Screwworm Larvae Identification: Lab Features and Signs

New World screwworm larvae are pale maggots, 6.5 to 17 mm long when fully grown, with rings of dark spines around every body segment and dark breathing tubes visible at the rear end. The CDC's laboratory bench aid (updated 19 February 2026) and the USDA APHIS pest card (May 2025) describe these features:

• Shape: tapered at the front and blunt at the back (CDC); tapered at both ends on the APHIS card.
• Spines: encircling bands of short spines with one, two or three points on each segment (CDC). Texas A&M AgriLife says the spines wrap around the body in a spiral, which is where the screw image comes from.
• Dark tracheal tubes: the two breathing tubes leading forward from the rear spiracles are pigmented dark brown to black across at least two full segments. In other flies that cause wound myiasis these tubes are clear or translucent (CDC).
• Rear spiracles: the rim (peritreme) around each spiracular plate is incomplete and does not enclose the button, and the three slits are roughly parallel and straight (CDC).
• Where they are: feeding in living flesh of a wound or body opening, not on dead meat. APHIS says they burrow in like a screw driving into wood and tear tissue with sharp mouth hooks, making the wound deeper and larger.

The CDC bench aid is written for third-stage larvae examined by a laboratory. A final identification is made by a lab, not from a photo.

Most maggots people find are not screwworms. The main difference is what they feed on and the colour of the breathing tubes.

• New World screwworm: feeds only on living tissue; dark tracheal tubes (CDC).
• Secondary screwworm (Cochliomyia macellaria): a close relative common in the south-eastern US, most abundant in states such as Florida and Louisiana. Its larvae eat dead and decaying tissue in wounds and do not have pigmented tracheal tubes (Wikipedia).
• Other blow fly and house fly maggots: found in garbage, compost and dead animals; the CDC notes their tracheal tubes are clear or translucent.

Maggots in a bin or on a carcass are ordinary fly larvae; see maggot identification and the larvae identification key. Maggots in a living animal's wound should be reported whatever they turn out to be.

In animals. Texas A&M AgriLife lists foul-smelling wounds with visible maggots, animals biting or licking at a wound, lesions in navels, ears, and dehorning or branding sites, and unusual restlessness or lethargy. USDA APHIS adds irritated behaviour, head shaking and a smell of decay.

In people. The CDC lists feeling or seeing maggots move in a skin wound or sore, or in the ears, nose, eyes or mouth; a wound that gets worse within days; foul-smelling discharge; bleeding from sores; and fever or chills from secondary bacterial infection. Anyone with these signs should see a doctor straight away.

Female flies lay eggs on or near wounds or on the moist membranes inside the nose, mouth and ears; the larvae hatch, burrow in and feed on living flesh for about 7 days before dropping off to pupate (CDC). The CDC says a wound as small as a tick bite can attract a female.

Human cases are uncommon and mostly involve people who tend livestock in affected areas or travel there, especially people with open wounds who sleep outdoors (CDC). The CDC also lists skin conditions that cause open sores, such as eczema or skin cancer, as a risk. Its prevention advice: keep every wound clean and covered, sleep indoors or under screens or bed nets, use an EPA-registered repellent, and wear long sleeves, long trousers, hats and socks.

Treatment is a job for a veterinarian or doctor, not a home remedy.

• In animals: Texas A&M AgriLife lists topical larvicides such as coumaphos or permethrin, cleaning the wound and removing the larvae, along with preventive wound care and sanitation.
• In people: the CDC's instruction is not to try to remove maggots or egg masses yourself, to contact a healthcare provider immediately and to mention recent travel.
• Across a region: the sterile insect technique, which releases sterile male flies (Texas A&M AgriLife). APHIS says the only sterile fly facility operating in North America is run jointly by USDA and Panama, and that USDA investment will bring production to about 500 million sterile flies per week.

For laboratory confirmation, the CDC asks for all suspected larvae to be placed in a leak-proof container fully submerged in 70% ethanol; 70% isopropanol or 5 to 10% formalin are acceptable if ethanol is not available.

The pest was eradicated from the United States in the 1960s and has since resurfaced in Central America and Mexico (Texas A&M AgriLife). The CDC says it has moved north from South America since 2023, and that on 3 June 2026 USDA confirmed the first US animal case in the current outbreak. As of the CDC page updated 11 August 2026, there were no locally acquired human cases in the United States; one travel-associated case was confirmed in August 2025 in a person returning from El Salvador.

USDA APHIS describes the current risk to animals and people in the United States as very low, and in September 2026 its news list included announcements that Texas had released New World screwworm infested zones. Check APHIS for the current map before relying on this summary.

• On a person: do not pull the maggots out yourself. See a doctor or go to urgent care straight away and mention any travel. The CDC says live specimens can be placed in rubbing alcohol and brought to the clinician.
• On livestock, a pet or wildlife: call your veterinarian and report immediately to your State Animal Health Official or the USDA Area Veterinarian in Charge (USDA APHIS). In Texas, APHIS lists 1-800-550-8242 for livestock cases, and Texas A&M AgriLife names the Texas Animal Health Commission for livestock and Texas Parks and Wildlife for wildlife.
• Keep specimens for the lab: a leak-proof container with 70% ethanol (CDC).
• Photograph the wound and a larva beside a ruler before handing it over; a photo helps the conversation but does not replace the lab.

The CDC stresses that human and animal infestations should both be reported immediately, because a human case can point to infested animals nearby.

Research and drafting for this page used AI assistance, and every fact was checked line by line against the sources listed below. Each number, date and quotation names the organisation it came from. The page does not replace a clinician, a veterinarian or a professional identification.