Answer to Case 803

Answer to the Parasite Case of the Week 803: Toxocara species, adult female. Humans are NOT a host for this life cycle stage of the parasite. Therefore, as Philip, Anastasia, Anatoly, HCLM Fan, and others mentioned, the most likely scenario is that this worm came from a family pet instead of the teenager. Less likely, the teenager could have swallowed the worm and its finding in stool represents spurious passage. Unfortunately, we were not able to get any additional history in this case. Hopefully this result was reassuring and helpful to the patient and family. 

Although this worm resembles the large human intestinal worm, Ascaris lumbricoides, it can be differentiated in this case by two main features:

First, this worm has pronounced lateral alae, which are not found in Ascaris lumbricoides. (Lateral alae are found in Enterobius vermicularis, but that worm is much smaller and can therefore be excluded).

This alone would have been sufficient to rule out a human parasite. However, we also have the eggs to confirm the identification of Toxocara sp. Note their thick outer wall and characteristic pitted shell:

Thanks again to those who wrote in with their comments. This case shows why it is helpful to be familiar with common zoonotic parasites. Additionally, the larvae of Toxocara can cause visceral larva migrans if the eggs are ingested by a human host. Therefore, identification may be helpful for the pet to be treated and reduce the household risk.