The New World screwworm (NWS), previously declared eradicated from the US, may be making a comeback. The pest that poses serious threat to livestock, wildlife, pets, and sometimes humans has been detected in northern Mexico, according to a press release from the US Department of Agriculture (USDA).

On August 26, the USDA and the Department of Health and Human Services confirmed a case of travel-associated NWS in an adult who had been in El Salvador. The CDC is conducting follow-up research, according to another press release, but the risk to public health is currently deemed very low. Previous human cases associated with travelers have been documented in past years, but all were considered isolated in the wake of local surveillance investigations, according to the press release.

Meet the Pathogen

The NWS is not actually a worm but a species of parasitic fly larvae that can cause myiasis, said Shirin Mazumder, MD, an infectious diseases specialist in Memphis, Tennessee, in an interview. “Infection occurs when screwworm flies lay eggs in open wounds or in body cavities lined with mucous membranes,” Mazumder said.

The larvae then hatch and use sharp mouth hooks to burrow into tissues, hence the screwworm name. The larvae can cause significant tissue destruction as well as infection, which distinguishes them among other larvae, according to the CDC. The NWS larvae may remain feeding on their hosts’ tissues for as long as 7 days. After this period, they leave the host and burrow into the ground, and an adult fly emerges anytime from 1 week to nearly 2 months later.

NWS is most often found in South America and the Caribbean, but cases have appeared in Central America in recent years. The majority of infestations occur in animals who have been exposed to the NWS fly, and the larval infestation is not contagious, according to the CDC.

What to Look for

“Symptoms of an NWS infestation include visible larvae in open wounds, poorly healing and painful skin wounds, and bleeding or foul odor from open sores,” Mazumder told Medscape Medical News. “It is also possible to feel the larvae move within the skin, eyes, nose, or mouth,” she said. Bacterial superinfection of a wound is also possible, and secondary bacterial infections could prompt chills or fever, she added.

Clinicians also can identify NWS by comparing distinctive structural features of the larvae, including mouthparts, skeleton, and cuticular spines, according to the CDC.

Who Is at Risk

Be alert to symptoms of NWS if individuals have a recent history of travel to the Caribbean and South America, where the NWS is endemic, as well as Central America, given the recent spread of the NWS there, Mazumder said.

According to the CDC, individuals are at greater risk if they travel to areas with endemic animal infestations of NWS and, while in these areas, spend time among livestock, sleep outdoors, and have an open wound, including a surgical wound. Individuals who are immunocompromised or suffering from malnutrition may be more vulnerable to NWS, as are the very young or very old. The size of the wound doesn’t matter, as the NWS fly may be attracted to a wound as small as a tick bite, or to odors in the eyes or nose.

Additionally, individuals with an underlying condition causing open sores, such as a dermatologic malignancy, are at higher risk, Mazumder noted.

Safety Precautions and Prevention

Preventative measures include cleaning and covering all wounds, even small wounds, when traveling to endemic regions, Mazumder told Medscape Medical News. “Using an EPA-registered insect repellent and limiting skin exposure by wearing long sleeves, pants, and socks are helpful,” she said. Counsel patients who may be traveling to high-risk areas to avoid sleeping outdoors if possible, and to use screens or bed nets, she said.

Although the risk for NWS to the general public is low, clinicians should remain vigilant to symptoms of NWS in patients who report travel to high-risk areas, the CDC states on its website. The CDC’s guidance for clinicians includes reminding all patients to keep wounds clean and covered, and to avoid livestock in rural areas of regions where NWS is endemic, if possible.

How Is NWS Treated?

The only way to treat NWS is to remove the larvae, sometimes through surgical excision, according to the CDC. Individuals infected with NWS should not remove or dispose of the larvae themselves. Instead, the larvae should be preserved for confirmatory diagnosis.

“Physicians should be aware that NWS myiasis is a notifiable disease and should report it to their local or state health department,” Mazumder told Medscape Medical News.

NWS infestations in animals or humans should be reported to public health immediately. Contact dpdx@cdc.gov for laboratory testing questions and instructions.

Mazumder had no financial conflicts to disclose.