Sunday, January 27, 2008

Parasite Case of the Week 6

Welcome to the Parasite C.O.W.
Every Monday I will post a new case. Please feel free to leave a comment or your thoughts regarding the diagnosis. Click on the images to enlarge. Enjoy!

The following cyst was found in a section of skeletal muscle and was submitted for hematoxylin and eosin stained-sections after formalin fixation and paraffin embedding.

Image 1 (top) - 100 times original magnification.
Image 2 (bottom) - 400 times original magnification.
Click on images to enlarge.

Name of disease caused by this parasite?
Causative agent?
How is this organism acquired?

Saturday, January 26, 2008

Answer to Case 6

Name of disease caused by this parasite? Cysticercosis
Causative agent? Taenia solium
How is this organism acquired? Through ingestion of EGGS. The adult worm is acquired through ingestion of larvae. This is a favorite test question.

Differential diagnosis

First, you can tell this is a cestode due to the presence of calcaneous corpuscles (small calcified bodies) which are unique to cestodes (arrows).

The presence of a single hooked protoscolex within the small folded cyst tissue is characteristic of T. solium. However, you should also consider the following when looking at a larval cestode in tissue. All can be found in muscle, as in this case.

1. Echinococcus species - expect to see multiple cysts. In E. granulosus, each cyst contains multiple hooked protoscoleces. (Note that T. solium cysticeri only have only one hooked scolex per cyst) Protoscolex formation is rare in E. multilocularis, whose cysts are usually sterile in humans.

2. Spirometra species - do not form protoscoleces, so no hooks or suckers will be seen. The larva is solid-bodied and is not surrounded by a bladder.

3. Taenia multiceps and Taenia serialis (coenurus) - Like E. granulosus, cysts contain multiple protoscoleces with typical hooks and suckers, but are surrounded by a thin wall without a laminated membrane.

Monday, January 21, 2008

Parasite Case of the Week 5

Many of you will be happy to see that there is NO arthropod in this week's case. Instead I have something a bit unusual - some movies. These are short films of what you might see in a "hot stool" - that is, one that comes fresh from the patient without any fixative. This method is commonly used in the tropics for diagnosis. Furthermore, you're bound to see questions regarding characteristic motility on Board exams and the like, so it's good to be familiar with the classic patterns.

The first movie shows the classic motion. The second movie is to demonstrate morphology. The organism in question has finally slowed down enough to allow a closer look.

1. Identify the organism
2. State the significance of finding this organism in a stool sample.

Movie showing characteristic motion

Movie showing morphology

Answer to Case 5

Congratulations to Heather who correctly identified this organism! This is indeed Trichomonas hominis; a non-pathogenic flagellate from a stool specimen. It has now been renamed as Pentatrichomonas hominis.

Here is her description which I include because it so perfectly describes the motion of the Trichomonads:

"This is more what I call the "Night at the Roxbury Guys" movement--you know, the head-side-twitch to the beat of the music thing. (Don't know what I mean? Check it out here:

Watch the YouTube video

This is characteristic motility of Trichomonas species. If in stool, then, this would be Trichomonas hominis, and in a genital source would be Trichomonas vaginalis. T. hominis is a commensal and considered nonpathogenic, while T. vaginalis is, of course, a problem."

Go and watch the youtube video (you don't have to watch the whole thing - just the first 5 seconds and you'll get the idea), and then check out the following. Alex from Rip and Read (also my husband) made this for me. Fun!


Monday, January 14, 2008

Parasite Case of the Week 4

Welcome to the Parasite C.O.W.
Every Monday I will post a new case, as well as the answer to the previous (you'll find it below the corresponding case). Please feel free to leave a comment or your thoughts regarding the diagnosis. Click on the images to enlarge. Enjoy!

Case 4
This microscopic arthropod is just a larva but is capable of spreading disease.

Name a disease it spreads.

Sunday, January 13, 2008

Answer to Case 4

Identification? Trombiculid mite.
The larvae are referred to as 'chiggers'. Not to be confused with 'jiggers' caused by the 'jigger flea' Tunga penetrans. My entomology professor gave me the following memory aid to distinguish them: Jiggers Jump (they're fleas), whereas mites don't.

Name a disease it spreads: This little mite spreads scrub typhus in its larval form.

There weren't a lot of responses to this case, and I admit that I'm not surprised. It's a challenging arthropod to identify. I put it in here to point out the main differences between mites and ticks. Both are arachnids with 6 legs in their larval form and 8 in their adult form. The bodies are not segmented which allows differentiation from insects. Mites are usually smaller than ticks, but this is not the most reliable feature to distinguish them. Both tick and mite larvae can be microscopic.

Here are two useful features for identification.
First, mites usually appear 'hairy' (due to presence of setae) whereas ticks do not. Note the fine hair-like structures on the above example. Second, ticks have a central piercing mouthpart called a toothed hypostome (arrow below).

Top: Example of a tick larva. The arrow points to the toothed hypostome (click on image to enlarge).

Note that this is absent in the mites. Also, note that the tick larva does not demonstrate any 'hairs'/setae. Otherwise, the 2 organisms are very similar.

mite mouth parts (note no toothed hypostome)

Other mites of medical importance include the scabies mite Sarcoptes scabiei, and the house-dust mite.

Monday, January 7, 2008

Parasite Case of the Week 3

Parasite Case of the Week 3
This is the third in a new series of parasite cases. Every Monday, I will post a new case, as well as the answer to the previous (you'll find it below the corresponding case). Please feel free to leave a comment or your thoughts regarding the diagnosis. Click on the images to enlarge. Enjoy!

This worm is approximately 1 cm in length.
Name an important clinical symptom.

Answer to Case 3

Answer: Ancylostoma duodenale adult worm
Disease: Hookworm infection; ankylostomiasis
The most important clinical symptom is anemia, resulting in high childhood morbidity in endemic areas. Heavy infections can result in cognitive delays, and severe growth retardation. In pregnant women, heavy infections can result in prematurity and intrauterine growth retardation.

The differential diagnosis includes hookworm infection with Necator americanus and rarely, Ancylostoma caninum, the common dog hookworm. They can be distinguished by their mouthparts: N. americanus has cutting plates instead of teeth and A. caninum has 3 pairs of teeth instead of 2.