For the Health of It
Screwworm Myiasis
Equine Disease Quarterly , January 2017 , Volume 26 , Number 1
18 • Walking On
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 ) may 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 screw-worms were extirpated , first from the USA 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 U . S . 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 race horses 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 USA screwworm free .
Contact : James W Mertins , MS , PhD ,
Entomologist James W Mertin @ aphis usda gov ( 515 ) 337-7919
USDA APHIS VS STAS ; National Veterinary Services Laboratories
Ames , Iowa