Myiasis is the term that describes the infestation of organs or tissues of humans or animals by fly maggots that, at least for a period of time, feed upon living, necrotic, or dead tissues or upon ingested food of the host. Many species of fly larvae that normally breed in decaying meat or carrion may infest traumatic skin wounds of cats and produce a condition known as facultative myiasis. Facultative myiasis resulting from infestation by flies of the Musca, Calliphora, Phaenicia, Lucilia, Phormia, and Sarcophaga genera should be distinguished from obligatory myiasis resulting from primary myiasis producing species.
Facultative myiasis is one of the oldest recorded diseases affecting man and domestic animals. Many human skin diseases in ancient times, although different in their cutaneous manifestations and etiologies, were grouped together under the catch-all term, "leprosy." Such was not the case for the presence of fly larvae (maggots) in the tissues or wounds of man or domestic animals. The presence of live maggots in a wound or their escape from a living being's body is a condition that could not be confused with any other syndrome. As early as 520 B.C., Herodotus, the "Father of History," described a case of facultative myiasis in a woman. "No sooner had she returned to Egypt, than she died a horrible death, her body seething with maggots while she was still alive." TheHolyBible alludes several times to maggots infesting human flesh. In Job 7:5, the afflicted Job states, "My flesh is clothed with maggots and clods of dust, my skin rotted and fouled afresh." In Acts 12:23, it is recorded that King Herod died 5 days after being smitten with gangrene, during which time maggots bred in the gangrenous mass. Historical descriptions, however, are not restricted to man. In the HortusSanitatis published in Antwerp, Belgium in 1521, there are woodcuts that imply that the authors were aware of the life cycle of flies; flies would swarm on maggot-infested meat and would attack a dead or dying animal (Greenberg, 1973). Reports of this syndrome in cats are rare, although facultative myiasis probably does occur frequently.
Facultative myiasis is often a condition of the skin. It is most commonly found around the perineal region or along the dorsal midline. Lesions appear as "punched out" ulcers. These ulcers frequently merge to produce larger ulcers with scalloped edges. Sections of skin may be undermined and the movement of large numbers of active larvae may be observed and felt (Prescott, 1984).
Adult Calliphora spp, Phaenicia spp, Lucilia spp, and Phormia spp, often referred to as blowflies or bottle flies, are metallic blue, green, or black in color. Adult Sarcophaga spp, often referred to as flesh flies, have a grey longitudinal striped thorax with a checker-board patterned abdomen. Keys are available for the first and second larval stages of the facultative myiasis-producing flies (Zumpt, 1965). The third stage larva of the facultative myiasis-producing flies is pointed anteriorly and possesses a broad, flattened posterior end. It may either have a smooth surface or be "hairy", possessing thorn-like, fleshy projections over most of its surface. The larva is approximately 10 to 14 mm long, pale yellow to grayish-white, but may possess a slight pink tinge. The anterior end bears a pair of oral hooks that are connected to the internal, chitinous, cephalopharyngeal skeleton. The posterior end of the larva exhibits one pair of stigmatic, stigmal, or spiracular plates. Each plate consists of 3 long, slender, parallel slits located within the spiracle. The different species of myiasis-producing flies may be differentiated by the cephalopharyngeal skeleton and by the shape of the spiracular plates. Of these two features, the structure of the larva's spiracular plate is the more important diagnostic characteristic.
LIFE CYCLE: The mouthparts of the adult flies are of a sponging type and are never used to lacerate tissues, rather they are used for imbibing liquid food. Solid food, feces, or necrotic tissue must be made fluid before it is sucked up by the fly. The fly disgorges saliva and crop contents; this mixture serves to liquefy the solid matter. The fly then sponges up the liquefied digestate using its mouthparts.
It is the larval stage that is important in facultative myiasis, because it is this stage that resides within the tissues of the host. Adult female flies lay clusters of light colored eggs in feces, carcasses, wounds, or soiled hair. While the female fly is selecting a suitable spot to lay her eggs, she feeds on the moist matter that may be present. If the female fly chooses feces or carrion as a feeding/egg laying site, her offspring will develop in a nonparasitic life style. However, if the female fly chooses a cat's contaminated wounds or soiled matted hair as an egg laying site, her offspring could develop to a parasitic existence.
