Myiasis is the infestation of vertebrate animals by larvae (maggots) of any species of fly. Some fly species are specialized to use amphibian, reptilian, or avian hosts, but most usually infest mammals. In horses, the typical and most widespread of such parasites are species of internal bot flies (Gasterophilus spp.), but many other maggots (e.g. blow flies and/or flesh flies) can facultatively infest equines externally. Although none of these typically poses a serious threat to hosts, screwworms are a separate and often dire exception.

Unlike the others, the New World screwworm, Cochliomyia hominivorax, is a specialized native blow fly and obligate parasite whose maggots voraciously feed only on living tissues of warm-blooded animals. It once occupied most of the Neotropical Region, including South Texas and South Florida, where it was historically the scourge of both wildlife and livestock. From its winter refuges, it spread variably northwards each summer and injured or killed thousands of victims yearly, causing substantial economic losses to animal agriculture.

In the mid-1900s, USDA scientists conceived and began field-testing an innovative method aimed at eradicating screwworms as pests. The so-called sterile insect technique (SIT) involved mass-rearing millions of adult flies in captivity, sterilizing them by exposure to radiation, and over-flooding wild screwworm populations with sterile insects to the point that most local, field-mated wild females produced inviable eggs. Within several generations of such pressure, local populations died out, and progressively screwworms were extirpated, first from the United States and eventually down to Panama by 2000.

Screwworms persist on a handful of Caribbean islands and in northern South America, but they are prevented from dispersing and re-infesting North America by continuous releases of sterile flies in an eastern Panamanian barrier zone. The last locally infested animal in the United States was seen in 1982 in Texas, and since then, dozens of screwworm incursions have been detected and dealt with on animals and humans entering the country from still-infested areas. Many of these cases involved racehorses or polo ponies entering from South America, with screwworms detected in quarantine facilities.

Currently, however, an active invasive population of screwworms has taken up residence in the Florida Keys. How these flies entered the country and from where is still a mystery, but the infestation is of particular concern because most of the known infested hosts have been endangered Florida Key deer on the National Key Deer Refuge. As of this writing, screwworms have killed approximately 10% of the Key deer herd, along with several local pet pigs, cats, and dogs.

Since the infestation was first discovered at the end of September 2016, personnel in a state/federal task force have instituted several strategies to contain and eliminate screwworms from the Keys, including monitoring and surveillance of fly and maggot activities to delimit the infested area, veterinary inspections of all animals leaving the Keys, preventative and curative treatments of animal hosts, and most importantly, local application of the SIT through release of over 25 million sterile flies flown in from Panama. Judicious continuation of these practices into early 2017 is expected to prevent the spread of screwworms, extirpate them, and once again, make the United States screwworm free.

CONTACT: James W. Mertins, MS, PhD, Entomologist——515/337-7919—USDA Animal and Plant Health Inspection Service Veterinary Services Science, Technology, and Analysis, National Veterinary Services Laboratories, Ames, Iowa

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London.