New World Screwworm

The Organism

New World screwworm is the larval stage of Cochliomyia hominivorax, a fly whose maggots feed on living tissue. This distinguishes it from many other blowflies that primarily colonize dead or decaying tissue. New World screwworm females can lay eggs on a wound in an otherwise healthy animal [1].

A female lays eggs in overlapping, shingle-like masses along the edge of a wound or in a body opening. The eggs generally hatch within about 12 to 24 hours. Larvae feed in the wound for several days, commonly about five to seven, before dropping to the ground and burrowing into the soil to pupate. Adult flies emerge after pupal development. The length of the full life cycle depends heavily on temperature and other environmental conditions; development is faster in warm conditions and slower in cool conditions [1].

Female screwworm flies generally mate only once, usually a few days after emerging. This reproductive behavior makes the sterile insect technique possible: large numbers of sterile males are released to mate with wild females, which then produce eggs that do not hatch. Successful eradication requires repeated releases, adequate geographic coverage, and surveillance to detect remaining wild flies [1].

Where It Lives

New World screwworm was eradicated from the continental United States through a coordinated sterile-insect program. Sterile fly releases began in the southeastern United States in 1958, and the fly was effectively eliminated from the country by 1966. The eradication program was later extended through Central America. By 2000, the fly had been eliminated north of the Darién Gap in Panama, where a sterile-fly barrier helped prevent its northward spread for many years [2].

That barrier eventually failed. Renewed detections occurred in Panama and Costa Rica in 2022, followed by spread into southern Mexico in November 2024 and subsequent movement northward [2].

The first US case in the 2026 outbreak was confirmed on June 3, 2026, in a three-week-old beef calf near La Pryor in Zavala County, Texas. The calf had larvae in its navel, was treated, and survived [2, 3].

Adult flies can disperse from their emergence sites to find hosts, while infested animals can transport larvae to new locations. The distance a fly travels depends on environmental conditions and host availability. Vehicles and animal movement can carry infestations much farther than an individual fly is likely to travel [1].

Temperature affects survival and development. Cold conditions can slow development and reduce survival, particularly during the pupal stage. However, freezing temperatures or soil temperatures below a particular threshold should not be assumed to eliminate every pupa under field conditions. Survival depends on temperature, exposure duration, soil conditions, and developmental stage [1].

How It Spreads

Every infestation begins when a female fly lays eggs on a suitable site. New World screwworm is not ordinarily transmitted directly from animal to animal or person to person. The larvae develop in the wound where the eggs were deposited [1, 4].

A wide range of wounds can attract egg-laying, including tick bites, cuts, wire or thorn scratches, branding and castration sites, dehorning wounds, and ear-tag wounds. Newborn navels and the reproductive tract and surrounding tissues of females after giving birth are important sites. Flies may also lay eggs near the nose, mouth, ears, eyes, and genital openings [1].

An infested wound can attract additional flies. As the wound becomes larger and more damaged, it may produce discharge and odors that attract further egg-laying. Repeated infestations can therefore increase the number of larvae and the severity of tissue damage [1].

Over longer distances, the fly spreads through adult dispersal and the movement of infested animals. Movement of animals with untreated wounds can transport larvae to locations far beyond the normal flight range of an individual fly [1].

Infection Pattern

Early in an infestation, the wound may show only subtle changes. It may become moist or slightly swollen, and the animal may show discomfort before larvae are readily visible. Over the following days, the wound can enlarge and deepen rather than healing. A bloody or watery discharge and a strong odor may develop. Larvae may be visible within the wound, often oriented with their anterior ends embedded in the tissue [1].

Affected animals may become restless or depressed, eat less, lose weight, produce less milk, separate from the herd, or seek shade. Severe tissue damage and secondary bacterial infection can complicate the infestation. Untreated or repeatedly reinfested animals may develop life-threatening disease [1].

The outcome depends on the size and location of the wound, the number of larvae, the animal’s condition, and how quickly treatment is provided. Historical reports of substantial fawn mortality on a South Texas ranch illustrate the potential wildlife impact, although those losses should not be interpreted as a prediction of mortality during the current outbreak [1].

