Case #494 - June, 2019

A 30-year-old stable worker from New Hampshire sought medical attention at a local health center for a painful bump with cellulitis on the side of her neck. The bump failed to go away with antibiotics, and when the wound was drained, the 3.5 mm long object pictured below was produced. Figure A shows the entire parasite, and Figure B shows a view straight down onto the posterior end of the organism. Of note: for Figure B, the terminal segment has withdrawn inside the preceding segment, making visualization of the diagnostic features very difficult. Only one of the paired spiracular plates is visible (inset).

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Figure A

Figure A

Figure B

Figure B

This was a case of myiasis caused by a bot fly larva. The small size and the presence of only two slits in each spiracular plate indicate that this is a 2nd instar larva, which makes precise identification difficult; most identification keys only consider mature 3rd instar larvae.

If in doubt about the identification beyond “myiasis, fly larva,” or “bot fly,” enlist the help of an entomological specialist, who may, with the specimen in hand, be able to make a more precise diagnosis. In this case, the specimen is a 2nd instar larva of a horse bot fly, Gasterophilus sp. In horses, the larvae crawl from the skin to the mouth, ultimately develop to the pupal stage in the intestine, are passed in feces and emerge as adults in the soil. In humans the larvae burrow into the skin directly and may cause “creeping eruption” as they migrate.

  • Occupational exposure to horses
  • Rows of stout, dark spines at the anterior margin of each body segment
  • Spiracular plate with no peritreme, slits curved rather than straight
  • The withdrawing of the terminal segment into the previous segment is a trait of 2nd instar Gasterophilus spp. larvae, from M.T. James (1947), The Flies that Cause Myiasis in Man.

CDC key for some flies of public health importance: icon

Images presented in the monthly case studies are from specimens submitted for diagnosis or archiving. On rare occasions, clinical histories given may be partly fictitious.

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Page last reviewed: July 19, 2019