primarily focused on reduced milk production and mammary gland involvement, recent evidence highlights severe respiratory and systemic manifestations in specific bovine populations. This study details the identification of the H5N1 virus in cull dairy cows exhibiting underlying disease, emphasizing the complex interplay between viral infection and host health.

Pathological and Histologic Findings

Researchers identified the HPAI A(H5N1) virus in four cull dairy cows that the U.S. Department of Agriculture had condemned. These animals presented with pneumonia and accompanying systemic changes. Histologic examination confirmed bronchopneumonia in three of the cows. Notably, the fourth cow exhibited embolic pneumonia and nephritis. Consequently, this study broadens our understanding of the virus's tissue tropism beyond the udder.

Furthermore, diagnostic testing using reverse-transcription real-time PCR (RT-qPCR) confirmed the presence of the virus in various formalin-fixed, paraffin-embedded tissues. Researchers also utilized immunohistochemistry and in situ hybridization to localize the virus. They detected influenza A virus in the pulmonary respiratory epithelium and the renal medullary tubules. Therefore, these findings suggest that the virus can achieve systemic distribution in compromised animals.

The Impact of Respiratory Coinfections

An essential aspect of managing

avian influenza in dairy cows

involves understanding the role of concurrent pathogens. A screening panel for common bovine respiratory pathogens revealed variable coinfections in the cows with bronchopneumonia. These included Histophilus somni, Pasteurella multocida, Mannheimia haemolytica, Mycoplasmopsis bovis, and bovine coronavirus.

Notably, the presence of these coinfections may influence viral replication and disease severity. The study highlights a critical need for further research into how the immune response and existing respiratory stressors facilitate H5N1 infection in cattle. In the Indian context, where dairy farming is often integrated with poultry and diverse livestock, these findings underscore the necessity for robust One Health surveillance.

Clinical Implications and One Health

The detection of H5N1 in non-mammary tissues like the lungs and kidneys indicates a risk for broader viral shedding. Although the risk to the general public remains low, the potential for zoonotic transmission to farmworkers is a priority for global health authorities. Consequently, clinicians and veterinarians should maintain a high index of suspicion when encountering unusual clusters of respiratory or systemic illness in dairy herds.

Frequently Asked Questions

What are the primary symptoms of H5N1 in dairy cattle?

While often associated with a drop in milk production and thickened milk, H5N1 can also manifest as respiratory distress, fever, lethargy, and systemic changes, particularly in cull cows or those with comorbidities.

How is avian influenza in dairy cows diagnosed?

Diagnosis typically involves RT-PCR testing of milk, nasal swabs, or tissue samples. In research settings, immunohistochemistry is used to identify the viral antigen within specific tissues like the lungs and kidneys.

Is the milk from infected cows safe for consumption?

Yes, pasteurization is effective at inactivating the H5N1 virus. However, health authorities strongly advise against the consumption of raw, unpasteurized milk from any herd suspected of infection.

Disclaimer: This content is for informational and educational purposes only. It does not constitute medical or veterinary advice. Practitioners should refer to the latest local and national guidelines for clinical practice and disease reporting.

References

1. Righter DJ et al. Detection of highly pathogenic avian influenza A(H5N1) clade 2.3.4.4b virus in cull dairy cows with underlying respiratory and systemic disease. J Vet Diagn Invest. 2026 Feb 13. doi: 10.1177/10406387261417354. PMID: 41685536.

2. Centers for Disease Control and Prevention. H5N1 Bird Flu: Current Situation Summary. 2024. Available from: https://www.cdc.gov/flu/avianflu/hpai/hpai-interim-recommendations.html

3. USDA APHIS. Detection of Highly Pathogenic Avian Influenza in Dairy Herds. 2024. Available from: https://www.aphis.usda.gov/animal-health/hpai-dairy-cattle

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