Myiasis-producing fly species are dipteran flies. The majority of species involved in facultative myiasis are members of the Calliphoridae family (blowflies). This fly family contains both facultative and obligatory myiasis-producing species.
Facultative myiasis-producing flies are normally saprophagous, developing in carrion and other decaying organic matter; however, they can opportunistically become parasitic in living animals. In contrast, obligatory myiasis-producing flies, including the Old World screwworm (Chrysomya bezziana) and the New World screwworm (Cochliomyia hominivorax), require a living host for larval development and cannot complete their life cycle in decaying organic substrates alone.
Calliphorids are medium-sized to large flies that often exhibit a metallic blue or green sheen. Some species are referred to as bottle flies, including green bottle flies (Lucilia spp) and blue bottle flies (Calliphora spp). Eggs are deposited on suitable organic substrates, such as carrion, wounds, and fleece contaminated by urine, feces, or exudates. After egg hatch, larvae develop and feed on host tissue, causing traumatic cutaneous myiasis. When mature, the larvae migrate away from the lesion and drop to the ground. The larvae pupate in soil, and subsequently new adults emerge. In the larval stages, the characteristics of the distinctive caudal spiracular plates and the cephalopharyngeal skeleton are unique for each species and are used for identification.
When larvae develop in the skin or tissue of living animals, the resulting infestation is termed myiasis. In sheep, cutaneous myiasis caused by blowflies is commonly referred to as fly strike, or simply strike, and infestations are described according to their anatomical location (eg, breech strike, body strike, poll strike, pizzle strike, or wound strike).
Fly strike in sheep is a dynamic process that follows a pattern of ecological succession; different fly species colonize the host sequentially as tissue damage and wound conditions change:
Primary strike flies are facultative myiasis-producing blowflies capable of initiating myiasis in healthy or minimally damaged tissue; the most important of these flies include the following:
Lucilia cuprina in Australia and South Africa
L sericata in Europe
L sericata and Phormia regina in North America
Secondary strike flies are attracted to the lesions created by these primary strike flies, typically colonizing only after myiasis becomes established and contributing to lesion expansion (eg, Calliphora spp).
Tertiary strike flies colonize the lesion and surrounding tissue as tissue damage progresses and wound exudates accumulate. Tertiary strike flies include the following :
Calliphora spp
Sarcophaga spp
Musca domestica
Advanced fly strike lesions contain larvae of multiple fly species.
Pathology of Facultative Myiasis-Producing Flies
In normal conditions, adult myiasis-producing flies 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 coat. A common site is the breech, where flies can be attracted to wool soaked with urine or feces. As adult female flies feed in these sites, they lay eggs, which hatch within 24 hours 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 strike or fly strike. The larvae irritate, injure, and kill successive layers of skin and produce exudates. (See .)
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 blowflies, 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 treatment, infested animals can die from shock, intoxication, histolysis, or infection. A peculiar, distinct, pungent odor permeates the infested tissue and the affected animal. Advanced lesions can contain thousands of maggots.
The sheep's body also can 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, sites where feet with footrot come in contact with fleece, and wounds.
As adults, myiasis-producing flies can be annoying 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, also known as limberneck in chickens, has been associated with ingestion of large numbers of larvae ofL caesar, Phaenicia sericata, and other species of flies. Clostridium botulinum multiplies in carrion, where it can 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.
Diagnosis of Facultative Myiasis-Producing Flies
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 might 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 perpendicular to the larval body. When the sliced caudal ends are placed cut surface down on a glass slide, covered with a coverslip, and examined under a compound microscope, a dichotomous key can be used to identify the genus or genera of flies within the wound (1). 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 can 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 due to Cochliomyia hominivorax or Chrysomya bezziana should be considered, depending on geographical area.
Identification of the genus and species of myiasis-producing flies from either adult flies or larvae (maggots) is complex and best left to an entomologist. Molecular methods can assist in identification; however, DNA-based barcoding based on cox1 does not always resolve closely related species, and its reliability should not be assumed for all groups of blowflies (2).
Treatment and Control of Facultative Myiasis-Producing Flies
Shearing affected wool
Applying fly strike dressing to the shorn area
Culling
Blowfly infestation of the breech can be effectively controlled for approximately 6–8 weeks 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.
