Question
Larva currens
Answer
Larva currens (“running larva”) is a distinctive, rapidly migrating cutaneous eruption caused by autoinfective larvae of Strongyloides stercoralis — a hallmark clinical sign of chronic strongyloidiasis, reflecting this parasite’s unique autoinfective life cycle, unlike any other human intestinal nematode.
Pathogenesis (basis of autoinfection): unlike most soil-transmitted helminths, S. stercoralis rhabditiform larvae can transform into infective filariform larvae within the host’s own intestine (rather than requiring an external soil maturation period), allowing these larvae to directly re-penetrate the intestinal mucosa or perianal skin, re-enter the circulation, and repeat the migratory cycle internally — this autoinfective capacity allows the infection to persist for decades in an untreated host, long after the person has left an endemic area, and underlies the risk of life-threatening hyperinfection/disseminated strongyloidiasis if the host later becomes immunocompromised (particularly with corticosteroid therapy or HTLV-1 co-infection).
Clinical appearance of larva currens: a pruritic, serpiginous (linear/snake-like), rapidly advancing urticarial track, typically arising around the perianal/buttock/thigh region (from larvae re-penetrating perianal skin during autoinfection) — distinguished from the cutaneous larva migrans caused by animal hookworm by its much more rapid rate of migration (several centimetres per hour, versus millimetres to a few centimetres per day for classic cutaneous larva migrans), giving rise to the name “larva currens” (running larva).
Diagnostic significance: this rapid migration rate is itself a useful clinical differentiating feature from cutaneous larva migrans (caused by Ancylostoma braziliense), and its presence should prompt specific testing for Strongyloides infection (stool examination — though often falsely negative due to low/intermittent larval output; serology; or, most sensitively, stool agar plate culture or the Baermann concentration technique).
Clinical significance for management: recognizing chronic Strongyloides infection (via larva currens or other clues) 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.