Many warm-blooded animals are susceptible, including livestock, pets, wildlife, and people. Surveillance figures through October 21, 2026, identified 59 US cases: 26 cattle, 16 sheep, seven goats, five dogs, four horses, and one elk [5, 8]. The elk was the wildlife case; the other 58 cases involved domestic animals.

What to Do If You Suspect Screwworm

Report a suspected infestation immediately. Do not wait for laboratory confirmation. Screwworm is treated as a foreign animal disease, and prompt reporting allows animal-health officials to investigate, arrange appropriate testing, and determine whether movement restrictions are needed [6, 7].

Do not move the affected animal or other animals off the premises unless directed by animal-health authorities. Moving an infested animal can transport larvae to a new location.

If sample collection is requested, follow the instructions of the state animal-health official or diagnostic laboratory. Larvae from deep within the wound may be more diagnostically useful than larvae on the wound surface, where secondary fly species may also be present. Do not delay reporting or veterinary treatment while attempting to collect a sample [1, 7].

Under the APHIS diagnostic procedures described beginning July 20, 2026, presumptive-positive classification is performed by APHIS-approved laboratories or entomologists, with confirmation by the National Veterinary Services Laboratories [6].

A veterinarian should examine the animal and coordinate treatment with animal-health officials. If practical, treatment should be performed in a location where larvae that leave the wound can be collected and destroyed rather than reaching soil and continuing development. Follow official instructions for disposal of larvae and contaminated material.

Inspect other susceptible animals on the premises, including livestock, dogs, cats, and horses. Pay particular attention to newborn navels, recent wounds, tick bites, surgical sites, and the hindquarters and reproductive tracts of animals that have recently given birth.

Diagnostic Differential

The most important differential is myiasis caused by other flies, including blowflies and flesh flies. Many other fly larvae colonize dead tissue, decaying material, or manure. New World screwworm larvae, in contrast, feed on living tissue and can cause progressive wound destruction [1].

The distinction is not always obvious in the field. Other flies may lay eggs in a wound that is already infested with screwworm, so more than one type of larva may be present. Finding maggots in a wound does not, by itself, establish screwworm infestation.

Expert identification is required for suspected cases. Do not rely on visual assessment by a producer or general field observer to rule out screwworm [1].

Diagnosis

Diagnosis depends on identifying the larvae. Second- and third-stage New World screwworm larvae have a cylindrical body with a tapered anterior end and a blunt posterior end. Dark spines arranged in rings around the body give the larvae a screw-like appearance. Features of the posterior spiracles and tracheal trunks can help distinguish screwworm larvae from other fly larvae. Identification requires appropriate expertise and, when necessary, laboratory examination [1].

Eggs are creamy white and are deposited in overlapping masses. Adult flies are metallic blue-green, with a reddish-orange head and three dark longitudinal stripes on the thorax [1].

The appearance of a larva can suggest screwworm, but official disease status depends on the designated diagnostic process. Under the APHIS procedures described beginning July 20, 2026, only APHIS-approved laboratories or entomologists classify cases as presumptive positive, and the National Veterinary Services Laboratories provide confirmation [6].

The USDA Agricultural Research Service laboratory in Kerrville, Texas, was identified in reporting about the first US wildlife detection. The October 21, 2026, wildlife case involved a hunter-harvested female elk in Pecos County, Texas [8].

Risk to Humans

People can be infested by New World screwworm in the same way as animals. A female fly may lay eggs on a wound, scratch, insect bite, surgical site, or near an opening such as the nose, ears, mouth, eyes, or genitals. The larvae then feed on living tissue. Screwworm is not transmitted directly from person to person [1, 4].

Infestations involving the eyes, mouth, sinuses, or ears can cause serious tissue damage and may become medically urgent. The severity depends on the location and extent of the infestation and how quickly treatment is provided [1].

One human case was reported in the United States in August 2025, involving a traveler returning from El Salvador. As of October 22, 2026, the CDC reported no human cases acquired in the United States [4, 9].

