Many dipteran flies produce larvae that must lead a parasitic existence and result in obligatory myiasis. Obligatory myiasis-producing flies can be broadly divided into species causing traumatic wound myiasis (screwworms), migratory myiasis (botflies), and furuncular myiasis, reflecting the characteristic location and behavior of larvae (maggots) within the host.
Only one fly in North America, Cochliomyia hominivorax (see New World screwworm), is a primary invader of fresh, uncontaminated skin wounds of domestic animals. Another species of screwworm, Chrysomya bezziana, is found in Africa and southern Asia, including Papua New Guinea. Both cause traumatic wound myiasis.
Obligatory myiasis-producing flies of veterinary importance covered in other chapters include the botflies causing migratory myiasis (see Gasterophilus spp in horses, Oestrus ovis in sheep, and Cuterebra spp in dogs and cats).
The human botfly (Dermatobia hominis) of Central and South America and the African tumbu fly (Cordylobia anthropophaga) of sub-Saharan Africa are oestrid flies that cause obligatory furuncular myiasis.
Chrysomya bezziana
(Old World Screwworm, Oriental Fly, Bezzi’s Blowfly)
Chrysomya bezziana is found in Africa, the Indian subcontinent, and southeast Asia from Taiwan in the north to Papua New Guinea in the south. This fly is not indigenous to Australia. Owing to its geography, C bezziana's most likely potential port of entry to the US is Hawaii.
Adult screwworms are usually not found in the field. The adult fly has a dark metallic green body with abdominal segments with narrow bands along the caudal margins. The legs are black or partially brown. The face is orange/yellow. The first larval stage can go unnoticed because of its small size (up to 3 mm) at the time of its molt to the second stage. The second stage is quite similar to the third but larvae are 4–9 mm long. The third-stage larvae are large, up to 18 mm. The body is composed of 12 segments with broad encircling bands of spinules. All three stages are maggot-like in appearance and have caudal spiracles that are unique to the species. The caudal end of the larva has its spiracular plate located in a deep cleft at the end of the eighth abdominal segment. The spiracular plates are large and well separated. The peritreme and the three breathing slits are wide.
The Old World screwworm fly, C bezziana, produces a particularly vile myiasis. Female flies are attracted to open wounds of humans and domestic and wild animals, laying their eggs in masses of 150–500 at the edge of wounds or near body orifices. Larvae develop to the third stage approximately 2 days after hatching. They burrow deep into the wound such that only their caudal ends are visible. The entire larval stage lasts 5–6 days. The pupal stage lasts 7–9 days in tropical conditions and longer in cooler environments. The adult flies emerge later to breed, locate a new host, and continue the cycle (1).
Female flies breed only once during their lifetime—a fact paramount in prevention and control. Under favorable conditions, there can be ≥ 8 generations per year (2).
Pathology of C bezziana
The larvae of C bezziana are obligatory wound parasites, never developing in carcasses or decomposing organic material.
Although female flies are attracted to open wounds, occasionally eggs are deposited on the unbroken, soft skin of various parts of the body, especially if contaminated by blood or mucous discharge. When the larvae hatch, they burrow into the flesh of the host, using their hooked mouthparts to scrape away at the tissues and lacerate the fine blood vessels. Larvae are voracious blood feeders. During the bloodsucking phase, only the caudal ends of the maggots with their blackish peritremes remain visible at the surface of the lesion, enabling the larvae to breathe. As many as 300 maggots have been observed in some wounds (1). (See and of screwworm myiasis in a cat.)
Top: Chrysomya bezziana myiasis in a cat; bottom: close-up of myiasis.
Courtesy of Dr. Han Hock Siew.
In untreated wounds, the destructive activity of the larvae can kill the animal within a very short time. Secondary infestation with facultative myiasis-producing flies can complicate treatment and control.
Diagnosis of C bezziana
Diagnosis is based on identification of the rarely observed adult flies. In suspect cases, the extracted larvae should be submitted to an entomologist for definitive diagnosis.
