What are Screwworms?

Screwworms usually infect livestock, and the economic impact on that industry can be enormous.

When people talk about screwworms, they are usually referring to the larvae (maggots) of the New World screwworm fly. The fly itself is about the size of a regular housefly but has orange eyes and a metallic-looking body that’s either blue, green, or gray with dark stripes.

The female is attracted to fresh open wounds on any warm-blooded animal. The wounds can range from a thorn scratch, insect, or tick bite mark to a gaping laceration. The females are also attracted to the newborn umbilicus of calves, castrated calves, dehorning, branding, or shearing wounds. The female screwworm will quickly lay 100-300 eggs on the dry perimeter of the wound.

In less than 24 hours, larvae hatch from the eggs and begin to feed on the open wound. It will take 1-2 weeks for the larvae to become fully developed, in that time they will consume a large amount of dead and live flesh.

Untreated screwworm infestations can be fatal.  The larvae will continue to feed on the animal and will eventually eat the host alive.

In 2007, a 12-year-old girl arrived in an emergency room in Connecticut complaining of extraordinary pain in her scalp. She had just returned from a trip to Colombia with her family and speculated that the pain was due to “sun poisoning.” Previous efforts to diagnose her scalp pain and what appeared as “fluid-filled bumps” at a local clinic in Colombia identified cellulitis, a bacterial skin infection, and she was given antibiotics and sent on her way. Yet the pain persisted and upon returning to Connecticut, the young patient and her family immediately went to the ER directly from the airport. To examine her scalp, shift the strands of her hair, and palpate the affected region, physicians had to administer intravenous morphine to bypass her discomfort and suffering.

On the right side of her scalp was a “5×5-cm area of swelling with multiple punched-out lesions oozing a foul-smelling, purulent exudate.” A computed tomography (CT) scan of her head confirmed the swelling. “Mobile larvae were identified,” every living person on earth’s nightmare.

The larvae were sent to the Connecticut Agricultural Experiment Station and were identified as the New World screwworm. The Latin name of this bug is Cochliomyia hominivorax. For those of us rusty on our Latin, “hominivorax” is roughly translated as “eater of man.”

An adult screwworm fly can travel up to 125 miles before laying eggs in a wound. Screwworms can also be
transported by animals and people travelling from infested
areas. Female screwworm flies are attracted to all warm-blooded animals. Wounds that might become infested include those caused by:

  • Tick bites
  • Castration or dehorning
  • Branding
  • Shearing
  • 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. It may be difficult to visualize the screwworm maggots at the wound surface, since only the breathing tubes of the maggot are exposed.

As the larvae feed on live tissue, they burrow into the tissue creating a deeper and wider wound. This deep burrowing is a distinctive feature of screwworms; other maggots are surface feeders on dead tissue. 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. The US agricultural office dealt with the issue of screwworms in the American South by sterilizing the males with radiation and then releasing them to mate unprofitably with their female counterparts.

The short lifespan of the females, and the valuable time wasted screwing around with a sperm-bereft screwworm, quickly eliminated the fly from the North American landscape. If screwworm is diagnosed, animal health officials will quarantine the animal until daily wound care and treatments with larvicides and insecticides have successfully eliminated the screwworm larvae. The USDA and CDFA investigate cases, and determine if additional control
measures, such as insecticidal treatment of the environment
or sterile fly release, are warranted.

Screwworms usually infect livestock, and the economic impact on that industry can be enormous. In Florida alone, the industry is worth well over 2.5 billion dollars per year. In addition to its impact on ranching, screwworm infestations require a lot of money to prevent or control.

This can cost local economies hundreds of millions of dollars. Preventive efforts in Texas alone are said to cost the state government and livestock industry around $561 million annually.

Further Reading


VeryWell Health
California Dept of Food and Agriculture
Entomology Today
Livestock Veterinary Entomology
Discover Magazine

Author: Doyle

I was born in Atlanta, moved to Alpharetta at 4, lived there for 53 years and moved to Decatur in 2016. I've worked at such places as Richway, North Fulton Medical Center, Management Science America (Computer Tech/Project Manager) and Stacy's Compounding Pharmacy (Pharmacy Tech).

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