Cochliomyia is a genus in the family Calliphoridae, known as blowflies, in the order Diptera. Cochliomyia are commonly referred to as the New World screwworm fly. There are four species in this genus: Cochliomyia macellaria, Cochliomyia hominivorax, Cochliomyia aldrichi, and Cochliomyia minima. The two main species are Cochliomyia hominivorax and Cochliomyia macellaria.
Cochliomyia hominivorax are known as the primary screwworm because their larvae produce myiasis and feed on living tissue. This feeding causes deep, pocket-like lesions in the skin, which can be very damaging to the animal host. Cochliomyia macellaria are known as the secondary screwworm because their larvae produce myiasis, but feed only on necrotic tissue. This species is forensically important because it is often associated with dead bodies and carcasses. Both C. hominivorax and C. macellaria thrive in warm, tropical areas.
In general, all Diptera have: three body regions (head, thorax, and abdomen); three pairs of legs; one pair of forewings used for flight; one pair of halteres which are modified hindwings; and one pair of antennae. When keying out a Dipteran specimen, it is important to first note whether bristles on the meron are present or absent. All species in the family Calliphoridae have bristles on their meron, plumose arista, and well-developed calypters. Both Cochliomyia macellaria and Cochliomyia hominivorax are metallic green to bluish green in major coloration, with setae on the dorsal surface of the stem vein, orange gena, pale white anterior spiracles, filiform palps, and three black longitudinal stripes (vittae) on the notum of the thorax. The species Cochliomyia macellaria have pale setulae on the fronto-orbital plate outside the row of frontal bristles while Cochliomyia hominivorax have dark setulae on the fronto-orbital plate outside the row of frontal bristles. The female Cochliomyia macellaria has a yellowish basicosta while the female Cochliomyia hominivorax has a brown basicosta. Cochliomyia macellaria are 6–9 mm in length. Cochliomyia hominivorax are 8–10 mm in length.
The larvae of both Cochliomyia macellaria and Cochliomyia hominivorax have a cylindrical body tapering anteriorly with 10 or more robust spines around the spiracular area, incomplete peritreme, an indistinct or absent button, and bands of small spines on each segment. The Cochliomyia hominivorax larvae have distinctly pigmented tracheal trunks. The Cochliomyia macellaria larvae do not have pigmented tracheal trunks; they have spines in a v shape on the anal protuberance and no oral sclerite. The mature third instar Cochliomyia hominivorax and Cochliomyia macellaria larvae can reach a length of 17 mm.
The general life cycle of Cochliomyia is similar to any other Diptera in that they are holometabolous. There are four stages: egg, larvae, pupa, and adult. This entire life cycle lasts an average of 21 days at prime conditions (preferably a warm, moist environment), and can last as long as two to three months in colder climates. Female Cochliomyia only lay eggs once in a lifetime and can lay anywhere from 100 to 400 in a clutch. Females usually lay their eggs on the edge of an open wound. The warm and moist conditions are the perfect combination of home and food source. The nasal, oral, or anal areas of a host are especially prone to Cochliomyia oviposition.
Larvae will hatch approximately 12–21 hours after the eggs have been laid. The larvae are a cream color. The Cochliomyia hominivorax larvae will dive head-first into whatever food source is nearest, and burrow deeper, eating into live flesh if available. This results in a pocket-like lesion that causes severe pain to the host. The Cochliomyia macellaria larvae will only feed on the necrotic tissue of a wound. After 5 to 7 days, the larvae will drop and move away from the food source to pupate. The larvae will burrow into the first layer of topsoil, beneath leaves or garbage and begin their pupation. The pupa is a dark brown color. This stage can last from 7 days at a warm temperature to as long as two months if the weather is much colder. The adults will emerge and spend a day or two finishing maturity. Adults will breed only once in their lifetime. Sexually mature adults choose to breed between 3–4 days after hatching from the pupa. Males mature rapidly, and will spend their time waiting and eating on nearby vegetation and the nectar of flowers. The females, however, are predacious, and will feed on the fluids from live wounds. The females can fly long distances to find a mate. The adult flies of this species will live approximately two to three weeks.
