A meadow vole in a moment of brief pause between running bouts
Raccoon Roundworm, Baylisascaris procyonis
The raccoon roundworm is a parasite that primarily infects raccoons (about 70% of adults and 90% of juvenile raccoons have the parasite – source), though dogs can serve as an alternate definitive host (definitive hosts are hosts for the adult life stage of the parasite and are required for producing the next generation). While the roundworm does not need an alternate host to complete its life cycle, it can and does regularly affect common prey animals, like rats, rabbits, mice, voles (see image from CDC’s page below for its life cycle). In the definitive host (again, raccoons), the eggs hatch in the intestine, and the larvae remain here through maturity. The roundworms reproduce and lay their eggs in the intestines as well. The eggs (females are prolific and can produce well over 100,000 eggs a day) are passed out the digestive tract. Ultimately the eggs will be ingested by another host and the cycle begins again.
Lifecycle of the Raccoon Roundworm (from the CDC’s website)