Anyone who finds larvae in a wound should seek prompt medical evaluation. Do not attempt to diagnose the larvae or manage a suspected infestation at home. Healthcare professionals should be told about relevant travel, animal, and geographic exposures. Clinicians who suspect screwworm should contact the appropriate local or state public-health authority and follow CDC and public-health guidance [4].

Keeping wounds clean and covered can reduce opportunities for flies to lay eggs. This is particularly important for people who have open wounds, recent surgical sites, or difficulty protecting themselves from flies.

Treatment and Control

Treatment requires removal of larvae, wound cleaning, and appropriate veterinary care. The wound may need to be explored and debrided to remove larvae and damaged tissue. A veterinarian may use an approved larvicidal product and prescribe antibiotics when secondary bacterial infection is present. Treatment may need to be repeated if larvae remain or the wound is reinfested [1].

Product selection matters. Not every insecticide or antiparasitic product is approved for every species, route, or use. Follow the current product label and official veterinary guidance, including meat, milk, and other withdrawal requirements.

As of September 3, 2026, the FDA listed 15 emergency use authorizations and three conditional approvals for screwworm products covering multiple animal species. No medicated feed was approved for screwworm in any species at that time [10]. Authorizations and approvals may change as the response develops.

Prevention depends on reducing opportunities for flies to lay eggs and detecting infestations early:

• When practical and consistent with animal welfare and management needs, avoid elective procedures that create wounds during periods of high fly activity in or near affected areas.
• Inspect newborn navels and recently calved or lambed dams frequently.
• Treat and protect wounds promptly, including cuts, tick bites, and injuries from fencing or handling.
• Examine dogs, cats, horses, and other animals as carefully as livestock.
• Follow movement restrictions and inspection requirements established by animal-health authorities.

Government control relies heavily on the sterile insect technique. Sterile male flies are released in large numbers so that wild females mate with sterile males and produce eggs that do not hatch. This approach requires sustained releases, surveillance, and coordination across affected regions [1, 2].

Movement controls may be imposed around confirmed cases. Texas response materials describe a 20-kilometer infested zone and movement requirements for animals leaving affected areas [3, 7]. The applicable zone boundaries, inspection procedures, treatments, and documentation requirements are established by animal-health authorities and may change during the outbreak.

APHIS has also described sterile-fly dispersal and production facilities in Texas and Mexico, including a facility near Moore Air Base and a facility in Hermosillo, Sonora [11]. Production capacity and geographic coverage depend on each facility’s operational status and stage of development.

What Screwworm Costs Producers

For producers in or near an infested zone, the economic impact includes both the disease itself and the regulatory response. Animals may require repeated examination and treatment. Producers may face additional labor, veterinary costs, product expenses, delayed sales, and losses from severe or untreated infestations.

Movement restrictions can have substantial effects on livestock operations. Animals may need inspection, treatment, documentation, or official authorization before leaving a regulated area. The exact requirements depend on the location and the applicable response order [7].

Texas cattle and calf sales totaled approximately $13.6 billion in 2024, illustrating the scale of the livestock industry potentially affected by screwworm [2]. Economic estimates for livestock and animal-product sales in the initial South Texas detection area and Lea County, New Mexico, similarly describe the scale of exposed agricultural production rather than direct losses caused by infestation.

The outbreak has also affected cross-border livestock trade. APHIS has described staged reopening procedures for cattle imports from Mexico [12]. Reopening under a defined import protocol does not necessarily mean unrestricted movement of all livestock through a port.

Where to Report a Case in the Southwest

Suspected New World screwworm should be reported immediately to animal-health authorities. Do not wait for laboratory confirmation. Reporting deadlines and procedures differ by state.

Federal

State animal-health officials coordinate with the USDA APHIS Area Veterinarian in Charge. APHIS provides the federal response and diagnostic framework for suspected foreign animal diseases [6].

People who suspect a human infestation should seek medical care and follow CDC and state or local public-health reporting guidance [4].

