screwworm

People have been eaten alive by screw worms.

Screw worms are parasites that enter the body through open wounds.

Depending on a parasite's reliance on its host, it is classified as either facultative or obligatory. Facultative critters feed on decaying tissue. Common maggots are facultative and are used medicinally to keep dead tissue from accumulating in lesions. Obligatories are much more ghoulish — they feed on living tissue.

Screw worms are obligatory.

Screw worm myiasis is the infestation of screw worms in living tissue. They are the larvae of flies; they are maggots.

While across the globe screw worms attach themselves most commonly to livestock, they're open to any warm-blooded creature.

Originally, screw worm flies were distributed through much of Africa, the Middle East, the Indian subcontinent, Southeast Asia and Papua New Guinea. They found their way to the United States in 1933, shifting seasonally from the southeast to the midwest following the waxing and waning cold fronts. Capitalizing on the northward-bound cold one spring they migrated to the Southwestern US. In early 1959 mounting numbers of dead livestock saw human efforts to eradicate the little monsters. Colder than normal winters helped things along, and by the end of 1961 the southeastern US was declared screw worm free. The next year people focused on the Southwest; by 1964 the US was clean.

Today screw worms in the Western Hemisphere are limited to parts of South and Central America. So far the old world hasn't been so lucky.

Screw worm flies deposit up to 400 eggs in a shingle pattern at the edge of a suitable wound. The eggs hatch in hours, revealing squirming, 2-millimeter larvae — there's your infestation.

Screw worms are densely packed at the site of infestation. Occupied wounds look like patches of white. As the worms feed, the wound grows. Secondary flies come around and lay their own eggs, accelerating the destruction of tissue. After a week of infestation a wound will grow three centimeters in diameter and five to twenty centimeters deep on a single egg mass.

After three days or so the larvae drop out of the wound. They're negatively phototophic, meaning they hate light, so they burrow and become little pupae. Depending on ambient temperatures, transformation into a mature fly can take a week to two months.

After emerging, young screw worm flies take two hours to stretch before flying. Once airborne, they subsist on water and nectar. Better. Until they find wounded animals and feed on the fluids seeping out.

Sexual maturity comes quick — three to five days. Males mate several times; females usually only once. Three or four days after mating, the female finds a good place to deposit her eggs. Males live about two weeks while females hold out for a month.

Symptoms of myiasis are nasty.

Most infestations are in livestock, particularly in the navels of newborn animals. Little movement can be observed because the worms are so tightly packed.

The sarosanguinous discharge produced by the infested body is pungent.

Afflicted livestock will lick the wound constantly, lose appetite, experience stunted growth, and fall to secondary infections. Those that succumb display liquefactive necrosis (play with prefixes for a second — liquefied dead tissue) and, of course, seething masses of larvae. Not uncommonly, the larvae will burrow into the body's natural cavities — sinus cavities in the head, the pleurae of the lungs.

It's the same in people, except for the wound-licking.

Differential diagnosis is easy — just compare a screw worm fly larvae to another blow-fly maggot. Sometimes the two appear in the same wound. Take a sample from the deepest part of the lesion with a pair of tweezers.

Treatment is equally simple. Cover the wound with larvacide two or three times a day. The larvae die, the wound heals quickly, screw worm flies never lay eggs there again.

Prevention is largely a matter of treating wounds. In areas where the worms are endemic livestock should be inspected every three or four days for infestation. Similarly, wound-generating practices like castration and de-horning can be scheduled for the winter, when the flies aren't as active. Dips in organophosphate insecticides prevent infestation for seven to ten days.

Eradication is not so simple.

Larvae are lab-raised. After about five days in pupal stage, they're exposed to 5000 to 7000 rads of gamma radiation. They emerge unharmed but sterile. They're released into the wild and mate, causing females to deposit unfertilized eggs. The life cycle breaks. After two years the buggers are gone.

Benjamin for Iraq
http://www.benjaminforiraq.org/embargo/Screw%20Worm%20Infections%20in%20Iraq.htm

Novy, James E. "Screwworm Myiasis."
http://www.vet.uga.edu/VPP/gray_book/FAD/scm.htm

SPC
http://www.spc.int/rahs/Manual/Multiple_Species/SWFE.HTM

Australian Government Department of Agriculture, Fisheries and Forestry
http://www.affa.gov.au/content/output.cfm?ObjectID=FC594E57-3C35-400D-909331739E5EE3DA

The screwworm, more formally known as the New World Screwworm, is the parasitic maggot of the Cochliomyia hominivorax fly, a type of blowfly. It infests mammals, including humans (hominivorax is Latin for `devouring of man') and differs from most maggots in that it eats living flesh rather than the decaying tissues of a dead animal. Freezing temperatures and prolonged cold weather kills the screwworm, so its range is restricted to warm subtropical or tropical regions, specifically the southern U.S, Mexico, Central America and parts of the northern South American countries, and seasonal invasion of temperate areas. As a pest to cattle, they have a devastating economic effect. The health threat to humans is also significant.

The Worm

Up to 400 eggs can be laid each time, and the infected wound then becomes even more attractive to other gravid flies, so re-infection occurs repeatedly until the animal dies. A single wound can contain as many as 3000 larvae. The screwworms become packed together in dense clusters with only their rear parts visible. They tend to burrow deeper when disturbed, but screwworms are ectoparasites and remain close to the surface of the host. Under magnification, they look much like smiley faces packed into a honeycomb.

Dropping Out

The Fly

Bye, bye screwworm

Kingdom:      Animal
Phylum:       Arthropoda
Class:        Insecta
Order:        Diptera (mosquitoes and true flies, with one pair of wings)
Suborder:     Cyclorrapha (flies)
Family:       Calliphoridae (blowflies)
Species:      Cochliomyia hominivorax (screwworm fly)