Human-to-Human Parasite Transmission: Routes & Prevention

Many parasites transmit directly or indirectly between humans. Key examples:

Protozoa

• Giardia lamblia: Cysts in feces → fecal-oral (water, food, hands, fomites). Daycare outbreaks common.
• Cryptosporidium: Oocysts → water (chlorine-resistant), food, person-to-person. Immunocompromised = severe.
• Entamoeba histolytica: Cysts → fecal-oral. Amebic dysentery, liver abscess.
• Trichomonas vaginalis: Sexual transmission. Urethritis, vaginitis.

Helminths (Worms)

• Pinworm (Enterobius): Eggs perianal → fingers → mouth (auto-infection) or fomites → others. Intense perianal itch. Household spread.
• Strongyloides: Skin penetration (auto-infection cycle). Hyperinfection in immunocompromised.
• Ascaris/Trichuris/Hookworm: Soil-transmitted (not direct human-human typically)

Ectoparasites

• Scabies (Sarcoptes): Mite burrows → skin-to-skin contact (prolonged), bedding/clothing. Intense nocturnal itch, burrows.
• Head/body/pubic lice: Direct contact, shared items (hats, brushes, bedding). Pruritus, nits.

Prevention

• Hand hygiene (soap, water, friction) — single most effective
• Safe water, food handling
• Treat index case + contacts simultaneously (pinworms, scabies, lice)
• Environmental decontamination (hot wash bedding, vacuum)
• Avoid sharing personal items
• Safe sex (Trichomonas)

Diagnosis

O&P exam (ova & parasites), Scotch tape test (pinworms), antigen PCR (Giardia, Crypto), skin scraping (scabies).