Hunters and Ranchers Beware Screwworm Re-emerges in Monroe County

Not since the 1960s have biologists seen screwworm widely present in the United States. Recently, wildlife officials in Monroe County, Florida, noted the loss of a number of endangered Key deer, and the United States Department of Agriculture confirmed the presence of screwworm in Key deer in Big Pine Key and surrounding islands. That federal agency and Florida’s Department of Agricultural and Consumer Services (“FDACS”) have declared an agricultural emergency and already mounted a response to ensure that the disease does not spread past the identified area in Big Pine Key. Cattle farmers, hunters, and other outdoorsmen, nonetheless, would be wise to inform themselves about these pests to help identify and prevent any spread of the disease to peninsular Florida.

Screwworms are actually fly larvae which are nested by an adult screwworm-fly in open orifices of warm blooded animals – notably, livestock, wild animals and pets. Even humans can be infected. In most cases, the adult fly nests the larvae in an open wound, a nostril, or, in the case of a newborn, a navel. The larvae live and develop by feeding off of the flesh of the animal. Within about seven days, the matured larvae exit the animal and burrow into the ground. They arise as adult screwworm-flies – equipped to mate and continue the cycle. An adult can produce thousands of offspring during its life cycle. They can have devastating effects on herds of game and livestock. It is important to know what to look for and who to call to help keep screwworms from invading your farm and hunting grounds.

Adult screwworm-flies share a similar (if not slightly larger) size and shape to normal house flies. Screwworm flies, however, have orange eyes and a metallic dark blue to blue-green or gray body. Notably also, they have three dark stripes running the length of their backs.[1] The larvae are cylindrical and have one pointed end and one blunt end. Screwworm eggs are creamy and/or white and are typically deposited in a shingle-like manner. If you notice an animal in obvious discomfort, separating itself from the herd, or, if livestock, suffering from decreased appetite or milk production, if safe to do so, take a closer look. If you find that animal has an open wound which is clearly infested with maggots or that the wound has a blood-tinged discharge and notice a foul odor, you may be dealing with screwworm. Infested animals, if left untreated, may die in 7-14 days.[2]

FDACS is working with their federal counterparts to prevent any spread outside of Monroe County. FDACS and wildlife officials, among other efforts, have already deployed the time-tested and very effective technique of releasing sterile male flies into the infested areas – stifling productive mating. USDA officials used this same technique in the late 50’s and early 60’s to virtually eliminate the screwworm’s presence in the United States. An animal checkpoint has also been established in Key Largo, and all pets are being inspected.

Given their prevalence in Mexico and the Caribbean, however, a resurgence of screwworm in the United States was always possible. While they can fly much farther during ideal conditions, generally the fly will not travel more than a couple of miles. Long distance spread is more likely when the infested animals are moved.

Veterinarians can assist pets and livestock suffering from the pest. Early detection can be successfully treated. If you think you have seen a case of screwworm infestation, please contact FDACS at 1-800-435-7352. For more information, please see Do your part to keep Florida’s livestock and wildlife safe and healthy.

About the Authors:
Cari L. Roth
is an attorney in Dean Mead’s Government Relations & Lobbying Team and Agribusiness Industry Team. She focuses her legal and legislative practice on state and local government issues, including land use, water and environmental law, growth management and infrastructure issues, and intergovernmental relations. Ms. Roth currently chairs the State’s Environmental Regulation Commission by Governor’s appointment. She may be reached at

Brian M. Stephens is an attorney in Dean Mead’s Real Estate department and Agribusiness Industry Team. He works with businesses and developers in various aspects of commercial and residential real estate, leasing, financing, land use, title claims and growth management. He may be reached at

[1] FDACS Website. Reportable Animal Diseases.

[2] Id.