Odors and associated moisture attract flies and stimulate oviposition, particularly during hot, humid weather. Urine staining of the crutch of Merino ewes can be virtually eliminated by removal of breech wrinkles in geographies where this procedure is allowed (Mules operation or mulesing) (3), and fecal contamination can be greatly decreased by docking tails at the third joint. Scouring should be controlled.
Development of fly strike requires the presence of gravid L cuprina flies, a susceptible host, and suitable environmental conditions. Sheep are at greater risk when the fleece becomes moist as a result of dermatophilosis, urine staining, fecal contamination or prolonged wet weather.
Fly strike is most likely to occur when temperatures range from approximately 17–40°C (63–104°F) and weather is calm, with winds generally < 30 km/hour (19 miles/hour) (4). Control programs therefore focus on decreasing sheep susceptibility rather then attempting to eliminate blowfly populations. Key preventive measures include effective worm control to decrease fecal staining, timely shearing and crutching, prevention and treatment of dermatophilosis, shearing before the onset of fly strike-risk period (typically spring, in many regions), prompt wound management, and genetic selection of sheep with decreased susceptibility to fly strike.
Chemoprophylaxis consists of wetting to complete fleece 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 weeks; however, where the primary fly is resistant (eg, L cuprina in Australia), it might last only 2–3 weeks (5). Before suitable agents are applied, all wool should be removed from the struck area and around it.
A product registered as a fly strike "dressing" is different from a preventative. A dressing kills remaining maggots and prevents restrike as the affected area dries and heals. A dressing rapidly kills and contains ivermectin or spinosyn. The organophosphate diazinon is not recommended as dressing because of prevalent resistance. Preventatives, such as dicyclanil and cyromazine, when applied alone, are not suitable as dressings on welfare grounds, because maggots will take up to 4 days to die. Ensure a product with a suitable withholding period is chosen.
Pearls & Pitfalls
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Burning or deep burying of the carcass can be a valuable general hygienic measure but has little effect on primary strikes. The main source of primary flies is the struck sheep. A genetic manipulation approach has been developed to control a strain of blowfly in Australia (6, 7); male flies are partially sterile but transmit a gene that causes blindness in female offspring (8).
Treatment and control measures for myiasis in dogs and cats are limited. If myiasis-producing larvae are detected in small animals, immediate treatment is necessary. The hair 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 can be difficult, and sedating or even anesthetizing the animal might be necessary. The lesion should be examined on successive days; adult flies lay eggs in the wound at different times, and larvae might not hatch synchronously.
Depressed, febrile, and prostrate animals should be treated according to their clinical signs. Ideally, culture and susceptibility studies should be performed on samples or scrapings of the wounds. If secondary bacterial or fungal infections are present, administration of broad-spectrum antimicrobials 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 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.
For More Information
Treatment of struck sheep. FlyBoss.
Also see pet owner content regarding flies and mosquitoes of dogs, cats, and horses.
References
Greenberg B. Flies and Disease I: Ecology, Classification and Biotic Associations. Princeton University Press; 1971.
DNA barcoding cannot reliably identify species of the blowfly genus Protocalliphora (Diptera: Calliphoridae). Proc R Soc B. 2007;274(1619):1731-1739. doi:10.1098/rspb.2007.0062
Phillips CJC. A review of mulesing and other methods to control flystrike (cutaneous myiasis) in sheep. Anim Welf. 2023;18(2):113-121. doi:10.1017/S0962728600000257
Greeff JC, Karlsson LJE, Schlink AC, Gilmour AR. Factors explaining the incidence of breech strike in a Mediterranean environment in unmulesed and uncrutched Merino sheep. Anim Prod Sci. 2018;58(7):1279-1288. doi:10.1071/AN16528
Kotze AC, James PJ. Control of sheep flystrike: what's been tried in the past and where to from here. Aust Vet J. 2022;100(1-2):1-19. doi:10.1111/avj.13131
Scott MJ. Development and evaluation of male-only strains of the Australian sheep blowfly, Lucilia cuprina. BMC Genet. 2014;15(Suppl 2):S3. doi:10.1186/1471-2156-15-S2-S3
Sandeman RM, Levot GW, Heath ACG, et al. Control of the sheep blowfly in Australia and New Zealand – are we there yet?Int J Parasitol. 2014;44(12):879-891. doi:10.1016/j.ijpara.2014.08.009
Black W, Alphey L, James A. Why RIDL is not SIT. Trends Parasitol. 2011;27(8):362-370. doi:10.1016/j.pt.2011.04.004
