06/06/2026
The second case of NWS was reported yesterday, in a month old calf, 5 miles from the first case.
I’ve had several people send pics of flies they suspected to be New World Screwworm, but every one of them so far has been a blow fly. I think the best thing is to write a post about how to tell them all apart…
Distinguishing the Invasive New World Screwworm from the Common Secondary Screwworm
The invasive NWS looks almost identical to the Secondary Screwworm (Cochliomyia macellaria), a native fly that is common and abundant throughout the United States.
The most noticeable difference between these two closely related species isn't how they look, but how they behave.
New World Screwworm is an “obligate parasite”. This means the female fly is attracted to fresh, open wounds on warm-blooded animals. She lays her eggs at the edge of the wound. When the larvae hatch, they burrow deep into healthy, living flesh, eating the animal alive from the inside out.
Secondary Screwworm is a scavenger (saprophage). It is attracted to carrion, decaying organic matter, or the dead, necrotic tissue found within existing neglected wounds. They do not feed on healthy, living tissue.
If you observe maggots tightly packed and tunneling deeply into clean, red, living flesh on a live animal, it should immediately be treated as a suspected case of New World Screwworm.
If you manage to trap or collect an adult fly, a close look under a magnifying glass or a high-resolution smartphone camera will reveal an important diagnostic feature located on the thorax (the middle body segment).
Both flies display a vibrant metallic blue-green body and a prominent reddish-orange or golden face. However, their dark back stripes are what tells them apart.
-New World Screwworm-Look at the three dark, longitudinal stripes running down its back. On a true NWS adult, THE CENTER STRIPE IS NOTICEABLY SHORTER THAN THE TWO OUTER STRIPES, FADING OUT ABOUT HALFWAY DOWN THE THORAX.
-Secondary Screwworm-On the common secondary screwworm, ALL THREE DARK STRIPES ARE ROUGHLY EQUAL IN LENGTH, EXTENDING FULLY FROM THE TOP OF THE THORAX DOWN TO THE BASE.
If you are inspecting larvae removed from a wound, their physical appearance also yields clues.
NWS larvae are shaped like a wood screw, featuring rings of dark spines around each body segment that help them anchor into live flesh.
Secondary screwworm larvae are more lightly spinose and lack these heavily defined, dark, continuous bands.
Also, notice the “Tracheal Trunks.” If you look through the translucent skin at the tail end of a mature NWS larva, you can clearly see two dark, heavily pigmented breathing tubes (tracheal trunks) extending well into the body. In the secondary screwworm, these internal tubes are pale or light-colored and far less visible.
The secondary screwworm is a permanent resident of the United States, with a stronghold in the warm, humid Southeastern states (stretching from Texas through Florida) and its populations surge during rainy seasons. It requires no regulatory action.
But, the New World Screwworm is a whole new ball of wax, so to speak. It is a reportable, regulated pest. Any uncontained detection in the US triggers an immediate federal and state quarantine response.
Never shrug off an unusual wound infestation. If you encounter larvae aggressively feeding on live tissue, or spot an adult fly with the characteristic short center stripe, contact your local veterinarian, state department of agriculture, or the USDA Animal and Plant Health Inspection Service (APHIS) immediately. Frontline awareness is the only way to keep this devastating parasite from gaining a permanent foothold once again.
And folks, please consider, it took up to 4 YEARS to eradicate them once the sterile flies were released. This isn’t going to clear up in a month or so.