Cochliomyia hominivorax, or the Primary screwworm, is a parasitic species belonging to the Calliphoridae family of flies. This species’ larvae are renowned for eating and infesting the flesh of living organisms, primarily warm-blooded animals such as cattle and other livestock. Their larvae cause myiasis, or an infestation of maggots, in lesions or other wounds and injuries that the host animal may have. Most fly strikes occur in part, due to such farming processes as branding, castrating, dehorning, and tailing of the host animal. These processes, along with barbed wire cuts and flea bites, lead to myiasis in the host animal. Navels of newborns also can be the sites of infestation. The hominivorax tend to reproduce only on the flesh of a living host. Unlike most other maggots, the hominivorax maggots will attack and consume healthy living tissue along with decaying tissue (hominivorax literally translates to “man eating”). The New World screwworm fly can grow to an average size of 8–10 mm (0.3–0.4 inch), and share many characteristics of the common house fly. About 12–21 hours after the larvae hatch, they crawl into the wound and immediately begin to feed and burrow into the flesh. The hominivorax larvae are responsible for Cochliomyia’s common name, the screwworm, because they possess small spines on each body segment that resemble a screw’s threads. After the larvae hatch from the eggs they will dive head first into whatever food source, and burrow deeper perpendicular to the skin surface eating into live flesh, again resembling a screw being driven into an object. The larvae will then continue to feed on the wound fluids and the animal’s tissue. After 5–7 days, the larvae grow and depart from the wound to burrow into the soil and pupate. The pupal period varies from a week to upwards of 2 months, depending on the soil temperature. Adults breed only once during their lifetime, which is close to 20 days.
The sterile insect technique was proposed by a pair of scientists, Edward F. Knipling and Raymond C. Bushland, and rapidly adopted by the United States Department of Agriculture in 1958. The technique centers on a unique reproductive handicap that prevents female hominivorax flies from reproducing more than once in their life-spans. The scientists reasoned that if it were possible to clinically sterilise and release huge numbers of male hominivorax as breeding time approached, fertile males might be out-competed and the majority of female flies would lay sterile eggs. Irradiating the males was the method used for sterilisation.
Because the agricultural industry was losing millions of dollars annually due to treatment and loss of fly-struck animals, this solution was quickly approved for testing. It was first applied on a large scale in the state of Florida, due both to the severity of the problem there and to the state’s unique island-mimicking geography, which allowed for relative isolation of the Florida hominivorax population. The eradication of Florida’s Primary screwworm population was completed in 1959. The program was then applied throughout the southern United States, and eventually adopted through much of Mexico in 1972 and parts of Central and South America. The hominivorax was completely eradicated from the southern United States in 1966 and from Mexico in 1991. Primary screwworm has been effectively eliminated in the United States and Mexico. Livestock there continue to be vulnerable, however, and strict laws regarding animal inspection and reporting of suspected infestations remain in place.
The largest documented infestation of hominivorax myiasis outside of the North American continent was found in sheep located in Northern Africa in the year 1989. The myiasis spread rapidly infecting numerous herds in the territory during the following months. The infested area was 25,000 square kilometers, and reached more than 2.7 million animals between the Mediterranean Sea and the Sahara desert. From July 1989 to April 1991 there were more than 14,111 documented cases of large-scale myiasis due to the Cochliomyia hominivorax species. Traditional control methods were unable to prevent this screw-worm spreading in Africa and therefore the sterile insect technique was used. For the African outbreak, sterile flies were made and shipped over from a factory in Mexico. Then the 1.26 billion sterile flies were released to mate in the infested 15,000 kilometer area. Soon after the fly population shrunk and became manageable. This successful program cost almost 100 million US dollars and several months to complete. This eradication was declared the most efficient and successful international animal health program in the history of the United Nations Organisation.
After this historical outbreak in Africa the sterile male technique was mastered and is now being used to lower the number of flies in areas of Central and Southern America. However, the inaccessibility of some areas that the fly inhabits, language differences, and the need of constant vigilance has slowed the eradication of this species.
