Cochliomyia hominivorax, commonly referred to as the New World Screwworm (NWS) or primary screwworm, is a parasitic fly that is native to parts of the Western Hemisphere. Unlike other blowfly species that feed on dead tissue and are important decomposers, NWS larvae feed only on live tissue. While any warm-blooded animal can be infested (including humans), cowsWhile "cows" can be defined to refer exclusively to female cattle, at The Open Sanctuary Project we refer to domesticated cattle of all ages and sexes as "cows." and other large mammalian farmed animalA species or specific breed of animal that is raised by humans for the use of their bodies or what comes from their bodies. species, as well as wildlife, are most often affected. Birds can be affected, but this is less common. NWS infestations cause extensive tissue damage and are usually fatal within 7-14 days if left untreated.
History
NWS was once widespread in the Americas, but was eradicated from the US in 1966. In subsequent decades, NWS was also eradicated from Mexico and most of Central America, but remained endemic in South America and the Caribbean. In 2006, COPEG (Panama-US Commission for the Eradication and Prevention of Screwworm) established the Darién Gap as the biological barrier to keep NWS out of Central and North America. A primary component of the control and eradication of NWS is sterile insect release programs, in which sterile male flies are released in designated areas. These sterile male flies mate with females, resulting in unfertilized eggs, reducing fly populations until they are eventually eradicated.
An NWS outbreak in 2016-2017 killed over 100 endangered Key Deer in the Florida Keys, but NWS was ultimately eradicated in the US again. In 2023, the biological barrier established in the Darién Gap was breached, and new outbreaks in Panama, Costa Rica, and Nicaragua raised concerns of an eventual return of this parasite to the US. Despite attempts to contain the outbreaks (with the strategic release of sterile male flies), outbreaks continued moving north in 2024 and 2025. In August of 2025, a travel-associated case of NWS was reported in a human in Maryland following their return from El Salvador. In June of 2026, the first case of NWS in almost a decade was reported in a calf in Texas, and additional cases in Texas and New Mexico were reported in the coming days. For the current status of cases in the US, please refer to the USDA’s dashboard, which you can find here.
Biology Of The New World Screwworm
After breeding, female NWS flies will seek out a warm-blooded animal to lay their eggs on, often laying eggs on the edge of a wound. These wounds can be as small as a tick bite. In addition to wounds, NWS flies may also lay their eggs near orifices and mucous membranes (eyes, ears, mouth, genitals, anus, or the umbilicus of neonates). A female NWS fly can lay between 200 and 300 eggs at a time and up to 3,000 eggs in her brief lifetime.
NWS hatch within 12-24 hours, and the larvae burrow down into the tissue to continue their development and feed on the animal’s living tissue for about 5-7 days, causing extensive damage. The presence of screwworm larvae feeding attracts other NWS females to the site to lay eggs, and other fly species may be attracted as well, further compounding the issue. By the third day, hundreds or thousands of larvae may be present. When NWS larvae reach their third larval stage, they fall to the ground to pupate in the soil. In about a week, an NWS fly will emerge to continue the cycle.
Adult NWS flies are similar in size or slightly larger than a house fly. They have red/orange eyes, a yellow face, and a metallic blue-green body. They have three stripes on their back, with the center stripe being shorter than the outer ones. They look very similar to the closely related Cochliomyia macellaria, or secondary screwworm, but unlike the NWS fly, the stripes on the secondary screwworm fly are all the same length. Secondary screwworm flies do not pose the same risk to animals as NWS because they do not lay their eggs on live animals.
Mature NWS larvae are about 17mm (⅔ inch) long and pale in color with raised spines that spiral around their body. The name “screwworm” comes from this spiral appearance and their feeding behavior of burrowing down into flesh like a screw into wood. Identification of larvae can be difficult for the untrained person since they resemble other fly larvae, so be sure to work with your veterinarian for proper sample collection and identification.
NWS flies thrive in environments that are hot and moderately humid (77– 86°F / 25-30°C with a relative humidity of 30–70%). However, previous cases demonstrate their ability to survive in less favorable conditions, including extremely hot and dry conditions in Arizona and cooler climates in northern US states. While cooler temperatures have a negative impact on NWS pupae and they are unable to survive a frost or soil temperatures that remain below 46°F for several days in a row, this does not mean NWS cannot be an issue in cooler climates. While NWS cannot survive year-round in the northern part of the US, interstate transport of infested animals has resulted in seasonal outbreaks in northern states in the past. In the US, NWS have previously been able to survive year-round in southern Florida, southern Texas, and Puerto Rico, but increasing global temperatures will likely result in additional areas becoming suitable for their year-round survival.
Clinical Signs Of NWS Infestation
As mentioned above, wounds as small as a tick bite can become infested. This, combined with the feeding behavior of these larvae, in which they burrow down into the flesh, can result in large, deep pockets of infestation concealed by the individual’s skin. Depending on the size of the wound opening, it may be quite difficult to detect an infestation unless you look closely for subtle movement of the larvae. It is important to note that in the early stages of an NWS infestation, the individual may act normally and may not be debilitated. However, the other signs may be similar to the typical myiasis seen with maggots (fly strike).
Besides noticing larvae or egg masses, other signs to watch for include bloody discharge and a foul odor coming from a wound, as well as signs of pain or irritability, including inappetence, head shaking, rubbing/scratching, biting or licking wounds, depression, restlessness or lethargy, and separating from their social group. You should also watch for wounds that deepen or enlarge over time. If these signs are noted, contact your veterinarian right away. With prompt treatment, recovery is possible, but in some cases, the damage caused by the larvae may be so extensive that euthanasia is recommended. Without treatment, infestations can become fatal due to the damage caused by the larvae or due to systemic secondary bacterial infections.
