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New World Screwworm

Mexico Confirms Case of New World Screwworm in Nuevo Leon: https://www.usda.gov/about-usda/news/press-releases/2025/09/21/mexico-confirms-case-new-world-screwworm-nuevo-leon

On September 21, 2025, Mexico’s National Service of Agro-Alimentary Health, Safety, and Quality (SENASICA) confirmed a new case of New World screwworm (NWS) in an animal transported to Sabinas Hidalgo, located in the state of Nuevo León, less than 70 miles from the U.S.-Mexico border. This is now the northernmost detection of NWS during this outbreak, and the one most threatening to the American cattle and livestock industry. Sabinas Hidalgo is located near the major highway from Monterrey, Nuevo Leon, to Laredo, Texas, which is one of the most heavily trafficked commercial thoroughfares in the world.

Why is this important?

(The following information is from the University of Arizona Cooperative Extension publication released this month. Read it here.).

Authors: Ashley Hall¹, Flavie Audoin², Andrew Brischke³, Elizabeth A. Greene⁴, Deborah L.H. Reed⁴, Katie Spanyers⁵, and Ashley Wright².

¹University of Arizona Cooperative Extension - Globe, AZ
²University of Arizona Cooperative Extension - Tucson, AZ
³University of Arizona Cooperative Extension - Kingman, AZ
⁴School of Animal and Comparative Biomedical Sciences, University of Arizona - Tucson, AZ
⁵University of Arizona Cooperative Extension - Prescott, AZ

New World screwworm (Cochliomyia hominivorax) is a fly larva, or maggot, that can burrow into the flesh of a living animal, unlike other fly larvae that only feed on dead flesh. New World Screwworm (NWS) can affect any livestock species, as well as pets, wildlife, birds, humans, and essentially any warm-blooded animal.

In the 1950’s and early 1960’s, NWS infestations resulted in devastating losses to cattle and wildlife in the southern United States. Economic impacts reached $50-100 million (USDA, 2025). In the 1960’s NWS was eradicated in the United States (US). Since then, the US and Panama have collaborated through the Commission for the Eradication and Prevention of New World Screwworm Infestation in Livestock (COPEG) to prevent NWS from spreading north of the Darién Gap by releasing sterile male flies. Since NWS female flies tend to breed once in a lifetime, this process decreases the NWS population.

In 2023, NWS was able to break this barrier and spread northward. As of late September 2025, NWS was detected as far north as Nuevo León, Mexico (Figure 1). COPEG continues to release sterile flies in Central America and Mexico, however sterile fly production is at maximum capacity at the current facilities. US livestock producers should be prepared to implement proactive management and husbandry practices in the near term as well as preventative measures to limit the potential impact an infestation will have on the livestock industry if and when a regional detection happens.

Figure 1. Current New World Screwworm detections as of September 2025.
Image credit: USDA-APHIS.

Identification
Adult NWS are roughly the size of a common house fly. The adult flies have orange eyes, metallic blue or green bodies, and three black stripes on their back (Figure 2). They are given the name screwworm because the larvae look like a screw. Larvae have a flat head with ridges that look like the threads of a screw along their body and are approximately 2/3 of an inch long when mature (Figure 3). They cause damage to skin tissue because larvae can latch on and tear through tissue with their sharp mouth hooks. As a result, wounds easily become larger as the screwworm gets deeper into tissue. When observed in a wound larvae present in a signature "pearl" formation, with the hind end of the larvae presenting to the outside of the wound and the mouth end positioned into the wound.

Figure 2. Adult New World screwworm fly. Photo: Kathleen Franklin, Flickr.

Figure 3. New World screwworm larva. Note ridges on the body
resembling threads of a screw. 
Image credit: USDA-APHIS.

Life cycle of the New World screwworm fly
The life cycle of NWS is determined by the climate of the region. In semi-arid environments like Arizona, the cycle can last approximately 24 days or longer, depending on environmental conditions. Female screwworm flies mate only once in their lifetime, which is why sterile male flies are released as an effective control mechanism. After mating, the female lays eggs on the edges of an open wound. In some cases, the eggs may become visible by the third day after being laid.

