Cochliomyia hominivorax - The Merck Veterinary Manual

Cochliomyia hominivorax
(Primary screwworm, New World screwworm)

Sterile Male Release Eradication Program

Cochliomyia hominivorax is distributed throughout the neoarctic and neotropical regions of the western hemisphere. As a result of massive state, federal, and international eradication programs, extant populations of C hominivorax are no longer found in the USA or Mexico; isolated reports are often traced to importation of infested animals from locations where the screwworm is still prevalent. Extant populations are found in Central and South America and in certain Caribbean Islands.

Cochliomyia hominivorax (screwworm) adult fly

Adult female flies lay batches of 200-400 eggs in rows that overlap like shingles in a mass on the edge of a fresh wound. After 12-21 hr, larvae hatch, crawl into the wound, and burrow into the flesh. The larvae feed on wound fluids and live tissue. After 5-7 days, grown larvae exit from the wound, fall to the ground, and burrow in the soil to pupate. The pupal period varies from 7 days to 2 mo, depending on the temperature. Freezing or sustained soil temperatures <46°F (8°C) kill the pupae. Adults breed only once during their lifetime, a fact used in biologic control. They usually mate when 3-4 days old, and gravid females are ready to oviposit when ~6 days old. In warm weather, the life cycle may be completed in 21 days. Only female flies feed and oviposit on wounds; males and younger, virgin females gather to mate in vegetation, especially flowering vegetation.

Pathology:

Newly infested wounds contain screwworm larvae of a single age; older, larger wounds may contain larvae of various ages and of different species of flies. The malodorous, reddish brown fluid produced in the wound usually drains and may stain the hair or wool around or below the wound. As annoyance increases, the infested animal seeks protection by retreating to the densest available shade. Even a small and relatively inconspicuous wound infested with screwworm larvae attracts not only more screwworm flies but also facultative myiasis-producing flies. Necrotic tissues attract even more flies. The wound can become greatly enlarged due to multiple infestation and, unless treated, usually results in death of the animal.

Diagnosis:

The parasitic larvae are tapered and have mouth hooks at the narrow end and breathing spiracles at the wide end. Body segments are ringed with spines. Fully grown larvae can be as long as 1.5 cm. Larvae are often identified by their “wood screw” shape and appearance and can be distinguished from the larvae of the facultative myiasis-producing flies by the darkly pigmented tracheal tubes on the dorsal aspect of the posterior end of third-stage larvae. These tubes can be easily visualized through the larval cuticle.

Adult screwworm flies are similar in appearance to other blow flies. They are bluish to bluish green, have a reddish orange head and eyes, and are slightly larger than a house fly. They are difficult to distinguish from other blow flies or bottle flies. Identification of adult screwworms is probably best left to an entomologist.

Treatment and Control:

Screwworm infestation must be reported to both state and federal authorities. C hominivorax has been eradicated from the USA but occasionally enters the country surreptitiously on imported animals. In the USA, if a wound is thought to be infested with screwworm larvae, appropriate samples should be collected and sent to eradication officials at P.O. Box 969, Mission, TX, 78572.

Screwworms in wounds can be killed by direct application of a wound dressing, called a smear. Such smears, which contain lindane or ronnel, may be difficult to find in the USA because of the eradication program. Smears are best applied with a 1-in (2.5-cm) paint brush and should reach all of the many pockets formed by the burrowing larvae in deep wounds. A thin layer should also be applied to the skin surrounding the wound to protect it from reinfestation. Wounds may also be treated with aerosol, dust, or foam formulations of coumaphos, lindane, or ronnel. To protect animals from infestation and also to kill larvae in small wounds that are difficult to detect, animals can be sprayed thoroughly with ronnel or sprayed with or dipped in coumaphos.

Sterile Male Release Eradication Program:

In 1958, the USDA initiated a program in the southeastern states to eliminate screwworms by the sterile male release technique. When reared artificially and exposed to irradiation shortly before they emerge from the pupae, male flies are sterile but able to mate. The female mates only once, and when mated with a sterile male, lays eggs that do not hatch. Therefore, release of sufficient numbers of sterile males in an area over a period of time leads to eradication. By 1959, screwworms had been eliminated from Florida. The program cost ~$11 million, whereas the fly and its treatments had been estimated to cost $200 million annually.

This program was expanded to cover the rest of the area involved in the USA and then, via a joint Mexico-USA agreement, to include most of Mexico. This, along with the use of screwworm attractant and an insecticide system that attracted and killed adults, led to eradication of screwworms from Mexico. There is interest in expanding this area throughout Central America and the Caribbean. However, until this has been achieved, constant vigilance by all who deal with animals in the southern USA and Mexico is necessary to detect an infestation quickly and to eradicate it before the flies reproduce and spread.