Veterinary Medicine Extension

University of California, Davis

Tulare CA 93274

July 27, 1999

Recently, screwworms have been found again in the United States despite the on-going sterile fly control programs to keep them far off down in Central America. In 1982, screwworms were eradicated from the United States. Re-infestation has been success ful prohibited by prevention strategies designed to keep cattle infested with the screwworm larvae from entering. However, a Basset Hound being imported from Panama into Texas was found to have a larvae- infested wound in its paw. Quick action by the exa mining veterinarian prevented spread from this dog to other animals or humans. One case was diagnosed recently in a military hospital in a U.S. person returning from Latin America. Soon, many US military will be returning from Panama as the Canal Zone is completely turned over to Panama. So the risk of re-entry is a real threat and we should be on the lookout for the screwworm larvae in all animal wounds.

Prior to the eradication program in the US, the annual livestock loss from screwworms was estimated to exceed $400 million annually. At that time in the southern US, almost every wound became infested with screwworm larvae unless the wound was specifi cally treated to prevent infestation. Left untreated, the flesh eating larvae could damage a steer severely enough to cause death in 5-7 days (Photo 2). To prevent these losses, laboratory-raised, irradiated, sterile screwworm flies were released in a ba rrier zone beginning in the 1950's. Millions of these sterile males were dropped by plane to mate with the fertile female flies. The resulting eggs would not hatch. By the mid-1960's, a self-sustaining screwworm population did not exist in the US. Howeve r, screwworm flies continued to cross over the Mexican border to infest our cattle. The final eradication effort involved moving the protective barrier slowly down through Mexico and Central America to Panama (Photo 1).

The screwworm fly is about twice as big as a common housefly. While these flies can travel up to 180 miles in a day, transportation of infested animal is the usual route of entry into an area. After mating, the female will lay her eggs at the edge of an open wound. One female can lay up to 400 eggs per day and they can hatch out in as little as 12 hours. Once they hatch, the larvae crawl into the wound. The larvae usually feed in a vertical position in the deeper portions of the wound. They are easil y seen as they grow to a half inch long (Photos 3, 5). The infested wounds have a very foul odor. Each larvae is equipped with a pair of sharp mouth hooks which they use to tear at their host's flesh (Photo 4). Typical dark liquid oozes from the infested wounds and is very attractive to other flies so wounds may become infested with thousands of larvae. Larvae feed on the wound for about 5-7 days before dropping to the ground to pupate. Adult flies hatch in about 7 days and can successfully mate in 2-3 days. Where present, flies can be found throughout the year in warmer climates such as California.

All types of wounds, even very small ones caused by tick bites, are attractive to the screwworm fly. Navel infestations in newborn calves is common as are infestation of adult cows (Photo 5). Wounds from dehorning, castration, docking, ear tagging and other common husbandry practices are also susceptible to attack. Tumors are also very attractive (Photo 6).

All livestock owners should be aware of the possibility of screwworm infestation in their animals. Any foul smelling wound oozing a dark liquid which contain fly larvae should suggest a screwworm problem. These wounds should be quickly brought to the attention of their veterinarian so that suspected larvae can be collected and submitted to the National Veterinary Services Laboratory in Ames, Iowa. Larvae can be placed in a container with 70% alcohol to preserve them for identification. By quickly ide ntifying any screwworm larvae, appropriate steps can be taken to prevent a return to the 1950's and the enormous losses which the livestock industry sustained do to screwworm infestations. Prompt attention to any wounds containing fly larvae is the key t o keeping this terrible livestock pest out of the United States.

Photo 1. Map showing the screwworm infested area from Panama down into South America.

Photo 2. Cow dying from multiple wounds infested with screwworms.

Photo 3. Screwworm larvae in an infested wound.

Photo 4. Closeup of screwworm showing the sharp mouth hooks used to tear flesh.

Photo 5. Screwworm larvae armed with sharp mouth hooks that they use to tear at flesh.

Photo 6. Cow with cancer eye infested with screwworms

Photo 7. Navel infestation of an adult cow.