Diagnosis can often be made by residence in or history of travel to an area endemic for C bezziana. If a wound is thought to be infested with larvae of C bezziana, samples should be collected and sent to appropriate eradication officials. A detailed manual is available from Animal Health Australia (Old World Screw-Worm Fly: Diagnostic Manual). Final identification is best left to an entomologist.
Treatment and Control of C bezziana
Removal of larvae from the lesion
Reporting, where indicated
Treatment of Old World screwworm myiasis involves killing the larvae in the lesions, promoting healing, and preventing secondary reinfestation with larvae of the facultative myiasis-producing flies. The extent of the lesions is determined by clipping the hair and removing as many larvae as possible. The larvae that are removed must be killed to prevent them from pupating and developing into adults. Larvae located deep within tissues must be extracted.
Ivermectin has demonstrated therapeutic and prophylactic efficacy against C bezziana infestation in cattle. Subcutaneous administration at dosages of 50, 100, and 200 mcg/kg produced complete control of infestations for 6, 12, and 14 days, respectively, while a single dose of 200 mcg/kg provided residual protection for 16–20 days(3). In field studies, cattle treated with ivermectin at the time of castration and branding experienced markedly fewer screwworm strikes than untreated animals (3). Later evaluation of insecticide formulations for potential use in Old World screwworm containment and eradication programs for Australia demonstrated the efficacy of several macrocyclic lactones and insecticidal products (4). Dicyclanil spray-on formulations and controlled-release ivermectin capsules provided prolonged protection against experimental infestations, whereas topical ivermectin and spinosad formulations were effective therapeutic treatments for established strikes.
In dogs, oral spinosad plus milbemycin registered for flea treatment is used for fast and spontaneous expulsion of Old World screwworm maggots in dogs. In a case series from Malaysia, all larvae were killed within 8 hours of treatment, with most larvae expelled spontaneously and the remainder easily removed without the need for general anesthesia or extensive mechanical debridement (5).
Preventive measures in domestic animals focus on prompt management of all wounds that should be properly dressed (umbilical, castration, shearing, and other husbandry-associated wounds); in addition, all elective surgical procedures should be avoided during the fly season. Regular inspection of susceptible animals for wounds that serve as oviposition sites for Old World screwworm flies, together with implementation of biosecurity measures in endemic and high-risk regions, is essential for prevention.
Female flies breed only once during their lifetime, which is an important consideration in the control of C bezziana. Pupal flies exposed to irradiation lead to sterile adults that can be released to breed with wild male and female flies. As a result, no viable offspring are produced in the wild.
Wolves (Warbles) of Small Animals
Larvae of the genus Cuterebra are often referred to as wolves, warbles, rabbit bots, or rodent bots. These fly larvae infest the skin of rabbits, squirrels, mice, rats, chipmunks, and occasionally dogs and cats. (For clinical findings, diagnosis, and treatment, see Cuterebra Infestation in Small Animals.)
Flesh Flies
Flesh flies are large (8–14 mm), nonmetallic flies with characteristic three longitudinal stripes on the thorax. Two species of veterinary importance, Wohlfahrtia magnifica and Wohlfahrtia vigil, are obligate myiasis-producing flies (see ; see also ). W magnificafemales are larviparous and deposit first-stage larvae directly onto or near wounds, mucous membranes, and natural body openings; in contrast, W vigil females typically deposit larvae onto intact skin.
Wohlfahrtia magnifica (spotted flesh fly). Top, front view; bottom, side view.
Courtesy of Dr. Stanislav Simin.
Side view of Wohlfahrtia magnifica (spotted flesh fly) in a culture jar. Note prominent red eyes on silver-white face.
Courtesy of Dr. Stanislav Simin.
W magnifica, the spotted flesh fly, causes traumatic wound myiasis in livestock and produces lesions that can resemble screwworm infestation. (See images of traumatic myiasis in sheep , , , and . Its distribution extends throughout the Mediterranean basin, Eastern and Central Europe, North Africa, and parts of Central and northern Asia.
Top: sheep with preputial traumatic myiasis caused by spotted flesh flies (Wohlfahrtia magnifica); bottom: view of the maggots within the lesion.
Courtesy of Dr. Stanislav Simin.
