New World Screwworm: What Shelters and Rescues Need to Know
New World Screwworm (Cochliomyia hominivorax) was confirmed in South Texas on June 3, 2026, marking its return to the U.S. for the first time since eradication in 1966. It's important not to panic, but to be prepared: New World Screwworm (NWS) is preventable and treatable. This guide helps you make simple, proactive adjustments to your protocols to protect your shelter population from NWS. It applies to all shelters and rescues — organizations near confirmed cases should adopt the protocols immediately, and those not yet affected should use this time to build a plan, since conditions can change quickly.
For comprehensive guidance, please view all resources on our Screwworm resource page, including:
What Is It?
Most fly larvae feed on dead tissue. Screwworm larvae are different: they burrow into living tissue through open wounds, surgical incisions, or mucous membranes. Infestations can progress quickly and can be fatal if left untreated. The female fly is strongly attracted to wounds, so any warm-blooded animal with an open wound is at elevated risk.
Who Needs to Pay Attention?
Areas of New Mexico and Texas with confirmed cases and other high-risk area animal shelters and rescues should treat this as an immediate action item. All organizations should be tracking any changes to travel and import restrictions and monitoring this interactive heat map for infected areas.
The AVMA has confirmed that travel makes it possible for NWS to be introduced into any U.S. state — not just states close to the southern border. The fly's seasonal range extends well north, potentially as far as Canada.
What This Means for Your Shelter
Check your animals.
Do daily hands-on wound inspections for every animal in your care. Look for enlarging wounds, foul odor, blood-tinged drainage, unexplained pain, head shaking, excessive licking, or visible larvae. If you find maggots, isolate the animal and contact your veterinarian immediately. Maggots should be submitted to your vet to confirm whether they are screwworms. Do not let discarded maggots fall onto the ground as this will cause spread. Euthanizing the host animal does not kill larvae. A euthanized animal with suspected or confirmed NWS is still a biosecurity hazard until the carcass is frozen or incinerated.
Stock the right medications.
Talk to your shelter vet about having appropriate medications on hand. The FDA maintains a current, updated list of approved and emergency-authorized products for NWS. Credelio/lotilaner, NexGard/afoxolaner, Credelio CAT, and nitenpyram are among the options currently in use but always important to confirm with your vet what is authorized at the time of treatment. See: FDA: NWS Information for Veterinarians
Every companion-animal medicine authorization is for treatment of an existing infestation, not prevention, so using these products as a means of prevention is an extrapolation. Texas A&M describes their preventive efficacy as “likely,” but these products have not been tested for efficacy when used for prevention. According to the FDA, monthly administration is highly regarded by veterinarians as a strong preventative measure against the parasite.
Apply protective ointment.
For animals other than cats, with an open wound, apply fly-repellent ointment (such as SWAT) around the wound to deter flies from laying eggs. Never put permethrin, pyrethroids, or DEET on cats. See more information on cats in the resource for Screwworms and Community Cats.
Screen every intake and transport.
Inspect all incoming animals for wounds or signs of infestation.
Review your fly control.
Now is a good time to assess how flies enter your facility and tighten any gaps. See resource: Fly Mitigation and Wound Management document for specific mitigation steps.
Transferring and Receiving Animals
Transport is safe and manageable with good protocols in place. Health certificate protocols for interstate transport exist to prevent disease spread. Stay vigilant on these examinations and make sure the veterinarians conducting them have access to these and other relevant resources.
If you’re transferring animals, remember: your attention to this now keeps your transfer pipeline healthy and moving, keeps your transfer partnerships strong, and keeps your lifesaving operations on track. Connect with your shelter veterinarian so you're ready to certify animals quickly and check in with state regulatory agencies for the receiving state regularly to stay compliant with evolving requirements; your receiving partners may not be aware of any incoming requirements. Keeping your transfer pipeline moving and healthy is essential to lifesaving, and a little preparation now avoids disruption later.
If you’re receiving animals from high-risk areas, inspect all animals immediately on arrival. If you find maggots, do not wait. Isolate the animal and call your vet. Even if your state has not yet issued formal guidance, build screwworm screening into your transfer intake process.
If You Operate Animal Control or Field Services in a High-Risk Area
Officers working in screwworm-affected areas should review current personal protective guidance at screwworm.gov before going into the field.
When dispatching calls from affected areas, ask the caller directly: “Does the animal have any open wounds, sores, or areas that appear to be causing distress?”
If yes, that call should be treated as an elevated response, since larvae can hatch within 24 hours and time between the call and officer arrival can directly affect the animal's outcome. Be sure to consult your local Health Department regarding your specific area’s response time recommendations.
On scene, officers should examine ears, mouth, paws, skin folds, and genital areas for foul-smelling wounds, active maggots, or a wound that is enlarging rather than healing are signs of infestation. Stray animals are especially at risk.
Reporting Cases
The New World Screwworm is a federally reportable foreign animal disease. If suspected, do not transport the animal across jurisdictions. Contact your state animal health authority and USDA-APHIS immediately, and check screwworm.gov for the latest outbreak updates.
Resources
- AVMA: New World Screwworm (clinical guidance, signs, treatment, reporting)
- USDA APHIS: NWS Response Playbook
- USDA APHIS: Standard Operating Procedure for Detection in Animals
- FDA: Approved and Authorized Drugs for NWS
- USAHA White Paper: NWS Preparedness, Prevention, and Response (March 2026)
- CDC: New World Screwworm Outbreak