Treatment
Thankfully, with prompt intervention, NWS infestations can be treated. Be sure to contact your veterinarian if you see signs of an infestation. Suspected and confirmed cases of NWS are reportable in the US, and your veterinarian will need to adhere to standard operating procedures developed by USDA-APHIS. Individuals with confirmed NWS infestations will need to be quarantined and undergo treatment, which involves the manual removal of eggs and larvae, wound care, and the use of an FDA-approved or authorized topical or systemic antiparasitic with larvicidal activity effective at eliminating NWS larvae. Analgesics and/or systemic antimicrobials may also be recommended depending on the severity of the infestation and wound. The individual will need to be re-examined 24 hours after treatment to confirm there are no live larvae remaining or to remove additional larvae and retreat the individual. The individual can be released from quarantineThe policy or space in which an individual is separately housed away from others as a preventative measure to protect other residents from potentially contagious health conditions, such as in the case of new residents or residents who may have been exposed to certain diseases. when no live larvae remain and they have been deemed NWS-free by an appropriate official, such as a State Animal Health Official, CDC Veterinary Medical Officer, or APHIS VS Veterinary Medical Officer.
Environmental decontamination will also be necessary to prevent ongoing issues. This includes the collection of any larvae that have been expelled into the environment and the use of an approved larvicidal spray to disinfect surfaces. To prevent residents from exposure to larvicidal residues, surfaces that have been sprayed should then be cleaned with soap and water. Following a confirmed case, weekly monitoring for one month following the affected individual’s release from quarantine is necessary.
Protecting Your Residents
While NWS is a very serious threat, the good news is that many of the practices sanctuaries already have in place can help with prevention and early detection. Close observation of residents plays a critical role in not just detecting signs of an infestation, but also detecting wounds that could attract NWS flies. If you have residents with known wounds, they should be even more closely monitored, and you should take steps to deter flies from the site as recommended by your veterinarian. In addition to wounds that are naturally sustained, individuals with wounds or incisions following a medical procedure (such as a neuter or spay procedure, removal of a mass, tooth extraction, etc.) or who were cut during shearing, as well as newly rescued individuals who arrived with wounds from tail docking, ear tagging, disbudding, or dehorning, are also at risk at these sites. Talk to your veterinarian about keeping wounds covered to help deter flies. Mothers who have recently given birth are at risk of genital infestation, and neonates are at a high risk of umbilical infestation. Close daily monitoring of high-risk sites is imperative, but keep in mind that infestation of mucous membranes and orifices is also possible, so every warm-blooded resident should be observed for possible signs of infestation.
Though new residents are not the only way NWS could come to your sanctuary, proper quarantine and a thorough intake evaluation of new residents remain critical components of welcoming new residents to your sanctuary.
To help reduce your residents’ risk of infestation, address hazards in their living spaceThe indoor or outdoor area where an animal resident lives, eats, and rests. that could result in injury, take steps to prevent ectoparasites (such as ticks, mites, or biting flies), and, particularly in areas with known infestations, consider delaying practices or procedures that could leave residents with a wound that puts them at risk. This may not always be possible and is a decision that should be made in collaboration with your veterinarian. When it is necessary to perform a procedure that will leave a resident with a wound that could attract NWS flies, be sure to work closely with your veterinarian to implement practices that can help mitigate this risk and closely observe the individual so early detection is likely in the event of an infestation.
While mammalian farmed animal species are most commonly affected, please keep in mind that birds can be infected as well, as can other warm-blooded animals such as dogs and cats. Therefore, we recommend the same close observation and preventative measures for all warm-blooded residents. When it comes to dogs, specifically, there are parasite preventatives that they may already be on that are protective against NWS larvae. Ask your veterinarian if you are unsure if their current preventatives are effective against NWS.
While the risk of NWS must be taken seriously, sanctuaries that provide individualized care and make close observation a daily practice are well-equipped to catch and respond to an infestation early, giving their residents the best possible chance of recovery.
SOURCES:
New World Screwworm Myiasis | CDC
What to Know About the New World Screwworm | NC State University CALS News (Non-Compassionate Source)
About Us | COPEG (Non-Compassionate Source)
Confirmed Detections Of New World Screwworm | USDAThe United States Department of Agriculture, a government department that oversees agriculture and farmed animals. APHIS (Non-Compassionate Source)
New World Screwworm PetAn animal who spends regular time with humans in their home and life for companionship or human pleasure. Typically a small subset of animal species are considered to be pets by the general public. Identification Card | USDA APHIS (Non-Compassionate Source)
New World Screwworm | Arizona Pest Management Center (Non-Compassionate Source)
New World Screwworm | Merck Veterinary Manual (Non-Compassionate Source)
NRCS Resources for New World Screwworm Preparedness and Response | USDA APHIS (Non-Compassionate Source)
New World Screwworm Webinar for Animal Industry | USDA APHIS (Non-Compassionate Source)
USDA Confirms Presence of New World Screwworm in the United States | USDA APHIS (Non-Compassionate Source)
New World Screwworm Fact Sheet | Texas A&M AgriLife Extension (Non-Compassionate Source)
New World Screwworm (NWS) | American Veterinary Medical Association (Non-Compassionate Source)
Standard Operating Procedures For NWS Detections | USDA APHIS (Non-Compassionate Source)
Rethinking LivestockAnother term for farmed animals; different regions of the world specify different species of farmed animals as “livestock”. Management To Consider Screwworm | Texas A&M AgriLife Extension (Non-Compassionate Source)
What To Know About New Age Screwworms And Dogs | American Kennel Club (Non-Compassionate Source)
