New World Screwworm

Screwworms are fly larvae (maggots) that feed on living flesh. These parasites can infect any warm-blooded animal. Screwworms enter wounds as small as a tick-bite and feed on living tissue in the area. If untreated, screwworm infestations can be fatal. In the US, Screwworm is a Foreign Animal Disease reportable within 24 hours of diagnosis. Screwworms have most recently entered the US in dogs that have been imported from areas of the world where the flies are endemic.

New World screwworm, Cochliomyia hominivorax, has been moving North again after years of being pushed down past the Darién Gap in Panama. While endemic in many regions of South America and some Caribbean Islands, the screwworm was recently found in Nuevo Leon, Mexico which is 70 miles off the US border.

While an adult screwworm fly is capable of flying for up to 10-14 days, they typically only fly short distances, 6-9 miles (10-15 km) when there are sufficient hosts in the area and suitable environmental conditions. Although the screwworm fly is capable of flying up to 125 miles, longer distance spread is more likely facilitated and propagated via transport on human and animal hosts.

Identification

New World Screwworm Larvae

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• Pale with encircling black spines
• No head capsule
• Long tapered appearance

Immature Larvae

• Larvae in myiasis cases may be recognized by the shape of their spiracles
  • Number of slits = instar
  • Sclerotized peritreme
  • Button
• Positioning of tubercles
• Spines at segment margins

	New World Screwworm Cochliomyia hominivorax	Secondary Screwworm Cochliomyia macellaria
Distribution	Americas and Caribbean	Americas and Caribbean
Abundance	Low densities in the environment	Common/abundant in the environment
Climate	Warm-temperate to tropical	Temperate to tropical
Feeding habit	Living, warm-blooded animals	Carrion, rarely living animals
Importance	Parasite of: Livestock Wildlife Pets Humans	Decomposer Forensics/time of death Pollinator Live animal infestation is rare
Adult Screwworm Flies

Infested animals may present with enlarging, draining, foul-smelling wounds and weight loss; they may isolate themselves and show signs of discomfort. Animals may die from secondary infection or toxicity in 7-14 days if not treated.

Wounds that might become infested include those caused by: Tick bites, castration or dehorning, branding, shearing, and sore mouth in sheep.

In addition, screwworms can infest the navels of newborn mammals, and mucous membranes of bodily orifices, including: nostrils and sinuses, eyes, mouth, ears, vulva, anus, prepuce, and urethral fossa.

A deer's head infested with screwworms

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Reporting

Screwworm infestation is a foreign animal disease and must be reported to CDFA or USDA animal health officials within 24 hours of diagnosis or suspicion for all livestock and pet.

Samples of eggs, larvae, or flies should be placed in 70% ethanol or isopropyl alcohol for transportation to the diagnostic laboratory.

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