– Dr. Michelle Arnold, Rminant Extension Veterinarian, University of Kentucky
Figure 1: NWS larvae are pale with encircling black spines and sharp mouth hooks. Accessed from “Update on New World Screwworm 2025”; Gleeson Murphy; Parasitology, Chemistry, Analytical Services (PCAS), USDA, APHIS, Veterinary Services.
The New World screwworm (NWS, Cochliomyia hominivorax) is a blow fly that is native to the Western Hemisphere. Unlike most species of blow flies, adult female screwworms do not lay eggs on dead and decaying flesh. Instead, they lay eggs on living mammals at the borders of fresh wounds or at the edge of body orifices. The larvae (maggots) feed on the host’s living flesh, causing extensive damage by tearing at the host’s tissue with sharp mouth hooks (see Figure 1). The term “myiasis” is used to refer to the infestation of wounds by fly larvae/maggots. The wound will become larger and deeper as more eggs hatch and larvae feed on the living tissue, which may result in secondary infection and death if left untreated. New World screwworm is a pest that poses a dangerous and significant agricultural, economic, and public health threat to livestock, wildlife, pets, and people.
Figure 2: Accessed from https://www.aphis.usda.gov/sites/default/files/nwspest-card.pdf
Adult New World screwworms are metallic blue or green blow flies about the size of a common housefly or a bit larger with three distinct stripes that run down the top (thorax) of the fly just behind the head with large orange eyes (see Figure 2). The Old World screwworm fly, (OWS, Chrysomya bezziana), is found in Africa, the Indian subcontinent, and southeast Asia. OWS and NWS are designated “the primary screwworms.” Cochliomyia macellaria (the secondary screwworm) is also a metallic blue blow fly with three distinct stripes that lives in the Americas and Caribbean, but the adult secondary screwworms primarily lay eggs on dead and decaying flesh (carrion). Although the secondary screwworm does not pose a threat to animal health, the adult flies and maggots look very similar to NWS, making confirmation of NWS myiasis much more challenging.
As mentioned previously, adult female NWS flies lay eggs on living mammals at the borders of wounds or at the edge of body openings on the mucous membranes. Almost any wound is attractive to the female, whether the wound occurred naturally (for example, from fighting, predators, thorns, disease, tick or insect bites) or was man-made (from shearing, branding, castrating, dehorning, tail docking, and/or ear-tagging). The most commonly infested wounds are navels of newborn animals and the vulva and perineum of their dams, especially if there was any trauma when giving birth. Flies are also highly attracted to velvet shedding in deer. Eggs may also be deposited on mucous membranes that enable maggots to invade through natural body openings including the nostrils and associated sinuses, the eye orbits, mouth, ears, and genitalia. One female fly can lay 200-300 eggs at a time and on average will lay 4 batches in her 10–30-day lifespan. Screwworm eggs are creamy and white and are deposited in a shingle-like raft on or near the edges of superficial wounds or body openings. This distinct pattern helps distinguish them from the eggs of other species of flies, which are generally not organized.
The eggs hatch within 12–24 hours and the larvae immediately begin to feed, burrowing head downwards into the living tissue. Larvae can usually be observed within the wound by the third day and are oriented with their rear ends up toward the surface. There can be as many as 200 maggots packed in deeply from a single infestation. Existing infestations often attract additional NWS female flies to lay their eggs, resulting in deep and gaping wounds that may contain hundreds or thousands of larvae in various stages of development. Large pockets of screwworms can also exist despite only small openings in the skin, but movement can be seen under the skin beneath the wound. Mature larvae can reach 17 mm in length (2/3 of an inch) and have spines that protrude from the body and wrap around in a spiral fashion, giving them the appearance of a wood screw and thus the name “screwworm”. Also, the name screwworm refers to the feeding behavior exhibited by the maggots as they burrow (screw) themselves deep into the wound. After developing through three larval stages in 5-7 days, the larvae leave the wound and drop to the ground, where they burrow into the soil to pupate. If the host dies before the larvae are mature enough to leave the wound and pupate, their survival is curtailed as they require living tissue as a food source. The duration of the life cycle in the ground is temperature and humidity dependent and can last from 7 days to 2 months, but the pupae cannot survive sustained temperatures below 46° F. The entire life cycle may be completed in as little as 3 weeks depending on temperature, moisture, and soil type.
