Screw Worms and the Spread of Disease
Author: Disabled World : Contact: Disabled World
Synopsis and Key Points:
There are two types of screw worm fly, Chrysomyia bezziana, and Cochliomyia hominivorax found predominately in tropical areas of the American continent.
Screw worm fly maggots are parasites of all wild and domestic animals, as well as birds and people. At this time, the fly is not present in the nation of Australia for example, although it is in Indonesia and Papua New Guinea and is considered to be the most serious exotic pest threatening the livestock and people of Australia. The status is due to the trouble with eradicating the screw worm, which requires extensive use of sterile male release, the wide host range, as well as the damage to food production in Northern Australia this pest is capable of.
There are two types of screw worm fly. One is the old world screw worm or, 'Chrysomyia bezziana,' which is endemic in nations north of Australia. The old world screw worm is found in the coastal swamps of Papua New Guinea adjacent to the Torres Strait and throughout much of Malaysia, Indonesia and the Philippines. The other type of screw worm fly is the new world screw worm fly or, 'Cochliomyia hominivorax,' which is found predominately in the tropical areas of the American continent. The new world screw worm has been eradicated from the southern-most states of America.
Screw Worm Disease in Animals
Screw worm disease is caused when fly larvae or, 'maggots,' burrow into living flesh of the host and feed until they are mature and dropping off of the host to pupate on the ground. Adult flies are free-flying and prefer warm and humid environments. The female screw worm fly is attracted to any moist discharge on warm-blooded animals or even people, such as wounds or natural body orifices. The female fly lays her eggs - usually between 100 and 200 of them, on the edge of all types of wounds such as:
- Tick bites
- Running eyes
- Peeling brands
- The naval of newborn animals
- Castration or de-horning wounds
The fly's eggs are laid in rafts beside a wound to permit easy access for the screw worm maggots when they hatch. Screw worm fly larvae hatch in approximately 12 hours and penetrate the living tissues surrounding the wound. The larvae create a cavernous and bad-smelling lesion up to 10-12 cm in diameter. The lesions are characterized by progressive liquefaction, haemorrhage and necrosis. A secondary bacterial infection, 'toxaemia,' as well as fluid loss contribute to the death of the creature affected.
The development of the larvae is complete within 5-7 days, after which they leave the wound and fall to the ground. The larvae pupate for between 2-60 days depending upon the temperature before the flies emerge. The optimal temperature range for fly activity is 20-30 C. The flies do not survive temperatures below freezing. In prime temperatures and humidity, the cycle may be completed in less than 20 days.
The Spread of Disease
Screw worm flies could arrive in a nation as an adult fly through a storm front from nations nearby, or on a plane or boat. It is also possible for maggots to enter a nation in an existing wound on an animal or person. Constant, vigilant surveillance is required. The mean distance screw worms can travel is 11 km and the furthest distance is 100 km with help from the wind. Survival of the flies at their destination depends upon the presence of animals or people they can infest.
If screw worm flies were to reach a nation with the right set of conditions, it could have a devastating effect on wild and domestic animals, as well as people. The flies would be hard to control, particularly as adults that are free-flying and may disperse over large areas. Spread of the fly could be rapid if it is not detected early. Control measures include the following:
- Zoning to define infected and disease-free areas
- Decontamination and disinfection of larval-contaminated areas
- Tracing and surveillance to determine the extent and distribution of the fly
- Quarantine and movement controls to prevent movement of infested animals
- Frequent mustering to treat animals that are affected with chemical pesticides or insecticides
Eradication programs have been successfully carried out in several nations using the sterile insect technique. People who travel overseas may be infested with screw worm fly larvae. Any maggot-infested wounds, particularly in a person who has just returned from abroad, should be reported to a physician or health care worker. Infested wounds should be cleaned and treated promptly to prevent the spread of the pest.
In the year of 1988, several adult old world screw worm flies were trapped in an empty livestock vessel in Darwin harbor. The vessel had just returned from delivering cattle to Brunei. In 1992, new world screw fly larvae were identified in a lesion on the back of the head of someone who had just returned to the nation of Australia from a visit to Argentina and Brazil.
Screwworm control and eradication in the southern United States of America
About Screw Worm Fly
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