Deer infested with NWS. Photo by the U.S. Department of Agriculture (USDA) Transboundary diseases are those that are not endemic in a region, but have significant risks if they were to enter, for example, the United States. Risks include economic and threats to human, animal, and environmental health. In the U.S., these diseases are referred to as foreign animal diseases (FADs). Examples include foot and mouth disease, bluetongue virus, and rinderpest, among others, including the recently re-emerging threat of New World screwworms (NWS). While many FADs are not zoonotic, they pose severe economic risks and threaten our food supply. NWS poses a genuine One Health concern, not only economically but also because of the animal and human health threats they present. Although rare in humans and occasionally found in birds, they can occur in wildlife, pets, and, most commonly, livestock.1 New World screwworms: Why we should care As of writing, there have been more than 144,000 animal cases and over 1,100 human cases of NWS reported in Mexico and Central America since 2024. One person in the U.S., returning from travel in El Salvador, was diagnosed with the condition. Thankfully, the fly remains undetected in the country. Still, continued One Health cooperation among all entities is warranted to prevent the reintroduction of this eradicated disease.2 An adult New World screwworm fly. Photo by the U.S. Department of Agriculture (USDA) While this condition was once present in the U.S., it remains a significant FAD and One Health concern. (See: “Risk factors for high-level concern for FADs” for more info.). Since 1966, excluding a small, easily controlled outbreak in the Florida Keys in 2017, NWS has been confined south of Panama, a biological barrier region. Before an outbreak in Chiapas, Mexico, on November 22, 2024, the organism was present in Cuba, the Dominican Republic, Haiti, and South American countries. Through international cooperation and the use of sterile insect technique, by releasing sterile males into the environment, females were eventually unable to lay eggs and died off, thereby eradicating the disease from the U.S., Mexico, and Central America.1,3–5 The recent encroachment of the organism back into Mexico has led to a robust U.S., Mexican, and other partner response to rapidly increase the production of sterile flies in hopes of pushing the species back, to achieve a safe buffer once again, not threatening U.S. borders, food supply, environmental contamination, and protecting people and animals alike. Risk factors for high-level concern for FADs15 Criteria for high-risk diseases (diseases may be reportable and or of crucial national security risks) warranting attention include those with: Significant risk of respiratory disease Vesicular lesions No treatment available or poor response if treatment is available Lumpy/pox conditions of the skin Unidentifiable etiology causing encephalitis Live larvae found on tissues (human/animal) Exotic flies, lice, mites, or related vectors Generalized unusual, inexplicable clinical signs Recent animal importation and contact noted (of animals and people) Travel history to distant locations Receiving goods or visitors from foreign regions with high-risk endemic diseases Diseases with both high morbidity and mortality rates High abortion rates with unknown etiology Small bugs, big bite Cochliomyia hominivorax, the NWS, represents the only fly species whose maggots feed on living, not dead tissue. The larvae of this fly species burrow into and feed on living tissue, causing not only painful injury, but also potentially fatal consequences. The flies are slightly larger than the common housefly. With three darkened stripes across their back, a green or blue body, and orange eyes, their larvae screw into a wound, noshing all the way.1 The NWS larva can burrow into and feed on living tissue, causing painful injuryand potentially fatal consequences. Photo by the U.S. Department of Agriculture (USDA) The female adult mates once and lays up to 3,000 eggs during her 10- to 30-day lifetime; the eggs hatch into larvae that feed for seven days, then drop to the ground, where they burrow into the soil and pupate. Within one to eight weeks, the pupa hatches and the adult emerges. Duration to hatching varies with environmental conditions. Adults are then drawn to open wounds, genitalia, umbilical regions in newborns, and orifices via smell. Even tick bites are sufficient to attract the female, who then lays her eggs in the wound (200-300 at one time).6 Clinical signs in livestock/pets Signs in animals may include:1,7 Visible evidence of maggots in open wounds or openings, e.g., genitalia, ears, nares, or umbilical region in newborns A smell of decay emanates from the animal Bloody or purulent discharge from the wound Shaking of the head Irritability Changes in appetite Depression Pain Herd animals self-isolating Because these organisms can