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A type of flesh-eating worm eradicated from the U.S. in the 1960s has been spotted again in Central America, prompting concerns about a potential northward spread, experts said during a call Thursday.

New World screwworm (NWS; Cochliomyia hominivorax) is a fly with a metallic blue or green body and orange eyes that lays hundreds of eggs on wounds or exposed mucous membranes. The eggs hatch into larvae with a screw-like body shape that burrow deeply into living flesh and consume it for up to 7 days. Infestations mostly occur in livestock, but can occur in humans and can even lead to death.

"There is an ongoing outbreak of New World screwworm currently in Central America with recent northward spread that poses a threat of reintroduction into the United States," said Rebecca Chancey, MD, of the CDC's National Center for Emerging and Zoonotic Infectious Diseases, during the call.

She noted that there are no new reports of New World screwworm in the U.S. among pets or people, and there are currently no restrictions regarding travel. "Most of the flies are in remote areas around cattle and other livestock, not typically where people are traveling for vacation," she added.

According to Chancey, NWS was successfully eradicated in the U.S. in the 1960s, followed by Mexico in the 1970s, and Central America in the early 2000s, primarily through the release of sterile male flies. Female flies only mate once over the course of their short life, and if the female encounters a sterile male fly to mate with, it eliminates her capability to reproduce.

As of October 10, four Central American countries have collectively counted over 30,000 animal cases -- and 114 human cases -- mostly in Panama, but also tracking north in Costa Rica, and Nicaragua and Honduras.

A handful of U.S. cases in recent years were "imported" largely after travel. In one, a 26-year-old woman on vacation in the Dominican Republic fell asleep on a beach after drinking and felt moving in her ear the next day. She removed a fly from her ear, and later began to feel pain. At the hospital, she underwent debridement, and was found to have "soft tissue larval infiltration extending to the temporal bone" and "tympanic membrane perforation without middle ear involvement," and was prescribed antibiotics.

In a case this year, an immunosuppressed man who traveled to the Dominican Republic returned to the U.S. with facial swelling, pain, and nasal bleeding. He ultimately had 100 to 150 larvae surgically removed from his nose and sinus cavity. He was also treated with ivermectin, though Chancey said surgical extraction should be prioritized over medical management "to address the extensive pain and local tissue destruction caused by the larva, as well as the need to remove the larva before they can contaminate the environment."

"These larvae, as mentioned, are pretty tenacious and hang on pretty tightly, so oftentimes, a great deal of force is required to remove them, so it's important to make sure that every single larva is removed," she explained.

Addressing clinicians, Chancey differentiated NWS from other obligate myiasis flies like sheep ked and horse bot fly. Compared with human bot fly larvae, for instance, NWS larvae are much smaller, with a tapered head and no black spines, though it can still be difficult to identify them.

People at increased risk for infestations may have open, ulcerated, or bacterial-infected wounds, compromised immune systems, or hyperkeratosis. Elderly and incapacitated people are also at increased risk. In addition, sleeping outdoors and working around infested livestock are risk factors.

Symptoms include pain, sensation of movement, and malodorous bloody discharge at the site of infestation. The larvae can cause extensive tissue invasion and destruction, and may be seen around the site, with the possibility of secondary bacterial infections. NWS can infest any mucus membrane or open body orifice. Orbital myiasis may need more aggressive treatment, and infestations of the head can be the most dangerous.

Chancey said the reason for the call was to alert physicians that they should make the CDC and other public health officials aware of any new cases, and to encourage people traveling to protect themselves. Travelers should keep wounds clean and covered; limit exposure of skin with long, loose-fitting clothing; use DEET-containing insect repellent; and sleep indoors.

Should clinicians encounter NWS larvae, they were advised to collect all larvae in a leak-proof container filled with 70% ethanol or isopropyl alcohol, or 5-10% formalin, and send them to a clinical lab for morphological identification, without putting any in the trash. They should also report the case to local or state health departments, and state animal health officials.

Releases of sterile male flies are ongoing in Central America, according to Chancey, along with field surveillance, health education campaigns, and a reporting line for the public.

"If the patients are able to protect themselves, and if we're able to monitor anybody that comes into the U.S. who may have had exposure, then hopefully we'll be able to keep an eye on any emerging situation here," she said.

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