New World Screwworm Update, July 2025
On July 9, 2025, United States Secretary of Agriculture Brooke L. Rollins suspended importation of live cattle, horse, and bison through United States (US) ports of entry along the southern border due to the continued and rapid northward spread of New World Screwworm (NWS) in Mexico. A phased reopening of the border had been announced on July 7, 2025. However, the finding of a case of NWS in Ixhuatlan de Madero, Veracruz in Mexico, put halt to the opening of the border. The new NWS case is 160 miles north of the previous cases and 370 miles south of the US border.
According to the United States Department Agriculture (USDA), NWS was eradicated from the US in 1966. Although a few cases were found until 1982. The last officially diagnosed case of NWS in Oklahoma occurred in 1976. Since 1982, only sporadic cases of NWS have been diagnosed, but these were from animals or people that were infested in another country and then entered the US.
During 2016-2017, there was a small outbreak of NWS in the Florida Keys. The first confirmed case was found in a Key deer. The USDA confirmed 136 wildlife and 9 domestic cases of NWS. With the exception of one case, all the cases were detected in the Florida Keys. The one exception was a dog in Homestead, Florida. This was the first case reported on the mainland in many years. The outbreak was contained by March 2017.
NWS were eradicated from the US by using the Sterile Insect Technique. The program began in the 1950s. The program released male screwworm flies that were sterile. Since female flies usually only mate once, females that mated with the sterile males produced non-viable eggs. After several years, the population was eliminated.
Currently, the USDA maintains the COPEG facility in Panama. This facility produces sterile male NWS flies that were previously released along the border of Panama and Colombia. This was done to prevent the migration of NWS from South America. On February 26, 2025, the United States Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) announced that the strategy was changing. The sterile male NWS flies will be released in certain areas in Mexico to push the pest back south. In addition to these changes, Secretary Rollins announced the USDA is investing $21 million in renovating an existing fruit fly production facility in Metapa, Mexico, to further the long-term goal of eradicating this insect. Also, the STOP Screwworm Act has been introduced in Congress which would build a domestic sterile fly facility in the US.
NWS adult flies are slightly larger than the common housefly. They have orange eyes
and a blue green body which has 3 stripes down the back. The females typically mate
once and lay their eggs along the edges of an open wound or in the mucous membranes
(mouth, nose, ears) of an animal. When the eggs hatch, the larvae burrow into the
host flesh and feed on living tissue and fluids. Wounds attract more flies which compound
the problem. Once the larvae mature, they drop to the ground, burrow into the soil,
and begin the pupal stage. In a few days, the adult flies emerge to repeat the life
cycle.
Animals that are infested with NWS will have a wound that has a foul-smelling odor and oozes a blood-tinged fluid. The wound will enlarge and deepen if left untreated. Occasionally, a wound will appear not to enlarge, but on closer examination the wound will have many tracts under the skin. As the lesions get worse, the animal will refuse to eat and separate from the herd. If treatment is not provided, the animal will die from tissue damage, toxemia, and/or from secondary bacterial infections.
The first step in treating NWS infections is to remove the larvae. Once the larvae are removed, the wound should be cleaned and any necrotic tissue removed. Next, an insecticide should be applied to the wound. Antibiotics may be necessary to treat secondary bacterial infections.
Before the eradication of NWS from the US, livestock producers spent a great deal of time trying to prevent NWS infestations. Navels of newborns required insecticidal treatment. If producers failed to do this, many of these babies would die due to infections. Procedures such as tail docking, castration, dehorning, or branding had to be done at certain times of the year when NWS were not a problem.
The message that Dr. Rod Hall, State Veterinarian of Oklahoma, would like livestock producers to remember is:
- Currently, the USDA has not found a case of screwworm in the US.
- Livestock producers should develop a basic understanding of the parasite.
- The possibility exists that Oklahoma might find a case this summer.
- Even if a case is discovered, Oklahoma beef, pork, and poultry continue to be safe for human consumption.
- Information about collecting and submitting samples on potential cases is being developed and will be announced soon.
- If cases are found in Oklahoma, Oklahoma Department of Agriculture, Food, and Forestry will be advising on the best treatment options.
Additional information about NWS may be found at the following websites: