Screwworm in Livestock
Screwworms were once among the most important pests of livestock in the world like Iraq, causing annual losses estimated at more than 700 million dollars in USA.
They are obligatory parasites, and under optimal environmental conditions the flies oviposit on almost every wound that occurs even abrasions as small as tick and fly bites.
It is important to distinguish between Screwworm and the secondary maggots, which frequently infest necrotic tissues.
Primary Screwworm myiasis should be diagnosed promptly so that livestock owners in the community may be alerted to protect their animals.
Myiasis:
Is the infestation of live vertebrate animals with dipterous larvae, which for at least a certain period feed on the host’s dead or living tissue, liquid body substances ,or ingested food .Depending on their reliance on the host, such larvae are classified as obligatory or facultative. Screwworms are classified as obligatory because they feed on live tissue. Screwworm larvae penetrate deeply into a wound of a warm-blooded animal and feed on living tissue and body fluid. Facultative larvae, which feed on dead tissue.
Biology:
Screwworms are the larvae of the blow fly, a member of the family (calliphoridae الذباب المعدني) .The bluish green screwworm fly is similar in appearance to the common blow flies, but differs in that it’s larval development occurs only in the tissues of living warm-blooded animals.
The screwworm fly deposits 200-400 eggs on the edge of a wound in rows that overlap like shingles. After about 12 hours incubation larvae circled with rows of dark spines are visible through the translucent shells and the egg mass has a gray appearance. Upon hatching, the larvae crawl into the wound and burrow into the flesh. The maggots feed on the wound fluids and live tissues and complete their growth 6-8 days .The larvae then drop from the wound and burrow into the soil to pupate, The pupal period varies from 8 days to 2 months depending on temperature.
A freezing or sustained soil temperature below (8°C) kills the pupae. The adult insects mate when 3 or 4 days, old and gravid females are ready to oviposit at 6 days of age. In warm weather, the life cycle may be completed in 21 days.
Full-grown screwworm larvae are about 12 mm long. Since the body is tapered with a pointed anterior end and a blunt posterior end they roughly resemble a screw.
Clinical Finding:
Screwworm feed as a group, burrowing deeply into the flesh .The wound fills with profuse reddish brown exudates that almost completely covers the larvae and may stain the hair or wool for several inches as it drains.
An aid in recognizing screwworm infestation in range animals is the change in the animal’s behavior. Even a small and relatively inconspicuous wound infested with screwworm attracts not only screwworm flies , but house flies and blow flies, which seek the wound primarily to feed on the exudates and are extremely annoying to the infested animal .As the annoyance increases, the infested animal seeks protection by retreating to the densest available shade.
Prognosis:
If an infestation continues without treatment for as long a 2 weeks, at a time when ovipositing flies are numerous the host animal will almost certainly be killed Wounds treated within 4 days after infestation usually heal within a month.
Bacterial infections often complicate screwworm infestation .In general, cattle are quite resistant to infection and the wounds usually heal promptly with good treatment, but sheep goats and horses frequently develop secondary infections.
Control:
The most important control measures are prevention of wounds when flies are active, the prophylactic treatment of unavoidable wounds, regular and frequent inspection of range animals to detect infestations promptly, and treatment of existing infestations to kill larvae and prevent reinfestation.
Breeding should be controlled so that young are not born during the fly season. The navel of the newborn animal is a preferred site for oviposition and should be treated promptly with a recommended screwworm prophylactic if young are born during the fly season.
Castration, dehorning, docking and branding should be carried out in cold weather. If surgery is performed during the fly season, the wound should be coated with a screwworm remedy and the treatment repeated as necessary until the wound has healed.
Flies and ticks should be controlled by the proper use of insecticides. Each animal on pasture should be seen twice weekly .Animal that are not with the herd, but are solitary and in dense shade or other dark places, should be examined closely.
Eradication Program:
In 1958,the United States Department of Agriculture(USDA)in cooperation with livestock authorities of the southeastern states, initiated a program to eradicate screwworms from the Southeast screwworm larvae were reared on artificial media and ,2 days before fly emergence ,the pupae were exposed to gamma irradiation at a dosage that caused sexual sterility but no other deleterious effects. Sterile flies were distributed over the entire screwworm infested region at the average rate of 400 male flies per square mile per week, sufficient to outnumber the native flies.
The female mates only once and when mated with a sterile male do not reproduce. There was a decline in the native population each generation until the native males were so outnumbered by sterile males that no fertile mating occurred and the native flies were eliminated. The last case in Florida was found in (June 1959) and sterile-fly releases were discontinued in November. The state east of the Mississippi have remained screwworm –free since that time except for brief infestation resulting from import of infested animals from western states.
In (1962) a more extensive program of eradication was undertaken in the southwestern U.S.A In most of the years since 1962,this program has eliminated almost all the losses caused by screwworm in these states. Native populations of flies were eradicated in the winter of 1963-1964 and again in the winter of 1964-1965.
Unfortunately the southwest has been periodically reinfected by fertile female flies from Mexico.
There is evidence that the flies are capable of flying over 200 miles and no natural barrier exits to block their northward movement during favorable seasons.
The goal of a Joint Mexico –Us program started in 1976 is the eradication of screwworm from both countries and the establishment of a permanent barrier is southern Mexico to protect against reinvasion .Veterinarians and livestock producer in the U.S.A and Mexico can greatly assist the eradication effort by promptly reporting all suspected cases of screwworm to regulatory officials .If myiasis is found, larvae should be preserved in 70% ethanol and forwarded for identification.
Treatment:
The USDA has developed 2 remedies, either of which can be used for the protection of wounds form screwworm infestation and for treating infestation (EQ335) is a mixture containing 3% Iindane in a gel base .Smear 62 is an older remedy that utilizes benzene as a killing agent and diphenylamine as a protectant against infestation or reinfestation by newly hatched larvae.Ronnel has also been incorporation into a wound dressing similar to EQ335. All 3 smears are best applied with a 1 inch(2.5 Cm)paint brush .Careful application is necessary to be sure that the smear reaches all the many pockets formed by the burrowing maggots in deep wounds .A thin layer should also be applied to the skin surrounding the wound.
Sprays and dusts may also be used for screwworm control.
The topical application of a dust containing 5% Coumaphos is an economical and effective means of killing the maggots in a wound. In regions where screwworm flies are very active, it is often advisable to spray livestock with either Coumaphos or Runnel as a prophylactic measure. If properly applied, these sprays will kill all larvae and protect animals from infestation for 2 weeks or longer.
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