cause significant wounds, even leading to mortality, and because livestock can be heavily impacted, the food supply is at great risk. When present in the U.S., the costs of livestock care and eradication were substantial. We need to take steps to ensure we don’t risk any losses (financial, medical, or food supply).3 For images of larvae in animals, check out the the United States Department of Agriculture (USDA) poster. A close-up shot of an NWS larva (maggot), with the first one showing its mouth hooks. Photo by the U.S. Department of Agriculture (USDA) Treatment No treatment to date has full U.S. Food and Drug Administration (FDA) authorization since the disease is still a FAD. However, conditional approval under the Emergency Use Authorization (EUA) for treatment has been achieved, as of December 17, 2025, for the following medications:8,9 Dectomax-CA1 (doramectin injection) for cattle Exzolt Cattle-CA1 Credelio Quattro-CA1 for dogs and puppies Credelio Cat (lotilaner chew) for cats and kittens Remember, drugs approved conditionally still require full licensure, and use is limited to a specific clinical indication, in this case, the treatment of NWS. It means reasonable safety and efficacy are presumed based on initial research, and more studies are ongoing.10,11 Prevention strategies Strategies on the national level to help prevent reintroduction of this organism include:13,14 The prohibition of livestock trading from the southern entry ports. This includes the prevention of live importation of horses, bison, and cattle from land via the Mexico-U.S. border. In cooperation with Mexico, the U.S. helped to open a sterile fly dispersal facility in Tampico, Mexico. Along the Mexico-Texas border, a sterile fly dispersal facility and a production center are slated to be completed at Moore Air Base, Mission, Tex., this year, to aid in the eradication process. The U.S. continues to assist in trapping, establishing, and re-evaluating animal movement protocols, and aiding in surveillance efforts in Mexico and in the U.S. neighboring regions of high risk. Strategies at the animal level1 Frequent vigilance and surveillance Inspect animals with recent travel history for NWS In newborns, treat umbilical cords with insecticide if animals are in high-risk regions or, at a minimum, ensure the umbilical region remains wound and discharge-free All wounds should be treated immediately Protect animals from ectoparasites, including fleas, ticks, mites, lice, and biting flies Ensure safe procedures and avoid those that may lead to open wounds Strategies for human NWS infection prevention2 Utilize proper insecticides on clothing and any equipment Utilize EPA safe repellents Cover open wounds in at-risk areas Sleep in enclosed tents/screened-in areas to protect from fly exposure Minimize skin exposure in the elements (long-sleeved shirts/pants with socks and a hat) How about NWS? The unicorn is always possible You observe a maggot in a wound on an animal, say a cat or rabbit, and naturally think of a cuterebra in most regions of the country. Although not in the U.S., and hopefully it will remain a FAD, keep NWS on your radar; the unicorn is always a possibility. Obtaining a complete history for all pets, including their travel history and all human contacts, remains paramount. Recognizing signs of concern and reporting any suspicions promptly could make all the difference. Do you need to remember the nitty-gritty about the organism? Of course not, but having it in the back of your mind, recognizing were you to identify it incorrectly and permit organism reproduction, you risk not just the health of the individual animal, but threaten our environment, livestock, wildlife, and even yourself and staff. While I think of rabies as the epitome of a global One Health disease, think of NWS as Central and South America’s current One Health disease of concern. NWS in people Those at highest risk for NWS are those with occupational exposure, residing in rural regions where the disease is endemic, those with open, unprotected wounds exposed to flies, and those who may be vulnerable, including those with immunosuppression, improper nutrition, and those who are very young or old. Symptoms in humans include:2,6 Visible maggots around orifices or open wounds Feeling movement in the same regions Painful sores/wounds that are not explainable and not healing Possible case of NWS, what do you do? If you suspect you have a case of NWS, you need to ensure you do the following:12 Contact your State Animal Health Official (SAHO) and the APHIS Veterinary Service (VS) Vet in Charge (AVIC). Submit samples to the National Veterinary Services Laboratories (NVSL) only AFTER consulting with the above representatives. Usually the official will take samples, not the identifying veterinarian. That being said, the collection of the larvae, with atraumatic forceps, from the deepest portion of the wound and placing them into a test