H5N1 Found in Mice Given Raw Milk from Infected Cows

-May 24, 2024

A colorized transmission electron micrograph reveals gold H5N1 avian influenza virus particles, flourishing within blue Madin-Darby Canine Kidney epithelial cells. The image, crafted by the CDC and enhanced by NIAID, sets the stage for alarming new findings.

Researchers have found that mice fed raw milk from dairy cows infected with the H5N1 influenza virus exhibit high viral loads in their respiratory organs. Published in the New England Journal of Medicine, the study raises significant concerns about the potential risks raw milk poses to both animals and humans.

Since its emergence in 2003, the H5N1 virus has spread across 23 countries, predominantly affecting wild birds and poultry, with around 900 human cases linked to close contact with infected birds. Recently, however, a highly pathogenic strain known as HPAI H5N1 has extended its reach to over 50 animal species. The United States reported an outbreak among dairy cows in Texas this past March, affecting 52 cattle herds in nine states and resulting in two human infections among farm workers.

In a bid to understand the risk posed by consuming raw milk from infected cows, scientists from the University of Wisconsin-Madison and Texas A&M Veterinary Medical Diagnostic Laboratory conducted an experiment. Five mice were given droplets of raw milk from infected cows. By day one, the mice showed signs of illness and were euthanized on day four for analysis. High virus levels were found in their nasal passages, trachea, and lungs, with lower levels in other organs—patterns consistent with H5N1 infections in other mammals.

Further experiments tested the virus's resilience to heat. Milk samples with high H5N1 levels were subjected to various pasteurization temperatures and times. Heating the milk to 63°C (145.4°F) for intervals of up to 30 minutes successfully eradicated the virus. At 72°C (161.6°F), virus levels were significantly reduced but not entirely eliminated after 15 to 20 seconds. The study's authors, however, caution that these conditions differ from industrial pasteurization processes, suggesting the need for further validation using commercial equipment.

Additionally, storing H5N1-infected raw milk at 4°C (39.2°F) for five weeks showed only a minor decline in virus levels, indicating the virus can remain infectious under refrigeration.

Despite these findings, the U.S. Food and Drug Administration (FDA) maintains that the commercial milk supply remains safe. An initial FDA survey of 297 dairy products from 17 states found no viable virus. Nonetheless, the FDA and USDA are conducting more extensive pasteurization validation studies to ensure safety under real-world conditions.

This research, funded by the National Institute of Allergy and Infectious Diseases (NIAID), underscores the need for vigilance and further study to protect public health in light of the persistent threat posed by H5N1.