In November 2024, Mexico reported its first case of New World Screwworm. Since then, the pest has spread northward and while not yet detected in the US, the threat to our livestock industry continues to grow.
With the coming warmer weather, we wanted to take this opportunity to remind you about the clinical signs and implications of NWS in the US.
New World screwworms (NWS) are larvae or maggots of the NWS fly (Cochliomyia hominivorax), that cause a painful and devastating condition known as NWS myiasis. NWS flies lay eggs in open wounds (tick bites, cuts, dehorning or branding wounds, etc…) or orifices of live tissue (nose, ears, umbilicus, or genitalia). These eggs hatch into dangerous parasitic larvae, and the maggots burrow or screw into live flesh with sharp mouth hooks feeding as they go. The wound can become larger, and an infestation can often cause serious, deadly damage. NWS primarily infest livestock, but can also affect mammals, including humans, and birds. Screwworm infestations are very painful, and animals may become depressed, stop eating, and isolate themselves from other animals or people. Without treatment, animals may die within a week of being infested. Adult screwworm flies are about the size of a common housefly (or slightly larger). They have orange eyes, a metallic blue or green body, and three dark stripes along their backs.
NWS infestations respond well to early treatment with ivermectin.
NWS is endemic in Cuba, Haiti, the Dominican Republic, and countries in South America. Since 2006, the United States and Panama have maintained a barrier zone in eastern Panama. This barrier zone prevents NWS from moving north from South America to screwworm-free areas in Central and North America. In 2023, APHIS confirmed an unprecedented number of NWS cases in Panama. Since then, cases have been detected in Costa Rica, Nicaragua, Honduras, El Salvador, Guatemala, Mexico, and Belize. Screwworms are very susceptible to freezing temperatures or long periods of near– freezing temperatures. These organisms are seasonal in some areas and can spread into colder climates during the summer. The ideal environmental conditions for survival and activity are temperatures of 25-30°C (77-86°F) and relative humidity of 30-70%.
NWS can threaten the livelihood of livestock producers. It can cause millions of dollars’ worth of production losses and economic damage. If you travel to regions affected by screwworm, know the requirements for returning to the United States.
Eradicating NWS is only possible through sterile insect technique. Sterile flies are released into an area where a known population has become established. The sterile male screwworm flies mate with fertile female screwworm flies, causing the population of screwworm flies to decrease until it eventually dies out.
New World Screwworm (NWS) is a Category I (Immediate Reporting) Foreign Animal Disease not only in Nebraska, but all states. Please notify NDA at 402-471-2351 or our local USDA office at 402-434-2300 if you suspect NWS in NE.
NDA relies on and thanks our veterinarians to report cases to help safeguard the animal industry in Nebraska.
References: USDA, CFSPH, Texas Animal Health Commission, Merck Veterinary Manual