The female flies are attracted to material containing protein, which is necessary for ovarian development. She will lay from 50 to 150 eggs in one batch and, in her life span, will produce from 1000 to 3000 eggs. The newly laid eggs are about 1 mm long. The first-stage larvae hatch from these eggs in 8 to 72 hours and begin to feed on the moist food in the contaminated wound. First stage larvae have underdeveloped mouthparts and must feed on liquid protein, which may be found in wound exudate. Without liquid protein, they cannot develop to second stage larvae. Second stage larvae demonstrate well developed mouthparts capable of scratching the skin; they then feed on the protein-rich fluid that exudes. This begins the pathologic sequence of events. The larvae grow rapidly to fully developed, third stage larvae. The larval developmental rate is dependent upon the amount and suitability of food, the temperature, and the degree of competition among the larvae present. Development from the egg to the third larval stage takes from a few days to a couple of weeks.
When the larva is ready to pupate, it usually drops off the host, crawls over or through the soil, and pupates beneath the soil surface. In some instances, however, the larvae of facultative myiasis-producing flies may pupate in the haircoat of the live animal. Under conditions of cold weather, pupation may become delayed and the larva may hibernate in the soil until warm weather prevails. At pupation, the larva loosens its skin and the skin turns brown and rigid. The resulting developmental stage is the pupa or puparium. During the summer months the pupal stage lasts from 3 to 7 days while hibernation usually occurs over the winter months. The adult fly emerges from the end of the puparium by alternately inflating and deflating the ptilinal sac within its head. It also uses this technique to move through the soil to the surface. As an adult, the female fly again has the choice of laying her eggs in feces, carcasses, wounds, or soiled, matted hair.
CLINICAL PRESENTATION AND PATHOGENESIS: For facultative myiasis to develop in any warm-blooded animal, some inciting lesion must be present in the animal's skin or haircoat. In many instances, the lesion is either in an area of fecal soiling, a neglected wound or other traumatic area, or where there is an ocular discharge (Wilkinson, 1985). The common characteristic of these situations is sustained, moist conditions within the haircoat. A study of fleece rot and fly strike in sheep revealed that oviposition by flies was found to be markedly affected by the availability of protein and by bacterial activity, especially that of Pseudomonas spp. Odors emanating from culture plates containing wool and these bacteria played an important role in fly attraction and oviposition (Merritt and Watts, 1978).
In time, the eggs hatch and the larvae emerge. It must be remembered that the first stage larvae have underdeveloped mouthparts and must feed on liquid protein. These mouthparts of first stage larvae do little to physically damage the skin. Second and third stage larvae have mouthparts capable of rasping the skin and inflicting considerable damage (Monzu, 1978). Throughout the infestation process, the later larval stages of facultative myiasis-producing flies move independently about the wound surface. The larvae of these flies ingest dead cells, exudates, secretions, and debris, but not live tissues. They irritate, injure, and kill successive layers of cells and provoke exudation. Large numbers of maggots rapidly consume dead cells and exudates. In time, the maggots tunnel through the thinned epidermis into the subcutis where they continue to feed upon dead cells and exudative debris. This erosive process forms tissue cavities up to several cm in diameter. Unless this process is halted by appropriate therapy, the infested animal may die from shock, intoxication, histolysis, or infection (Kimberling, 1988).
Close examination of the early lesions reveals a moist dermatitis, varying numbers of small maggots, and a peculiar, distinct, pungent odor. The epidermis is thin and the skin is inflamed, and appears reddened and tender. Advanced lesions may contain thousands of maggots, some of which have produced cavitations in the subcutaneous tissues. The lesion can become quite large and the tissue destruction quite extensive. Fly larvae may even destroy portions of musculature and invade body cavities. By this time, infested animals are usually depressed, febrile, and usually prostrate (Hendrix, 1991).
DIAGNOSIS: The diagnosis of maggot infestation in cats can be easily made by a layperson as maggots can be observed in an existing wound or among soiled matted hairs. As mentioned in above, people have been doing this for centuries. A history of traumatic injury or surgical intervention may alert the owner or veterinarian. Most cases of facultative myiasis in small animals occur in geriatric patients with fecal or urinary incontinence that result in soiling of the haircoat. In those breeds of cats with long, thick haircoats that become matted or soiled by feces or urine, the diagnosis may not be made in an early stage of infestation; the infested animal may become depressed, febrile, and prostrate before a problem is recognized. In such cases, the peculiar, distinct, pungent odor characteristic of facultative myiasis will also be present.
Cats infested with larvae of the facultative myiasis-producing flies may often ingest these larvae during the grooming process. Fly larvae have been known to pass through the gastrointestinal tract in an undigested state; owners or veterinarians might therefore suspect facultative myiasis, although diagnosis by this method is a rare occurrence. Such larval passing may occur because of pseudomyiasis, which occurs when a free roaming cat ingests carrion that contains maggots and the maggots then pass via the feces in an undigested state.
