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For Animal Shelters

New World Screwworm: What Animal Shelters & Rescues Need to Know

Many of you have seen recent headlines: New World Screwworm (Cochliomyia hominivorax) has arrived in Texas from Mexico. Here’s what that means for animal shelters and rescues, and what you can do right now.

What is it?

Unlike most flies whose larvae (maggots) feed on dead tissue, screwworm larvae burrow into open wounds on living animals, causing severe damage that can be fatal if untreated. The pest was eradicated from the U.S. in 1966, but its return to border states warrants attention.

No need to panic — but animal shelters and rescues in border states should remain vigilant and act now:

Inspect and protect.

Check all animals in your care for open wounds. Apply fly-repellent ointment like SWAT around any wounds to deter infestation. Collect any maggots from wounds and submit to a veterinarian to determine if they are screwworms.

Stock up.

Keep NexGard or Simparica Trio on hand. Both are approved for treatment and are effective against screwworm infestations. Nitampyram (Capstar or other similar products) is also effective as a one dose treatment. Consult your veterinarian for dosing guidance.

Fly control matters.

Review your fly mitigation plan and make sure you’re minimizing fly access throughout your canine kennel area, facility or vehicle. See tips on controlling your fly population below.

Screen all intakes and transports.

Prioritize screening over quarantining. Examine every incoming and outgoing animal for wounds or signs of infestation.

Transporting pets out of border states?

A few states have banned the import of pets from impacted areas such as New York, Pennsylvania, and Florida, but most states are simply requiring health certifications confirming animals are screwwormfree 3–5 days before transport. Get ahead of it by coordinating with your veterinarian now, so you’re ready when requirements go into effect. Check-in regularly with receiving shelters in other states regarding potential importation rule changes. Failure to follow guidelines may result in further restrictions.

Receiving pets from border states or Mexico?

Check all pets immediately on arrival for wounds and maggots. If maggots are found, submit to a veterinarian to determine if they are screwworms.

By staying vigilant and working together, we can keep the animals in our care safe.

Additional educational resources:

Mitigation Tips

With the recent confirmation of New World Screwworm (NWS) in South Texas, animal shelters should think about mitigation in layers, much like mosquito or vector control.

The biggest risk factor is…

…the specific NWS female fly finding an open wound, surgical incision, draining skin lesion, tick bite, ear wound, or moist body opening where eggs can be deposited. Female flies are strongly attracted to wounds and mucous membranes.

You should already have NexGard, Credelio, or Nitenpyram (Capstar) in your plan, and you should implement the following shelter mitigation priorities:

1. Aggressive Wound Management (Priority)

Conduct daily hands-on wound checks. Cover wounds whenever practical and keep all wounds clean and dry. Isolate pets with:

    • Surgical incisions
    • Bite wounds
    • Hot spots
    • Draining abscesses
    • Ear injuries
    • Skin infections
    • Tick attachment wounds

2. Move All High-Risk Pets Indoors

If shelters have limited indoor space, prioritize:

    • Post-surgical animals and recent spay/neuter patients
    • Puppies/kittens
    • Senior pets
    • Pets with dermatologic disease
    • Pets recovering from trauma

3. Physical Fly Exclusion

If your facility is open air:

    • Increase air flow with fans
    • Install fine insect screening around recovery kennels
    • Add screened shade structures
    • Add fly curtains at kennel entrances
    • Install portable screened kennel pods

4. Develop a Suspect Case SOP

Every shelter near the border should have a written procedure. If suspicious larvae are found:

    • Isolate animal to treat
    • Photograph lesion
    • Contact shelter veterinarian
    • Administer Nexguard/ Credelio
    • Notify State Animal Health Official
    • Preserve larvae samples if instructed
    • Document animal origin and movement history

5. Intake Screening

Include a visual wound inspection upon intake and consider adding NWS questions to intake forms. Has the animal:

    • Traveled from Mexico?
    • Recently crossed the border?
    • Been imported?
    • Been found near ports of entry?
    • Had recent wounds? Been treated for maggots?

6.Daily Surveillance Program

Train staff and volunteers to immediately report:

    • Enlarging wounds
    • Foul-smelling wounds
    • Bloody wound drainage
    • Unexplained pain
    • Head shaking
    • Excessive licking
    • Visible larvae

Credits

This information was made possible through a collaboration with the Bissell Pet Foundation‘s Animal Incident Management Initiative. Learn more about the Bissell Pet Foundation’s response to New World Screwworm.

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