Voles Running in Circles: a possible case of raccoon roundworm

So it was both with delight (and a touch of sadness for the vole) that I got to witness this firsthand. Immediately, I started making observations about the vole’s behavior:

  • always running clockwise
  • circles are about 6-12″ in diameter
  • running bouts lasting from 5-120 seconds with short pauses between
  • at rest, the vole was leaning down to the right (in the same direction it was running)
  • often coming to a rest when it ran into a leaf. It would then bury its head into the duff for a short while before beginning its endless circles
  • seems constrained by the large depression it was in (a pothole on a dirt road) and as it hit a wall of the the pothole it would veer back the other way

Just as I was about to leave the vole ran up a short slope and tumbled over backwards. It laid on its side breathing heavily for about 30 seconds. When I got closer it squeaked loudly then began running again, but it was far more erratic this time and kept tumbling upside down, exposing its white belly.

A meadow vole in a moment of brief pause between running bouts

A possible explanation

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)

Alternate host of the roundworm

The next host may very well be a raccoon, but eggs may also be picked up and ingested by a small mammal or bird. These paratenic hosts, though not a necessary stage, act as carriers that effectively disperse eggs to new habitats and make transmission to a definitive host more likely. In cases like our poor vole, the eggs hatch in the digestive tract. The larvae cease development and burrow into the animal’s tissues where they encyst, remaining dormant until the paratenic host is eaten by a raccoon. Once in the raccoons intestines, the roundworm can leave dormancy and complete its lifecycle.

The vole laying on its side a brief break in its bouts of intense activity (Intervale, Burlington)

A similar scenario

Like many other parasites with intermediate hosts, the parasite disrupts the normal behaviors of the intermediate host, increasing the likelihood that the animal will be captured and ingested by the definitive host. While I’m not positive that this meadow vole was indeed infected with raccoon roundworm (voles can “waltz” – source), the behavior sure did remind me of the behavior of killifish infected with the trematode parasite, Euhaplorchis californiensis. The parasite causes the killifish to rapidly flip over, pointing their white bellies up to the sun. This makes it significantly more likely that the fish in consumed by a predatory bird. The bird is the terminal host of the parasite, which lays its egg in the bird. The eggs are then passed out by the bird in the scat, which are then ingested by snails (source). You can think of this as similar to flowers that can either be pollinated by wind – relying on chance to find another flower to pollinate – or by animals – relying on an animal vector to bring pollen directly from one flower to another.

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