Toxocara species
Toxocara species, eggs
Toxocara canis and Toxocara cati eggs are shed in the stool of the definitive host (dog or cat respectively) and may be accidentally ingested by a human. Each egg then hatches to release a larva which migrates throughout the human body causing visceral larva migrans. The larvae cannot mature in humans, and humans are therefore dead-end hosts. Eggs are not seen in human stool specimens, but may be seen in soil that is contaminated with the feces of an infected dog or cat.
Toxocara sp. Adult and Eggs
This case was submitted from an otherwise asymptomatic teenager. No further history was provided. This roundworm was identified as the adult female Toxocara species. Humans are NOT a host for this life cycle stage of the parasite. Therefore, this worm likely came from a family pet. 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:
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.
We also have the eggs to confirm the identification of Toxocara sp. Note their thick outer wall and characteristic pitted shell.
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.
