New World Screwworm: Understanding the Risk and Current Status in the U.S.

The New World screwworm (Cochliomyia hominivorax) is a parasitic insect that poses a severe threat to livestock and wildlife, but it is currently eradicated from the United States. While social media reports occasionally suggest the presence of the pest in states like Alabama, Florida, or Georgia, the U.S. Department of Agriculture (USDA) confirms that the species has been absent from the country for decades, with the exception of an isolated outbreak in Florida in 2016.

What is the New World screwworm?

The New World screwworm is the larval stage of a fly that feeds on the living tissue of warm-blooded animals. Unlike common blowflies that lay eggs on decaying carcasses, female screwworm flies deposit eggs in open wounds or mucous membranes of living hosts. According to the USDA Animal and Plant Health Inspection Service (APHIS), once the eggs hatch, the larvae burrow into the host’s flesh, causing deep, painful wounds and potentially leading to death if left untreated. The parasite can affect cattle, deer, pets, and, in rare instances, humans.

Is the screwworm currently in the United States?

No, the New World screwworm is not currently established in the United States. The U.S. successfully eradicated the pest through a decades-long effort utilizing the Sterile Insect Technique (SIT), which involves releasing mass-reared, sterilized male flies to disrupt the reproductive cycle of wild populations.

The last major domestic outbreak occurred in 2016 in the Florida Keys, which affected the endangered Key deer population. The USDA, in coordination with the Florida Department of Agriculture and Consumer Services, declared the eradication successful in March 2017 after months of intensive monitoring and sterile fly releases. Since that time, there have been no confirmed reports of the screwworm establishing a population in any U.S. state.

How to distinguish verified reports from misinformation

Claims circulating on social media regarding widespread screwworm infestations in the American South often lack verifiable evidence or institutional backing. When assessing the risk of agricultural pests, officials advise relying on the following sources:

• USDA APHIS: The federal authority for monitoring and managing foreign animal diseases.
• State Departments of Agriculture: Agencies such as the Florida Department of Agriculture and Consumer Services maintain active surveillance programs for invasive species.
• University Extension Services: Local agricultural extensions often provide localized, science-based guidance on livestock health.

Historical context of eradication

The United States eliminated the screwworm as a public health and agricultural threat through a consistent, multi-national effort. The program, which began in the mid-20th century, effectively pushed the screwworm barrier south. Today, a permanent barrier is maintained in Panama by the Commission for the Eradication and Prevention of Screwworm (COPEG), a joint venture between the U.S. and Panamanian governments. This facility continues to produce sterile flies to prevent the northward migration of the pest from South America.

Key Facts Summary

Livestock owners who suspect an infestation of any foreign pest are encouraged to contact their state veterinarian or local USDA office immediately. Rapid reporting remains the primary defense against the reintroduction of the New World screwworm.