Question
Autoinfection
Answer
Autoinfection is a mechanism of parasitic infection in which a parasite’s own offspring (larvae/eggs) produced within or on the host are able to directly re-infect the same host, without needing to first pass through the external environment or an intermediate host — allowing the infection to persist and even amplify indefinitely in a single host, unlike most helminth infections, whose worm burden gradually declines without repeated external re-exposure.
Classical example — Strongyloides stercoralis: uniquely among human intestinal nematodes, S. stercoralis rhabditiform larvae can transform into infective filariform larvae within the host’s own intestine, which then directly re-penetrate the intestinal mucosa or perianal skin, re-enter the circulation, and repeat the migratory (lung-tracheal-intestinal) cycle internally — this autoinfective capacity allows Strongyloides infection to persist for decades in an untreated host, long after leaving an endemic area, and underlies the risk of life-threatening hyperinfection/disseminated strongyloidiasis if the host becomes immunocompromised (particularly with corticosteroid therapy or HTLV-1 co-infection), since autoinfection can then proceed unchecked, dramatically amplifying the worm burden.
Other examples of autoinfection in parasitology: Enterobius vermicularis (pinworm) — a milder form of autoinfection occurs via hand-to-mouth transfer of eggs from scratching the itchy perianal area, allowing direct re-ingestion and reinfection without requiring external soil maturation, contributing to the persistence/recurrence of infection within a household.
Clinical significance: recognizing chronic Strongyloides infection is important before initiating corticosteroid or other immunosuppressive therapy in a patient with relevant epidemiological exposure history, since this can trigger fatal hyperinfection syndrome — screening/empirical treatment (ivermectin) is often recommended in such at-risk patients before starting immunosuppression.