Case of the Week 504

For this week's case, I'm going to take advantage of whole slide imaging technology via the cloud. The specimen is a section of large bowel obtained at autopsy. The prosectors were surprised to see several small worm-like objects attached to the bowel mucosa and therefore submitted representative sections for processing and microscopic examination. Here is a representative view of what they found:

To see the entire slide and zoom in on the diagnostic areas, click HERE.
Note that you don't need to download any special software to view this slide. You can use the "+" sign to zoom in, or simply use the track wheel on your mouse. It's pretty slick! Special thanks to Dr. Andy Norgan who made "Helminths in the Cloud" possible.

Answer to Case 531

Answer to Parasite Case of the Week 531: Sarcoptes scabei, var. hominis, the human "itch" mite. Seen here are mites, eggs and fecal pellets (scybala).

Old one nicely described the biology and morphology of these arthropods:

Sarcoptes scabei occur in a number of host species. Primarily in swine here in Minnesota but occasionally in humans. The male mites range in size from 213-285 μm long by 162-240 μm wide and the female mites range from 300-504 μm long to 230-420 μm wide. Sarcoptes are round to ovoid when viewed from the back; when viewed from the side they are ventrally flattened and dorsally rounded (similar to a turtle). They possess short stumpy legs, and have no internal or external respiration apparatus (stigmata or tracheae). The ventral surface contains a number of chitinized plates called apodemes, the dorsal surface is partially covered by wide-angled, V-shaped-spines (>). The cuticular surface is sculptured into numerous parallel ridges which superficially resemble human finger prints, and the anus is at the posterior end of the mite (this is the characteristic used to differentiate Sarcoptes from Notoedres which has a dorsal anus and sometimes infests humans) The morphology of the developmental stages of Sarcoptes varies. You can, however, differentiate the adult stages from other mite species using easily recognized characteristics. The last segment (tarsus) of legs 1, 2, and 4 on males and legs 1 and 2 on females have a long, unjointed empodium or stalk with a small sucker-like pad at its end. These stalks are diagnostic for Sarcoptes. You can see these suckered stalks in the wonderful video [in this case].

Here are some images from previous cases that show some of these features:

Mite shown in cross-section within the stratum corneum in a hematoxylin and eosin (H&E) stained tissue:

As noted by Bernardino Rocha, the scabies mite is an efficient miner. That's a neat way to think about this interesting arthropod. Sarcoptes scabei spend their entire lifecycle on or in their human host. The female burrows into the outer most layer of the epidermis (the stratum corneum) to create a short burrow called a molting pouch. The male penetrates this pouch and mates with the female. Interestingly, a female only mates once, and then remains fertile for the rest of her life. She leaves the molting pouch after mating to find another suitable location to create a permanent burrow.  Bernardino further notes that "dispersed along the burrows are eggs, hatched larvae, and excrements."

According to the CDC, "Scabies mites generally do not survive more than 2 to 3 days away from human skin." However, the mite in this case was still 'alive and kicking' 5 days after collection, so clearly there is a range in the survival time away from the host, which may have important implications for scabies prevention and control measures.

I'll leave you all with this lovely poem from Blaine Mathison:

You go to the doctor when your groin starts to itch

so he scraps some of your skin into a petri dish

and after a thorough microscopic examination

he tells you scabies is the cause of your sensation

to which you reply, 'Son of a ...'

Answer to Case 504

Answer: Trichuris trichiura, the human whipworm.

It's hard to top Blaine's excellent description of the diagnostic features for this case:
"Trichuris trichiura. If the eggs in utero were not enough, there is also stichocytes and bacillary bands (best seen in the anterior end of the worm within the tissue), polymyarian/coelomyarian musculature, and a strongly nucleated hypodermis. The size and egg morphology rules-out Capillaria philippinensis, which would also be in the intestinal tract and have trichuroid features of stichocytes and bacillary bands."

William Sears also pointed out a helpful feature: "it is telling that the thinner head part is embedded in the tissue and the larger tail is not embedded consistent with whipworm." Besides the eggs, the location in the tissue is one of the key features. Here are some of these diagnostic features:

Eggs with polar plugs (arrows; below):

As many of you may know, the whipworm gets it's name from it's shape - a skinny end and a thicker end, giving the worm a 'whip-like' shape. Unlike what is typically expected, however, the skinny end is anterior - not posterior. When my students find this puzzling, I ask them what they would rather try to do - insert the thin, needle-like end of the worm into the colonic mucosa, or the fat end? I also ask them what makes more sense - having the end with eggs hanging free in the lumen or caught up in the mucosa? This usually helps them conceptualize the worm's morphology, and I think they tend to remember it as well. When it comes to trying to remember the location in the gut, I ask my students what the dreaded complication of whipworm infection is. They always remember that it is rectal prolapse! Once they remember involvement of the rectum, it's easy for them to remember that it's the large bowel rather than the small bowel that is typically involved.

Thanks again to everyone who wrote in, and to my Clinical Microbiology fellow, Dr. Andy Norgan, who came up with the cool programming to rapidly and nearly-seamlessly display whole-slide digital images.  For those of you who couldn't get the embedded link to work for you, try this one instead:
https://s3.us-east-2.amazonaws.com/mcr-id-path/slides/BEOEOz_wVUmkRf2L2oG2sg.html