Answer to Case 209

Answer:  Visceral larva migrans caused by Toxocara spp.

The images of the liver show a single section of a larval nematode within a granuloma.  The larva is surrounded by multinucleated giant cells and an outer rim of lymphocytes.  Based on this image, the main differential diagnosis is toxocariasis and balisascariasis.  Other nematode infections that should be considered also inlude gnathostomiasis, ascariasis due to Ascaris suum, and capillariasis due to C. hepatica. 

The morphologic features that allow the diagnosis of Toxocara spp. from these other nematodes are:
1.  Only a small larval form is seen.  The diameter of Toxocara canis is 18-21 microns, while the diameter is T. cati is 15-17 microns.  In comparison, the diameter of Gnathostoma and Ascaris larvae (especially third stage larvae) is usually much larger, and larvae are not seen in Capillaria hepatica infections.
2.  Lack of a patent intestine in Toxocara.  The other organisms mentioned have a patent intestine, even when only present as larvae.
3.   Adult forms are not present.  In capillariasis, adults, and often characteristic eggs, are seen.
4.  The presence or absence of lateral alae may be helpful in the differential.  In this case, no cross-sections were identified and so it is not possible to determine if alae are present.  However, here is the break-down of relevant larval worms and whether or not they have alae:

Toxocara spp. - lateral alae present, single ala on each side
Baylisascaris spp. - lateral alae present, single ala on each side
Ascaris spp. - lateral alae present, single ala on each side
Gnathostoma spp. - no lateral alae
Capillaria hepatica - larvae not seen in human liver; no alae in adult worms

Finally, the presence of this larva in the liver and the young age of the host is classic for toxocariasis.

The image below the histopathology photos shows the characteristic egg of Toxocara spp.  Note the classic thick pitted shell. Inside is a coiled larva, indicating that it is infectious. These eggs are never seen in human feces but are commonly seen in the feces of un-wormed dogs and cats.  The eggs of T. canis are slightly larger (75-85 microns in diameter) than those of T. cati (65-75 microns in diameter) and the shell is more finely pitted in the latter.