Answer to Case 467

Answer: Schistosoma sp. miracidium. Given the location in urine, the likely species is S. haematobium.

This case from Idzi Potters and the Institute of Tropical Medicine Antwerp shows the characteristic morphology of a form we rarely get to see in the laboratory - the motile ciliated miracidium that hatches from an egg when exposed to water. Note the interesting motility provided by the circumferential cilia.

You can recreate this in the lab by performing the 'hatching test' when you find Schistosoma eggs in stool or urine. There are instructions for the hatching test in most standard parasitology texts, although the process is somewhat time consuming.

In this case, the contamination of the urine specimen with toilet water and the delay in reaching the laboratory likely provided the stimulus and time needed for the eggs to hatch and release miracidia.

The primary differential diagnosis in this case is species of the free-living ciliate, Paramecium, that may be seen in fresh water. Idzi kindly provided the following videos of a Paramecium sp. so that we can appreciate the differences between them and schistosome miracidia. Paramecium spp. may range from 50 to 300 micrometers in length and therefore may overlap with the size range of S. haematobium miracidia (approximately 150 micrometers). They are also covered in circumferential cilia. The primary differences that we can appreciate at this magnification is that they are ovoid, lack an apical papilla (pointed apical end), and have rapid spiraling motility.

One reader suggested that the organisms seen in this case were miracidia from zoonotic schistosomes. This interesting suggestion prompted Idzi and I to do a little research! Idzi was able to find some very helpful studies to show that miracidia are unlikely to survive for more than 24 hours, and therefore couldn't have come from eggs that were passed by another animal and made it into the toilet water. According to Maldonado et al. (Biological studies on the miracidium of Schistosoma mansoni. Am J Trop Med 1948;28:645-657), the average life span of S. mansoni miracidia is 5 to 6 hours. Similarly, Lengy (Studies on Schistosoma bovis [Sonsino 1876] in Israel. Larval stages from egg to cercaria. Bull Res Counc Israel 1962;10:1-36) found that by 24 hours, all of the eggs in their study had hatched and all miracidia were dead. Ozgur Koru also noted that Schistosoma haematobium eggs will quickly hatch once being exposed to water (within 15 minutes). Based on these data, our conclusion is that the miracidia must have come from eggs in the patient's urine that hatched en route to the lab. Luckily the specimen reached the lab in time for the miracidia to be observed in their motile state.