Screwworm Outbreak
Authors: Claire Alexander, Kamea Lessoway, Umama Mohsin, Benjamin Onyango
Current Outbreak
New World screwworm (NWS; Cochliomyia hominivorax) is a dangerous pest insect that infects wounds and vulnerable tissues of both animals and humans. Eradication efforts and successes have occurred over more than 50 years. It was first declared eradicated from the United States in 1966 (U.S. Department of Agriculture, n.d. -a). It has returned several times since, most recently in early June 2026. The current outbreak of NWS in North and Central America began in 2023.
On July 3, 2023, Panama declared a state of emergency after NWS crossed the Darién Gap between Panama and Colombia (Lee 2025). By the end of 2024, every country in Central America had experienced screwworm cases, and screwworm larvae were identified in a cow at the border between Mexico and Guatemala in November 2024 (Lee 2025).
The current outbreak in the United States began in Zavala County, Texas (U.S. Department of Agriculture, n.d. -b). Since June 3rd, 53 cases have been recorded, 3 of which are still active as of October 10, 2026. 51 cases were reported in Texas and two were reported in New Mexico; these cases have been in sheep, cattle, and goats, and several cases have also been reported in dogs. NWS can infect any warm-blooded animal, including livestock, wildlife, pets, and humans.
The current outbreak can be tracked at https://www.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/current-status/us-confirmed-cases-new-world.
Humans and Public Health
Human health and animal health are deeply connected. Previous eradication of NWS in Central and North America was a major victory for public health, animal health, and the economy. The current reemergence raises concerns about animal health and animal diseases that can affect humans and have potentially devastating economic impact, serving as a reminder of the interconnected nature of animal and human health (Walsh 2026).
Transmission of NWS to humans is rare but dangerous in those infected. The first human case in the United States associated with the current outbreak in the Americas was reported on August 4th, 2025, in Maryland, in a person who had traveled to Guatemala. The patient made a full recovery, and there was no known transmission to another person or animal (Pryce 2025).
Infection in humans is unlikely unless working around livestock, or more rarely, around pets, that are infested. Several cases in the current outbreak have been in dogs. Risk to pets is considered low, unless the animals are in counties with NWS cases. Monitor your pets daily for signs such as wounds, foul odours, or unusual behavior (Liz Szabo 2026).
Current Govt. Responses
Screwworm populations are controlled by using the sterile fly technique. Because female flies only mate once in their lives, sterilizing male flies is an effective way to reduce screwworm reproduction (Denton 2026). This form of autocidal control earned Edward Knipling and Raymond Bushland the 1992 World Food Prize (Denton 2026).
The United States Department of Agriculture (USDA) has released a five-pronged plan to respond to screwworm, including building new partnerships, conducting surveillance, and expanding sterile fly production (Usda.gov, n.d.). Currently the United States and Mexico can produce roughly 100 million sterile flies each week; roughly 600 million sterile flies will need to be released each week to be effective (Matza 2026). The United States has recently completed a new sterile fly dispersal center at the Moore Air Force Base in Texas, and a more advanced sterile fly production center that can release 300 million flies weekly is being built at the base over the coming years (Usda.gov 2026; Usda.gov, n.d.). The United States is also partnering with Mexico to expand its capacity of sterile fly production (Usda.gov, n.d.).
The development of the outbreak will depend partially on border controls as the United States, Mexico, Canada, and other nations open and close their borders to certain domestic animals. This will also impact patterns of trade, as beef cattle are often raised in Mexico and slaughtered in the United States (Polansek and Garrison 2026).
This recent emergency occurred after substantial cuts in budgets and personnel made to the US Agency for International Development (USAID) and the USDA under the Trump administration, limiting their capacity to respond (Armour 2026). As part of the USAID cuts, more than $300 million was cut from the United Nations’ Food and Agriculture Organization (FAO), which monitors zoonotic diseases like screwworm and protects global food security (Armour 2026). This was particularly devastating to South America because the region is continuously under threat by the New World screwworm (Food and Agriculture Organization of the United Nations 2026). With a decrease in preventative measures, the risk of undetected transmission increased.
It is important to note that recent budget cuts are not the sole cause of the epidemic. Joseph F. Annelli, an accomplished veterinary and agricultural scientist, emphasizes that this issue “reflects broader and longer-term vulnerabilities that predate any single administration” (Macdonald 2026).
Agricultural Safeguards
Agricultural safeguards are a key component of prevention and control. Good zootechnical processes and preventive measures can help control the disease. This includes veterinary certification and quarantine of animals destined for travel (WOAH – World Organisation for Animal Health 2025). There is, however, also an illegal cattle trade run by organised criminals. It is unlikely these livestock are checked, treated, and quarantined before transportation. Ecologist Jeremy Radachowsky says this has contributed to the return of NWS to Central America (Saltman 2026).
About Screwworm
Infestation with New World screwworm, a form of myiasis, begins when a female fly smells an open wound, even as small as an insect bite, and lays her eggs (CDC 2026; Zarra Aldrich 2026). Other targets include body orifices, such as the inward opening of the eye (medial canthus), mouth, nose, and genitals.
NWS maggots target domestic and wild animals and can also infect humans. Although they only mate once, females can lay eggs in multiple batches of up to 300 eggs (Zarra Aldrich 2026).
NWS larvae or maggots have mouth hooks that they use to burrow into and eat living flesh (Zarra Aldrich 2026). Any itching or disruption only makes them burrow deeper into the wounded tissue, hence the name “screwworm”. After ~7 days, the larvae fall off and pupate in the ground to become adult screwworm flies (CDC 2026). Screwworm can kill domestic animals. However, it is not considered a food hazard, and infected animals can recover completely.
Treatment and Prevention
Symptoms of a New World screwworm infection include wounds or unexplained sores that get worse over time, bleed, and/or cause pain. Infected people or animals may feel the maggots moving, and they can sometimes be visible. Wounds caused by NWS maggots often smell foul, and opportunistic bacteria often cause secondary infections, which may require antibiotics. People living in areas impacted by New World screwworm can reduce their risk by sleeping indoors, covering open wounds, and using EPA-registered bug spray. Clothing should be loose-fitting, long-sleeved, and sprayed with 0.5% permethrin (Cdc.gov, n.d.).
Screwworm is prevented in livestock by spraying or dipping animals in insecticide and cleaning open wounds (WOAH – World Organisation for Animal Health 2025). Wound-generating procedures such as dehorning put animals at higher risk (Denton 2026).
Individuals should never remove screwworm larvae from themselves because any disturbance encourages the maggots to burrow deeper. In humans, screwworm larvae should be removed by a medical professional, sometimes through surgery (Cdc.gov, n.d.). Affected animals should be quarantined or isolated, and their wounds should be cleaned, disinfected, and treated with larvicide (WOAH – World Organisation for Animal Health 2025).
Historical Context
The mainland United States eradicated New World screwworm in 1966. In Puerto Rico, NWS was eradicated in 1975 (Vargas-Terán, n.d.).
The Screwworm Eradication Program led by the United States helped eradicate New World screwworm from many Central American countries in the past 60 years. This program operated largely through the release of sterile males screwworms. The first partner was Mexico, whose government took a major role in designing the Mexico-United States Commission for the Eradication of the Screwworms. The United States contributed monetarily because eradicating screwworm from Mexico would also prevent its recurrence in the United States. Screwworm was also eradicated from Central American countries during the 1990s and in the early-mid 2000s (U.S. Embassy in Costa Rica, n.d.).
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