Don’t Eat the Wildlife!


“Adventures with a Botfly”

Caution: this is a little graphic. If you don’t have a strong stomach you probably want to skip this post!

vole with botflies
Botfly larvae that were embedded in the skin of this vole. They’re huge! Click on image to see more detail, if you dare!

A few weeks ago my dog, Fritz, caught and killed a little vole in the backyard. Not surprising considering they’re running all over the place, and my dog is a rat terrier mix (programmed to hunt small game).

I really don’t want him eating what he kills so I rushed over to take it from him and dispose of it elsewhere. From a distance I could see it was probably a vole, but when I picked it up I had the surprise of my life!

On the back end of the little animal, generally around it’s butt, were large growths with black centers. Being the curious sort that I am I decided to look at it a little closer on the picnic table. As I poked and squeezed a little I could see that there was actually some mass inside the tissue. Squeezing more I forced out a couple dark, larvae-like bodies. A little more squeezing pushed out several smaller, white ones.

Gross, gross, gross! The poor little animal was infested with some sort of insect larvae!

But, the most shocking part of this was the SIZE of these larvae. As you can see from the image the dark ones are fairly large, compared to vole itself. If you were to compare this mass to our body size I’d say it was something like a small watermelon! I can’t imaging how this little animal managed it’s life with this kind of encumbrance hanging out it backside. That’s probably why it was moving slower and consequently more susceptible to being caught by the dog.

I was only able to snap one pic with my phone before the battery died, but one is plenty in this case.

Not knowing what this could possibly be infesting the little vole I started doing some research. Turns out these are…

Botflies in Rodents

Having had several horses in the past I knew about intestinal botfly infestation in horses, and how to keep up with a regular worming schedule to prevent them taking hold in the gut. Because horses are grazers they are prone to ingesting several kinds of worms and parasites from the ground they feed on. This is particularly a problem when you have several horses grazing a small patch of ground where they poop. The term from microbiology is the “fecal-oral” route of exposure. (Although botflies generally lay their eggs on the leg hair of horses, where the horse can them scratch with their teeth, thus getting into their mouth and GI.)

And I had heard of cutaneous or dermal botflies in tropical regions, particularly on humans. If you have a strong stomach do some image research – it’ll probably put you off from visiting any tropical region for quite some time.

But, in North American there is a variety of botfly (Cuterebra spp.) that specifically targets laying their eggs in rodent burrows. The eggs then hatch when CO2 levels increase, indicating the presence of the host. This species of botfly target white-footed field mice, but can be found on any small mammal from voles to chipmunks to rabbits.

A really good rundown of the insect infestation process and life cycle can be found at Mountain Lake Biological Station, U. of Virginia: Rodent Bot Fly Larvae.

I saved one of the larvae in a jar, hoping to see what it would hatch into. But, alas, it died after about a week. Apparently it has to complete it’s final stage in the soil, not a plastic jar.

And here are pictures of the adult botflies.


Parasites can find and feed on a host in the most disgusting ways. I’ll stick with eating domesticated animals that are cared for with routine anthelmintics, thank you.

This probably underscores the importance of being vigilant, as a farmer, in caring for the animals on my farm. Just because they’re in a nice, fenced-in pasture doesn’t mean they’re immune from parasitic attacks.

Yay for dewormers!

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