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Fly strike

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Synonym(s): Myiasis, Blow strike, Maggot infestation, Cuterebriasis

Introduction

  • Cause: fly larvae causing tissue damage. In the UK it is commonly caused by Lucilia spp (greenbottle fly) and in the USA Wohlfahrtia vigil, the flesh fly. Cuterebra spp can also cause myiasis. In reptiles, maggots of botflies, Phormia spp, Lucilia spp, Calliphora spp and Cistudinomyia cistudinis can cause myiasis.
  • Signs: depression, collapse, presence of fly larvae, open wounds, death.
  • Diagnosis: observation of fly larvae/maggots.
  • Treatment: fluid therapy, analgesia, supportive care, remove maggots, flush wounds, systemic antibiosis, address underlying causes.
  • Prognosis: depends on the severity; good if treated very early on.

Presenting signs

  • Depression.
  • Collapse.
  • Presence of fly larvae.
  • Open wounds/tissue damage.
  • Malodorous smell.
  • Anorexia.
  • Dragging of bottom.
  • Unexplained bleeding.
  • Diarrhea.

Acute presentation

  • Collapse.
  • Shock.
  • Hemorrhages under skin and red-purple mucous membranes (reptiles with septicemia).
  • Death.

Geographic incidence

  • UK during warmer humid months.
  • USA during warmer humid months.
  • Worldwide.

Age predisposition

  • Geriatric animals with mobility issues may be more predisposed to fly strike if they are unable to groom on a regular basis and have reduced mobility.

Breed/Species predisposition

  • Chelonians are more commonly reported in the literature with myiasis, compared with other reptiles. This is likely due to traumatic shell injuries with open wounds present (hit by car, strimmer/garden equipment, dog attacks).
  • Injured wild lizards and turtles, especially box turtles (USA) Box turtle are frequently seen with open wounds and maggot infestation. Injured released terrapins (UK) also may be susceptible.

Cost considerations

  • Cost of initial consultation which is often an emergency/out of hours fee.
  • Cost of initial stabilization which often requires intense medical and/or surgical treatment of wounds.
  • Risk of anesthesia/stress/sedation for initial debridement depending on severity; this may not always be doable conscious.
  • Ongoing cost of hospitalization/medication for secondary problems.
  • Cost of investigations of cause; fly strike will often reoccur if the cause of the problem has not been addressed.

Special risks

  • Animal may be in shock.
Will need stabilizing before sedation/general anesthesia to debride and clean wounds.

Pathogenesis

Etiology

  • Fly larvae causing tissue damage. In the UK it is commonly caused by Lucilia spp (greenbottle fly) and in the USA Wohlfahrtia vigil, the flesh fly. Cuterebra spp can also cause myiasis.
  • Pharyngeal myiasis with flesh fly larvae has been reported in two lizards (common ameivas) (rare).
  • Maggots of botflies, Phormia spp, Lucilia spp, Calliphora spp and Cistudinomyia cistudinis can cause myiasis.

Predisposing factors

General

  • Warm, humid weather.
  • Animals kept outdoors in suboptimal conditions and health.
  • Unobserved/untreated diarrhea or excessive fecal soiling.
  • Wounds/broken skin Traumatic injuries.
  • Prolapse of tissues through vent aperture.
  • Hatching reptiles with exposed unabsorbed yolk sac.

Specific

  • Environmental conditions of at least 60% humidity and 9-11°C/48.2-51.8°F for fly larva development.

Pathophysiology

Myiasis by Lucilia spp

  • Flies lay eggs in open wounds or soiled skin/fur.
  • Eggs → L1 larvae. L1 maggots do not cause tissue damage.
  • L1 larvae → L2 → L3.
  • Larvae bury deeper into tissues.
  • Extensive tissue damage, secondary infections, and potentially fatal toxin release.

Myiasis by Cuterebra flies

  • Flies lay eggs on skin.
  • Developing larvae migrate through subcutaneous tissues or enter the subcutis through body openings.
  • Subdermal cysts seen with breathing hole caused by botfly.
  • Larvae 1-3 cm in length can be visualized within the lesion.

Timecourse

Myiasis by Lucilia spp

  • Eggs → L1 larvae (within 12 h). These are not harmful.
  • L1 larvae → L2 → L3 (within 3 days).

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed Papers

  • Recent references from PubMed and VetMedResource.
  • Mitchell M A & Diaz-Figueroa O (2004) Wound management in reptiles. Vet Clin Exot Anim (1), 123-140 PubMed.
  • Harkewicz K A (2001) Dermatology of reptiles: A clinical approach to diagnosis and treatment. Vet Clin Exot Anim (2), 441-461 PubMed.

Other sources of information

  • Carpenter J W, Klaphake E, Gibbons P M & Sladky K K (2019) Reptile Formulary. In: Mader’s Reptile and Amphibian Medicine and Surgery. 3rd edn. Eds: Divers S J & Stahl S J. Elsevier, USA. pp 1191-1211.
  • Eatwell K & Hedley J (2019) Parasitology. In: BSAVA Manual of Reptiles. 3rd edn. Eds: Girling S J & Raiti P. BSAVA, UK. pp 411-422.
  • Fraser M A & Girling S J (2019) Dermatology. In: BSAVA Manual of Reptiles. 3rd edn. Eds: Girling S J & Raiti P. BSAVA, UK. pp 257-272.
  • Kischinovsky M, Divers S J, Wendland L D & Brown M B (2019) Otorhinolaryngology. In: Mader’s Reptile and Amphibian Medicine and Surgery. 3rd edn. Eds: Divers S J & Stahl S J. Elsevier, USA. pp 736-751.
  • Wright K & Raiti P (2019) Breeding and Neonatal Care. In: BSAVA Manual of Reptiles. 3rd edn. Eds: Girling S J & Raiti P. BSAVA, UK. pp 70-88.