By Jill Pease
June 3, 2026 update: USDA confirms the detection of New World screwworm in the U.S.
As animal health officials brace for the possibility of screwworms entering the U.S. and infecting livestock, a new University of Florida project is designed to address a critical food safety question: How long should animals be kept out of the food supply chain after receiving antiparasitic drugs?
A team led by Zhoumeng Lin, Ph.D., an associate professor in the Department of Environmental and Global Health at the UF College of Public Health and Health Professions, seeks to answer that question with the development of novel computational models that are faster and more efficient than current approaches to determining drug withdrawal intervals for food animals.
The U.S. Department of Agriculture is supporting the work as part of a rapid response funding program designed to fill critical knowledge gaps in protecting food and agricultural supply chains.
New World screwworm is a parasitic fly whose larvae feed on the living tissue of warm-blooded animals. Female flies lay eggs in open wounds and other body openings, including ears, eyes, nose and mouth. Larvae burrow into flesh and can cause serious injury and even death. Screwworm infestations have been found in livestock, wildlife, pets and, rarely, humans.
“New World screwworm is a re-emerging pest,” said Lin, a member of UF’s Center for Environmental and Human Toxicology and Center for Pharmacometrics and Systems Pharmacology. “Any new disease outbreaks affecting our national food supply will produce serious impacts on food and agricultural systems and food safety.”
Health officials eradicated the parasite from the U.S. in the 1960s, but over the decades, New World screwworm has moved northward from South America, where it is endemic. In April 2026, the USDA Animal and Plant Health Inspection Service confirmed the presence of New World screwworm in a calf in Mexico, just 60 miles from the U.S. border.
The Food and Drug Administration has authorized the antiparasitic drugs ivermectin and doramectin for emergency use to treat and prevent New World screwworm in livestock. While these drugs are effective against infestations, veterinarians currently lack guidance on how long they persist in food animals’ systems following extralabel uses. If excessive residues remain when animals are slaughtered, the drugs could enter the food supply chain.
Current methods for determining drug withdrawal periods in food animals require researchers to euthanize healthy animals, conduct residue experiments in various tissues and analyze the data using biostatistical approaches consistent with FDA or European Medicines Agency guidelines.
“This traditional method is time-consuming, resource-intensive and ethically challenging,” Lin said. “And a more important limitation is that the derived withdrawal period can only be applied to the specific administration regimen. If a veterinarian uses a different dose, treatment interval or number of dosings, the label withdrawal period will no longer be applicable.”
Computational models offer a better alternative, Lin said. Physiologically-based pharmacokinetic, or PBPK, models describe the absorption, distribution, metabolism and excretion of a drug in the body using mathematical equations, and they can be used to predict the concentration of a drug following different therapies.
Using datasets on drug depletion in food animals from the Food Animal Residue Avoidance Databank, or FARAD, a USDA-funded university-based consortium that includes UF, Lin and his team will develop PBPK models to predict withdrawal intervals for ivermectin and doramectin in cattle, swine, sheep and goats, and ivermectin in broiler chickens and laying hens.
The results will provide veterinarians with recommendations that are fast, accurate and adaptable to different routes of drug administration, doses and dosing frequencies.
“In the event of widespread outbreaks of screwworm, veterinarians will have to use ivermectin and doramectin in a variety of food animal species in an extralabel manner,” Lin said. “FARAD can apply our PBPK models to help predict scientifically-based appropriate withdrawal intervals to ensure that meat products derived from food animals treated with ivermectin or doramectin are safe for human consumption.”
The research team includes co-investigator Fiona Maunsell, BVSc, Ph.D., a clinical associate professor in the Department of Large Animal Clinical Sciences at the UF College of Veterinary Medicine; Jim E. Riviere, D.V.M., Ph.D., a consultant and distinguished professor emeritus at North Carolina State University; and collaborators Lisa A. Tell, D.V.M., Dipl. ABVP (avian) and ACMZ, a distinguished professor at University of California, Davis; Ronald E. Baynes, D.V.M., Ph.D., a professor at North Carolina State University; Jennifer L. Davis, D.V.M., Ph.D., a professor at Virginia Tech; and Majid Jaberi-Douraki, Ph.D., a professor at Kansas State University-Olathe.