TREATMENT: The literature on treatment and control measures against facultative myiasis in sheep is voluminous, but such is not the case for the condition in cats (Prescott, 1984, Hendrix,1991). Should the larvae of facultative myiasis-producing flies be detected in small animals, immediate therapy is necessary. The extent of the lesion is determined by clipping the animal's haircoat, thus removing many larvae that are present in the hair. However, removal of the maggots from existing deep tissue pockets may prove to be difficult. Sedation or anesthetize may be required to allow physically extraction of the larvae from subcutaneous locations. The practitioner should examine the lesion for the presence of fly larvae on successive days; it must be remembered that adult flies lay eggs in the wounds at different times and that hatching of larvae may not be synchronous. Dead tissues should be debrided and pockets exposed or adequately drained.
Depressed, febrile and prostrate patients should be treated symptomatically (intravenous fluids, nutritional support). Ideally, culture and sensitivity examinations should be performed on wounds. If secondary bacterial or fungal infections are present, they must be treated. Administration of broad-spectrum antibiotics would be a wise decision.
Various insecticides have been used to kill the adult flies and their larvae within wounds in sheep. These include the chlorinated hydrocarbons and the organophosphates including chlorfenvinphos, diazinon and bromophos ethyl. More recently, the synthetic pyrethroids (permethrin, cypermethrin and cyprothrin) have been demonstrated to be safe, effective insecticides for use in sheep dips. Analogous insecticides may be used to treat the larvae of facultative myiasis-producing flies in small animals.
EPIZOOTIOLOGY: Facultative myiasis is usually observed in weakened or grossly ignored cats. It usually occurs during hot weather and is always associated with neglected wounds or with haircoats that have become matted and stained with urine or feces. There is no age or sex predilection, however this condition does tend to occur more commonly in long-haired cats (Prescott, 1984).
HAZARDS TO OTHER ANIMALS: Facultative myiasis may develop in any warm-blooded animal. A prerequisite is some inciting lesion in the animal's skin or haircoat (Wilkinson, 1985).
HAZARDS TO HUMANS: Likewise, neglected lesions in humans may be subjected to infestation by the myiasis-producing flies (Miller etal., 1990). There is an additional hazard to humans, the repulsion when one observes or smells a maggot infested wound.
CONTROL/PREVENTION: The best control techniques against the facultative myiasis-producing flies are always preventive (Zumpt, 1965). The veterinarian should educate the client concerning the immediate treatment of all skin wounds. The client must be aware that the animal must be confined in a fly-free area. The cat's haircoat has to be kept clean of urine or feces and should not be allowed to become matted. Contaminated wounds and matted haircoats soaked in urine or feces rapidly attract the adult myiasis-producing flies. Recognition of this fact can help the client protect the animal from attack by myiasis-producing flies (Hendrix, 1991).
Greenberg B. 1973. In: FliesandDisease, volume II, 1st edition. Princeton University Press. Princeton. pp 11-18.
Harwood RF and James MT. 1979. In: EntomologyinHumanandAnimalHealth, 7th ed. Macmillan. New York. pp 37-38, 248-251, 255-266, 296-318.
Hendrix CM. Facultative myiasis in dogs and cats. 1991. Comp Cont Ed Prac Vet 13:86-96.
Kimberling CV. 1988. In: JensenandSwift'sDiseasesofSheep. Lea & Febiger. Philadelphia. pp 308-312.
Merritt GC and Watts JE. 1978. An in-vitro technique for studying fleece-rot and fly strike in sheep. Austral Vet J 54:513-516.
Miller KB, Hribar LJ, and Sanders LJ. 1990. Human myiasis caused by Phormiaregina in Pennsylvania. J Am Pod Med Assoc 80:600-602.
Monzu N. 1978. Some basic facts about primary blowflies. J Agric, W Austral 19:93-95.
Prescott CW. 1984. In: ParasiticDiseasesoftheCatinAustralia. 2nd edition. University of Sydney, Post-Graduate Foundation in Veterinary Science. New South Wales. p 76.
Wilkinson, GT. 1985. In: ColorAtlasofSmallAnimalDermatology. Williams & Wilkins. Baltimore. p 64.
Zumpt F. 1965. In: MyiasisinManandAnimalsintheOldWorld. Butterworths. London. pp 1-267.