New World screwworm (Cochliomyia hominivorax) is a parasitic fly whose larvae burrow into and feed on the living tissue of warm-blooded animals. After a 60-year absence, on June 3, 2026, USDA-APHIS confirmed an active infestation in Zavala County, Texas, the first domestic detection since the 1960s. For pet owners and livestock owners in or near the outbreak zone, knowing what to look for and when to call your veterinarian is the most important step you can take.
Unlike most blowfly species that feed on necrotic or dead tissue, NWS larvae feed exclusively on living, healthy tissue. Female flies are drawn to any open wound, body orifice or moist tissue and lay 200–400 eggs at a time. Larvae hatch within 24 hours and use hook-like mouthparts to spiral deeper into tissue as they feed, causing wounds to enlarge rather than heal.
Because NWS is a federally reportable foreign animal disease, any suspected case needs to be reported to USDA-APHIS and your state animal health official right away. Laboratory confirmation is not required before reporting.
All warm-blooded animals are susceptible to NWS infestation. Cattle are historically the most commonly affected species, particularly calves (navel wounds in the first days of life), post-partum cows (vulvar injuries after delivery) and animals following castration or dehorning. Horses are highly susceptible given their frequent outdoor exposure and open wounds. Dogs and cats, especially those spending time outdoors in outbreak areas, are at risk through any wound, bite injury or minor skin abrasion.
In active outbreak conditions, even wounds as small as a tick bite site can serve as an entry point. Minor skin breaks that you might otherwise leave alone are worth a closer look right now.
Early recognition is what makes the difference. Contact your veterinarian immediately if you observe any of the following in an animal in or near the outbreak zone.
Not all wounds carry equal risk. NWS flies are drawn to sites that are moist, warm and produce biological scent, so those are the places to check first when you're examining animals in outbreak areas.
For livestock and horses, prioritize the umbilicus (navel) in newborns of any species, castration and dehorning sites, post-partum vulvar injuries in cows and mares, nasal passages, ears, periorbital regions and the sheath or prepuce in intact males. Any recent surgical wound or traumatic injury is also a high priority.
In dogs and cats, pay close attention to bite wounds, fight injuries and any wound on the face or genital area that appears to be expanding or producing an unusual smell. For sheep and goats, direct physical inspection is essential — fleece can conceal early wound development entirely.
If something looks or smells off, your first call is to your veterinarian. Keep the affected animal separate from others in the household or herd, as movement can spread larvae through soil contamination. Do not transport the animal until you have guidance from your veterinarian or authorities.
Because NWS is a federally reportable disease, reporting to USDA-APHIS and your state animal health official is required, not just recommended. Report before laboratory confirmation, not after. In Texas, you can reach USDA-APHIS Veterinary Services at (512) 383-2400 and the Texas Animal Health Commission at (800) 550-8242.
Before your veterinarian begins treatment, larval samples need to be collected and submitted for laboratory confirmation. The AVMA's sample collection guidance covers the specifics: use forceps and disposable gloves to collect larvae from multiple sites within the wound, place them in 70% ethanol, photograph the wound and submit to the National Veterinary Services Laboratories (NVSL) with a completed Parasite Submission Form. Your veterinarian will walk you through this process.
From there, your veterinarian will determine appropriate antiparasitic therapy based on your animal's species and the current FDA Emergency Use Authorizations (EUAs). For secondary bacterial infections, systemic antibiotics and topical wound care are typically part of management as well — your veterinarian will prescribe what is appropriate.
Mixlab is a full-service veterinary pharmacy that ships to all 50 states and has filled prescriptions for animals across 300+ species — from dogs and cats to horses, cattle, sheep and exotic wildlife. Whether your NWS concern involves a companion animal or a working herd, Mixlab can fulfill the individual prescriptions written by a veterinarian to address the protocol.
Veterinarians can fulfill the complete NWS treatment protocol through a single Mixlab order, shipped free to all 50 states. Mixlab currently carries the following FDA EUA approved NSW treatment options:
Additionally, Mixlab can help with secondary infections and wound care:
Browse NWS treatment medications at Mixlab
New World screwworm (Cochliomyia hominivorax) is a parasitic fly whose larvae feed on the living tissue of warm-blooded animals — unlike most blowfly species, which feed on dead or necrotic tissue. Female flies are attracted to open wounds and body orifices, laying hundreds of eggs that hatch within 24 hours. NWS is a federally reportable foreign animal disease in the United States.
As of June 2026, USDA-APHIS has confirmed NWS in Zavala County, Texas — the first domestic detection in 60 years. For the most current outbreak updates, including affected areas and response efforts, visit the USDA-APHIS website. If you are in or near the affected region, it is worth talking with your veterinarian now about what to watch for in your animals.
Yes. Dogs and cats are susceptible to NWS through any open wound, bite wound, fight injury or minor skin abrasion. Outdoor and free-roaming animals are at greatest risk. Any dog or cat in or near the outbreak zone with a wound that is enlarging, producing an unusual odor or showing visible larvae should be evaluated by a veterinarian without delay.
The key difference is the tissue type affected. Most blowfly species (secondary myiasis) are associated with wounds that already contain necrotic tissue and tend to cause relatively stable wound conditions. NWS larvae actively burrow into living, healthy tissue, causing wounds to expand rapidly and deeply. Rapid wound enlargement and the active, head-down burrowing behavior of the larvae are the main field differentiators — though definitive identification requires laboratory confirmation.
The FDA has issued Emergency Use Authorizations for specific antiparasitic medications for NWS-affected animals. For dogs, veterinarians may discuss NexGard under the current EUA; for cats, NexGard COMBO; for cattle, ivermectin injectable per the EUA label - all available through Mixlab with a valid prescription. Authorization guidance is evolving, and your veterinarian will determine what is appropriate and currently authorized for your specific animal. Do not administer any medication without veterinarian direction.
NWS can infest humans through open wounds, though human cases are uncommon. When handling or examining an animal suspected of NWS infestation, wear protective gloves and avoid direct contact with wound tissue or larvae. Human exposure concerns can be directed to the CDC at parasites@cdc.gov.
New World screwworm has returned to US soil for the first time in 60 years, and the most important things animal owners can do right now are to know the signs and act quickly. A foul-smelling wound, visible and active maggots, a wound that is enlarging rather than healing and unexplained behavioral changes are the primary signs to watch for. If you observe any of these in an animal in or near the Texas outbreak zone, call your veterinarian right away and report to USDA-APHIS before waiting for laboratory confirmation. Mixlab is here to help with FDA EUA-authorized NWS treatment protocols, wound care and secondary infection management solutions for veterinarians across all 50 states, with free shipping.