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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 newly born animals, castration or other surgical sites).

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 (in which they undergo 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 northern 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.

There is growing concern among some scientists that screwworms may become an issue in the US again soon. 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 and 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.

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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 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. 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, a red face, and three stripes running down their body and along their back.

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 (e.g., USDA Animal and Plant Health Inspection Service/APHIS), so 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 and save any fly eggs as well. Save larvae/eggs by putting them into 70% ethanol or isopropyl alcohol (do not use formalin). 

Treatment, Control, and Prevention

Seek veterinary care immediately. Treatment involves removing eggs, screwworms, and other maggots from the wound as carefully as possible. 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 may not be possible. 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 it can damage healthy tissues.

The US Food and Drug Administration (FDA) granted an Emergency Use Authorization for the isoxazoline insecticide lotilaner to treat infestations in dogs and cats. In addition to lotilaner, other isoxazolines effective in dogs for screwworm treatment include afoxolaner, fluralaner, and sarolaner (given at standard monthly doses). Nitenpyram is another insecticide that can kill screwworm larvae after two treatments given six hours apart. This is safe for both dogs and cats. Spinosad is also a potential option (extra-label per the US FDA), but it tends to be only about 80% effective in dogs, and its effectiveness in cats is unclear. Additionally, antibiotics and pain medication may be necessary.

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.