Could carrion birds help spread prion diseases?

The term “murder of crows,” which apparently is the collective noun referring to a flock of the birds, could have a quite literal meaning.  A new report from USDA researchers indicates that carrion birds such as crows could be responsible for helping distribute prions – the infectious agents in transmissible spongiform encephalopathy (TSE) diseases including chronic wasting disease (CWD), scrapie, and BSE.

The researchers, working at the National Wildlife Research Center in Fort Collins, Colorado, hypothesized that TSE prions could pass intact through the digestive tract of crows. This would suggest crows or other scavenger birds could play a role in the spread of TSE diseases, as their feces potentially could contaminate food sources consumed by mammals, particularly in the case of cervids such as deer and elk, which are susceptible to CWD.

To test their hypotheses, they fed 20 crows mouse tissue infected with a form of mouse-adapted scrapie. Five crows served as a control group and received mouse tissue from uninfected mice.

Within four hours, the crows fed the infected tissue passed prions in their feces. The researchers injected healthy mice with an extract from the crow feces, and all the mice receiving the material from crows fed infected tissue soon exhibited signs of scrapie. The mice injected with extract from the crows fed normal tissue did not show signs of the disease.

The researchers conclude that the particular prions used in the study can remain infectious after passing through the digestive system of a crow. They acknowledge that further research is needed to determine whether prions from CWD-infected deer or BSE-infected cattle could be spread via crow feces in a natural environment.

BSE in cattle, of course, is exceedingly rare, and dead cattle typically are not left for scavengers to consume. CWD on the other hand, affects wild animals such as deer, elk and moose, which, if they die of the disease, likely attract carrion birds. CWD, while not common, has gradually spread over a wide range of North America. The means of transmission of CWD has remained a mystery, but these research findings could help explain how the disease spreads.

The research article is published in PLOS One, a peer-reviewed, open-access journal, and the full article is available online.

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