Equis ISSN 2398-2977


Synonym(s): New World screw-worm


  • Infestation with screw-worm fly (SWF) can result in serious disability and economic loss.
    The SWF is an obligate parasite of warm-blooded animals. Domestic pets, livestock and humans are susceptible to strike
  • Cause: infection of pre-existing wound by the New World screw-worm fly,Cochliomyia hominivorax.
  • Signs: intense discomfort, malodorous wound, bleeding or oozing of serosanguinous fluid; repeated strike   →   scarring; weight loss.
  • Diagnosis: identification of larvae and fly associated with wound.
  • Treatment: wound management, systemic parasiticides.
  • Prognosis: depends on site and duration of infection as well as effective ongoing parasite control.
    There are two known species of the screw-worm fly. They have different geographical ranges but similar biology; the New World SWF occurs in Central and South America, while the Old World SWF,Chrysomya bezziana, occurs in tropical and subtropical Africa, the Middle East, India, South-East Asia and Papua New Guinea


Predisposing factors

  • Geographical location - tropical, subtropical climate.
  • Moist mucous membranes, eg preputial opening, vulva, conjunctiva.
  • Extensive farm management system   →   minimal supervision or inspections of animals.


  • Broken skin or mucous membrane near body orifice:
    • Routine husbandry procedures, eg castration.
    • Umbilicus.
    • Tick or other insect bite.


  • Gravid female fly seeks wound   →   up to 250 eggs laid on edges of wound   →   larvae hatch within 12 - 20 hours   →   penetrate deep into wound   →   cause tissue destruction and feed on blood   →   after 6-7 days development larvae fall to ground and pupate in soil.
  • Presence of larvae attracts other gravid females.
  • Minimum life cycle takes 20 days, but can extend to several months in cold weather.
    The biology and pathogenicity of Old World SWFChrysomya bezzianais very similar


  • Tissue destruction begins with the migration of newly hatched larvae into deeper tissues and continues as long as larae are present.


  • Obligate parasite of warm-blooded animals, eg horse, cattle, sheep, goats, pigs, dogs, cats and humans.
  • Adults:
    • Usually low numbers.
    • Prefer moist well-shaded areas, eg woods.
    • Can survive in intense heat and low humidity provided vegetation is available for cover.
    • Optimal temperature 20-30°C.
    • Will not move at <10°C.
    • Can persist in environment in absence of livestock if wild animals present.
    • Female range 25-50 km.
    • Lifespan about 21 days.
    • Females mate only once.
    • Lay up to 250 eggs.
  • Larvae:
    • Hatch within 12-20 hours of being laid at edge of wound.
    • Burrow into tissues.
    • Feed on blood.
    • Complete development 6-7 days.
    • Drop off host during night.
    • Penetrate soil to 2 cm.
  • Pupae:
    • Require soil that is moist but not wet.
    • Minimum pupation about 1 week, but can take 60 days.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Moya-Borja G E, Muniz R A, Umehara O, Goncalves L C, Silva D S and McKenzie M E (1997)Protective efficacy of doramectin and ivermectin against Cochliomyia hominivorax.Vet Parasitol72(1), 101-9 PubMed.
  • Lindquist D A, Abusowa M and Hall M J (1992)The New World screw-worm fly in Libya- a review of its introduction and eradication.Med Vet Entomol6(1), 2-8 PubMed.
  • Reichard R E, Vargas-Teran M and Abu Sowa M (1992)Myiasis - the battle continues against screwworm infestation.World Health Forum13(2-3), 130-8 PubMed.

Other sources of information

  • Geering W A, Forman A J and Nunn M J (1995)Screw-worm fly.In:Exotic diseases of animals.Australian Government Publishing Service, Canberra. ISBN 0-644-33513-0.
  • Knottenbelt K D and Pascoe R R (1994)The integumentary system.In:diseases and disorders of the horse.Wolfe Publishings, Mosby-Year Book Europe. ISBN 0-7234-1702-4.