tube/vial with 70 percent alcohol as part of your treatment protocol, may be reasonable. Keep them at room temperature.7 Treat and quarantine any animal of concern. Re-evaluate the animal 24 hours after treatment. If live larvae remain, retreatment is warranted, and the larvae should be removed. Once clear of the organism, a State or Federal Veterinarian will clear the animal of NWS and release the quarantine. Proper cleaning and disinfection of any contaminated areas is then necessary Continued surveillance of the property where the affected animal lives, as well as any other animals/people in the vicinity is necessary. Erica Tramuta-Drobnis, VMD, MPH, CPH, is the CEO and founder of ELTD One Health Consulting, LLC. Dr. Tramuta-Drobnis works as a public health professional, emergency veterinarian, freelance writer, consultant, and researcher. She is passionate about One Health issues and believes pet health, food safety, agricultural health, and more can address the interconnection of human, animal, and environmental health. References USDA: Animal and Plant Health Inspection Service. New World Screwworm. USDA: Animal and Plant Health Inspection Service. December 22, 2025. Accessed December 31, 2025. https://www.aphis.usda.gov/livestock-poultry-disease/cattle/ticks/screwworm Centers for Disease Control & Prevention (CDC). New World Screwworm Outbreak. CDC: New World Screwworm. December 29, 2025. Accessed December 31, 2025. https://www.cdc.gov/new-world-screwworm/situation-summary/index.html USDA National Agricultural Library. The New World Screwworms: Selections from the Screwworm Eradication Collection. USDA National Agricultural Library. Accessed December 31, 2025. https://www.nal.usda.gov/exhibits/speccoll/exhibits/show/stop-screwworms--selections-fr/introduction Nolan RS. Mexico screwworm case triggers US emergency response. American Veterinary Medical Association. December 20, 2024. Accessed December 31, 2025. https://www.avma.org/news/mexico-screwworm-case-triggers-us-emergency-response USDA: Animal and Plant Health Inspection Service. Mexico Notifies United States of New World Screwworm Detection. USDA: Animal and Plant Health Inspection Service. Accessed December 31, 2025. https://www.aphis.usda.gov/news/agency-announcements/mexico-notifies-united-states-new-world-screwworm-detection Centers for Disease Control & Prevention (CDC). Clinical Overview of New World Screwworm. CDC: New World Screwworm. November 5, 2025. Accessed December 31, 2025. https://www.cdc.gov/new-world-screwworm/hcp/clinical-overview/index.html USDA: Animal and Plant Health Inspection Service. New World Screwworm: Be Aware and Prepare (Information for Veterinarians). Published online April 2025. Accessed December 31, 2025. https://www.aphis.usda.gov/sites/default/files/factsheet-nws-private-veterinarians.pdf FDA’s Center for Veterinary Medicine. FDA Issues Emergency Use Authorization for Credelio CAT to Treat New World Screwworm in Cats. FDA. November 21, 2025. Accessed December 31, 2025. https://www.fda.gov/animal-veterinary/cvm-updates/fda-issues-emergency-use-authorization-credelio-cat-treat-new-world-screwworm-cats FDA’s Center for Veterinary Medicine. New World Screwworm: Information for Veterinarians. FDA. December 17, 2025. Accessed December 31, 2025. https://www.fda.gov/animal-veterinary/safety-health/new-world-screwworm-information-veterinarians FDA Office of the Commissioner. Emergency Use Authorization. FDA: US Food and Drug Administration. September 26, 2025. Accessed December 31, 2025. https://www.fda.gov/emergency-preparedness-and-response/mcm-legal-regulatory-and-policy-framework/emergency-use-authorization FDA’s Center for Veterinary Medicine. Conditional Approval Explained: A Resource for Veterinarians. FDA: US Food and Drug Administration. May 29, 2024. Accessed December 31, 2025. https://www.fda.gov/animal-veterinary/resources-you/conditional-approval-explained-resource-veterinarians USDA: Animal and Plant Health Inspection Service. Standard Operating Procedure for Possible Detections of New World Screwworm in Animals. Accessed December 31, 2025. https://www.aphis.usda.gov/sites/default/files/aphis-sop-detection-nws-in-animals.pdf USDA: Animal and Plant Health Inspection Service. Announcements: New World Screwworm. USDA: Animal and Plant Health Inspection Service. Accessed December 31, 2025. https://www.aphis.usda.gov/news/releases American Veterinary Medical Association (AVMA). USDA unveils Texas screwworm facility, eradication strategy. AVMA News. July 1, 2025. Accessed December 31, 2025. https://www.avma.org/news/usda-unveils-texas-screwworm-facility-eradication-strategy USDA’s Animal and Plant Health Inspection Service (APHIS). NVAP Reference Guide: Notifiable Diseases and Conditions. USDA: Animal and Plant Health Inspection Service. November 17, 2025. Accessed December 31, 2025. https://www.aphis.usda.gov/nvap/reference-guide/emergency-management/notifiable-diseases
