Dimorphic fungi: "Mold in the Cold, Yeast in the Heat (Beast)" — change form with temperature
Temperature-Dependent Dimorphism
The fungi that switch between mold and yeast forms — and where they live
At room temperature (25°C): mold (hyphal) form — found in soil/environment. At body temperature (37°C): yeast form — causes disease. Key dimorphic pathogens: Histoplasma capsulatum (Mississippi/Ohio River valleys — bat/bird droppings), Blastomyces (Great Lakes), Coccidioides (Southwest US — spherules in tissue), Sporothrix schenckii (rose thorns — sporotrichosis). Paracoccidioides (South America). Treat with itraconazole (mild) or amphotericin B (severe).
Cryptococcus: "Soap bubbles in brain." India ink stain shows capsule. Meningitis in AIDS patients (CD4 <100).
Cryptococcus neoformans
The encapsulated yeast that causes the most common AIDS-related meningitis
Cryptococcus neoformans: found in pigeon droppings and soil. Large polysaccharide capsule — antiphagocytic virulence factor. India ink stain: clear halo around yeast. Latex agglutination test: detects capsular antigen in CSF. Presentation: subacute meningitis (headache, fever) in HIV+ (CD4 <100). CSF: India ink +, very high opening pressure. Treat: amphotericin B + flucytosine induction, then fluconazole maintenance.
Tinea = dermatophyte (ringworm). Named by body location: capitis (scalp), corporis (body), pedis (athlete's foot), cruris (jock itch), unguium (nails).
Dermatophyte Infections
Superficial fungal infections — same organisms, different location names
Three genera: Trichophyton, Microsporum, Epidermophyton. Infect keratinized tissue only (skin, hair, nails). KOH prep: hyphae visible. Tinea pedis (athlete's foot): most common — interdigital. Tinea unguium (onychomycosis): yellow, thickened nails. Tinea capitis in children: treat systemically (griseofulvin or terbinafine). Topical clotrimazole/miconazole for skin. Nail: oral terbinafine.
Toxoplasma
Toxoplasma: "cats + pregnant women + AIDS." Brain ring-enhancing lesions. Reactivation when CD4 <100.
Toxoplasma gondii
The parasite that lives harmlessly in healthy adults but devastates the immunocompromised
Definitive host: cats (oocysts in feces). Humans: ingestion of oocysts or tissue cysts (undercooked meat). Healthy adults: asymptomatic or mono-like illness. Danger zones: (1) Primary infection in pregnancy → congenital toxoplasmosis (chorioretinitis, hydrocephalus, intracranial calcifications). (2) Reactivation in AIDS (CD4 <100) → encephalitis, ring-enhancing lesions. Treat: pyrimethamine + sulfadiazine. TMP-SMX prevents reactivation.
Giardia
Giardia: "backpacker's diarrhea" — contaminated water. Trophozoite has two nuclei = "owl eyes." Flagella-propelled.
Giardia lamblia
The most common intestinal parasitic infection in the US
Fecal-oral transmission via cysts in contaminated water (streams, lakes — resistant to chlorine). Cysts → small intestine → trophozoites attach to mucosa (ventral disc) → malabsorption. Symptoms: foul-smelling fatty diarrhea (steatorrhea), bloating, no blood/mucus. Diagnosis: stool O&P exam (teardrop trophozoites with 2 nuclei), antigen test. Treatment: metronidazole or tinidazole. Prevention: filter/boil backcountry water.
Schistosomiasis
Schistosoma: only trematode with separate sexes. Penetrates skin in freshwater. Eggs cause granulomas.
Schistosoma species
The blood fluke that causes chronic liver and bladder disease in the tropics
Cercariae released from snails → penetrate human skin in freshwater. Adult worms live in blood vessels (not gut). Eggs deposited → granuloma formation → pathology. S. mansoni/japonicum: portal hypertension, hepatosplenomegaly, esophageal varices. S. haematobium: bladder (hematuria, squamous cell carcinoma risk). Acute: Katayama fever (immune complex). Treatment: praziquantel (all species). No snails in US — travel history key.