Top: sheep hoof traumatic myiasis caused by Wohlfahrtia magnifica; bottom: sheep hoof with Wohlfahrtia magnifica maggots in traumatic myiasis. Note the adult fly (yellow arrow) next to the lesion with the maggots.
Courtesy of Dr. Stanislav Simin.
Tail and vaginal traumatic myiasis caused by spotted flesh flies (Wohlfahrtia magnifica) in sheep. View of the maggots within the lesion.
Courtesy of Dr. Stanislav Simin.
Vaginal traumatic myiasis caused by Wohlfahrtia magnifica. View of the maggots within the lesion.
Courtesy of Dr. Stanislav Simin.
W vigil causes cutaneous furuncular (boil-like) myiasis of young wild mammals (fox, rabbit, mink) in North America, particularly in southern Canada and the northern part of the US. Infestation occurs in the skin of healthy animals, particularly the unbroken skin of the young, where larvae develop within localized skin swellings (as with botflies) rather than extensive traumatic wounds.
Larval stages are maggot-like in their appearance and have caudal spiracles unique to the species. The third larval stage is 1.5–2 cm long. Its caudal end is narrow, and it is covered with many irregular rows of small spines. The oral hooks are strongly developed. (See .)
Development to the infective third larval stage is usually completed in 5–14 days. The parasites then drop to the ground and pupate; pupation time depends on the season of the year and the temperature. When cold weather approaches, the pupation period is greatly prolonged.
Pathology of Flesh Flies
Traumatic myiases caused by W magnifica are primarily recognized in sheep; however, goats, cattle horses, wildlife, and occasionally humans can become infested. In sheep, lesions occur predominantly around the hooves, vulvar and perineal region in ewes, and preputial region in rams. The resulting wounds can resemble screwworm infestation.
Cases of W magnifica myiasis are strongly seasonal, typically appearing in spring, peaking during summer and early autumn, and disappearing in winter (6).
Furuncular (botlike) myiases caused by W vigil are characterized by localized swellings containing one or more larvae, often associated with a central opening (2–3 mm in diameter) and serous exudate. They primarily affect young wild mammals, particularly rabbits, foxes, and mink but are reported in other small mammals, including dogs and cats. W vigilhas also been isolated from the skin of young children, particularly infants (7).
The penetration of the skin by the larvae, their development in the subcutaneous tissues, and secondary bacterial infection produce intense irritation and inflammation. Attempts by the animal to remove the larvae or relieve the irritation tend to aggravate the condition.
Diagnosis of Flesh Flies
Adult flesh flies are nonparasitic and as a result will probably not be found by owners or veterinarians. They are large grayish flies, about twice the size of a house fly.
The presence of a dermal boil-like swelling with a central opening can lead to a tentative diagnosis of myiasis due toW vigil, whereas extensive traumatic wound myiasis with visible larvae might indicate W magnifica and can resemble screwworm infestation (open, enlarging wound containing numerous larvae). A definitive diagnosis can be made only after extraction and identification of a typical larva. A tentative diagnosis can often be made by a history of either residence in or travel to a geographical area endemic for W vigil or W magnifica.
The identification of adult flies and their associated larval stages should be left to an entomologist.
Treatment and Control of Flesh Flies
Extraction of larvae from the skin and lesion
Wound management
Prophylactic fly control in endemic areas
Elimination of W vigil reservoirs
Control of W magnifica involves eliminating the larvae from lesions, promoting wound healing, and preventing reinfestation by both W magnifica and facultative myiasis-producing flies. The extent of the lesion should be determined by clipping the surrounding wool or hair and removing larvae, including those deep within tissues. Removed larvae must be destroyed. Organophosphate and pyrethroid insecticides can be effective for killing larvae but generally provide limited residual protection against reinfestation. Prevention relies on prompt wound management, strategic use of approved insecticides, and regular monitoring during fly season. Prophylactic treatments of sheep with registered dicyclanil formulations can provide effective protection (8).
Larvae of W vigil must be extracted from the skin. In suspect cases, the extracted larva should be submitted to an entomologist for definitive diagnosis. Applying heavy oil, liquid paraffin, or petrolatum jelly to the opening of the lesions will occlude the airway of the larvae.
