12 February 2009

Snails and deer poop

Parelaphostrongylus tenuis (brainworm) is a nematode that infects the brain of the white-tailed deer (Odocoileus virginianus). Although the nematode doesn’t usually cause any apparent illness in deer, there is quite a bit of interest in its life history and transmission, because it can also infect caribou, reindeer, sheep, goats, etc., often leading to the death of those animals.

I have some interest in P. tenuis because terrestrial gastropods are the intermediate hosts of the nematode’s larvae. The nematode larvae are found in deer feces and the snails are believed to get infected with the larvae when they crawl over the feces. Presumably, the nematodes pass on to the deer or the other animals when they ingest infected snails accidentally during browsing (I don’t think the deer or the other susceptible animals eat snails intentionally).

Terrestrial gastropods don’t seem to mind to supplement their diets with feces whenever the opportunity arises (for example, read this post and also this one). So, expectedly, several studies have examined the consumption of deer feces by snails. The most recent study that I am aware of is by Garvon & Bird, published in 2005. (Again, I haven’t been that interested in this subject to have spent time searching for more recent papers.)

Garvon & Bird provide a brief review of the previous reports in the literature: one study claimed that the snails were repelled by fresh deer feces, but not weathered ones, while another study claimed that snails were repelled by deer feces containing P. tenuis larvae. The results of Garvon & Bird introduce further confusion. They found that the snail Anguispira alternata was attracted to fresh deer feces whether or not they contained P. tenuis larvae, although the snails seemed to prefer larvae-free feces to those containing larvae.

All of this popped in my mind late yesterday afternoon when I was passing by some deer poop in the woods. So I stopped to investigate.


But my poking around with a stick produced no snails. Perhaps I will have better luck in the spring and the summer.

Garvon, J.M. & Bird, J. 2005. Attraction of the land snail Anguispira alternata to fresh faeces of white-tailed deer: implications in the transmission of Parelaphostrongylus tenuis. Canadian Journal of Zoology 83:358–362.


John said...

I'm wondering if the deer acquire nematodes by (accidentally or deliberately) eating other deer's feces.


Interesting thought, but I don't know if the deer eat feces.

beetlesinthebush said...

Next time your stirring through some poop, keep an eye out for Geotrupes or Onthophagus beetles ;-)


I didn't notice anything moving. If I see any insects next time, I will certainly take some pics.

Anonymous said...

I seem to recollect a study on a deer population on an island, which took to biting the heads and wings off of sea bird chicks. The researchers surmised that the low levels of calcium on the island led to this behaviour. It may be when deer are growing antlers they may deliberately eat snails for their calcium, thus making the nematodes very happy.

vanessa cardui said...

Dog feces, left behind by their unkind owners, were found to be strong magnets for banana slugs (sorry, not sure which species) as we hiked the northern CA redwoods. We noticed the fresh feces on our way up a short hike, then found the pile covered with slugs on the way down, about 30 to 45 minutes later.

Also, reliance on accidental ingestion of the snails by herbivores for completion of the nematode life cycle seems like a tenuous grasp on continuity for the worms.


vanessa cardui: next time you come across slugs on feces, please take some pictures.

You are making a good point there about the snail-nematode-deer cycle.