Answer to Case 670

Answer to the Parasite Case of the Week 670: Enterobius vermicularis (pinworm) adults within the lumen of the appendix. This is a classic ectopic location for pinworm. While there is some evidence that they may cause appendicitis in this setting, it could just be an incidental coincidence, given how common  both appendicitis and pinworm infection are!

For those of you not familiar with the gross appearance of the appendix, I've included an annotated version below for clarification. You can also compare the size of the appendix to the rest of the intestinal tract from this Mayo Clinic image.

Note that the wall of the appendix is a bit thickened here due to inflammation and edema. The patient had her appendix removed because of appendicitis. The worms are in the relatively small lumen, which gives you some idea of their size. 

The small size of the appendix will allow you to rule out Ascaris lumbricoides here, since the adult worms would be about the same size as the inside of the appendix. Having said that, Ascaris will occasionally migrate into the appendix and possibly lead to appendicitis, so it's an important consideration! 

The other worm to consider in this case is Trichuris trichiura (whipworm), would can also occasionally be seen within the appendix. While it's hard to rule out whipworm in this case, there are occasional worms in which a narrow 'pin-shaped' tail can be seen, which is characteristic of female pinworms. 

Whipworms have a broad, rounded tail in comparison.