New World Screwworm

Cochliomyia hominivorax, the New World screwworm (NWS), is an important threat because of reemergence events in 2025 and earlier. After decades of successful eradication from the U.S. and Mexico, new outbreaks have been documented in Panama (2022), Costa Rica (2023), and Mexico (2024–2025), and bioclimatic models indicate high invasion potential into the southern US, particularly Texas and Florida. The impact of deadly NWS on animal health has proven to be devastating. In Panama alone in 2023, NWS detections rose from approximately 25 cases per year to greater than 6,500 cases just that year, and in the US in 1976, before NWS was eradicated, 1,488,256 cattle and 332,600 sheep and goats were infested with NWS in Texas.

Critical dispersal points in Mexico include Chiapas, Campeche, Tabasco, and Veracruz, and regions of high livestock and wildlife density are at greatest risk. The species is endemic throughout South America and the Caribbean, and genetic studies show the highest diversity in Trinidad and Tobago and evidence of ongoing gene flow between Caribbean and South American populations.

Favorable climates, increased livestock and wildlife densities, and lack of awareness among human populations will likely facilitate further northward expansion and increase the frequency and severity of outbreaks in areas where the New World screwworm was previously eradicated.

Bioclimatic modeling predicts the re-emergence of NWS in suitable habitats in the southern US, as well as establishment of this parasite in new geographic regions because of increasing global temperatures. The recent availability of a chromosomal-scale reference genome supports ongoing research into the biology and control of this parasite.

The economic impact of myiasis is substantial; annual losses are estimated at $3.6 billion in South America as a result of livestock disease, death, and decreased productivity.

Adult females lay 200–400 eggs on wound edges; larvae hatch within 10–21 hours, feed on live tissue for 5–7 days, then pupate in soil. The life cycle can be completed in as little as 21 days in warm climates.

The presence of larval feeding attracts other females to lay their eggs, leading to additional infestations. Untreated larval infestations lead to death of the infested animal within 7–14 days. Neonate animals are at major risk, and death always occurs if egg laying is not prevented.

Myiasis caused by C hominivorax is a zoonosis affecting livestock, wildlife, dogs and other pets, and birds and humans on rare occasions. The larvae of this parasitic fly feed on the flesh of wounds of any warm-blooded animal, preferring larger mammals over birds and humans, as well as mucous membranes that have small wounds. The spread of NWS is a direct result of animal movement, both legal and illegal, as well as of infestation of local wildlife populations.

Human cases of NWS are reported wherever the parasite exists. Occasionally human cases are reported in the US in individuals who have returned from travel to NWS-infested areas.

Diagnosis of NWS myiasis relies on morphological identification of larvae, characterized by their pigmented tracheal tubes and distinct segmental spines. Suspected NWS larvae must be collected and placed in a leak-proof container with 70% ethanol (enough to fully submerge and kill the larvae). Alternatively, > 70% isopropanol may be used. Specimens must be submitted to the state animal health agency or CDC when a human case for testing. Identification of larvae is most often done by visual taxonomy; however, advances in molecular epidemiology also allow rapid determination of outbreak origins using single nucleotide polymorphism (SNP) panels and genetic databases, which is critical for surveillance and control.

Odorant-based traps are the mainstay of surveillance, and research into molecular targets to improve attractant specificity and sensitivity is ongoing.

Screwworm infestations are reportable to veterinary authorities, and rapid response is essential to prevent spread. Suspected animal or human cases should be reported to state animal health departments, state animal health officials, and APHIS. The CDC may be contacted at dpdx@cdc.gov for laboratory testing inquiries and instruction for telediagnosis and physical specimen submissions. Clinical inquiries may be directed to parasites@cdc.gov.

Treatment of livestock is limited to wound care (larvae removal, debridement, prevention of reinfestation), and three available topical pyrethroid sprays are currently labeled for use against screwworm larvae but should only be used when larvae are present and not as a preventative.

Treatment options in dogs include a single oral dose of either nitenpyram (1.43–4.42 mg/kg), which induces rapid larval expulsion within 6 hours, or lotilaner (minimum 20.5 mg/kg), which achieves 100% efficacy within 24 hours. 

Oral spinosad/milbemycin is also highly effective; these drugs have synergistic activity in expelling screwworm larvae. Oral afoxolaner and milbemycin are effective but act more slowly. Topical lindane and ronnel, previously used in wound dressings, are now less available or restricted because of regulatory concerns. Coumaphos remains in use in some regions, typically as a topical formulation.

Recognizing and reporting suspected infestations are crucial to maintaining eradication of NWSs. Advice from the USDA-APHIS training module on NWS includes the following:

• NWS larvae can be found deep within wounds.

• Field diagnosis of the maggot is not recommended; the focus should be the characteristics of NWS infestation.

• Suspicious cases should be reported immediately to the State Animal Health Official and USDA Veterinary Official. 

• Both local and state veterinarians should be alerted if infection is suspected.

• Samples should be collected and submitted, using the following procedure: Collect maggots from all areas of the wound, including the deepest parts of the wound, and preserve the specimens in alcohol (ethyl or isopropyl), not formalin.

The sterile insect technique (SIT), in which NWS flies are reared in mass and the pupae are sterilized with gamma radiation before they are mass released to let the male flies mate with the wild females. This program is the cornerstone of area-wide eradication and outbreak response and the only way of achieving New World Screwworm eradication. This program is successful only because the female flies only mate once in their lifetime. Mating wild females with sterile males prevents the development of future larvae as the eggs will be nonviable.

C hominivorax poses a substantial threat to food supply and safety through direct livestock losses, decreased animal productivity, and potential contamination of animal products that reduce the supply of meat and other animal-derived goods to consumers. The risk of introduction into new areas is heightened by global transport and climate change, with implications for both animal and human health.

Additional studies are needed to clarify NWS zoonotic transmission dynamics, quantify food safety impacts, evaluate sterility techniques, and assess the regulatory status and safety profiles of older insecticides used across different countries.

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