Screwworm

  We realize that this is likely an inappropriate time of the year to publish an article on Screwworm.  But because it was recently in the news that positive and false positive cases were reported in the U.S., we will print the following article which was plagiarized from a USDA/APHIS article found on the world wide web as a reminder of its potential for entrance.  The U.S. has spent millions of dollars in successfully eradicating Cochliomyia hominovirax; if they are reintroduced, it could well be a veterinarian who would make the first identification.  The diagnosis of this screwworm occurrence is classified as an exotic myiasis and, therefore, a reportable disease.  Being aware that it would be rare to find this parasite in Indiana, we are also cognizant of the importance of identifying it early if it should arrive here so that immediate eradication actions can be taken.

  A screwworm infestation is caused by larvae of the fly Cochliomyia homino-virax.  These larvae can infest wounds of any warm-blooded animal, including human beings.  The screwworm fly is about twice the size of a regular house fly and can be distinguished by its greenish-blue color and its large reddish-orange eyes.  Infestations can occur in any open wound, including cuts, castration wounds, navels of newborn animals, and tick bites.  The wounds often contain a dark, foul-smelling discharge.  Screwworm larvae distinguish themselves from other species by feeding only on the living flesh, never dead tissue.  Once a wound is infested, the screwworm can eventually kill the animal or human, literally eating it alive.

After mating, the female screwworm fly fly lays her eggs in open wounds.  One screwworm fly can lay up to 400 eggs at a time and they can hatch into larvae in as little as 12 hours.  A single adult female can lay as many as 2800 eggs during its 31-day lifespan.  The screwworm larva grows inside the wound to greater than one-half inch within 5 to 7 days of entering the wound.  The full-grown larva then drops from the wound, tunnels into the soil, and forms an immobile protective case that houses the pupa.  An adult screwworm fly emerges from the pupa.

Screwworms are eradicated through a form of biological control.  Millions of sterile screwworm flies are raised in a production plant located in Tuxtla Gutierrez in the southern Mexican State of Chiapas near the Isthmus of Tehuantepec.  During the pupal stage of the fly’s life cycle, the pupae are subjected to gamma radiation.  The level of radiation is designed to leave the fly perfectly normal in all respects but one: it will be sexually sterile.  Thus, when the artificially raised flies are released into the wild to mate with native fly populations, no offspring will result from the matings.  These unsuccessful matings lead to the gradual reduction of native fly populations.  With fewer fertile mates available in each succeeding generation, the fly, in essence, breeds itself out of existence.  The screwworm fly can travel up to 180 miles in several days and under warm, favorable conditions can complete a life cycle in as few as 3 weeks. Left untreated, screwworm-infested wounds lead to death.  Multiple infestations can kill a grown steer in 5-7 days.

  As early as 1825, western States reported serious screwworm problems.  Infestations spread to the Southeast by the 1930’s.  Losses to livestock producers exceeded $400 million annually.

  A plan for eradicating the pest began in the early 1950’s, when USDA’s Agricultural Research Service developed a new control method.  Under this method, laboratory-raised flies sterilized by gamma rays are spread by aircraft over infested areas.  As millions of sterile flies flood an area, the sterile males mate with fertile female flies.  The resulting eggs do not hatch.  This sterile insect technique was tested in a field trial on the Dutch island of Curacao in 1954 and then used operationally in Florida by 1957.  By 1959, screwworms had been eradicated from the Southeast.

  The sterile insect technique was next applied to the more extensively infested Southwest starting in 1962.  Self-sustaining screwworm populations were eliminated from the United States by 1966.  A barrier zone of sterile flies was set up along the 2,000-mile-long U.S.-Mexican border to prevent reinfestation from Mexico.  However, constant reinfestation from migrating flies or larvae carried by animals, which are then transported by people, remained a  problem.  The United States-Mexico Joint Commission was formed in 1972 between Mexico and the United States with the goal of eliminating the pest from Mexico and pushing the barrier to the Isthmus of Tehuantepec, just north of Guatemala.  A new sterile screwworm plant at Tuxtla Gueierrez, Chiapas, Mexico, was dedicated in 1976.  With a production capacity of more than 500 million sterile flies per week, it replaced the former production plant in Mission, Texas, which was closed in January 1981.  APHIS also is cooperating with Central American countries and Panama in efforts to eradicate screwworms from those countries and establish and maintain a barrier of sterile flies at the Darien Gap between Panama and Colombia.

  As a result of these cooperative efforts, Mexico was officially declared free of screwworms in 1991, Belize and Guatemala in 1994, and El Salvador in 1995.  In addition, Honduras is considered technically free, with no pest detections since January 1995.  Currently, screwworm program officials are focusing their efforts on eradicating the pest from Nicaragua and Costa Rica.  APHIS hopes to begin eradication activities in Panama, the final frontier of the program, in 1997.  Eradication activities include regulation of cattle movement, wound treatment, and the release of sterile flies.  To date, the program has been very successful.

  The screwworm program’s resources were strained by introduced outbreaks that occurred in Mexico in 1992 and 1993.  Of the 66 cases that were identified, the closest one was only 121 miles from the U.S. border.  While these outbreaks were eliminated, the resources that APHIS spent in eradicating them pushed back the entire Central American program by 12 to 18 months.

  As part of the screwworm program’s overall strategy, a new sterile fly rearing facility in Panama will be established to replace the existing one in Mexico.  The Panamanian government has agreed to provide land for the new facility.  Establishing the new facility well outside the area where screwworms have been eradicated will reduce the risk of reinfestation through an accidental release of fertile flies.