Cottage Q&A
QUESTION
I have enclosed pictures of a very social chipmunk my family befriended at our cottage last summer. In
August, we noticed a strange swelling on its side. Over the following weeks, we watched the spot move and
show a distinct hole in the middle. What is this problem? Does it pose any risk to humans?
ANSWER
Your chippy friend is infected with a parasitic warble, the larval stage of a bot fly, explains Mark Engstrom, senior curator for mammals at the Royal Ontario Museum in Toronto. The larvae of bot flies (family Oestridae) are parasites on a wide variety of mammals. However, the various species are host specific, so this rodent bot fly (genus Cuterebra) cannot infect humans. (There is a species of bot fly that infects humans in the tropics, he adds. But you won't encounter it in cottage country.)
"This fly's life cycle is interesting," Engstrom says, "if a bit unpleasant from the chipmunk's perspective." The female fly typically lays her eggs along runways or at entrances to the rodent's burrow. The eggs hatch into larvae in response to a sudden increase in temperature and moisture when a rodent passes by, then enter their victim through the nose or mouth, or a small cut in the skin. Eventually, a larva migrates to a location under the skin, where it causes a "warble," or swelling, that contains a breathing hole such as the one you observed. When mature, 20 to 40 days after the original infection, a leathery, black larva crawls out, pupates in the soil, emerges as an adult bot fly, and mates to begin the cycle all over again.
Compared with other parasites, fully grown bot fly larvae are huge in proportion to their host - 20-42 mm long by 7-10 mm wide. Though the swelling is undoubtedly uncomfortable, says Engstrom, and may even prevent the chipmunk from moving around freely, it's not necessarily a major health risk. Once the larva crawls out, the animal will be relieved of its burden - however, the hole the larva leaves behind can become infected, causing more problems than the warble itself. There's no easy way to remove a warble, though, so we humans can't do much more than cheer on the chipmunk, and hope for the best.
* Published in the November/December 2005 issue of Cottage
Life
