New World Screwworm Threatens Florida Again

In 2016, a devastating threat crept its way to the Florida Keys, impacting several species of animals, including the endangered Key deer. The culprit was Cochliomyia hominivorax, also known as the New World screwworm (NWS) fly. More than 130 endangered Key deer died before the parasite was eradicated. 

Now, headlines look all too familiar, warning that the flesh-eating maggot has reappeared too close to the United States for comfort.  

What Is the New World Screwworm? 

The screwworm fly looks deceptively like a common housefly. But instead of laying eggs in rotting food or manure, it targets the living. Its larvae, or maggots, hatch inside wounds or mucous membranes of warm-blooded animals - including humans - and feed on live flesh. 

Infected animals may appear lethargic, lose appetite, and develop weeping, foul-smelling wounds. If left untreated, infestation can lead to death. 

“This fly is an obligate parasite,” explained Dr. Christopher Lee, a public health veterinarian who has studied screwworm spread and response. “If the maggot is not inside of an animal, it’s likely not NWS.” 

The 2016 Outbreak 

According to Lee, the 2016 screwworm outbreak was likely caused by a smuggled pet, an infected dog from Cuba, where the parasite is still endemic. Although that theory was never officially confirmed, it offered a plausible explanation for how the parasite slipped past border control. 

Crews from the USDA, Florida Department of Agriculture, and U.S. Fish and Wildlife Service worked around the clock to treat wounds, trap flies, and halt the spread. “Thanks to rapid detection and an aggressive eradication response, the parasite was eliminated before it could spread widely,” said Lee.  

In 2016, therapid response teams managed to contain the outbreak before it spread inland, but experts now warn that the next wave won’t be so easily stopped.  

The New World Screwworm Today 

“Today’s situation is very different,” said Lee. “We’re not just looking at another isolated incident. This time, it’s a march - a steady parasitic advance.” 

As the current outbreak evolves, U.S. agencies are tracking a slow but relentless northward spread of screwworm through every Central American country. In response, the U.S.-Mexico border has been closed to livestock imports. “The NWS incursion will bring a wave of parasites that will more likely establish residency here,” said Lee. “The level of threat is high.” 

Why Florida Is at Risk 

While experts expect initial cases in Texas, New Mexico, Arizona, or California, Florida’s climate and ecology make it especially vulnerable to spread. 

“Florida’s warm, humid climate and abundance of wildlife make it an ideal environment for Cochliomyia hominivorax to thrive,” said Lee. “NWS will likely devastate pets, livestock, and wildlife. Human cases will also occur, and homeless populations will be particularly vulnerable.” 

In the past, the parasite has caused staggering economic damage. In the 1930s and 1940s, U.S. ranchers reported annual losses of $5 to $10 million. By the 1950s and 60s, that number had ballooned to as much as $120 million per year, according to USDA data. 

Today, wildlife, livestock, and public health agencies are preparing accordingly. And as the climate warms and international animal movement increases, experts caution that screwworm may not be the last parasitic threat Florida faces. 

The Response: Sterile Flies and Surveillance 

The gold-standard method for screwworm control - sterile insect technique - involves breeding and releasing millions of sterile male flies to collapse wild populations. But rebuilding infrastructure for mass production takes time. 

Today, only one center in Panama (COPEG) is actively releasing sterile NWS flies. Two more facilities, one in Mission, Texas, and one in Tuxtla Gutierrez, Mexico, are being retooled to support the effort. But Lee warns that full-scale capacity may still be “a couple years away.” 

Meanwhile, the National Veterinary Services Laboratory (NVSL) is receiving a surge of suspected screwworm samples, many of which turn out to be harmless caterpillars or moth larvae. 

How to Spot It and Stop It 

There is currently no FDA- or USDA-labeled treatment for NWS in animals or people. Prevention is key.