Answer to Case 814

Answer to the Parasite Case of the Week 814: Toxocara sp. egg, most likely T. canis based on size.

As noted by several readers, Toxocara eggs are spherical to subspherical, and have a thick, coarsely-pitted shell. HLCM Fan shared the following story: "I remember a professor in college telling us the eggs of Toxocara look like golf balls, but nobody there ever saw one (a sport for only the privileged here), so when I teach, I say the look like styrofoam balls (a big ball made of tiny balls)." 

Note the characteristic pits in the shell of this embryonated Toxocara egg:

The video is particularly effective in showing the outer surface.

According to the CDC, T. canis is presumed to be the most common species to infect humans. This is likely due to several factors: transplacental transmission ensures that many puppies are infected from birth; dogs, especially puppies, shed enormous numbers of eggs; and dogs often contaminate areas where children play. However, recent molecular studies have shown that T. cati is probably an underrecognized cause of human infection. 

Eggs of Baylisascaris procyonis, the raccoon round worm, may also be found in soil and have a similar appearance to those of Toxocara, but have a finely mammillated or granular outer surface. Their size range is slightly smaller than that of Toxocara eggs, but there is significant overlap.

Finding Toxocara (and Baylisascaris) eggs in a child's sandbox is a significant finding, as both can cause visceral larva migrans VLM if the eggs are ingested by a human host. Following ingestion, the eggs hatch to release a larva that migrates throughout tissues, including the liver and lungs. Clinical manifestations range from asymptomatic infection to severe disease, with symptoms depending on the number of larvae, the organs involved, and the host's immune response.

Given the potential significance of VLM, it is important to prevent environmental contamination by keeping sandboxes covered when not in use, to prevent them from being used as latrines by infected dogs, cats, and racoons (!) In this case, there is no practical way to reliably decontaminate the contaminated sandbox, since the eggs are extremely resistant to environmental conditions and can remain infective in soil or sand for months to years after embryonation. It's best to completely remove or replace all of the sand.

Thanks again to Idzi Potters and ITM for donating this great case!