Answer to Case 521

Answer: Dicrocoelium dendricitum eggs

This is relatively rare find in human stool specimens. The eggs are small (35-45 µm long by 20-30 µm wide), thick-walled, and often brown due to bile staining. They are shed in a fully embryonated state, with the embryo often easily seen within the egg (these look like a skull to me!)

D. dendricitum is such a fascinating parasite. Its life cycle typically involves as ruminant, a snail and an ant. The whole life cycle is nicely illustrated by the CDC DPDx group HERE. Here is the accompanying text: 

Ruminants are the usual definitive hosts for Dicrocoelium dendricitum, although other herbivorous animals, carnivores, and humans can serve as definitive hosts. Embryonated eggs are shed in feces. The eggs are ingested by a snail. Many species of snail may serve as the first intermediate host, including Zebrina spp. and Cionella spp. When the miracidia hatch, they migrate through the gut wall and settle into the adjacent vascular connective tissue, where they become mother sporocysts. The sporocysts migrate to the digestive gland where they give rise to several daughter sporocysts. Inside each daughter sporocyst, cercariae are produced. The cercariae migrate to the respiration chamber where they are shed in slime ball from the snail. After a slime ball is ingested by an ant, the cercariae become free in the intestine and migrate to the hemocoel where they become metacercariae. Many ants may serve as the second intermediate host, especially members of the genus, Formica. After an ant is eaten by the definitive host, the metacercariae excyst in the small intestine. The worms migrate to the bile duct where they mature into adults. Humans can serve as definitive hosts after accidentally ingesting infected ants.

What this nice narrative does not mention are the following fun facts:
1. While in the ant intermediate host, the parasite takes control of the ant's nervous system and controls its actions. It directs the ant to climb to the top of a blade of grass every night and clamp down tightly to the blade with its mandibles. At dawn, the ant goes back to its normal activity in its colony. The recurs night after night, with the poor parasitized ant residing on the blade of grass until it is eaten by the grazing definitive host! What a clever way for a parasite to ensure the continuation of its own life cycle. This story reminds me of the effect that Toxoplasma gondii has on its rodent host (a story for another day).
2. As Old One reminded us, "D.d is beautiful critter. So flat you can clearly see all it's major organ systems without the benefit of stains. So thin it is commonly called the Lancet fluke. These dimensions allow D.d to dwell within the biliary ducts where they can allow normal bile flow in small numbers or block the flow with greater numbers."  For this reason, the fluke is less likely to cause symptoms in its host.
3. When humans serve as the definitive host for D. dendricitum, eggs of the parasite may be seen in the stool. However, eggs may also be seen in the stool when humans inadvertently eat the infected liver of another definitive host such as a cow; in this scenario, the presence of eggs would be considered spurious passage or 'pseudo-parasitism'.

This brings us to the question that Idzi posed to us: What is the possible clinical relevance of finding these eggs in a human stool specimen? Luis and Nema both pointed out that it is necessary to rule out spurious egg passage due to ingestion of infected liver. To accomplish this Nema suggested collecting repeat stool specimens (instructing the patient not to eat any more liver!) If follow-up specimens are negative, then this would provide evidence that the patient was not actually parasitized. A related note is that liver ingestion is not common in many parts of the world, so getting a good dietary history (including possible ingestion of ants!) is very helpful.

I'll now leave you with a lovely story from Blaine:
So, there is this old story, or so I am told, about ants getting ready for the winter. They gathered up food stores all day, including slime balls produced by snails. Then along came a lazy grasshopper, which had spent the summer playing and not preparing for the winter. When the winter came, the grasshopper begged the ants for something to eat, even a slime ball!!! The ants refused. In the Spring, the ants had an insatiable urge to climb to the tops of blades of grass and ended up getting eaten by cattle and sheep! The grasshopper did not get eaten by the livestock. The moral of this story? Play all day and you are guaranteed not to become cow chow!