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:
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.
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