WBR0084
| Author | PageAuthor::Kalsang Dolma (Reviewed by Rim Halaby, Will Gibson, and Yazan Daaboul) |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Microbiology |
| Sub Category | SubCategory::General Principles |
| Prompt | [[Prompt::A 28-year-old female immigrant presents to the emergency department (ED) for diffuse pruritus and a foreign body sensation in her right eye for the past 2 hours. The patient states that she returned from a rainforest trip in western Africa 8 months ago. The patient has no significant past medical history. She takes no medications and has no known allergies. Physical examination is remarkable for a mild eyelid swelling and non-tender localized angioedema in the right forearm. Dim slit-lamp examination of the the right eye shows a thread-like worm in the subconjunctival space. Initial laboratory work-up in the ED reveals eosinophilia. Peripheral smear and PCR confirm the diagnosis. Which of the following vectors is most likely associated with the infectious agent responsible for this patient's condition?]] |
| Answer A | AnswerA::Black fly |
| Answer A Explanation | [[AnswerAExp::The black fly is the vector for onchocerciasis (fever blindness) caused by Onchocerca volvulus.]] |
| Answer B | AnswerB::Soft tick |
| Answer B Explanation | [[AnswerBExp::The soft tick is a vector for relapsing fever.]] |
| Answer C | AnswerC::Ixodes tick |
| Answer C Explanation | [[AnswerCExp::Ixodes tick is the vector for lyme diseae and babesiosis.]] |
| Answer D | AnswerD::Deer fly |
| Answer D Explanation | [[AnswerDExp::Deer fly (or Deerfly) is the vector for Loa loa.]] |
| Answer E | AnswerE::Sandfly |
| Answer E Explanation | [[AnswerEExp::Sandfly is the vector for leishmaniasis.]] |
| Right Answer | RightAnswer::D |
| Explanation | [[Explanation::Loa loa is a human filarial nematode (roundworm) whose adult form migrates in the subcutaneous and deep connective tissues. Microfilariae, ingested by mango flies, horse flies, or deer flies (Chrysops spp.), may be transmitted to humans following a bite by the infected flies. The time for the Loa Loa to mature requires approximately 6-12 months, but a prolonged latency (several years) is not uncommon.
Loa loa is called the "eye worm" because it can cross the conjunctivae, creating an uncomfortable "foreign sensation in the eyes" among infected patients. Additionally, diffuse pruritus and a Calabar swelling, a non-tender angioedema that is often observed in the forearms, may be seen in approximately half of the cases. The diagnosis is suspected when patients report a history of travel to endemic African regions, consistent clinical symptoms, and the presence of eosinophilia. The diagnosis is then confirmed by high Loa-specific antibody titer and a peripheral blood smear that often shows the microfilariae. PCR is the gold standard for diagnosis. Administration of oral ivermectin or diethylcarbamazine (DEC) is the optimal pharmacologic therapy for Loa loa. However, surgical removal of the parasite as it passes through the corneal conjunctiva or the nasal bridge may be necessary. Cho H, Lee Y, Shin S, et al. Sunconjunctival Loa loa with Calabar swelling. J Korean Med Sci. 2008;23(4):731-3.
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| Approved | Approved::Yes |
| Keyword | WBRKeyword::Loa Loa, WBRKeyword::Infectious disease, WBRKeyword::Deerfly, WBRKeyword::Deerflies, WBRKeyword::eye worm, WBRKeyword::Worm, WBRKeyword::Zoonotic, WBRKeyword::Microbiology, WBRKeyword::Vector |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |