Parasitic Infections — Pinworm, Giardia, Toxoplasmosis

A pregnant woman asks whether to rehome the cat. The answer is no — but cleaning the litter box has to stop, and here's exactly why.

Pinworm (Enterobius vermicularis): the most common helminth infection in the United States, predominantly affecting school-age children. Transmission: fecal-oral via ingested eggs, with autoinoculation (child scratches perianal area, transfers eggs to mouth). Eggs are viable on surfaces for 2–3 weeks. Presentation: intense perianal pruritus worse at night (female worm migrates to the perianal area to deposit eggs during sleep). Diagnosis: the scotch tape (cellophane tape) test — press clear adhesive tape against the perianal skin first thing in the morning (before bathing or toileting) to collect eggs, then examine under microscopy. Do NOT rely on stool ova and parasite examination (O&P) — pinworm eggs are rarely found in stool. Treatment: mebendazole or albendazole as a single dose; repeat in 2 weeks (to kill newly hatched worms from surviving eggs). Treat the ENTIRE household simultaneously. Environmental measures: wash all bedding and underwear in hot water, keep fingernails short, emphasize hand hygiene before meals and after toileting. Giardia (Giardia lamblia/duodenalis): the most common parasitic cause of diarrhea in the US. Protozoan transmitted fecal-orally via contaminated water (streams, lakes — 'backpacker's diarrhea'), daycare settings, and food. Highly resistant cysts survive standard chlorination. Presentation: foul-smelling, greasy, floating stools (steatorrhea from fat malabsorption), bloating, flatulence, abdominal cramping, and nausea. No blood or mucus in stool. Diagnosis: stool O&P (may require 3 specimens collected on different days, as cyst shedding is intermittent) or stool antigen testing (more sensitive). Treatment: metronidazole or tinidazole — CRITICAL patient teaching: avoid ALL alcohol during treatment and for 48–72 hours after completion (disulfiram-like reaction causing severe nausea, vomiting, flushing, and tachycardia). Toxoplasmosis (Toxoplasma gondii): intracellular protozoan parasite. Cats are the definitive host; humans acquire infection from cat feces (especially while cleaning litter boxes), undercooked or raw meat (particularly pork and lamb), and contaminated soil or unwashed produce. Most immunocompetent adults are asymptomatic or have mild mononucleosis-like illness. DANGER populations: pregnancy and immunocompromised (especially HIV/AIDS with CD4 count <100). Congenital toxoplasmosis: classic triad of hydrocephalus, intracranial calcifications (diffuse, unlike CMV which has periventricular calcifications), and chorioretinitis. Can also cause seizures, developmental delay, and stillbirth. In HIV/AIDS: ring-enhancing brain lesions on MRI — the most common CNS mass lesion in AIDS. Treatment: pyrimethamine plus sulfadiazine plus leucovorin (folinic acid to prevent bone marrow suppression from pyrimethamine). Prophylaxis with TMP-SMX for HIV patients with CD4 <100. Prevention is the primary nursing focus for pregnancy: avoid changing cat litter (if unavoidable, wear gloves and wash hands), cook meat to safe internal temperatures (no rare or medium-rare), wash all fruits and vegetables thoroughly, wear gloves when gardening, and wash hands after handling raw meat or soil.

Key Distinctions

Pregnant women do NOT need to rehome their cats — they need to avoid the LITTER BOX. This is the most commonly tested teaching point. The scotch tape test for pinworm is performed in the MORNING before bathing — not after bathing and not via stool sample. Metronidazole-alcohol interaction (disulfiram-like reaction) applies to giardia treatment — this is a high-yield pharmacology point shared with other metronidazole indications. Toxoplasmosis intracranial calcifications are DIFFUSE; CMV calcifications are PERIVENTRICULAR — this is a classic radiology distinction tested on NCLEX.

Clinical Pearl

Cat stays, litter box duty goes. That's the whole toxoplasmosis-in-pregnancy teaching in one sentence.