W vigil often infects young mink.
Rabbits and rodents are implicated in maintaining populations of W vigil. Control of rabbit and rat populations should be considered in management of W vigil on farms.
African Tumbu Flies
(Mango Fly, Skin Maggot Fly, Ver du Cayor, Worms of Cayor)
The African tumbu fly, Cordylobia anthropophaga, is responsible for furuncular (boil-like) myiasis in both humans and animals in Africa, particularly in sub-Saharan regions. Another furuncular myiasis in animals and in humans is the human botfly, Dermatobia hominis, which is endemic to tropical and subtropical regions of Central and South America.
The adult African tumbu flies are nonparasitic and as a result are not observed by owners or veterinarians. They are stout, compact flies, 6–12 mm long. They are light brown, with diffuse blue-gray patches on the thorax and dark gray on the caudal part of the abdomen. The face and legs are yellow. The second- and third-stage larvae are the stages usually observed in the animal’s skin.
Second-stage larvae of African tumbu flies are slightly club-shaped and exhibit large, black cuticular spines directed caudally and distributed irregularly over segments 3–8. Segments 9–11 are almost bare, compared to the preceding segments. The segments have a few rows of small, pale spines caudally. Segment 12 is densely covered with these spines. Segment 13 is indistinctly demarcated, lacking spines but possessing two pairs of short processes. Each tracheal tube opens through two slightly bent slits. The second-stage larvae are 2.5–4 mm long. The size of advanced second-stage larvae varies greatly, as does the size of third-stage larvae. Fully mature larvae are 1.3–1.5 cm long. The body is cylindrical with 12 identifiable segments. Curved spines that are directed caudally are densely arranged at least up to segment 7; the last 5 segments can be either partially or densely covered with spines. (See and .)
Dog myiasis caused by larvae of Cordylobia anthropophaga (African tumbu flies), blowflies found primarily in sub-Saharan Africa.
Courtesy of Dr. David Modry.
After fertilization, female African tumbu flies produce 100–500 banana-shaped eggs, usually depositing them in dry, shady, sandy soil that has often been contaminated by urine or feces. Eggs are never deposited on the skin of the host. Eggs hatch after 1–3 days, and the larvae are initially 0.5–1 mm long. Larvae can survive up to 15 days while waiting for a host and can penetrate the host in as little as 25 seconds. After penetration, larvae reside in a cavity in the dermis and hypodermis. This cavity communicates to the external environment by means of a central breathing pore, which corresponds to the caudal end of the larva with its spiracles. A single larva is found in each cavity, within which the larva develops to the second and third stages. Larvae take 7–15 days to mature and then emerge through the breathing pore and drop to the ground, where they pupate. Adult flies emerge 10–20 days later, and the cycle begins again (9).
Rats and dogs are the usual definitive hosts; however, humans, mice, monkeys, mongooses, squirrels, leopards, boars, antelopes, cats, goats, pigs, rabbits, guinea pigs, and chickens can be infested.
Pathology of African Tumbu Flies
Clinically, infestation with African tumbu flies is characterized by a small erythematous papule that appears 2–3 days after larval penetration. Within days, the papule enlarges until it becomes a nodule that resembles a boil (furuncle), hence the description furunculoid myiasis. At the center of the nodule is a pore through which serous fluid oozes. This fluid can be hemorrhagic or purulent and contains larval feces.
Dogs with thin, soft skin seem to be more suitable hosts for larval development than dogs with thick skin. Preferential sites of infestation are the feet, genitals, tail, and axillae. In endemic areas, mild infestations in dogs do not produce clinical distress. Massive infestation can induce marked swelling and edema, especially if larvae are in close proximity to each other. Larvae can penetrate deep into tissues and cause considerable damage and even death.
Diagnosis of African Tumbu Flies
The presence of a dermal swelling with a central opening can lead to a tentative diagnosis of myiasis due to C anthropophaga. A definitive diagnosis can be made only after extraction and identification of typical larvae. The identification of adult flies and their associated larval stages should be left to an entomologist.
