Myiasis (Fly Strike) in Animals

Myiasis (Fly Strike) in Animals: Classification, Pathogenesis, Clinical Signs, Treatment & Prevention

Myiasis (fly strike) is a parasitic infestation caused by the larvae (maggots) of certain dipteran flies that invade the tissues of living humans and animals. It is an important condition in veterinary parasitology because it can lead to severe tissue damage, reduced productivity, secondary bacterial infections, and even death if left untreated. This article provides a comprehensive overview of myiasis, including its classification, epizootiology, pathogenesis, clinical signs, treatment, and preventive measures in animals.

The term myiasis was introduced by Hope (1840). Later, Zumpt (1965) defined myiasis as the infestation of live humans and vertebrate animals by dipteran larvae that, for at least part of their life cycle, feed on the host’s dead or living tissues, liquid body substances, or ingested food.

Classification of Myiasis

1. Based on the egg-laying habit of flies
2. Based on the type of parasitism
3. Based on the area in which the strike occurs
4. Based on clinical manifestation
5. Based on etiology

1. Based on the Egg-Laying Habit of Flies

• Primary Myiasis Producer: It initiates the process of strike (the act of egg deposition) by laying eggs on live animals. The larvae develop in wounds that are autolytic and undergoing early bacterial decomposition. E.g., Lucilia and Calliphora.
• Secondary Myiasis Producer: These do not initiate the process of strike but lay eggs on existing wounds produced by primary flies. Increased putrefaction attracts a greater number of secondary myiasis producers. E.g., Chrysomya and Sarcophaga.
• Tertiary Myiasis Producer: These flies arrive during the healing phase of the wound, further damaging it, and may also attack dead carcasses. E.g., Musca and Fannia.

2. Based on the Type of Parasitism

• Obligatory/Specific Myiasis Producer: The larvae require a living host for the completion of their development. They live in or on the host. E.g., Chrysomya bezziana.
  • Endoparasitic Type: Larvae of Hypoderma, Oestrus, and Gasterophilus spp.
  • Ectoparasitic Type: Larvae of the Congo floor maggot (Auchmeromyia spp.)
• Facultative/Semispecific Myiasis Producer: The larvae can live on living tissue or as free-living organisms on dead organic matter or decaying wounds. E.g., Lucilia and Sarcophaga.
• Accidental Myiasis Producer: The larvae are accidentally ingested by animals along with food. They survive in the alimentary canal. E.g., Musca.

3. Based on the Area in Which the Strike Occurs

Breech (crutch) strike, tail strike, pizzle strike, poll strike, body strike, wound strike, and belly strike.

4. Based on Clinical Manifestation

• Nasopharyngeal/Ophthalmomyiasis: Larvae of Oestrus ovis.
• Gastric/Intestinal Myiasis: Larvae of Gasterophilus equi.
• Urogenital Myiasis: Larvae of Fannia spp.
• Cutaneous (Dermal or Subdermal) Myiasis: Larvae of Hypoderma spp.
• Furuncular Myiasis: Larvae of Hypoderma spp. and Cordylobia spp.
• Sanguinivorous Myiasis: Larvae of the Congo floor maggot (Auchmeromyia spp.).

5. Based on Etiology

• Calliphorine (Blow Fly) Myiasis: Caused by the larvae of Lucilia, Calliphora, and Chrysomya spp.
• Screw Worm Myiasis: Caused by the larvae of Callitroga and Chrysomya spp.

Epizootiology

Factors influencing the occurrence of calliphorine myiasis in sheep are classified into three groups.

• Factors Controlling the Prevalence of Flies: It is seasonal. Flies are abundant during late spring (between winter and summer) and late summer. Their prevalence depends on temperature, humidity, and the abundance and suitability of food.
• Factors Controlling the Susceptibility of Sheep: Various types of strike occur depending on the part of the body affected, namely breech, crutch, tail, pizzle, poll, body, and wound strike.
• Factors That Make Sheep Attractive to Flies:
  • Two factors are important:
  • Sheep factor (“S” factor): Attracts flies.
  • Putrefaction factor: Stimulates oviposition.

Pathogenesis

The putrefactive odor from moist wool soiled by urine and feces attracts flies for oviposition. The soiled wool causes skin irritation, and bacteria further aggravate the condition, resulting in wounds. These wounds provide a suitable substrate for larval development.

The larvae of primary blow flies initiate the strike and create favorable conditions for secondary flies. They secrete proteolytic enzymes that digest and liquefy the host tissues and then feed on the predigested material. Only after this process are secondary larvae able to develop in these lesions, and these secondary blow fly larvae are predatory on primary blow fly larvae.

Large wounds produced by secondary flies may form deep tunnels within the tissues and beneath the skin. The central portion of the lesion may heal with the formation of a thick scab, while the action of the larvae extends outward. The odor emanating from the lesion attracts additional flies to deposit eggs, allowing further batches of primary and secondary larvae to develop.

Maggot-infested wounds are strongly alkaline, making them highly attractive to blow flies.

