A study has identified the presence of normal prion proteins in milk from humans, cows, sheep, and goats.
"Prion Protein in Milk" appeared in the December issue of PLoS One, an online journal from the Public Library of Science, available at www.plosone.org.
Infectious prion proteins cause transmissible spongiform encephalopathies in many species, including scrapie in sheep. In view of a recent study showing evidence of prion replication in the mammary gland of sheep with scrapie and mastitis, the authors of the new study conclude that the presence of normal prions in milk implies a possibility that milk from animals with TSEs serves as a source of infectious prions.
The study found that the absolute amount of normal prion proteins in milk differed among species, with sheep milk containing more than human milk. The study also identified prions in homogenized and pasteurized off-the-shelf milk. Even ultrahigh temperature treatment only partially diminished the concentration.
The authors note that scientific groups, risk assessment agencies, and public health organizations have debated the TSE risk for milk and milk products throughout the past decade. Epidemiologic and bioassay data have not provided evidence that milk harbors prion infectivity. Bioassays of the milk, colostrum, or udder of cows with BSE have not as yet detected infectious prions.