A tentative diagnosis can often be made by a history of either residence in or travel to a geographic area endemic for C anthropophaga. However, the parasite has also been diagnosed in travelers and their accompanying pets from geographical areas where the parasite is not found.
Treatment and Control of African Tumbu Flies
Larval extraction
Sanitation and management of vermin
Larvae can be removed by coating the breathing pore with a thick, viscous compound, such as heavy oil, liquid paraffin, sticking plaster, or petrolatum jelly. Clogging the pore causes the larva to become hypoxic and leave the cavity in search of oxygen. Light pressure at the edge of the lesion also aids in larval removal.
Lidocaine hydrochloride can be injected into the furuncular lesion to facilitate larval extraction with thumb forceps. Surgical excision is usually unnecessary and unwarranted while the larvae are alive but is used to remove dead or decaying larvae. Great care should be taken during the extraction process to avoid rupturing larvae in situ and causing potential allergic reactions, although anaphylaxis has not been reported. Antimicrobials should be prescribed.
Pearls & Pitfalls
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Adult flies that are indoors should be killed. Larvae should be removed from animals entering the house and destroyed. All rats should be killed and burned. Prevention of an infestation depends on cleanliness and regular disinfection of the animal’s sleeping quarters. In the case of valuable animals (eg, Angora rabbits), flies can be kept out of rabbit pens using wire gauze.
Because the adult female flies lay eggs in sandy soil contaminated by feces or urine, the parasite can be controlled in the pet’s environment by prompt removal of the pet’s feces and by covering urination sites on the premises with a layer of dirt.
For More Information
Animal Health Australia. Response strategy: screw-worm fly. Australian Veterinary Emergency Plan, version 5. 2020.
Also see pet owner content regarding flies and mosquitoes of dogs, cats, and horses.
References
Humphrey JD, Spradberry JP, Tozer RS. Chrysomya bezziana: pathology of Old World screw-worm fly infestations in cattle. Exp Parasitol. 1980;49(3):381-397. doi:10.1016/0014-4894(80)90073-9
Animal Health Australia. Response strategy: screw-worm fly. Australian Veterinary Emergency Plan, version 5. 2020. https://animalhealthaustralia.com.au/wp-content/uploads/dlm_uploads/AUSVETPLAN-Manuals_Response_SWF-1.pdf
Spradbery JP, Tozer RS, Drewett N, Lindsey MJ. The efficacy of ivermectin against larvae of the screw-worm fly (Chrysomya bezziana). Aust Vet J. 1985;62(9):311-314. doi:10.1111/j.1751-0813.1985.tb14913.x
James P, Wardhana A, Brown G, Mayer D, Urech R. Prophylactic and therapeutic efficacy of Australian-registered insecticide formulations against Old World screwworm (Chrysomya bezziana) infestation. Aust Vet J, 2017;95(8):265-272. doi:10.1111/avj.12609
Han HS, Sharma R, Jeffery J, Noli C. Chrysomya bezziana (Diptera: Calliphoridae) infestation: case report of three dogs in Malaysia treated with spinosad/milbemycin. Vet Dermatol. 2017;28(2):239-e62. doi:10.1111/vde.12403
Remesar S, Otero JL, Panadero R, et al. Traumatic myiasis by Wohlfahrtia magnifica in sheep flocks from southeastern Spain: prevalence and risk factors. Med Vet Entomol. 2022;36(1):30-37. doi:10.1111/mve.12548
Francesconi F, Lupi O. Myiasis. Clin Microbiol Rev. 2012;25(1):79-105. doi:10.1128/cmr.00010-11
Sotiraki S, Stefanakis A, Hall MJR, Graf JF. Field trial of the efficacy of dicyclanil for the prevention of wohlfahrtiosis of sheep. Vet Rec. 2005;156(2):37-40. doi:10.1136/vr.156.2.37
Mihalca AD, Mitrea IB, Rabei ŞO, Lolokote S, Modrý D, Beugnet F. Efficacy of afoxolaner (NexGard®) for the treatment of myiasis caused by the zoonotic tumbu fly, Cordylobia anthropophaga, in domestic dogs under field conditions. Vet Parasitol. 2026;347:110866. doi:10.1016/j.vetpar.2026.110866