The lesions are highly irritating, causing affected animals to reduce feed intake and become weak and emaciated. The immediate cause of death is probably toxemia due to the absorption of toxic substances from the lesions or, in some cases, septicemia.

Financial losses due to strike result from the decreased value of fleece and reduced meat and milk production.

Clinical Signs

• Affected sheep are aloof, i.e., they stand with their heads down, do not feed, and attempt to bite the affected area.
• When the lesion is located around the tail or on the buttocks, the animal stamps or jerks its hind legs and wags its tail.
• Examination reveals a patch of discolored, grayish-brown, moist wool with a foul odor.
• Maggots may be found in the wool attacking the skin, and they crawl into the surrounding area when disturbed.
• In the later stages, there is an inflamed, ragged wound from which a foul-smelling exudate drains into the wool.
• The larvae burrow into the tissues, with only their posterior extremities projecting.
• Body temperature may be elevated.
• Malnutrition, reduced milk production, and death may occur in severe cases.
• A condition known as “lightning strike“ may occur, in which the affected sheep dies within a few days. This is probably due to the introduction of bacteria, which subsequently cause toxemia and septicemia.

Treatment

• Clip the wool to expose the lesion.
• Remove the maggots manually using forceps.
• Clean the wound with potassium permanganate solution.
• Apply oil of turpentine to the lesion. The larvae embedded in the tissues may emerge, allowing them to be removed and destroyed.
• Insert camphor pellets, or apply a mixture of camphor in neem oil superficially.
• Hydrogen peroxide solution may be applied to remove the scab and necrotic tissue.
• An insecticidal dip solution may be used on maggot-infested wounds.
• Proflavine and Gammexane cream may also be applied to maggot-infested wounds.
• Drugs such as ivermectin, closantel, and doramectin may be administered.

Prophylaxis

Attempt to Render Sheep Less Attractive to Flies

• Selective Breeding: Eliminate sheep with a narrow breech and excessive skin folds from breeding programs.
• Surgical Removal of Breech Fold (Mule’s Operation): A crescent-shaped area of skin on either side of the urogenital region of the sheep is removed. The resulting scar tissue flattens the skin folds, thereby minimizing the accumulation of excreta.
  • Chemical Mulesing: A 40% phenol solution is applied along the prepuce of young wethers after shearing or crutching. This produces a dry encrustation that heals with scar formation, eliminating skin folds in the affected area of the prepuce. A similar treatment can also be applied to the breech area.
• Docking: Docking the tail of lambs behind the fourth coccygeal vertebra instead of the usual second coccygeal vertebra reduces the incidence of strike appreciably.
• Crutching: This consists of clipping the wool around the tail and breech. It is a very useful measure that promotes dryness in this area.
• Dagging: Removal of fecally soiled wool.
• Annual shearing of sheep.
• Treatment of Sheep with Insecticides: Insecticides may be applied as dips, sprays, jetting, or fogging. Examples include 0.6% BHC as a dip, 0.05% dieldrin + 0.016% HCH as a dip, and 0.5% aldrin as a dip.

Directed Against the Flies Themselves

• Spraying of insecticides.
• Proper disposal of carcasses.
• Destruction of larvae and pupae.
• Insect Growth Regulators (IGRs):
  • Cyromazine: Prevents house fly larval development in chicken droppings beneath caged laying hens. It should be incorporated into the feed as a 1% premix (500 g/ton of feed) for 4–6 weeks or until the fly population is under control. Cyromazine interferes with molting and pupation without directly affecting chitin synthesis. It passes through the chicken into the manure, where it prevents the emergence of house flies.
• Biological Control:
  • Spores of Bacillus thuringiensis – larvicide.
  • Spores of entomopathogenic fungi parasitize adult Oestrus ovis flies.
  • Mormoniella vitripennius parasitizes the pupae of blow flies.
• Sterile Insect Technique (SIT) or Sterile Male Technique (SMT): Male flies are sterilized by irradiation. This technique is based on the fact that these flies mate only once; therefore, mating with sterilized males prevents reproduction. This method has been successfully used for the eradication of Callitroga hominivorax in the United States.
• Genetic Control: A strain of blow fly has been developed in which females are blind and cannot survive in the field, while males are partially sterile but transmit the genes responsible for blindness to female offspring. This system is known as the Female Killing System or the Genetically Impaired Female Technique (GIFT).
• Baits, Traps, and Targets: A screw-worm adult suppression system (SWASS) has been developed for Callitroga hominivorax. It consists of an insecticide (2% dichlorvos) combined with a synthetic odor attractant known as Swormlure to attract and kill adult flies. Traps baited with synthetic chemicals or carrion have also been developed to attract and kill sheep blow flies. Colored targets treated with insecticides, IGRs, or chemosterilants have been used to attract, kill, or sterilize adult flies.
• Immunological Control (Blow Fly Vaccines): Vaccination of sheep with a partially purified extract of larvae of Lucilia cuprina results in a marked reduction in larval growth when the larvae feed on sheep.
• PM 44: It is a concealed antigen obtained from the peritrophic membrane of the larval gut of Lucilia cuprina. It has been investigated for vaccination against blow fly strike in sheep in Australia under experimental trials.