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New World Screwworm

On October 3, 2016 USDA announced the confirmation of New World screwworm (Cochliomyia hominivorax) in Key deer from the National Key Deer Refuge in Big Pine Key, Florida. New World screwworm had been eradicated from the United States more than three decades ago, and this was the first infestation in Florida in 50 years.  USDA partnered with US Fish and Wildlife Services, Florida Department of Agriculture and Consumer Services, and Florida’s Monroe County to prevent the spread of screwworm and to eradicate the screwworm flies from the affected Florida Keys, preserve the endangered Key deer, and protect domestic animals and pets on the Keys.

A New World Screwworm Story Map, by USDA APHIS

Click the link below to see an interactive story map of the USDA’s history of eradicating the infestation and the continuing efforts to keep screwworm out of the US.

What is Screwworm?

New World screwworm disease is an infestation with the larvae of the New World screwworm fly (Cochliomyia hominivorax) that lives off the flesh of living mammals and, less commonly, birds. During the past century the presence of New World screwworm cost the U.S. livestock industry an average of $20 million annually. 

The eradication strategy consists of the following elements:

Sterile Fly Release  

Sterile fly release

USDA is using a proven method for eradicating New World screwworm fly populations that takes advantage of the fly’s own biology.  Female screwworm flies mate one time in their 21-day life span.  We release approximately 3 million sterile flies twice a week in the affected area of the Florida Keys.  As the population of sterile screwworm flies increases the population of fertile screwworm flies decreases until the population dies out.

USDA will continue releasing sterile flies until there have been no new clinical cases and the population of New World screwworm flies has been eliminated. This eradication strategy typically takes 4-6 months to complete.

USDA has released over 40 million sterile flies since the strategy was implemented in early October 2016, and screwworm cases in Key deer are declining

Learn more about APHIS IS’ Sterile Fly Program.

Screwworm Fly Surveillance

APHIS International Services are working with colleagues from Panama to monitor the success of the screwworm fly eradication strategy.  The team captures flies from across the Florida keys and determines if they are sterile or fertile.

No fertile flies have been found outside the infested zone in the lower Florida keys, an early indicator of successful implementation of the eradication strategy.

Animal Health Checkpoint

To prevent New World screwworm from leaving the keys on pets or livestock a mandatory animal health checkpoint has been established at Mile Marker 106 in Key Largo.

Travelers with livestock or pets are required to stop at the checkpoint and report any signs consistent with screwworm. A veterinarian is assigned to the checkpoint to examine all suspect animals. The checkpoint is also a source of community outreach and education about New World screwworm, the local infestation, and the eradication strategy, and preventive measures that can be taken to protect livestock and pets.

More than 6,000 animals have travelled through the checkpoint and none have been affected by New World screwworm.

Animal Reports and Response

USDA and Florida Department of Agriculture respond to reports of suspect wildlife, livestock (including poultry) and pets with clinical signs consistent with New World screwworm infestation. There have been no cases in livestock in the Keys or on the mainland, and no cases have been reported outside of the infested Key islands.

Key Deer Monitoring
and Response

The U.S. Fish and Wildlife Service (FWS), USDA, and the Florida Department of Agriculture and Consumer Services have made significant efforts to protect endangered Key deer and other wildlife from infestation. The response effort has been to locate infested deer and administer medication, treat mild and moderate wounds by trained wildlife veterinarians, and in advanced cases euthanize deer for the animal’s welfare and to protect the remaining healthy deer and other wildlife.

On October 20, 2016 FWS personnel and trained Refuge volunteers began administering doramectin, an anti-parasitic medicine, to treat Key deer as a preventative measure. Service biologists and a team of more than 170 trained volunteers continue to administer oral doses of the anti-parasitic medication doramectin to Key deer.

Additionally, 27 self-medication stations have also been erected in backcountry areas inaccessible to staff and volunteers. These self-medication stations will become the main method of doramectin administration over time.

FWS provides current statistics on the total number of therapeutic doses of doramectin administered to Key deer, the number of Key deer euthanized or deceased from screwworm infestation, and the date of the last Key deer death due to screwworm.  Visit the FWS Key deer refuge screwworm response update page for this information.


Report suspect animals. Please report mammals and birds with the following symptoms – irritated behavior, head shaking, the smell of decay, evidence of fly strike – to the agency hotlines below.

  • Key deer and wildlife: U.S. Fish and Wildlife Service Key Deer hotline (888) 404-3922, ext. 7.
  • Livestock and pets: Florida Department of Agriculture & Consumer Services (800) HELP-FLA (800-435-7352).

Don’t take screwworm home with you.  Visiting the Keys? The screwworm fly can spread by transportation inside vehicles.  On your trip home, check your car for flies and remove them before driving home.

Report a historical screwworm case.  If you live on or visited the Florida Keys this spring or summer and you observed an animal with signs consistent with screwworm, please report it at This information will assist the USDA and FDACS in identifying the possible point of re-entry of screwworm into the United States.

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