Research is currently being performed to develop a synthetic odor bait to replace the use of wounded animals as bait for the female Cochliomyia hominivorax. The previously used method of wounded animals to serve as bait for the female hominivorax has fallen out of favor. The synthetic bait is formulated to mimic natural wound fluid from animals. The female hominivorax adult flies are attracted to animal wounds in order to obtain a protein meal and also to oviposit. The synthetic bait could be used at research stations that monitor hominivorax in eradicated regions and to help decrease the hominivorax populations in infested regions.
Cochliomyia macellaria, or the Secondary screwworm, is a flesh-eating fly whose larvae consume only necrotic tissue, either that of carrion or of an animal or human host (myiasis). This important distinction between macellaria and hominivorax was not understood for much of medical history; myiasis of humans and animals was viewed as universally disastrous. However, as medical understanding of the process of tissue breakdown and infection progressed, it began to be observed that wounds with specific types of maggot infestation actually had a decreased severity and duration of infection. This progressed to the point of macellaria being applied in some cases as surgical maggots. However, the negative connotation surrounding the word “screwworm” has persisted, and the largely harmless macellaria are often blamed for myiasitic attacks that hominivorax are actually responsible for. This should not be interpreted to mean that macellaria is not an avid consumer of flesh; Cochliomyia macellaria are routinely among the first colonisers of carrion, and in forensic cases have long had a habit of literally consuming evidence. The macellaria are especially abundant on corpses and carrion in warm, direct sunlit areas. Fortunately, with the recent advent of molecular evidence, macellaria maggots removed from a body and boiled to sterility can now provide vital information regarding a victim and determining a post mortem interval. Forensic entomologists can use various extraction methods to test the composition of the alimentary canal of the larvae to determine if the victim had any drugs or mind-altering substances in their system before they were killed. It is important, however, for the forensic entomologist to determine whether the Old World Screwworm, Chrysomya rufifacies, is present in the maggot masses on the body. This is because Chrysomya rufifacies is usually after the macellaria in the succession of colonising a body and the rufifacies second and third instar larvae are facultatively predacious. This could result in a PMI being off by a few days at the most if the rufifacies were to predate upon all of the macellaria larvae.
Cochliomyia macellaria have the stereotypical metallic green body of the genus, and the larvae are extremely similar to that of hominivorax. The most effective way to differentiate the two is to note the absence on macellaria of the distinctive pigmented tracheal tubes, as well as the presence of a v-shaped pattern of spines on the anal protuberance, and the lack of an oral sclerite.
Interaction between humans and macellaria outside of accidental secondary myiasis has been largely unremarkable, with the notable exception of the early use of macellaria in surgical maggot therapy. However, given the medical stigma surrounding the screwworm fly, and given the slight potential for even macellaria to secondarily infest tissue beyond the desired extent of medical use, it has currently fallen out of favor with the medical community, which prefers the predictable Lucilia sericata. Nonetheless, macellaria has been and remains a concern for farmers, veterinarians, and public health officials.
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Cochliomyia hominivorax are primary, obligate parasites in the larval stage, and as a result of this are capable, unlike their Secondary screwworm counterparts, of initialising the penetration of the skin barrier to create an entry wound. Despite this, they are most commonly seen as colonisers of previously existing wounds, and will frequently be hatched from eggs laid at the perimeter of the wound. Once the infestation commences, a dark brown or reddish-brown discharge will begin leaking from the wound, sometimes accompanied by an unpleasant smell as the flesh begins to become necrotic. This is often the first sign in both livestock and human victims that something is amiss, and will often initialise consultation of a professional. As the infestation increases, the victim will begin to experience escalating tissue irritation, and in the case of domesticated animals may be observed to become withdrawn, listless, and anorexic.
Once the process of clinical diagnosis begins and the condition of myiasis is recognised, the larvae are fairly easy to identify. Their overall body structure resembles the spiraled screw shape their common name is based on, a shape fairly unique within parasitic larvae. The cranial end of the larvae will possess two sharply curved hooks, generally dark in color, and distinctive spiracle patterns will be observed at the caudal end. The most identifying features, and the easiest way to differentiate between hominivorax and macellaria, are the prominent darkened tracheal tubes which are visible in the final third of the larval body, and are often visible both with a microscope and to the naked eye.
