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Adult screwworm blowfly

New World screwworms are flesh-eating larvae (maggots) of the blowfly, Cochliomyia hominivorax. These flies commonly infest livestock. Other warm-blooded animals, including dogs, cats, wildlife (e.g., deer), and even people, can also be infested. The female fly lays her eggs near open wounds or body entrances (e.g., the umbilical cord site or belly button of newborn animals, castration or other surgical sites, near the entrance to the ears, near the mouth or nostrils).

Screwworm eggs hatch within approximately 24 hours, and the larval stage of the fly (screwworms) begins feeding on living, healthy tissue. This is unlike many other types of fly larvae, which feed on dead and decaying tissue. After about seven days of feeding, larvae fall off the animal, pupate (undergoing a transformation from maggot to fly), and develop into adult flies.

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Where Do Screwworms Live?

New World screwworms live in Central and South America, as well as on some Caribbean islands. They were also common in the southern US but were eliminated in the 1960s. Small outbreaks have occurred in the US since then, but for the most part, the US has remained free of major infestations.

Unfortunately, as of June 2026, New World screwworms have reemerged in the US. Another type of screwworm species that acts like New World Screwworms is Old World Screwworm (Chrysomya bezziana). This species of blowfly is mainly found in Africa, India, and Southeast Asia.

What Does a Screwworm Infestation Look Like?

Usually, an infested animal has an open wound of some kind. These wounds will have an intense smell, often leaking pus or bloody fluid, and enlarge quickly as screwworms feed and grow. Some animals may be very sick, showing signs of poor appetite, hiding, and/or not wanting to get up and move around. Screwworms will be present within and throughout the wound area. Other types of fly larvae may also be present, feeding on dead/decaying tissues. Tiny clusters of white fly eggs may be visible at the edges of wounds, and adult flies may land, hover, and/or fly off the animal frequently.

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How to Identify New World Screwworms

Ideally, identification of screwworms or the adult C. hominivorax fly should be made by a veterinarian (often with the help of a parasitologist or entomologist, someone highly specialized in parasites or insects). Many species of adult blowflies and their larvae appear similar to the untrained eye.

Eggs: New World screwworm eggs are about 1mm long, cream to white in color, and are usually laid in shingle-like rows or groupings along the border of an open wound. Eggs laid by other species of blowfly are not usually as well-organized in a distinctive pattern. 

Larvae/Maggots: First-instar (stage of development) screwworm larvae eclose or hatch from the eggs eight to 12 hours after being laid. They grow to about 2mm long. Larvae molt to second- and then third-instar screwworms.

At these developmental stages, screwworms are cylindrical, pointed at one end and blunt at the other, and have rings of tiny, dark brown spines encircling their bodies. The name screwworm comes from the larvae’s similar appearance to a wood screw as well as their ability to tunnel deeply into a wound. Mature screwworms grow to about 15 mm long on average, with a range between 12-17mm. 

Mature screwworm larvae also have two distinctive dark stripes along their backs called tracheal trunks.

Adult Flies: Adult C. hominivorax flies have a metallic bluish-green body, orange-to-red eyes, and three stripes running along their backs.

Wound Types: A major identifying feature of screwworms that should be a warning flag for anyone, even if they can’t be identified otherwise, is that they eat healthy tissue. So even if you can’t tell for certain whether an animal is infested with less-harmful maggots or screwworms, only screwworms eat healthy tissue. In the US, if screwworms are discovered, they must be reported to state health officials and/or the United States Department of Agriculture (USDA) Area Veterinarian-in-Charge. It is best to involve a veterinarian as soon as possible to help the animal and properly identify and report the infestation.

Because time is of the essence, taking pictures of eggs (i.e., how they are deposited on the wound and the egg structure itself), larvae, flies, and wounds to send to a veterinarian, parasitologist, or entomologist may help identify screwworm faster. It will also help document the infestation.

Screwworms can be removed from the animal to help with identification. Use gloves and forceps or tweezers, and avoid damaging the larvae by pinching or pulling forcefully during removal. Remember, other types of fly larvae may be present, feeding on dead/decaying tissue in the wound, so try to remove larvae in the deepest part of the wound to make sure you are getting screwworm larvae. Remove flies and any fly eggs as well, if possible. Save larvae/eggs and flies by placing them in 70% ethanol or isopropyl alcohol (do not use formalin). 

Destroy any eggs, flies, or larvae removed from the wound that you don’t save for identification.

Treatment, Control, and Prevention

Seek veterinary care immediately. Treatment involves carefully removing eggs, screwworm larvae, and other maggots from the wound. Avoid damaging healthy tissue or damaging larvae, which could further contaminate the wound. Screwworm infestations can go very deep beneath the skin, so removal of all larvae is extremely difficult. The wound will need to be flushed/irrigated and cleaned with appropriate, animal-safe cleansers (e.g., sterile saline, sterile Lactated Ringer’s solution, dilute povidone-iodine/betadine, dilute chlorhexidine). Don’t use peroxide or alcohol, as they can damage healthy tissues.

Prevention involves treating any open wounds quickly and keeping wounds covered to prevent flies from laying eggs. Insecticides used for monthly ectoparasite prevention (e.g., flea prevention) in dogs and cats (e.g., lotilaner) that are appropriate for treating screwworms can be used regularly as preventive therapies. Female flies tend to mate only once. An eradication program involving the release of sterile (i.e., barren) male flies is ongoing to help control the population.