Answer to Case 785

Answer to the Parasite Case of the Week 785: Oestrus ovis larva.

This is a fascinating case of a probable 2nd instar stage of Oestrus ovis, commonly known as the sheep nasal botfly. O. ovis, can occasionally cause infection of the eye (ophthalmomyiasis) or, less commonly, the nose and sinuses (rhinomyiasis) in humans. Human infection is an accidental zoonosis and results from deposition of first-instar larvae by adult flies, typically in the ocular or nasal mucosa. Human cases are most prevalent in Mediterranean and other subtropical regions, with seasonal peaks in summer and spring. 

Most infestations are self-limited as larvae rarely progress beyond the first instar in humans. Therefore, this is a very interesting presentation of what appears to be a 2nd instar larva involving the nose and/or sinuses. 

Diagnosis is based on clinical suspicion and examination of the larvae. First-stage larvae are small (approximately 1–2 mm) and mostly translucent As noted above, this is the most common form seen in humans. 

Second-stage larvae are larger (up to 7 mm), more robust, and display increased segmentation, with the body becoming more opaque and the cuticle developing small spines. The oral hooks are more prominent, and the posterior spiracles begin to show more complex structure. This is what I believe this specimen to be.

Third-stage larvae are the largest (up to 21 mm), cylindrical, and have a thick, heavily pigmented cuticle with pronounced transverse bands of spines and well-developed oral hooks in their mature form; the posterior spiracles are fully formed and more sunken into the body. Also, the body is distinctly segmented, and takes on a brown color in the mature form.

Check out these two publications for some great photos of the different stages:

b105_pp382-387.pdf

Prevalence Rate and Molecular Characteristics of Oestrus ovis L. (Diptera, Oestridae) in Sheep and Goats from Riyadh, Saudi Arabia

Thanks to all who wrote in on this interesting case, and to Idzi for donating it! Special thanks to Blaine Mathison for his input on larval stage.

Case of the Week 784

This week's case was generously donated by Rebecca Black, a Veterinary Medicine student training in Minnesota, USA. She was able to help this poor little kitty get the care it needed. What is your diagnosis? Does this organism pose a risk to humans?

Answer to Case of the Week 784

Answer to the Parasite Case of the Week 784: Myiasis causing fly larva, most likely Cuterebra species. 

Thankfully, Rebecca Black and her colleagues were able to remove the larva from this poor kitty! 

As noted by Florida Fan and others, we don't have the posterior spiracles to make a definitive identification. However, based on the presentation, geographic location, dark color, and overall appearance, we can still make an identification of Cuterebra sp. third instar stage larva.

Note the dark color and small spines covering most of the body. 

If you are interested, you can read more about this infection in animals at the following excellent sites:

Cuterebra Infestation in Small Animals - Integumentary System - Merck Veterinary Manual

Companion Animal Parasite Council | Cuterebriasis

Genus Cuterebra - Rodent and Lagomorph Bot Flies - BugGuide.Net

The usual hosts of Cuterebra are rodents and lagamorphs. Flies lay eggs on vegetation or woody ground debris, often near the opening of rodent burrows. The eggs hatch to release a first instar stage larva when there is exposure to increased temperature, indicating the presence of a nearby host. The larva then enters host through any natural body opening such as the mouth or existing wounds. It will then migrate to the subcutaneous tissues and enlarges rapidly. It creates a pore from which it can breathe and eventually exit the host, usually in 3-6 weeks. 

As you all know, my interest is in human infections, and therefore I posted this case as both a pet lover and a human medical parasitologist. Cuterebra infections (cuterebriasis) can rarely occur in humans, and as with other forms of myiasis, removal of the larva is curative.

Thanks again to student veterinarian, Rebecca Black, for donating this case! 

Case of the Week 783

This week's case was generously donated by Dr. Richard Bradbury. The following were seen in a stool specimen from a middle-aged man with diarrhea. Preparations are a concentrated wet prep and trichrome stained permanent mount. Object measure 10-15 micrometers long. What is your identification?

Answer to Case 783

Answer to the Parasite Case of the Week 783: Chilomastix mesnili cysts and trophozoites. Note the classic morphology:

C. mesnili is a non-pathogenic flagellate and therefore not the cause of this patient's diarrhea.

Thanks again to Dr. Bradbury for donating this great case!