Screwworm Is the Parasite Found in Paradise

Florida Outbreak Triggers Alarm and a Federal Response

Amid the palm trees, sandy beaches, and ideally coastal waters of the Florida Keys, an unwanted parasitic invader called the New World screwworm fly (Cochliomyia hominivorax) wreaked havoc on local pets, livestock, and game in the summer of 2016.

At the height of the outbreak, between 10% and 15% of the endangered Key deer population had to be euthanized (killed) to prevent the further spread of this painful and potentially deadly fly-borne infestation.

What made the 2016 outbreak all the more concerning is that C. hominivorax has long been considered eliminated from the United States due to rigorous cross-border insect control efforts.

Close-up of screwworm fly
Time Life Photos / Getty Images

What Are Screwworms?

When people talk about screwworms, they are usually referring to the larvae (maggot) 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 fly causes problems when it lays its eggs on the edge of a wound (or sometimes on the border of the mouth, nose, or anus) of a mammalian host. Humans are sometimes affected, but this is rare.

Once laid, the eggs will hatch into larvae within a day and immediately begin to consume the surrounding tissues for food. This process is called myiasis (commonly referred to as a maggot infestation).

However, unlike most maggots that live off of dead tissues, screwworms sustain themselves by consuming live and dead tissues. This causes painful open sores as the larvae burrow into deeper layers of skin and muscle. The larvae then go into the next stage of development, becoming a dormant pupa, before finally emerging from the wound as a fully formed fly.

The infestation process—from the laying of an egg on the host to the emergence of a mature screwworm fly—takes around a week. However, in cooler weather, the process has been known to take up to two months.

But, the problem doesn't start and end with a single wound. As a maggot infestation develops, the festering wound becomes attractive to other screwworm flies. As more eggs are laid in the same wound, an animal can become sicker and sicker and eventually die. Newborns are particularly vulnerable.

Where Screwworms Are Found

Screwworms are commonly found in South America and in parts of the Caribbean. Countries commonly affected include Argentina, Bolivia, Brazil, Colombia, Ecuador, Haiti, Jamaica, Paraguay, Peru, Uruguay, and Venezuela.

Screwworm larvae or flies can sometimes be transported into the United States through infested livestock or pets. Even luggage from international travelers poses a risk.

This is why rigorous inspections are conducted whenever animals are transported from countries where screwworm infestations are common. Or, why your luggage may be thoroughly inspected after returning from parts of South America or the Caribbean.

In addition to the New World screwworm fly, there is also the Old World screwworm fly (Chrysomya bezziana) found in tropical parts of south and southeastern Asia, Africa, and the Middle East.

Outbreaks in the US

Screwworm flies are thought to have arrived in the United States from South America in the 1800s. With aggressive insect control efforts, screwworms were largely been considered eliminated in the U.S. since the mid-1980s. Even before then, no self-sustaining populations had been seen since 1966, and no cases have been reported outside of Texas since the 1970s.

That's not to say there has not been the odd isolated incident. In May 2010, for example, a pet dog was found by veterinarians to have carried the infestation from Venezuela to Florida. The area was thoroughly treated to ensure the infestation was contained.

Before 1966, screwworms were an enormous problem for ranchers. The insects were able to thrive in states like Florida, Texas, Arizona, and California that enjoyed year-round warm climates. While the flies were largely dormant in the winter, the onset of spring, summer, and fall allowed the insects to creep further and further northward with each generation—in some cases, up to 100 miles per generation.

Until improved insect control efforts were instituted, screwworm flies could be found as far north as the Canadian border.

The outbreak in the Florida Keys, which reached its height in the summer of 2016, was finally reined in after the Department of Agriculture instituted aggressive control efforts in September, including the culling of deer. On March 23, 2017, the outbreak was declared controlled.

Economic Impact

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.

Insect Control

Screwworms were originally eliminated from the United States by releasing sterile male screwworm flies into endemic areas back in the 1950s. The mating of sterile male flies with fertile female flies results in fewer eggs being laid, reducing the sustainability of the population until it finally collapses.