Treatment of the victim can be time-consuming and, due to the high incidence of secondary infection, frustrating, but with decisive treatment, a surprisingly positive result is often achieved in all but the worst cases. The obvious first step is the manual removal of the maggots, generally using tweezers or forceps to seize the larva at the posterior end as the spiracles emerge to allow respiration. Once all larvae have been removed, a topical antibiotic smear will be applied, often with an oral antibiotic accompaniment. Necrotic tissue may need to be debrided, which can be a painful process. A loose dressing is essential to allow continued fluid drainage from the wound.
As with many things, prevention is the best cure. Any open wound, even one so small as a blister, is a potential infestation site, and should be treated accordingly with approved pesticides. At times of the year when livestock are most vulnerable (castration, birthing, etc.), daily inspections should be conducted when possible. Infestations detected early are quite treatable, but fatalities can and do result from advanced infestation, particularly in sheep and newborn calves.
Cochliomyia macellaria, or Secondary screwworms, appear only around an existing wound (wound in this case being defined as including the opened cord area of newborn livestock) and therefore practice facultative myiasis. They are attracted most strongly to infected wounds due to the strong odor of the wound’s discharge, but, as with hominivorax, even a small blister or abrasion can serve as an infestation site. Flies can also be induced to lay their eggs in hair or wool that is matted thickly and stained with any bodily fluid.
As with hominivorax, the first sign of an infestation is often the irritated surface of the wound, producing many of the same symptoms (although macellaria does not ingest the live tissue). The larvae can be diagnosed and identified by a trained professional based on larval size, shape (again following the spiraled screw-shape), and ultimately on spiracle pattern. Unfortunately, these larvae lack the distinctive pigmented tracheal tubes that mark hominivorax.
Once a screwworm infestation has been confirmed, rapid treatment is essential. The physical removal of all maggots is the first step, and is often accomplished by the same methods as discussed above. The incidence of secondary infection is considerably lower in macellaria infestations due to their lack of damage to living tissue and, in fact, in a neglected animal, macellaria may actually be beneficial from an infection-control standpoint, as the maggots will diligently remove only infected and necrotic tissue. A topical antibiotic smear should then be applied, again often reinforced with oral or injected antibiotic support, and a loose dressing should be placed in such a way as to allow drainage. If the victim in question is to be replaced in the same environment where the infestation was originally contracted, preventative measures in the form of a general insecticide application to the victim’s body, with an emphasis on the wound area, should be considered mandatory. As victims in macellaria strike may have severe wounds that have been in existence over a period of time, supportive fluids should also be considered. It should be noted that macellaria is also a possible vector of dysentery.
As the Secondary screwworm does not have the biological quirk of single-lifetime breeding that characterises hominivorax, no widespread control methods are currently in place beyond the usual pesticide spraying done for general fly control. However, due to its being associated with facultative myiasis, its impact on livestock in the United States is not nearly as severe. It is important to remember that macellaria can and is often also associated with human infestation in poorly run medical facilities and areas stricken by poverty; vigilance is therefore essential.
In a general note on prevention of all Diptera infestations, including hominivorax and macellaria, a great deal of trouble can be avoided by insuring sanitary living conditions for livestock and humans. Open garbage accumulation, contaminated standing water, and general exposed filth all serve to attract flies, and also serve as good locations for oviposition, which greatly increases the chances of an infestation. Exposed wounds also attract flies in general, and should never be neglected, no matter how insignificant they seem, for even a short amount of time.
Both Cochliomyia hominivorax and Cochliomyia macellaria thrive in tropical areas which are warm and humid.
Cochliomyia macellaria is the most common Cochliomyia species found in North America. This species is distributed throughout northern South America, Central America, Caribbean Islands, United States, and southern Canada.
Cochliomyia hominivorax was distributed throughout the northern South America, Central America, Caribbean Islands, and the United States prior to the use of the Sterile insect technique, after which it was eliminated from the U.S. and Mexico. However, the bordering Central American countries serve a challenge to keep the species eliminated since these countries still have populations of this fly. Many of these countries continue to implement elimination programs.
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