A 34-year-old man comes to the physician because of progressive swelling of the left lower leg for 4 months. One year ago, he had an episode of intermittent fever and tender lymphadenopathy that occurred shortly after he returned from a trip to India and resolved spontaneously. Physical examination shows 4+ nonpitting edema of the left lower leg. His leukocyte count is 8,000/mm3 with 25% eosinophils. A blood smear obtained at night confirms the diagnosis. Treatment with diethylcarbamazine is initiated. Which of the following is the most likely route of transmission of the causal pathogen?

Deposition of thread-like larvae into the skin by a female mosquito

Penetration of the skin by hookworms in feces

Penetration of the skin by cercariae from contaminated fresh water

Deposition of larvae into the skin by a female black fly

Ingestion of encysted larvae in undercooked pork

A 42-year-old man presents to a free dermatology clinic, complaining of itchy skin over the past several days. He has no insurance and lives in a homeless shelter. The patient has no significant medical history. Physical evaluation reveals 2 mm erythematous papules and vesicles on his back and groin, with linear excoriation marks. Careful observation of his hands reveals serpiginous, grayish, threadlike elevations in the superficial epidermis, ranging from 3–9 mm in length in the webbing between several digits. What should be the suggested treatment in this case?

No medication should be administered, only proper hygiene.

Ectoparasites and vector-borne diseases for USMLE Step 3: High-Yield Microbiology Lessons

Arthropod Vectors - The Unwanted Guests

Mosquitoes: Transmit key viral & parasitic diseases.
- Anopheles: Malaria
- Aedes: Dengue, Chikungunya, Zika, Yellow Fever
- Culex: West Nile Virus, St. Louis Encephalitis
Ticks: Arachnids carrying bacterial & protozoal pathogens.
- Ixodes scapularis: Lyme disease, Anaplasmosis, Babesiosis. 📌 Remember L.A.B. (Lyme, Anaplasmosis, Babesiosis).
- Dermacentor: Rocky Mountain Spotted Fever (RMSF).
Other Vectors:
- Louse: Epidemic typhus (R. prowazekii)
- Flea: Plague (Y. pestis)
- Tsetse fly: African Trypanosomiasis
- Sandfly: Leishmaniasis
- Reduviid bug: Chagas disease (T. cruzi)

⭐ The Ixodes tick can co-transmit Lyme disease, Anaplasmosis, and Babesiosis. Suspect co-infection with atypical presentations or severe illness.

Tick-Borne Diseases - A Walk in the Woods

Ixodes scapularis tick life cycle and hosts

Vector: Primarily Ixodes scapularis (deer tick) in Northeast US; Dermacentor for RMSF; Amblyomma for Ehrlichiosis.

Disease	Pathogen	Key Features	Treatment
Lyme Disease	Borrelia burgdorferi	Erythema migrans (bull's-eye rash), AV block, Bell's palsy	Doxycycline
Babesiosis	Babesia microti	Hemolytic anemia, jaundice, fever. "Maltese cross" tetrads.	Atovaquone + Azithromycin
Anaplasmosis	Anaplasma	Flu-like, thrombocytopenia, leukopenia. Morulae in granulocytes.	Doxycycline
Ehrlichiosis	Ehrlichia	Flu-like, thrombocytopenia, leukopenia. Morulae in monocytes.	Doxycycline
RMSF	Rickettsia rickettsii	Rash on palms/soles, spreading centrally.	Doxycycline

📌 Lyme mnemonic: "B.A.K.E. a Key Lyme Pie" - Bell's palsy, Arthritis, Kardiac block, Erythema migrans.

Mites & Lice - The Itchy & Scratchy Show

Scabies (Sarcoptes scabiei)
- Intense pruritus (worse at night) from Type IV hypersensitivity. Burrows in interdigital spaces, wrists, axillae.
- Transmission: Prolonged skin-to-skin contact.
- Rx: Permethrin 5% cream (entire body, neck down), oral ivermectin. Treat all household members.
- 📌 Itching so bad you want to 'Scab-ies' your eyes out!
Lice (Pediculosis)
- P. humanus capitis (head), P. humanus corporis (body), Pthirus pubis (pubic).
- Nits (eggs) attached to hair shafts; pruritus from saliva.
- Rx: Topical permethrin, pyrethrins, malathion.

⭐ Body lice (Pediculus humanus corporis) are vectors for diseases like epidemic typhus (Rickettsia prowazekii) and trench fever (Bartonella quintana).

Scabies mite: skin burrow, eggs, excrement, and adult mite

Other Vectors - Fleas, Flies, & Foes

Flea (Xenopsylla cheopis)
- Yersinia pestis (Plague): Gram-negative rod causing buboes (painful lymph nodes), fever, and sepsis. 📌 Classic "safety-pin" bipolar staining.
- Rx: Streptomycin or gentamicin.
Sandfly
- Leishmania spp.: Intracellular protozoa. Presents as cutaneous (ulcers) or visceral disease (kala-azar: fever, hepatosplenomegaly, pancytopenia).
- Dx: Amastigotes in macrophages.
Other Flies
- Tsetse Fly → Trypanosoma brucei (African sleeping sickness)
- Deer Fly → Loa loa (Loiasis)

⭐ Plague: Yersinia pestis can be transmitted via flea bites, aerosols (pneumonic), or direct contact. Its high mortality and potential for aerosol spread make it a major bioterrorism agent.

Yersinia pestis with bipolar staining

High‑Yield Points - ⚡ Biggest Takeaways

Scabies (Sarcoptes scabiei) causes intensely pruritic burrows, especially in finger webspaces; treat with permethrin.

Ixodes ticks are the vector for Lyme disease (Borrelia), Babesiosis, and Anaplasmosis.

Dermacentor ticks transmit Rocky Mountain Spotted Fever (Rickettsia rickettsii).

Chagas disease (Trypanosoma cruzi), from the Reduviid bug, classically causes cardiomyopathy and megacolon.

Malaria (Plasmodium) is transmitted by Anopheles mosquitoes; Leishmaniasis is from sandflies.

Aedes mosquitoes transmit key arboviruses like Dengue, Chikungunya, and Zika.

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