Answer to Case 610

 Answer to Parasite Case of the Week 610: Brachylaima cribbi, a parasitic trematode found only in Australia to date. B. cribbi infects land snails and slugs as first/second intermediate hosts, and employs a wide range of mammals, reptiles, birds and amphibians as definitive hosts. The first human infections were published in 1996 by Dr. Andrew Butcher, who also wrote an excellent review on this parasite. Dr. Butcher recently passed away, and so I am dedicating this post to him and his important work. Humans become infected after ingesting undercooked snails. The main symptoms that have been reported with infection are watery, mucoid diarrhea, abdominal pain, anorexia, and weight loss. 

The eggs of this parasite are quite interesting in that they are small (only ~30 μm long), have an inconspicuous operculum, and are flattened on one side. They also have an abopercular knob or thickening. The eggs are usually fertile when seen in stool, with a well-developed miracidium. However, infertile eggs have also been seen in chronic infections; they are smaller and lack an internal miracidium. We can see both fertile and infertile eggs in this case:

You can really appreciate the infertile nature of an egg in this image:

Some readers suggested that this parasite might be Clonorchis sinensis or Opisthorchis sp., given the small size and presence of an operculum. However, we can exclude these parasites based on the lack of travel and the asymmetric flattening of one side. The flattened side of the eggs likely led some to suggest that this is Enterobius vermicularis; however, this parasite be excluded by the small size and presence of an operculum. The operculum also allows us to exclude artifacts and parasite mimics such as pollen and mushroom spores. 

So in many ways, a fascinating case! Thanks again to Dr. Bradbury for donating this case.