A parasite twice eradicated from the United States threatens to return amid a Central American outbreak that is spreading northward, according to a briefing by the Centers for Disease Control and Prevention (CDC).
The New World screwworm (NWS) fly (Cochliomyia hominivorax) is about the size of a common house fly. Its larva consumes living flesh after hatching from an egg deposited into open wounds or mucous membranes of its host. That host typically is livestock, but the screwworm fly also can deposit eggs in humans and other animals. There have been three cases of NWS infestation in human travelers to the U.S. since 2014.
“A female fly can lay hundreds of eggs in a single wound, which can hatch out into hundreds of larvae, causing extensive tissue destruction, debilitation and a risk of death,” said Rebecca Chancey, M.D., medical officer for the CDC’s Division of Parasitic Diseases and Malaria at the National Center for Emerging and Zoonotic Infectious Diseases, during a Clinician Outreach and Communication Activity (COCA) call last week.
One such wound is particular to newborns.
“The umbilical wound is a wound site that the flies favor,” entomologist Mark Fox, M.S., said.
Children generally face the same risk of NWS infestation as adults, provided they take sufficient precautions in an area where the fly is present, Dr. Chancey said.
Those areas include Panama, Costa Rica, Nicaragua and Honduras. As of Sept. 28, Panama had recorded 18,553 animal cases and 79 human cases during the outbreak that began in August 2022, according to the Panama-United States Commission for the Eradication and Prevention of Screwworm.
Dr. Chancey advised people traveling to an area where NWS is present to keep skin wounds clean and covered, limit the amount of skin exposed, use insect repellent containing DEET, treat clothing and gear with 0.5% permethrin insecticide, and sleep indoors or in a room or tent with screens.
Symptoms of NWS infestation vary by body location but include:
- visible larvae in or around open sores;
- pain;
- sensation of movement inside the body;
- foul-smelling, bloody discharge;
- tissue invasion and destruction; and
- secondary bacterial infections with fever or chills.
Tissue destruction and/or secondary infections can lead to death, which is more likely when the larvae are in deep structures of the head, Dr. Chancey said.
Indicators of infestation to doctors include patient travel to South America, the Caribbean or the countries in Central America with re-emergence, combined with the presentation of symptoms above.
Treatment requires extraction of all NWS larvae in the patient’s body, debridement of necrotic tissue and wound care. Larvae should be collected in a leak-proof container with 70% ethanol and sent to a clinical lab for identification. Larvae should never be disposed of in the trash.
Suspected human cases should be reported to local and state health department officials as well as the CDC at [email protected]. Diagnostic assistance from the CDC is available at [email protected].
Panama had been the geographical barrier between South America, where NWS is endemic, and Central and North America. A facility in the Darién Gap produces sterile male screwworm flies that disrupt the lifecycle.
“They’re gamma-irradiated to be sterile and then dropped out of an airplane over the jungle,” Fox said. “If a wild female encounters one of these sterilized males, this will use up her one opportunity to mate in her lifetime, and she will never be able to produce eggs.”
No concrete reason for the northward spread has been given, and Dr. Chancey said it is likely “multifactorial.” She noted that surveillance of NWS is limited in hard-to-reach areas across Central America, and livestock often are transported through unmonitored or insufficient checkpoints.
NWS was first eradicated in the U.S. in 1966. An outbreak local to deer in the Florida Keys was discovered in 2016 and eradicated the next year.
