Screwworm Is the Parasite Found in Paradise

Florida outbreak triggers alarm and a federal response

Amid the palm trees, sandy beaches, and idyllic 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.

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

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.

This article details what screwworms are, where they are most commonly found, and what can be done if or when there is an outbreak.

What Are Screwworms?

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 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 both 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. It's one reason 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 an 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 have 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 haven't been isolated incidents. 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, which enjoy 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.

Florida Outbreak of 2016

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.


Similar plans have been instituted to control 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.


Screwworms outbreaks, like one that took place in the Florida Keys in 2016, are caused when the New World screwworm fly lays its eggs on mammals near moist orifices or open wounds. The emerging larvae (maggots) feed on the tissues, causing painful sores and infections.

Screwworms flies mainly live in warm, tropical climates. During outbreaks, public health officials will often respond by releasing sterile male screwworm flies to mate with the female flies. Without the means to effectively reproduce, the screwworm population eventually collapses.

Game and wildlife are mainly affected by screwworm outbreaks, but pets can also be targets of infestation. Human infestations are rare but have been known to occur.

10 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.
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By Megan Coffee, MD
Megan Coffee, MD, PhD, is a clinician specializing in infectious disease research and an attending clinical assistant professor of medicine.