11 May 2026
In 2024 and 2025, the livestock sector was shaken by extensive H5N1 outbreaks in U.S. dairy cattle, particularly involving the B3.13 and D1.1 genotypes. While cattle have been the focus of recent surveillance, a new study published in Science Advances warns that small ruminants like sheep may be just as vulnerable, and could play a unique role in the virus's ecology.
To understand how these viruses behave in small ruminants, the research team, led by Dr. Yohannes Berhane, inoculated sheep with two distinct genotypes: D1.1 (H5N1) and A6 (H5N5).
The study explored two primary exposure routes:
- Aerosol Exposure: Nonlactating sheep were exposed to the virus via a nebulizer. This resulted in a relatively mild, self-limiting respiratory infection, and low-level viral shedding.
- Intramammary Inoculation: In lactating ewes, researchers introduced the virus directly into the right mammary gland via the teat canal. The results were far more clinical: ewes developed severe mastitis, producing milk containing visible clots, blood, and exceptionally high viral loads.
Perhaps the most significant discovery was the role of the suckling lamb. While only the right mammary gland was initially infected, the researchers left the left gland uninoculated to see if the virus would spread. They found that suckling lambs, after consuming infected milk from the right side, inadvertently transferred the virus to the previously healthy left mammary gland. This "mouth-to-teat" transmission resulted in the left gland developing mastitis and shedding high levels of the virus as well. The study underscores the need for increased monitoring of small ruminant flocks, especially in mixed-species farms where sheep and poultry may interact. Just like in infected dairy cows, the milk from infected sheep harbors high levels of infectious virus. This poses a direct risk to humans in regions where raw milk consumption is common.
Alkie, T. N., Embury-Hyatt, C., Signore, A. V., et al. (2026). Mammary and respiratory infection of sheep with H5Nx clade 2.3.4.4b viruses with milk-mediated transmission to lambs. Science Advances, 12(19), eaed1287.
See: USA: H5N1 HPAI in a herd of goats
H5N1 found in UK sheep
