USA – The U.S. Department of Agriculture (USDA) has recently completed a pivotal animal challenge study investigating the transmission and progression of the H5N1 virus in dairy cows. 

Conducted by a multi-disciplinary team from the USDA Agricultural Research Service (ARS), the USDA Animal and Plant Health Inspection Service (APHIS), and Iowa State University, the study has shown that laboratory-induced infections in dairy cows can replicate clinical signs observed in naturally infected animals on farms.

Understanding how the virus infects cows and progresses within them is crucial for developing effective therapeutics, such as vaccines. 

Although the study size was limited, successfully validating a reliable challenge model is essential in addressing the critical knowledge gaps necessary to prevent the spread of H5N1. The USDA has plans to extend these studies with more animals and additional endpoints.

The outbreak of H5N1 in dairy cows was first detected in a Texas cow on March 25, 2024. As of July 29, there have been 172 confirmed cases in 13 states. 

According to the Centers for Disease Control and Prevention (CDC), the risk to human health remains low, with only four cases of H5N1 reported in people exposed to infected dairy cows.

The USDA study, detailed in a forthcoming paper published in a peer-reviewed journal, demonstrates that field infections can be replicated under laboratory conditions. The research, carried out in a Biosafety Level-3 (BSL-3) agricultural facility in Ames, Iowa, assessed two routes of infection: respiratory and intramammary.

In the respiratory study, four yearling heifers were exposed to an aerosol mist containing the virus. These heifers became infected but exhibited minimal clinical signs, such as sporadic nasal discharge. In the intramammary study, two non-pregnant lactating cows were inoculated via teats and udders. 

These cows displayed significant symptoms within 48 hours, including reduced appetite, decreased rumen motility, a drop in milk production, and thick, yellowish milk, akin to viral mastitis symptoms observed in naturally infected cows.

ARS study leader Amy Baker and her team noted that their experiments were initiated to clarify the route of infection and transmission among cows. The study’s findings indicate that respiratory transmission is not the primary route of infection within farms. 

This conclusion aligns with observations that within-farm spread occurred without a consistent pattern following the movement of live lactating cows between farms.

Knowledge gaps persist 

Ongoing studies aim to address several knowledge gaps, including the virus’s tropisms within the body over time, the mechanisms of virus transmission between animals, the efficacy of HPAI vaccines in dairy cattle, and the best practices for disinfection, particularly of milking machines.

This research is part of a broader effort by the USDA, CDC, National Institutes of Health, Food and Drug Administration, and other partners to safeguard human and livestock health and ensure food supply safety. 

By better understanding H5N1 transmission and progression in dairy cows, the USDA aims to advance strategies for preventing and controlling outbreaks, thereby protecting both animal and public health.

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