What Is New World Screwworm and How Does It Affect Humans
New World screwworm (Cochliomyia hominivorax) is a parasitic fly whose larvae feed on living tissue of warm-blooded animals and humans. The female lays eggs in open wounds or mucous membranes, and the hatched larvae burrow into healthy tissue, causing severe, progressive damage. Human cases are rare but documented in regions with livestock outbreaks, especially where wound care is limited or access to medical facilities is delayed. The condition requires prompt medical removal of larvae and wound management to prevent secondary infection and tissue loss. The United States Department of Agriculture (USDA) and the Animal and Plant Health Inspection Service (APHIS) lead surveillance and eradication campaigns targeting screwworm in livestock populations to reduce spillover risk to humans. For detailed biological and epidemiological background, refer to the USDA APHIS screwworm overview at USDA APHIS Screwworm Information.
Transmission to humans typically occurs when open wounds, sores, or damaged skin are exposed to screwworm flies in endemic areas, including parts of Central America, South America, and the Caribbean. The larvae secrete enzymes that digest living tissue, and infestations can expand rapidly if untreated, leading to extensive skin and soft-tissue destruction. Historically, screwworm caused significant livestock losses and occasional human morbidity in the southern United States prior to eradication campaigns completed in the 1960s using the sterile insect technique. The sterile insect technique involves releasing large numbers of sterilized male flies to suppress wild populations, a method that has been applied across the Americas through regional cooperation. The World Organization for Animal Health (WOAH) maintains updated distribution maps and outbreak notifications relevant to human exposure risk. See the WOAH screwworm status page at WOAH Screwworm Disease Card.
Symptoms, Diagnosis, and Treatment of Human Screwworm Infestation
Early symptoms include a painful, swollen wound that worsens despite standard wound care, often with visible larvae or serous discharge. Patients may report a sensation of movement in the wound, localized inflammation, and secondary bacterial infection. Diagnosis is confirmed by clinical examination and identification of larvae, which are large, segmented, and equipped with distinctive mouth hooks used to anchor in tissue. Healthcare providers in endemic areas are trained to differentiate screwworm infestation from other wound myiasis causes, such as blowflies or other parasitic flies, based on larval morphology and travel history. The Centers for Disease Control and Prevention (CDC) provides clinical guidance on diagnosing and managing myiasis cases, including screwworm, through its parasitic diseases resources. For clinical guidance, visit the CDC parasitic diseases diagnostic page at CDC Myiasis and Wound Infestation Guidance.
Treatment involves complete mechanical removal of all larvae from the wound, followed by thorough debridement of devitalized tissue and irrigation with antiseptic solutions. Wound care includes topical and systemic antibiotics to prevent secondary bacterial infection, and patients are monitored for signs of cellulitis, sepsis, or necrotizing soft-tissue infection. In severe cases, surgical intervention may be required to excise extensively damaged tissue and reconstruct affected areas. Anti-parasitic agents such as ivermectin are sometimes used as adjunctive therapy, but mechanical removal remains the cornerstone of effective treatment. Public health agencies emphasize that early presentation to a healthcare facility significantly reduces the risk of complications and long-term tissue damage. The Pan American Health Organization (PAHO) publishes region-specific alerts and treatment recommendations during screwworm outbreaks affecting both animals and humans. See PAHO health alerts at