Wednesday, October 5, 2011

A parasite more disgusting than ticks

** Warning: This post is going to maybe too gross for some people, so if you really are not too cool with parasites you may just want to skip this one (hehe).

The little white-footed mice that are the focal host of most of my research aren’t just hosts for ticks. They are the habitat for a multitude of macro- and microparasites. For instance, in our research we have collected three kinds of ectoparasites, ticks, fleas and mites. Many other researchers have studied intestinal worms, both acquired trophically (through eating infected food, like crickets), and by simple contact with worm eggs in the habitat. They can carry many vector-borne pathogens, bacteria and other microbes carried by ticks and fleas, such as Borrelia burgdorferi, the causative agent of Lyme disease, Bartonella, a flea-borne bacteria, and Babesia, a tick-borne protozoa similar to the causative agent of Malaria. And one of the more disgusting parasites that uses these mice as hosts is the bot fly.

You may have heard of bot flies in association with people coming back from the tropics with weird bumps, to then have a squishy larva pop out of them a few weeks later (see NBC story here). I had heard of them infecting primates and other tropical vertebrates, but didn’t know they were around in temperate areas as well. But, in our 2009 field season we were finding mice with what looked like tumors or extra testes (they were usually in their lower torso area). The size of the growth in relation to the mouse’s body size is pretty significant, like half the size of its torso in some cases (see below). We really had no idea what these were, so we took some pictures and sent them to John Whitaker, a mammal expert at Indiana State University. He identified the growths as bot flies right away. This year I saw a few more mice with similar infections, and one of the other researchers in my lab did too. And being nerdy scientists, we did some online searching to find out more about this gross parasite that is infecting our animals.

One of the mice we captured in 2009 with a botfly infection. Notice how big it is compared to the size of its body!

The most useful source we found was a Catts 1982 paper in the Annual Review of Entomology. It starts out with a great line:

“Maggots of cuterebrid bot flies are conspicuous, repulsive, and frequently encountered cutaneous parasites of mammals in the New World.”

I don’t know if I’ve seen the word “repulsive” in an academic paper before.* This went through a detailed overview of the life history of this parasite and what is known about different aspects of this larval and adult biology in the Americas. A drawing of a typical bot fly life-cycle is below, and is simply the adult fly lays eggs (1000-3000 per female, depending on species) usually close to host burrows, those eggs hatch into microscopic larvae when they sense and increase in temperature (a potential host), the larva get in to the host through eyes, nose, mouth or open cuts on the skin, they then migrate through the host’s body over a few days and then settle in the abdomen of the host to develop into the final larval stage. They create a “warble pore” through the host’s skin to breathe out of while developing. The larva then emerged out of the pore, develops into a pupae under leaf litter and can overwinter in this stage, and then emerges as an adult. Adults do not eat at all, they only mate and lay eggs.

Illustration of bot fly lifecycle from Catts 1982. The host is on the right side, and two alternate pathways are shown for infection (direct contact with larvae or females laying eggs on host, which is less common).

Yes, this is pretty gross and you may have visions of “Aliens” in your head right now. But surprisingly, it seems like hosts don’t suffer too many long-term effects of the infection. The Catts paper summarizes studies that found hosts have lower than usual protein levels during infection but seem to make up for the loss by eating more. The warble pore usually heals completely in a couple days with little infection, possibly due to compounds in the mucus the larva surrounds itself in.

Last week I did my last field sampling of the year and found a mouse with a really huge bot fly infection. I took a picture to show Angie, the other researcher who had found these infections while doing some mammal trapping. I said it would be really interesting if I caught the same guy again to see if the larva emerged, since it was so big already. 

Animal 595 with a big bot fly infection. The dark circle is the warble pore made by the developing larva. Hopefully you can see how huge this infection is at the time.
And, low and behold, I caught that same mouse the next day and the larvae had emerged. 
Same mouse, no bot fly. The pore is bloody from the larva emerging and his torso looks much more normal-shaped. But the mouse doesn't look too happy!
The little guy did not look like he was in good shape, go figure since a parasite the size of his torso just squirted out of him. When I released him after doing all the handling of the other animals, he just kind of sat on the ground, instead of running away quickly like they usually do. He was all hunched up and kind of wobbled as he walked away. I was even able to reach down and grab him off the ground, which I would never be able to do with a healthy animal. I am thinking he must have been in some significant pain after that lava emerged. I am hoping he is ok, after reading that the wounds heal pretty quickly. Poor little guy! But I guess since I do study parasites, I should be glad that I am studying a host that is so desirable for all these different things, and emphasizes that the host really is a community and that ecology matters at so many different levels.

* If you want to take a look at the paper, there is a lot of other somewhat non-academic language used. It’s kind of funny to see what you could get away with in older publications.


  1. Hej,

    I found three infected mice last week here in Germany. One with three larvae and 2 with one larvae each. I let them in the trap until I finshed with the plot (all three specimens were caught on one of 12 plots, each with 80 traps) to take photos. In the end I found three emerged larvae, which I collected. Now I hope, that they will hatch to determine the species :-)


    1. wow, cool! thanks for sharing your experience :)

  2. I Googled the phrase "mouse parasites" and came across this post. This past October I set a mouse trap in my kitchen in suburban Chicago after seeing some signs of mouse activity. I was in my basement at around 1:00AM when I heard the trap go off. About an hour later I went to check on the trap. As I expected there was a dead mouse in the trap. What I did not expect to see however was what looked like a black jelly bean with somewhat pointed ends moving next to the mouse. I was grossed out to say the least. I picked up the unknown creature with a wad of toilet paper and flushed it away. Now I know what it was. I guess when the mouse was killed the bot fly larva sensed the decline of body temperature in its host and decided to depart via its "warble pore" for warmer digs. It never got more than an inch away from its previous home. Now knowing what that thing was in no way makes it any less gross. In fact, knowing that there was a mouse carrying a bot fly larva in my kitchen makes it even more disgusting!!

  3. Just found one in Spencer, Indiana- so cool, and gross....