H5N1 Avian Flu Infects Cows with 10 Viral Particles

A study published in Nature Communications reveals that as few as 10 viral particles of H5N1 bird flu can infect dairy cows. According to senior author Andrew Bowman, a professor of veterinary preventive medicine at The Ohio State University, the virus targets mammary glands rather than the respiratory system, which has complicated efforts to contain the disease.

The research identifies a “major paradigm shift” in how the virus operates in cattle. Unlike typical influenza, which is respiratory, H5N1 shows a specific affinity for cow mammary glands.

To date, 1,053 outbreaks of the B3.13 virus genotype have been confirmed in dairy herds across 17 states. Federal data currently shows confirmed infections in Texas, Utah, and Idaho.

How does H5N1 infect dairy cows?

Researchers found that the smallest tested dose of 10 particles caused productive infections. While these low doses resulted in fewer clinical signs than higher doses, the cows still shed milk containing high concentrations of viral particles.

The virus replicates to high titers within the mammary glands. Once a cow is infected, she may produce this high-viral-titer milk for more than a week, according to Bowman.

The study noted that cows have four separate mammary glands with a common blood supply. In the animals tested, researchers did not see significant viral movement between these glands, which may influence how sick an individual cow becomes.

Did You Know? Since March 2024, when H5N1 was first reported in U.S. dairy cattle, a national milk testing strategy has been used to control outbreaks by halting the movement of infected herds.

Why is the transmission method still a mystery?

Scientists have struggled to pinpoint exactly how the virus spreads from cow to cow. Several experiments conducted at one of six specialized Biosafety Level 3 facilities in the U.S. failed to replicate the spread.

Ohio State researcher takes part in CDC bird flu study

Researchers transferred contaminated milking equipment between cows twice daily for 14 days, but healthy cows remained unaffected. Similarly, bottle-feeding calves high-viral-titer milk led to minimal virus detection and inflammatory signs, suggesting feeding doesn’t spread enough material to establish infection.

Tests on airborne transmission also yielded minimal results. Lactating cows given intranasal doses of the virus did not get sick, and chickens co-housed with them remained healthy after 17 days.

Bowman suggests the sterile conditions of the lab may not mirror daily farm life. He notes that because roughly 20% of cows on farms become clinically ill, a limited number of tests may not be enough to capture the transmission route.

Expert Insight: Samantha Carter notes that the shift from respiratory to mammary infection fundamentally changes the risk profile for livestock handlers. If the virus bypasses the airways and concentrates in milk, traditional respiratory precautions may be insufficient, placing a higher premium on the sterilization of milking machinery.

What are the risks to the retail milk supply?

Early this year, researchers reported that 36.3% of retail milk samples tested positive for H5N1 particles. However, Bowman stated that pasteurization is inactivating the virus, rendering the commercial supply safe.

Despite this, the potential for “spillover” remains a concern. Bowman described the transition of the virus from a waterfowl’s intestine into a cow’s mammary gland as a “head scratcher” that scientists are still trying to solve.

What may happen next?

Scientists may continue to investigate milking equipment as the most likely route of transmission. Because the virus replicates to high titers in mammary glands and is shed in milk, common contact equipment could still be the primary vector.

Future efforts could focus on changing milking or farming practices to limit cow-to-cow spread. Bowman warns that further spillover events are likely to happen, meaning evidence-based recommendations for prevention are a critical next step.

Frequently Asked Questions

Can H5N1 be found in store-bought milk?
Yes, early testing showed 36.3% of retail milk samples were positive for H5N1 particles, but pasteurization inactivates the virus.

Is the bird flu in cows a respiratory illness?
No. According to the study, the virus has an affinity for mammary glands rather than the airways, which represents a major shift from typical influenza.

How many states have seen H5N1 outbreaks in dairy herds?
Outbreaks of the B3.13 genotype have been confirmed in 17 states, with recent confirmed infections in Idaho, Utah, and Texas.

Do you think current farming regulations are sufficient to handle unexpected viral spillovers in livestock?