In dealing with its outbreak in 2016, Florida repeated the same process, releasing around 188 million sterile flies from 35 release stations over the course of six months. In addition, local insect control experts recruited around 200 volunteers to coax the local deer population to eat treats infused with insecticidal medications.

A quarantine was also instituted to prevent potentially affected pets or livestock from leaving the Florida Keys. The dipping or spraying of pets with non-toxic insecticides was also conducted on a voluntary basis.

Research

Similar plans have been instituted to rein in mosquito-borne diseases like Zika, dengue, and chikungunya in hotbed regions. This includes a newer procedure known as the Wolbachia-incompatible insect technique (IIT).

Wolbachia is a bacteria that some insects need to reproduce. By rearing male mosquitoes in Wolbachia-free environments—and then releasing them into endemic areas—mosquito populations can be controlled without the need for widespread insecticide programs.

It is unclear whether the same intervention can be applied to the New World screwworm fly. Research is ongoing.

Pets and Screwworms

Screwworms can affect pets and stray animals in the same way as game and livestock. During the Florida outbreak of 2016, screwworm infestations were identified in cats, dogs, and even pigs.

When a screwworm infection occurs, topical insecticides like imidacloprid, fipronil, and selamectin be applied to the wound for two to three days. The eggs, larvae, and pupa can then be removed with tweezers. The oral insecticide Capstar (nitenpyram) has also proven effective.

Even so, the treatment of a screwworm infestation can be painful and lead to an infection if the wound is not kept sterile. For this reason, the treatment of screwworms in pets or livestock should be carried out by a licensed veterinarian.

Was this page helpful?
Article Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Skoda SR, Phillips PL, Welch JB. Screwworm (Diptera: Calliphoridae) in the United States: response to and elimination of the 2016-2017 outbreak in Florida. J Med Entomol. 2018 Jul;55(4):777-86. doi:10.1093/jme/tjy049

  2. Altuna M, Hickner PV, Castro G, Mirazo S, Perez de Leon AA, Arp AP. New World screwworm (Cochliomyia hominivorax) myiasis in feral swine of Uruguay: One Health and transboundary disease implications. Parasit Vectors. 2021;14:26. doi:10.1186/s13071-020-04499-z
  3. Batista da Silva JA, Moya-Borja GE, Queiroz MMC. Factors of susceptibility of human myiasis caused by the New World screw-worm, Cochliomyia hominivorax in São Gonçalo, Rio de Janeiro, Brazil. J Insect Sci. 2011;11:4. doi:10.1673/031.011.0114

  4. Center for Food Security and Public Health. Screwworm myiasis. Updated January 2016.

  5. Department of Agriculture. New World screwworm ready reference guide—historical economic impact. Updated December 2016.

  6. American Veterinarian Medical Association. Screwworms found in Florida. November 2, 2016.

  7. Gutierrez AP, Ponti L, Arias PA. Deconstructing the eradication of new world screwworm in North America: retrospective analysis and climate warming effects. Med Vet Entomol. 2019 Jun;33(2):282-95. doi:10.1111/mve.12362

  8. Scott MJ, Concha C, Welch JB, Phillips PL, Skoda SR. Review of research advances in the screwworm eradication program over the past 25 years. Entomol Experment Applic. 2017 Sep;164(3):226-36. doi:10.1111/eea.12607
  9. Pagendam DE, Trewin BJ, Snoad N. Modelling the Wolbachia incompatible insect technique: strategies for effective mosquito population elimination. BMC Biol. 2020;18:161. doi:10.1186/s12915-020-00887-0

  10. Han HS, Chen C, Schievano C, Noli C. The comparative efficacy of afoxolaner, spinosad, milbemycin, spinosad plus milbemycin, and nitenpyram for the treatment of canine cutaneous myiasis. Vet Dermatol. 2018 May 6;29(4). doi: 10.1111/vde.12548