Latest study from the Journal of Virology shows that prion disease spreads in sheep via mother’s milk. Sheep were infected with a common retrovirus causing mastitis and with misfolded prions. After collecting milk from animals, it was given to lambs, that had never been exposed to prions. The lambs were diagnosed with prion disease after two years, which suggested that there was a high level of prion infectivity in milk.

Bovine spongiform encephalopathy (BSE), also known as mad-cow disease, is a neurodegenerative disease in cattle that causes a spongy degeneration in the brain and spinal cord. The infectious agent in BSE is a prion, which causes native cellular protein to deform into the infectious state. Deformed prions are further infecting the native proteins in an exponential cascade. This results in forming a dense plaque fibers, degeneration of physical and mental abilities, and death. The same degenerative neurological disorder in humans is called Creutzfeldt–Jakob disease (CJD). It was believed that both BSE and CJD were transmitted through consumption of infected brain or organ meat.

Study from the Journal of Virology is a first demonstration of causing clinical symptoms in a natural host for prion disease through prions secreted from an inflamed organ. Recent research had suggested that human-to-human transmission of prions has occurred via blood transfusions. In 2004 a new report published in the Lancet medical journal showed that CJD can be transmitted by blood, but there is no test to determine if a blood donor is infected while in the latent phase of CJD. Prions accumulate in lymphoid tissues before invading the central nervous system. Inflammation can cause lymphoid follicles to form in other organs, such as liver and kidney, which leads prions to invade organs that normally do not harbor infection. This led to a hypothesis that prions in inflamed mammary glands could be secreted into milk.

After publishing the results of the sheep study, there are several concerns. If a common virus in a sheep with prion disease can lead to prion contamination of milk, the same milk could transmit infection to other animals. Moreover this situation might be particularly likely on factory farms, where mastitis may be common. Humans with variant Creutzfeldt-Jakob disease might accumulate prions in inflamed organs, and could also secrete prions. And the last, probably the most important question – is there a possibility of prion infection in humans through drinking BSE cow milk? All we can do is just waiting for an answer.

Source:
1. C. Ligios, M. G. Cancedda, A. Carta, C. Santucciu, C. Maestrale, F. Demontis, M. Saba, C. Patta, J. C. DeMartini, A. Aguzzi, C. J. Sigurdson. Sheep with Scrapie and Mastitis Transmit Infectious Prions through the Milk. Journal of Virology, 2010; 85 (2): 1136 DOI: 10.1128/JVI.02022-10.
2. American Society for Microbiology (2011, January 19). Prion disease spreads in sheep via mother’s milk. ScienceDaily. Retrieved January 20, 2011, from http://www.sciencedaily.com­ /releases/2011/01/110119191350.htm.

Author: Karolina Kłoda