Friday Parasite: How is a sheep like a rat?

                  

They both get infected by microscopic parasitic protists that form cysts inside their tissues. The parasites aren't reproducing inside these tissues -- they're waiting for their host to get eaten by the parasite's final carnivorous host. But Sarcocystis tenella isn't the creepy brain-encysting behavior-changing parasite that Toxoplasma is to rats. Instead, it forms cysts inside muscle (especially in the heart and esophagus), and waits for the sheep to die of natural causes. Canines that scavenge off the dead sheep eat the S. tenella cysts with the meat and get infected.

Photograph by S.J. Upton, Kansas State University.