1. General Considerations

   1. Myiasis is the infestation of organs and tissues by larvae (maggots) of dipterous flies (order Diptera) that feed on necrotic or living tissue of the host.
   2. Calliphorine myiasis (blowfly strike) is caused by larvae of facultative flies of the genera listed below.
   3. Screwworm myiasis is caused by larvae of the obligatory flies Cochliomyia hominivorax and Cochliomyia macellaria (they will not breed in carcasses).
   4. Myiasis is most frequent in late spring, early summer and early fall, times that correlate with the increased proliferation of flies.
   5. Blowfly myiasis is especially a problem in sheep in most areas of the world.
   6. Predisposing factors for myiasis are accidental or surgical wounds, rainy weather, and virtually any dermatosis causing exudation and associated bacterial infection.
   7. In contrast to larvae of calliphorine myiasis, larvae of screwworm myiasis penetrate into living tissues. After penetration, they liquefy the tissue extending the lesion considerably.

2. Cause

   1. Flies of Calliphorine Myiasis:
      1. Lucilia spp., Calliphora spp., Phormia spp., and Chrysomyia spp.
   2. Flies of Screwworm Miyasis:
      1. Cochliomyia hominivorax and Cochliomyia macellaria.

3. Clinical Signs

   1. Calliphorine Myiasis:
      1. Calliphorine myiasis is an important disorder of all large animals, especially sheep.
      2. Merino sheep are predisposed because they have highly developed skin folds.
      3. Wound strike may occur on any wounded part of the body.
      4. Breech, crutch and tail strike follow soiling of these areas from diarrhea and urine scald.
      5. Pizzle strike follows soiling of the sheath of rams with urine.
      6. Poll strike follows fight wounds or intertrigo (fold dermatitis) due to deep head skin folds.
      7. Body strike affects the dorsum, especially the withers, and usually follows prolonged wet weather and resultant fleece rot.
      8. Affected animals usually stand with their heads down, do not feed properly, and are restless.
      9. Lesions consist of foul-smelling ulcers, often with scalloped margins and a “honey-combed” appearance that are filled with maggots.
      10. Lesions are usually pruritic and painful.
      11. Animals may die of toxemia and septicemia.
   2. Screwworm Myiasis:
      1. Screwworm myiasis is a serious disease of all large animals.
      2. It typically develops in sites of wounds from shearing, dehorning, castration, docking, fighting, wire cut, ear marking, and branding. Moreover, the navel and mouth of newborn animals, eye lesions in cattle, and tick-bite lesions can be sites targeted by the flies.
      3. Lesions are as described for calliphorine myiasis but they are more severe, painful and pruritic and have a fouler smell.
      4. Severe infections and death (toxemia, septicemia) are common.
      5. Humans can also be affected.

4. Diagnosis

   1. History and clinical signs.
   2. Visualization of the larvae on skin, hair and lesions.

5. Treatment

   1. The financial losses from myiasis can be astronomic and result from wool and hide damage, weight loss, decreased milk production, and death.
   2. Treatment of individual animals includes:
      1. Thorough wound cleaning and debridement.
      2. Application of a topical insecticide (e.g. malathion or coumaphos) in ointment form.
      3. Other symptomatic therapy (e.g. topical antibiotics) as needed.
   3. Routine spraying or dipping with organophosphates and pyrethroids is helpful for controlling adult flies and larvae.
   4. Doramectin 1% and ivermectin 1% have shown to be effective in the prevention and treatment of myiasis in cattle.
   5. A study comparing the efficacy of dicyclanil and cyromazine (insect growth regulators) for the prevention of blowfly strike in the U.K. revealed that both products were efficacious but dicyclanil consistently showed significantly longer periods of protection than cyromazine.
   6. Isoxazolines such as fluralaner, afoxolaner, sarolaner and lotilaner have shown to be effective in the treatment of myiasis in cattle, dogs and cats.
   7. Selected breeding has been used in attempts to produce sheep with flatter and plainer breeches.
   8. Clipping the wool around the tail and in the breech is very effective in reducing tail and breech strike.
   9. Clipping the wool from around the belly and sheath of rams is very effective in reducing the incidence of pizzle strike.
   10. Surgical procedures should be avoided during fly season when possible.
   11. In certain countries (e.g. Brazil), the release of sterilized screwworm flies is used in the control of myiasis, typically in conjunction with a parasiticidal.
   12. Screwworm myiasis is a reportable disease in the U.S.

References

Anziani OS, Flores SG, Moltedo H et al. Persistent activity of doramectin and ivermectin in the prevention of cutaneous myiasis in cattle experimentally infested with Cochliomyia hominivorax. Vet Parasitol. 2000; 243-247.

Correia,TR, Scott FB, Verocai GG, et al. Larvicidal efficacy of nitenpyram on the treatment of myiasis caused by Cochliomyia hominivorax (Diptera: Calliphoridae) in dogs. Vet. Parasitol 2010; 173(1): 169-172.

Cutolo AA, Perier N, Menz I, et al. Efficacy of afoxolaner (NexGard®) on the treatment of myiasis caused by the New World screwworm fly Cochliomyia hominivorax (Diptera: Calliphoridae) in naturally infested dogs. Vet Parasitol Reg Stud Reports 2021; doi: 10.1016/j.vprsr.2021.100569.

da Costa AJ, de Souza Martins JR, Borges FA et al. First report of the efficacy of a fluralaner‑based pour‑on product (Exzolt® 5%) against ectoparasites infesting cattle in Brazil. Parasit Vector 2023; 16: 336.

Lonsdale B, Schmid HR, Junquera P.  Prevention of blowfly strike on lambs with the insect growth regulator dicyclanil.  Vet Rec 2000;  147(19): 540-544.

McGraw TA ,Turiansky GW. Cutaneous myiasis. J Am Acad Dermatol 2008; 58(6): 907-926.

Scott DW. Parasitic Diseases.  Large Animal Dermatology. 1st edn. Philadelphia, PA: W.B. Saunders, 1988;  248-251.

Scott DW. Color atlas of farm animal dermatology. Ames, IA: Blackwell Publishing, 2007; 30.

Scott DW, Miller WH Jr. Equine dermatology. 2nd edn.  Philadelphia: W.B. Saunders, 2011; 238-239.