Research suggests the H5N1 bird flu virus targets mammary glands rather than the respiratory tract, complicating efforts to contain outbreaks in the US.
RT’s Three Key Takeaways:
- Low Infectious Dose: Researchers found that only 10 viral particles of the H5N1 bird flu are enough to cause a productive infection in dairy cows.
- Mammary Gland Affinity: The study indicates that the virus primarily targets the mammary glands rather than the respiratory tract, representing a significant shift in understanding how the disease manifests in cattle.
- Transmission Uncertainty: While the virus replicates at high concentrations in milk, experiments involving contaminated milking equipment and respiratory exposure did not result in clear transmission, leaving the exact method of spread between cows unconfirmed.
Just 10 viral particles of the H5N1 bird flu that caused hundreds of influenza outbreaks in US dairy cattle can cause infection in cows, according to a study from The Ohio State University published in Nature Communications.
The research also suggests why the outbreaks have challenged scientists and livestock handlers. The virus appears to have an affinity for cow mammary glands rather than the airways, which represents a shift in how influenza typically behaves.
“Initially, we had no idea that cows could even be infected with influenza, let alone that the mammary gland was involved. That in and of itself was a major paradigm shift: It’s not respiratory,” said Andrew Bowman, professor of veterinary preventive medicine at The Ohio State University, in a news release.
Mammary Gland Involvement
Researchers tested the effects of varied levels of viral particle inoculations into individual cow teats. Results showed that the smallest dose of 10 particles resulted in productive infection. While these cows showed fewer clinical signs compared to those receiving higher doses, they still shed milk containing high concentrations of viral particles.
The study noted that cows produce high-viral-titer milk for more than a week once infected. Bowman noted that while pasteurization inactivates the virus for healthcare consumers, the high concentration in raw milk remains a concern for farm-level transmission.
“How it spreads from cow to cow becomes a very important question. We need to understand if there’s a way to change milking practices or farming practices, whatever it is, to limit cow-to-cow transmission because we think spillover is going to happen again. It’s just a matter of time,” said Bowman, in a news release.
Transmission Routes Remain Unclear
Several experiments aimed to identify how the disease spreads, but many common routes did not yield clear results. Researchers transferred contaminated milking equipment from an infected cow to a healthy cow twice daily for 14 days, but the healthy cows remained unaffected.
Additionally, bottle-feeding calves milk with high viral concentrations led to minimal virus detection, suggesting this method may not spread enough material to establish an infection.
The study also looked at respiratory paths. Lactating cows given an intranasal dose of the H5N1 virus did not get sick, and the virus was absent in their milk. These cows were co-housed with chickens that remained healthy for 17 days, suggesting that airborne transmission may be minimal.
Despite these results, Bowman suggested that milking equipment remains a likely suspect due to the high titers found in milk and the common contact between equipment and mammary tissue.
To date, 1,053 outbreaks of the originally detected virus genotype have been confirmed in dairy herds across 17 states. These have been largely managed through a national milk testing strategy and movement restrictions on infected herds. Current data from the US government suggests small numbers of infections are currently confirmed in Idaho, Utah, and Texas.