Once hatched, the larvae screw into the wound and go through three stages of development. Unlike other species of flies that may lay eggs in or around a wound, the larvae of the NWS feed on living tissue, not dead/decaying tissue. This is what makes a case of screwworm infestation much more serious than a case of simple flystrike. In the final developmental stage, the mature larvae leave the host animal, fall to the ground, and burrow underground. Once in the ground they form a hard protective case known as a pupa. Inside this hard case, they transform into adult flies. Once adults emerge, they can easily spread and infect new hosts.

New World screwworm pupae thrive in warmer climates, where their pupation is more successful. Pupae will not survive if exposed to soil temperatures below 46°F for several consecutive days or after frost. Temperatures below 50°F have a significant effect on survival. Adult screwworm flies survive best in hot, moderately humid environments typically between 77– 86°F with a relative humidity of 30–70%. High temperatures above 104°F in dry environments reduce the reproductive capabilities of these flies. However, previous outbreaks in Arizona have shown that they can successfully survive and reproduce in our climate, thriving particularly in areas where there are the “Three W’s”, i.e. woods, water, and warm-blooded animals.

The Arizona Livestock Incident Response Team (ALIRT) issued a BOLO (Be On the Look Out) notice for New World screwworm in July 2005. Read it here: https://extension.arizona.edu/sites/default/files/2025-07/Az2149_2025_ALIRT_BOLO_NWS_final.pdf

Clinical Signs of infestation
Maggots in wounds or other body openings (like nose, ears, umbilicus, or genitalia) of any warm-blooded animal (including birds and humans). These flies are attracted to wounds as small as a tick bite but thrive on open wounds or surgical sites. The egg masses may be around or in the wound; larvae may be visible by the third day of infestation, and because they burrow into and feed on live flesh, NWS maggots are often found deeper in the wounds or openings and the wound site will be enlarged, draining, and have a characteristic decaying smell. Other species of maggots usually appear closer to the surface of the wound. Screwworm infestations are very painful, so animals may become depressed, stop eating, and separate themselves from other animals or people.

Prevention
The most effective prevention is prohibiting the reintroduction of New World screwworms to the United States. The Arizona Livestock Incident Response Team (ALIRT) is initiating a Pilot New World Screwworm Surveillance Project which involves distributing NWS larvae collection kits to veterinarians, ranchers, Arizona Department of Agriculture and University of Arizona Cooperative Extension personnel, to assist in submitting suspect maggot larvae to the Arizona Veterinary Diagnostic Laboratory for early identification. Positively identified screwworm larvae will be reported to the federal agriculture authorities. Anyone may report suspected cases of screwworm, and we need everyone to be on the lookout. This disease is serious and can have catastrophic impacts on US agriculture, and animal and public health. Catching it early and eliminating it quickly is vital.

What can the general public do?  Steps you can take to avoid infestation:
1) Be alert for NWS symptoms in pets and livestock.

2) Quarantine any suspect animals, and work with your veterinarian to send larvae samples and treat the animals.

3) Ensure that pets or livestock traveling internationally are inspected for screwworm.

4) Check your vehicle for screwworm flies if you are in an NWS-infested area.


In areas where NWS is found, the following measures should be implemented:
1) To the extent possible, eliminate or delay performing wounding procedures such as dehorning, branding, shearing, ear notching, tail docking and castration.

2) Since untreated umbilical cords of newborn animals and foot lesions are commonly infested sites, immediately treat all wounds with approved insecticides; it may also be prudent to follow up with precautionary spraying of animals with insecticide before transport.

Treatment
Animals infested with NWS should be treated according to their veterinarian's recommendations. People with a suspected NWS infestation should seek immediate medical treatment following the Centers for Disease Control and Prevention (CDC) guidelines.

Collecting a Specimen
The University of Arizona Cooperative Extension Offices and AZDA have limited free Arizona Livestock Incident Response Team (ALIRT) sampling kits and online submission forms available for Arizona livestock owners (Figure 4). New World screwworm is federally reportable, and it is required to notify both AZDA and USDA of suspected cases immediately. Read more about collecting samples and submitting information in our publication. Currently, resources and testing services are only available to livestock producers, not the general public.

If domestic pets develop maggot infestations, take them to the emergency vet immediately and follow their instructions.

Figure 4: Arizona Livestock Incident Response Team (ALIRT) NWS Surveillance kit.
Photo: Betsy Greene.

For more information on the ALIRT New World screwworm surveillance project, go to https://extension.arizona.edu/alirt.