Answer to Case 538

Answer to Parasite Case of the Week 538: Toxocara species
Another case with lots of great discussion! There was quite a bit of debate on whether or not this was T. cati vs. T. leonina. This differentiation is potentially important since the former is commonly associated with human larva migrans (visceral, ocular and neural), whereas the latter only rarely is. As noted by Old One, a number of morphologic characteristics can be used to differentiate between the two, including the worm size, male spicule morphology, egg morphology and shape of the cervical alae. The cervical alae are short and wide in T. cati, and long and narrow in T. leonina. As the cervical alae in this case also appeared to be long and narrow, several readers were favoring T. leonina. However, the angle of the photographs made it difficult to really characterize the alae. Thus Old one suggested that we look at the eggs. Therefore, here is a new photograph for your consideration:

The scalebar represents 50 µm, putting the egg at ~70 µm long, within the size range of both species. Importantly, it has a pitted shell which is characteristic of T. cati. In comparison, the shell of T. leonina is smooth. Thus we have a nice case here of Toxocara cati! Mystery solved 😊.

This discussion still doesn't address Idzi's follow up question of what potential risk is posed to humans from contact with the infected cat and these worms. The adult worms themselves are not a risk to humans, but mature eggs, if ingested, can hatch to release larvae that cause larva migrans. While we clearly have at least 1 gravid female here, it is important to note that the eggs are shed in an unembryonated state and need to mature in the environment for 4 or more days (longer at lower temperatures) before becoming infectious to humans. Therefore, there is no immediate risk to humans in this case. However, eggs that may have been shed into the environment from the infected cat MAY pose a risk if accidentally ingested. Thus I would recommend carefully cleaning the litter box (wearing gloves, followed by meticulous hand washing) and getting the cat treated asap! As always, hand washing after soil contact (e.g. gardening) is also prudent.

Thank you all for the great comments!