New World screwworm was eradicated from the US, Mexico and Central America by repeatedly releasing sterile male flies that mated with wild female screwworms to produce unfertilized eggs. This “sterile insect technique” (SIT) was effective because males mate with multiple females while the female mates only once. This led to a reduction in screwworm numbers and eventually complete elimination from the US in 1966. This approach, along with regular active surveillance and animal inspections to prevent entry of any animal infested with larvae, proved highly successful and pushed the fly population south of Panama to the Darién Gap (along the border between Columbia and Panama) in 2006. APHIS maintains the only NWS pupae sterilization facility in North America known as COPEG, which stands for “Panama-United States Commission for the Eradication and Prevention of Screwworm”. Located in Pacora, Panama but managed jointly by USDA and Panama, COPEG produces, sterilizes, and releases 20 million pupae per week for maintenance of the barrier with the capacity to increase to 100 million pupae per week during an outbreak.
Figure 3: Screwworm movement northward beginning in 2023. Accessed from “Update on New World Screwworm 2025”; Gleeson Murphy; Parasitology, Chemistry, Analytical Services (PCAS), USDA, APHIS, Veterinary Services
The Darien Gap quarantine line held with only occasional cases in areas west of the Panama Canal until 2023, when the New World screwworm became reestablished north of the Panama Canal (Figure 3). There have been over 6,500 cases in 2024, and on 11/22/2024, Mexico notified APHIS of a positive detection. On February 26, 2025, APHIS announced it was shifting sterile fly dispersal to the northern most point in Mexico of the current outbreak. As of May 2025, the fly had moved as far north as Veracruz, Mexico. On May 11, 2025, U.S. Secretary of Agriculture Brooke Rollins suspended live cattle, horse, and bison imports from Mexico through U.S. ports of entry along the southern border since the most probable pathway for NWS to enter the country is through infested individuals or animals arriving at border crossings or interior ports of entry. APHIS is investing $109.8 million to keep the pest from spreading into North America with the goal of eradicating NWS in Central America and Mexico and re-establishing the biological barrier in the Darien Gap. The last time NWS was in the US was the Florida Keys outbreak in 2016-2017: 5 dogs, 2 pet pigs, 2 cats, 1 raccoon, and 135 Key Deer were affected (15% of the Key Deer population died) during the outbreak and it cost $3.2 million to finally eradicate the fly. Texas is at high risk for NWS because of its border with Mexico but what about Kentucky? Every state is at risk due to international travel of people, pets, and livestock. If you suspect NWS (see Box 1 below), immediately report any suspicious wounds, maggots, or infestations to a local accredited veterinarian, your State Animal Health Official, or a USDA veterinarian. Definitive field identification of screwworm adults and larvae can be difficult, even with a microscope, and can only be confirmed by submitting specimens for expert diagnosis. A USDA accredited veterinarian or state/federal authority will take the necessary samples and send them to NVSL for identification. The affected animal will be held in quarantine until official parasite identification is obtained and, if confirmed, treatment is administered until no larvae remain in the wound. Treatment is generally by application of organophosphate insecticides into infested wounds, both to kill larvae and to provide residual protection against reinfestation. A list of effective pesticides is available at https://www.aphis.usda.gov/sites/default/files/pesticides-for-nws.pdf. Preventive measures include the spraying or dipping of susceptible livestock with organophosphate compounds and, more recently, use of avermectins (ivermectin, doramectin) as subcutaneous injections to animals ‘at risk’. For more information, APHIS maintains a dedicated website at: https://www.aphis.usda.gov/livestock-poultry-disease/cattle/ticks/screwworm