Documented Incidence in the Southwest

As of November 4, 2026, USDA reported 113 confirmed US cases since the first detection on June 3: 111 in domestic animals and two in wildlife, across Texas, New Mexico, and Arizona [19, 20]. Of these, 55 were active [20]. Sixty-one cases were confirmed in the 30 days ending November 4 [19]. The first wildlife case, a hunter-harvested female elk in Pecos County, Texas, was confirmed on October 21 [8]; the second was an elk in Jeff Davis County, Texas [20].

These numbers describe the outbreak at that reporting date. The USDA APHIS dashboard provides updated case counts, affected counties, and regulatory information as the outbreak develops.

Arizona

Arizona’s first case was confirmed on October 28, 2026, in a dog on a ranch in Cochise County. A veterinarian had examined the dog on October 26 and found a wound on its back heavily infested with larvae [22, 23].

A second Cochise County case, in a horse near Hereford, about 45 miles from the first, was reported on November 3 [22]. USDA data compiled through November 4 also list detections in Greenlee County [19, 20].

A 12.4-mile infested zone is required around each detection site, restricting the movement of live animals within it [23]. Officials are releasing sterile screwworm flies in response to the Arizona cases [22].

Reports also described more than 100 active cases in Sonora, Mexico, and increased cattle imports through Arizona ports [18]. These figures reflect the reporting period rather than a continuing fixed level of disease activity or trade.

APHIS has described sterile-fly releases associated with a facility in Hermosillo, Sonora [11]. The operational status and coverage of sterile-fly releases may change during the response.

New Mexico

Thirty cases were reported in New Mexico from June through November 4, 2026, tripling within a week [21]. Recent cases were identified in Doña Ana, Grant, Luna, Otero, Hidalgo, and Catron counties, in cattle, horses, dogs, and a domestic cat. A screwworm fly was also trapped in Doña Ana County [21].

New Mexico’s infested zone covers all of Doña Ana, Grant, Luna, Otero, and Hidalgo counties and parts of Catron, Sierra, Lincoln, and Chaves counties. Animals must be inspected and, if necessary, treated before moving into or out of the zone [21].

Earlier cases included a dog in Lea County in June, a horse in Grant County on September 17, a horse and a calf in Grant County on October 15, and a cow in Otero County on October 21 [5, 16].

Texas

Fifty-four cases were reported across 18 counties through October 21, 2026, concentrated in Crockett, Brewster, Pecos, Edwards, and Terrell counties [5, 7].

As of November 4, animal movement restrictions applied in 16 Texas counties: Brewster, Crockett, Culberson, Edwards, El Paso, Hudspeth, Jeff Davis, Pecos, Presidio, Reeves, Schleicher, Starr, Sutton, Terrell, Val Verde, and Zapata. Wild screwworm flies had been detected in five USDA traps, four of them in October [20].

Texas response reports also described infested zones covering parts of 12 counties [7]. Zone boundaries and surveillance findings are updated as the response progresses.

