What exactly is screwworm and why is it different from other maggots?

New World screwworm is the larval stage of the blowfly Cochliomyia hominivorax, and it is an obligate parasite of living tissue rather than a scavenger of dead or decaying flesh, which sets it apart from the maggots most owners have seen on roadkill or in garbage (USDA APHIS, 2024). The larvae hatch from eggs laid in clusters at the edge of a wound or in a moist body opening, then burrow inward using rows of backward-facing spines that give the species its "screw" name and make the maggots difficult to flush or pull out intact (Merck Veterinary Manual, 2023). A single infestation can involve hundreds of larvae feeding simultaneously, and because they consume live muscle and connective tissue rather than just necrotic debris, the wound keeps enlarging even as it appears to be granulating and healing on the surface.

The species was declared eradicated from the United States in 1966 and from all of North America north of the Darién Gap in Panama by the early 2000s, using a sterile-insect-release program still maintained jointly by USDA APHIS and Panama (USDA APHIS, 2024). Since 2022, however, the containment line has moved north through Central America, and by 2025 cases were confirmed in southern Mexico, prompting USDA to temporarily suspend live cattle, bison, and horse imports from Mexico at various points and to expand surveillance along the Texas border (USDA APHIS, 2025). For dog owners in the continental U.S. who have not traveled with a pet to an affected region, current risk remains very low, but that geographic picture is actively changing.

What does a screwworm wound actually look like on a dog?

Owners typically first notice a wound that seems to be getting worse instead of better: it enlarges over 24 to 48 hours, produces a foul, distinctive odor, and weeps serosanguinous fluid even if it started as a minor scrape, tick-bite site, or surgical incision (Merck Veterinary Manual, 2023). On close inspection, tiny cream-to-white larvae, often described as looking like grains of rice with dark hooked mouthparts, may be visible moving within the wound margins, and adult metallic blue-green flies with three dark stripes on the thorax may be seen landing repeatedly on the site. Dogs frequently show pain out of proportion to the wound's apparent size, along with excessive licking, whining, or reluctance to be touched near the area.

Because screwworm flies are drawn to any break in the skin or moist orifice, infestations are also reported around the nose, eyes, ears, mouth, genitalia, and umbilicus, particularly in newborn puppies whose umbilical stumps have not fully dried (USDA APHIS, 2024). A wound that was clean at a suture recheck two days earlier but now has a wider gap, undermined edges, and a strong odor warrants same-day veterinary evaluation rather than a wait-and-see approach, especially if the dog has recently traveled through or resided in a region with confirmed screwworm activity.

Which dogs are actually at risk, and does travel history matter?

For the large majority of U.S. pet dogs, the risk of screwworm exposure is currently minimal because the fly's established range does not yet extend into the continental United States outside of isolated, rapidly contained detections (USDA APHIS, 2025). Risk rises sharply for dogs with any open wound, recent surgical incision, tick or insect bite, or umbilical remnant that spend time outdoors in or near regions with active screwworm populations, since gravid female flies can detect wound odor from a considerable distance and lay eggs within hours of a wound occurring. Dogs that travel internationally for work, breeding, hunting, or relocation through Mexico or Central America carry meaningfully higher risk than dogs that stay within screwworm-free areas.

Livestock exposure patterns are informative for pet owners too: cattle and other livestock with fresh brands, dehorning wounds, or castration sites in affected regions have historically shown the highest infestation rates, which is why USDA APHIS restricts animal movement from outbreak zones and requires inspection before interstate or international transport (USDA APHIS, 2024). Any dog owner planning to travel with a pet to, or adopt a dog from, an area with confirmed screwworm activity should ask their veterinarian about pre-travel wound checks and post-travel monitoring.

How do veterinarians confirm it's screwworm and not just a regular maggot infestation?

Diagnosis starts with physical examination of the wound and collection of larvae for species identification, since screwworm larvae have distinctive dark, pigmented tracheal tubes visible near the posterior end and prominent body spines that differentiate them from common blowfly maggots that feed only on dead tissue (Merck Veterinary Manual, 2023). Because New World screwworm is a foreign animal disease of major agricultural significance in the United States, veterinarians who suspect a case are required to report it immediately to USDA APHIS and their state animal health official, who can arrange confirmatory identification through USDA's National Veterinary Services Laboratories (USDA APHIS, 2024).

This reporting requirement exists specifically because screwworm is not endemic here; a single confirmed case triggers an epidemiological investigation into the animal's travel history and surveillance of the surrounding area, similar to protocols for other reportable foreign animal diseases. Owners should not feel this reporting step reflects negatively on them, it is standard federal procedure designed to protect livestock, wildlife, and other pets from a reintroduction of an eradicated pest.

How is a screwworm infestation actually treated?

Treatment centers on physically removing all larvae from the wound under veterinary care, often requiring debridement of damaged tissue, thorough wound irrigation, and application of a larvicidal or antiparasitic product labeled for the purpose, since larvae embedded deep in tissue tunnels are not reliably removed by irrigation alone (Merck Veterinary Manual, 2023). Systemic antiparasitic medications such as ivermectin-class drugs may be used under veterinary direction alongside topical treatment, and broad-spectrum antibiotics are frequently indicated because the wound bed is heavily contaminated with bacteria by the time larvae are found.

Pain management is a core part of treatment given how much tissue destruction can occur even in wounds that looked minor at onset, and dogs may need sedation for thorough wound exploration and cleaning. Follow-up rechecks are important because new eggs can be laid on an unhealed wound within a day if the animal remains in an area with active adult fly populations, so environmental fly control and wound covering are part of the treatment plan until the site is fully healed (USDA APHIS, 2024).

What can owners do at home to protect wounds and prevent screwworm in the first place?

Daily wound checks are the single most useful home habit: any cut, abrasion, tick-bite site, or surgical incision should be inspected at least once or twice a day for increasing size, new odor, discharge, or visible larvae, particularly for dogs in or recently returned from a region with confirmed screwworm activity (Merck Veterinary Manual, 2023). Keeping wounds clean, dry, and covered with a properly fitted bandage or e-collar to prevent licking reduces the odor cues that attract gravid flies, and prompt veterinary attention for any wound, rather than home first-aid alone, is advised when travel history raises risk.

USDA APHIS recommends that anyone traveling with animals to or from affected regions inspect them closely before and after travel and report any suspicious wounds with larvae to a veterinarian or state animal health official right away rather than attempting to treat what looks like screwworm at home (USDA APHIS, 2025). For breeders, keeping umbilical stumps on newborn puppies clean and dry, and monitoring them closely in the first week of life, is a practical extra precaution in higher-risk areas.

What's the long-term outlook for a dog after a screwworm infestation is treated?

Prognosis is generally good when a screwworm infestation is caught early and treated aggressively, since the underlying wound heals through standard veterinary wound-management principles once the parasitic burden is removed; the bigger risk comes from delayed recognition, which allows destruction to progress toward deeper structures such as muscle, cartilage, or in severe orifice cases, the sinuses (Merck Veterinary Manual, 2023). Untreated infestations in livestock and wildlife have historically caused death from tissue destruction, secondary infection, and fluid loss, which is exactly why USDA maintains active eradication and surveillance infrastructure rather than treating screwworm as a manageable endemic parasite (USDA APHIS, 2024).

Dogs that recover typically do not have long-term susceptibility issues beyond the scarring from the original wound and its treatment, and there is no lasting immunity concern the way there might be with some infectious diseases; the ongoing risk factor is environmental, not physiological, meaning a fully recovered dog is just as vulnerable to reinfestation if it develops a new wound while still in or traveling through an active outbreak area.