Screwworm: The Flesh Eating Parasite And How You Could Prevent It

On August 25, 2025, the U.S. Department of Health and Human Services (HHS) reported the country’s first travel-related human case of New World screwworm (NWS) myiasis, a parasitic infection caused by the larvae of the flesh-eating screwworm fly (Cochliomyia hominivorax).

This case represents a notable development amid an ongoing outbreak in Central America and Mexico, sparking concerns over the parasite’s possible impact on U.S. public health and livestock. The infection was identified in a Maryland resident who had recently traveled to El Salvador, although early reports from some beef industry sources suggested Guatemala as the point of exposure.

The Centers for Disease Control and Prevention (CDC), working alongside the Maryland Department of Health, confirmed the diagnosis on August 4, 2025, using telediagnosis, an expert review of images of the larvae submitted for analysis. Officials have stated that the patient has made a full recovery and stressed that the overall risk to the wider U.S. population remains “very low,” with no evidence of further transmission to people or animals.

What Is The New World Screwworm?

The New World screwworm is a parasitic fly whose larvae (maggots) attack and consume the living tissue of warm-blooded animals, including livestock, wildlife, pets, and in rare cases, humans.

Adult female flies can deposit as many as 400 eggs at once in open wounds, as well as in the nose, mouth, or eyes. Within a few days, these eggs hatch into hundreds of larvae equipped with sharp mouth hooks that bore into flesh, leading to intense pain, significant tissue destruction, and, if left untreated, potentially deadly secondary infections.

The term “screwworm” comes from the larvae’s twisting, screw-like movement as they feed, which can cause wounds to expand rapidly. Although they primarily pose a serious threat to cattle, devastating herds and inflicting billions of dollars in economic damage, human infections, while uncommon, are described as “very painful” and demand immediate medical care.

Causes Of New World Screwworm

The primary cause of New World screwworm (NWS) infestations is exposure to the parasitic fly Cochliomyia hominivorax and its eggs or larvae. Here are the key factors that lead to NWS myiasis (infestation):

Presence Of Open Wounds Or Sores

The female screwworm fly is attracted to open cuts, scrapes, or even small skin abrasions on warm-blooded animals, including livestock, wildlife, pets, and sometimes humans. She lays eggs (up to 400 at a time) on these wounds or around moist openings like the nose, mouth, or eyes.

Egg Hatching And Larval Development

Once the eggs hatch (usually within 12–24 hours), the larvae (maggots) penetrate healthy living tissue rather than feeding only on dead tissue like some other fly species. Their screw-like burrowing action leads to severe tissue damage and further attracts more flies.

Poor Wound Care Or Sanitation

In environments where wounds are not properly cleaned or protected, the risk of infestation increases. This is especially common in rural or agricultural settings where animals may be exposed to untreated injuries.

Environmental And Geographic Factors

NWS flies are native to tropical and subtropical regions of the Americas. Outbreaks are often linked to warm, humid climates that favor fly reproduction and survival.

Transmission via Travel

Human cases are rare and often occur when individuals travel to endemic areas (Central and South America) and return with untreated wounds that become infested.

Symptoms Of New World Screwworm

The signs of infection can be extremely painful and may include:

• Persistent wounds or sores on the skin that fail to heal or worsen over time
• Bleeding from affected sores
• Sensations of larvae moving within open wounds or inside the nose, mouth, or eyes
• Visible maggots present in the wounds or in the nose, mouth, or eyes
• A strong, unpleasant odor coming from the infected wounds or affected areas such as the nose, mouth, or eyes

What Are The Treatments?

Managing this condition usually requires immediate medical intervention. Treatment often includes surgically removing all visible larvae, thoroughly cleaning and disinfecting the affected wounds, and administering antibiotics.

The hundreds of larvae need to be carefully extracted by hand, and the wounds meticulously disinfected to prevent further damage. Antibiotics are typically prescribed to address secondary bacterial infections, while pain relief measures are necessary in more severe cases.

For animals, additional treatments such as topical or systemic insecticides are used to kill any remaining larvae and reduce the risk of re-infestation.

With prompt and comprehensive care, patients can recover successfully; however, if left untreated, the infestation can result in severe tissue destruction and even death.