THE MERCK VETERINARY MANUAL
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Facultative Myiasis-Producing Flies

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Facultative myiasis-producing flies of veterinary importance covered elsewhere in the Manual include Gasterophilus spp in horses (see Gasterophilusspp in Horses), Oestrus ovis in sheep (see Sheep Nose Bot), and Cuterebra spp in dogs and cats (see CuterebraInfestation in Small Animals). The following larval dipterans are often referred to as facultative myiasis-producing flies: Musca domestica (the house flies); Calliphora, Phaenicia, Lucilia, and Phormia spp (the blow flies or bottle flies); and Sarcophaga spp (the flesh flies). Their adult stages are synanthropic flies, ie, they are often associated with human dwellings and readily fly from feces to food. Larval stages are usually associated with skin wounds of any domestic animal that have become contaminated with bacteria or with a matted hair coat contaminated with feces. In the larval stages, the characteristics of the distinctive posterior spiracular plates and the cephalopharyngeal skeleton are unique for each species and are used for identification.

The life cycle of M domestica is a representative example of that of the filth-breeding flies (see Filth-Breeding Flies). Several species of blow flies cause myiasis in sheep. Primary flies in the USA and Canada are Phormia regina and Protophormia terraenovae (the black blow flies) and Lucilia sericata (the green bottle fly). L illustris, Cochliomyia macellaria (secondary screwworm), and some others are usually secondary invaders. L cuprina is the most important primary fly in Australia and South Africa; L sericata in Great Britain; and L cuprina, L sericata, and Calliphora stygia in New Zealand.

In normal conditions, adult flies of these genera lay their eggs in feces or in decaying animal carcasses. In facultative myiasis, the adult flies are attracted to a moist wound, skin lesion, or soiled hair coat. A common site is the breech, where flies may be attracted to wool soaked with urine or feces. As adult female flies feed in these sites, they lay eggs, which hatch within 24 hr if conditions are moist. Larvae (maggots) move independently about the wound surface, ingesting dead cells, exudate, secretions, and debris, but not live tissue. This condition is known as fly strike or strike. The larvae irritate, injure, and kill successive layers of skin and produce exudates. Maggots can tunnel through the thinned epidermis into the subcutis. This process produces tissue cavities in the skin that measure up to several centimeters in diameter. Once established, strikes can spread rapidly and attract more blow flies, secondary as well as primary. Mild strikes can cause rapid loss of condition, and bad strikes can be fatal. Unless the process is halted by appropriate therapy, the infested animal may die from shock, intoxication, histolysis, or infection. A peculiar, distinct, pungent odor permeates the infested tissue and the affected animal. Advanced lesions may contain thousands of maggots.

The body of the sheep also may be struck. This is usually associated with soaking rains that cause the development of fleece rot, often characterized by discoloration due to Pseudomonas spp or dermatophilosis. Other sites are the horns of rams, wool around the prepuce, sides where feet with footrot come in contact with fleece, and wounds.

As adults, these flies can be pestiferous in veterinary clinics, farms, or poultry operations. The flies are vomit drop feeders and fly from feces to food, spreading bacteria on their feet and from their disgorged stomach contents.

These fly larvae have also been associated with toxic effects in chickens. Botulism (see Botulism), also known as limberneck in chickens, has been associated with ingestion of large numbers of larvae of Lucilia caesar, Phaenicia sericata, and other species of flies. Clostridium botulinum multiplies in carrion, where it may be picked up by fly larvae breeding in that medium and then passed on to chickens that eat the maggots. Dead animals should be quickly and safely disposed of, preferably by incineration.

Strikes should be diagnosed early; behavior of sheep is a good indicator of myiasis. Affected animals become depressed, stand with their heads down, do not feed, and attempt to bite the infested areas. Screwworm may be suspected if the larvae are associated with wounds.

The species of myiasis-producing flies can be definitively identified by closely examining the larvae. The extreme caudal ends of several third-stage larvae infesting the wound should be sliced using a scalpel blade held perpendicularly to the larval body. When the sliced caudal ends are placed cut surface down on a glass slide, covered with a coverglass, and examined under a compound microscope, a dichotomous key can be used to identify the genus or genera of flies within the wound. The unique spiracular plates are distinct for a particular genus, much like a human fingerprint. Several specimens should be examined because more than one genus may be present within the lesion. The first larvae to hatch in the lesion often create a favorable medium attractive to flies of other genera. Also, the possibility of obligatory myiasis caused by Cochliomyia hominivorax (see Cochliomyia hominivorax) or Chrysomyia bezziana (see Chrysomyia bezziana) should be considered, depending on geographic locale.

Blow fly infestation of the breech can be effectively controlled for ~6–8 wk by tagging or crutching (ie, wool is shorn between the legs and around the tail). Complete shearing controls outbreaks involving other parts of the body. Wool removed from around the head and the prepuce can prevent strike in these areas. Urine staining of the crutch of Merino ewes can be virtually eliminated by removal of breech wrinkles (Mules operation), and fecal contamination can be greatly reduced by docking tails at the third joint. Scouring should be controlled. Odors and associated moisture attract flies and stimulate oviposition, particularly during hot, humid weather.

Chemoprophylaxis consists of wetting to complete saturation of susceptible areas with suitable insecticidal and larvicidal preparations, such as the organophosphate insecticides or cyromazine, a specific larvicide in dips and sprays. Jetting is the most efficient procedure—insecticide is forced into the fleece, usually locally to the breech and along the back and head, under high pressure. Protection can last 6–8 wk, but where the primary fly is resistant (eg, L cuprina in Australia), it may last only 2–3 wk. Weekly application of agents such as ronnel (2.5%) under pressure to wounds until healed can be highly beneficial, particularly for screwworm infestation. Before suitable agents are applied, all wool should be removed from the struck area and around it.

Burning or deep burying of the carcass may be a valuable general hygienic measure but may have little effect on primary strikes. The main source of primary flies is the struck sheep. A genetic manipulation approach has been used to control a strain of blow fly in Australia; male flies are partially sterile but transmit a gene that causes blindness in female offspring.

Treatment and control measures for myiasis in dogs and cats are limited. If these larvae are detected in small animals, immediate therapy is necessary. The hair coat should be clipped to determine the extent of the lesion and to remove many of the larvae present in the hair. Removing maggots from existing deep tissue pockets may be difficult, and sedating or even anesthetizing the animal may be necessary. The lesion should be examined on successive days; adult flies lay eggs in the wound at different times, and hatching of larvae may not be synchronous.

Depressed, febrile, and prostrate animals should be treated according to their clinical signs. Ideally, culture and sensitivity studies should be performed on samples or scrapings of the wounds. If secondary bacterial or fungal infections are present, administration of broad-spectrum antibiotics is advisable.

With respect to prevention, owners should be educated about the effectiveness of treating all skin wounds. Animals with skin wounds should be confined to fly-free areas. The hair coat should be kept clean of urine or feces and should not be permitted to become matted. Contaminated wounds and matted hair coats soaked in urine or feces rapidly attract adult myiasis-producing flies. The control of adult flies in the field and the destruction of their breeding places are excellent preventive measures. All areas should be free of opened garbage cans and decaying carcasses or carrion.

Last full review/revision August 2013 by Charles M. Hendrix, DVM, PhD

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