References

1. Spickler AR. Screwworm myiasis. Center for Food Security and Public Health, Iowa State University, January 2025. https://www.cfsph.iastate.edu/Factsheets/pdfs/screwworm_myiasis.pdf↑ a b c d e f g h i j k l m n o p q r s t u
2. First US cases of New World screwworm detected. American Farm Bureau Federation, Market Intel, 9 June 2026. https://www.fb.org/intel/markets/first-u-s-cases-of-new-world-screwworm-detected↑ a b c d e
3. USDA confirms presence of New World screwworm in the United States. USDA APHIS, 3 June 2026. https://www.aphis.usda.gov/news/agency-announcements/usda-confirms-presence-new-world-screwworm-united-states↑ a b
4. New World screwworm outbreak: situation summary. US Centers for Disease Control and Prevention, updated 22 October 2026. https://www.cdc.gov/new-world-screwworm/situation-summary/index.html↑ a b c d e f
5. US New World screwworm tracker. GenomicEpi, compiled from the USDA APHIS dashboard, accessed 23 October 2026. https://www.genomicepi.com/outbreaks/new-world-screwworm/↑ a b c d
6. New World screwworm emergency response. USDA APHIS, modified 6 October 2026. https://www.aphis.usda.gov/animal-emergencies/nws↑ a b c d e
7. New World screwworms. Texas Animal Health Commission, accessed 23 October 2026. https://www.tahc.texas.gov/emergency/nws.html↑ a b c d e f g h
8. USDA confirms first detection of New World screwworm in US wildlife. Michigan Farm News, 22 October 2026, reporting USDA APHIS and Texas Parks and Wildlife Department statements. https://www.michiganfarmnews.com/about/news-media/usda-confirms-first-detection-new-world-screwworm-us-wildlife↑ a b c
9. HHS and USDA confirm singular traveler-associated New World screwworm case. USDA Release No. 0206.25, 26 August 2025. https://www.usda.gov/node/65519↑ a b
10. Animal drugs for New World screwworm. US Food and Drug Administration, updated 3 September 2026. https://www.fda.gov/animal-veterinary/safety-health/animal-drugs-new-world-screwworm↑
11. Sterile fly production and dispersal facilities. USDA APHIS, modified 21 October 2026. https://www.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/stop-screwworm/sterile-fly-production-dispersal↑ a b
12. Current status of New World screwworm. USDA APHIS, modified 23 October 2026. https://www.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/stop-screwworm/current-status↑
13. Arizona Administrative Code § R3-2-402: Mandatory disease reporting by veterinarians and veterinary laboratories. https://www.law.cornell.edu/regulations/arizona/Ariz-Admin-Code-SS-R3-2-402↑
14. New World screwworm update. Arizona Veterinary Diagnostic Laboratory, University of Arizona. https://azvdl.arizona.edu/news/new-world-screwworm-update↑
15. 21.30.4.9 NMAC: New Mexico reportable disease list. New Mexico Livestock Board. https://www.srca.nm.gov/parts/title21/21.030.0004.pdf↑
16. New Mexico reports 2 more New World screwworm cases. Source: New Mexico, 16 October 2026. https://sourcenm.com/2026/10/16/new-mexico-reports-2-more-new-world-screwworm-cases/↑ a b
17. 4 TAC § 45.3: Reportable and actionable disease list. Texas Animal Health Commission. https://www.law.cornell.edu/regulations/texas/4-Tex-Admin-Code-SS-45-3↑
18. Arizona officials urge caution as New Mexico reports 2 new cases of New World screwworm. KJZZ Fronteras Desk, 20 October 2026. https://www.kjzz.org/fronteras-desk/2026-10-20/arizona-officials-urge-caution-as-new-mexico-reports-2-new-cases-of-new-world-screwworm↑
19. Vilhauer L. USDA reports 61 New World screwworm cases in 30-day period. High Plains Journal, 5 November 2026. https://hpj.com/2026/11/05/usda-reports-61-new-world-screwworm-cases-in-30-day-period/↑ a b c d e
20. U.S. New World screwworm cases climb sharply to 113, with nearly half still active. Texas Public Radio, 4 November 2026. https://www.kedt.org/2026-11-04/u-s-new-world-screwworm-cases-climb-sharply-to-113-with-nearly-half-still-active↑ a b c d e f
21. O’Hara M. New World screwworm cases in New Mexico triple, spread to Catron County. Searchlight New Mexico, 4 November 2026. https://searchlightnm.org/new-world-screwworm-cases-in-new-mexico-triple-spread-to-catron-county/↑ a b c d
22. Kravinsky N. Arizona reports 2nd New World screwworm case in Cochise County. KJZZ Fronteras Desk, 3 November 2026. https://www.kjzz.org/fronteras-desk/2026-11-03/arizona-reports-2nd-new-world-screwworm-case-in-cochise-county↑ a b c d
23. Second case of New World screwworm confirmed in Cochise County. KGUN 9, 3 November 2026. https://www.kgun9.com/news/local-news/second-case-of-new-world-screwworm-confirmed-in-cochise-county↑ a b