Low infectious dose: H5N1 strain B3.13 infected the mammary glands of lactating Holstein cows at doses as low as 10 TCID50. Infected glands shed high viral titers in milk (up to >10¹² TCID50/mL).
No transmission via milking equipment: Sentinel cows exposed to contaminated milking equipment over 14 days in a high-biocontainment lab did not become infected.
No co-housing transmission: Sentinel cows co-housed with infected cows did not contract the virus.
Route-dependent severity: Direct mammary gland infusion caused severe mastitis and systemic illness; respiratory exposure and feeding infected milk to calves resulted in mild or no clinical signs.
Chickens not infected: Chickens co-housed with intranasally inoculated cows showed no infection.
Study Design
The study, published in Nature Communications, used a dairy cattle-derived H5N1 B3.13 virus. Researchers infused varying viral doses (10, 100, 1,000 TCID50) into individual mammary quarters, monitored clinical signs, measured viral titers and RNA, and attempted transmission via shared milking equipment and co-housing.
Implications
While the mammary gland is highly susceptible, the failure to reproduce transmission under controlled conditions suggests that other factors (e.g., environment, farm management) are critical for spread in real-world settings.