Answer: Dirofilaria immitis; the Dog Heartworm
Congratulations to everyone who wrote in with the correct answer to this case!
Dirofilaria spp. cause both subcutaneous and pulmonary infections in humans, with D. immitis being largely responsible for the pulmonary infections in the U.S. and worldwide. In dogs, the adult worms live in the right side of the heart and cause debilitating disease. The adults produce unsheathed larvae which circulate in the blood and are transmitted to other hosts through the bite of an infected mosquito. Humans are accidental hosts that also acquire infection through a mosquito bite. However, the worms cannot live in the human heart and quickly die and are passively transported to the lungs where they wedge in a small vessel and produce an infarct. Eventually the lesion heals as a granulomatous coin lesion which can mimic a primary or metastatic tumor. It's easy to forget that there is filarial disease in the U.S.!
On resection, the worms are found in various stages of degeneration with a surrounding inflammatory response. They can be recognized by their size (150 - 300 microns in diameter) and thick multi-layered cuticle with lateral internal cuticular ridges (arrows, image below). The internal organs degenerate quickly and are more difficult to identify.
To demonstrate the variable appearance of Dirofilaria spp. in tissue, here a number of other images from different cases. The appearance of the internal structures are variable, but note the internal cuticular ridges in all cases.
Most of you realized the Valentine's Day connection with the Heart worm. Maybe next year I'll be able to find a fun heart-shaped cross-section of a worm for you.
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