Insights on H5N1 Avian Flu Spread in Dairy Cattle

While the occurrence of H5N1 avian flu in dairy cattle and commercial poultry remains low in the United States, researchers are still exploring how the virus spreads on farms. Recent findings this week have focused on two potential avenues: contamination by house flies and a phenomenon referred to as “milk snatching.”

In the past month, the US Department of Agriculture (USDA) has reported a single case in poultry—a game bird farm in Pennsylvania—and two instances in dairy cattle, involving herds from California and Arizona.

The reduction in detections of the virus seems to correlate with a seasonal decline in cases. A recent monthly update from the World Organization for Animal Health (WOAH) indicated a decrease in poultry outbreaks in June, with only 15 reported. However, new outbreaks in wild birds are increasing, particularly in Europe.

Initially, it was believed that the movement of infected cattle and contaminated equipment were the main factors facilitating the virus’s spread in dairy cattle. Yet cases continued to emerge despite restrictions on interstate cattle movement and enhanced biosecurity measures, raising ongoing questions and prompting scientific investigations into additional factors at play.

Fly Findings Spark Discussion

This week, Raj Rajnarayanan, a computational biologist at the New York Institute of Technology’s College of Osteopathic Medicine, pointed out that sequence samples from a house fly in California had recently been shared on GISAID, the Global Initiative on Sharing All Influenza Data. The virus from this fly belonged to the same subtype affecting cattle.

Rajnarayanan noted on social media that identifying the virus in house flies implies that these insects could potentially transport the virus across farms. While there’s no evidence indicating they spread H5N1 to humans or animals, it underscores the need for improved biosecurity on farms.

Mike Coston, who has been documenting avian flu developments for years, referenced a 2006 report from Japan suggesting that blow flies near poultry outbreaks were capable of carrying the virus on their bodies. He mentioned that, although these flies aren’t classically “infected,” they could be among numerous factors contributing to the spread of the highly pathogenic avian influenza virus. He also highlighted a more recent study indicating that blow flies in a Japanese crane colony could acquire the virus from deceased birds or contaminated feces.

Coston raised the possibility that other local animals, such as rodents, and elements like “poultry dust” might also play roles in this transmission. He expressed concern that unless we understand better how HPAI spreads in the wild and between farms, our efforts to manage these outbreaks will remain quite limited.

‘Milk Snatching’ and Infection Routes in Cows

In another development, a research team from China has explored how the H5N1 virus might infect mammary glands in dairy cattle. They published their findings earlier this month in the National Science Review.

The study concluded that when the virus is introduced through the nose, it primarily reproduces in the mouth and respiratory tracts. In contrast, when inoculated into the mammary gland, it replicates exclusively in that area, indicating that the virus’s natural pathway to infect the mammary glands is likely through the teat.

The team theorized that some lactating cows engaging in “milk stealing”—whether by self-nursing or mutual nursing—could be a means for H5N1 to spread to mammary glands. Calves are also capable of transmitting the virus through nursing. The researchers noted that bovine oral tissues have a high concentration of sialic acid receptors, which might promote viral infections through contaminated feed and water, allowing for prolonged replication.

In additional experiments, the researchers tested two vaccines on lactating cattle: an H5 inactivated vaccine and a hemagglutinin-based DNA vaccine. Remarkably, both vaccines provided complete protection against H5N1, even after direct high-dose inoculation into the mammary gland.