Screwworm in Dogs: Symptoms, Risks, and the New Treatment's Side Effects

In the summer of 2023, a case of New World Screwworm in a dog that had not traveled outside the U.S. would have been an anomaly. 

Today, the situation has changed.

The U.S. Food and Drug Administration (FDA) has issued its first-ever Emergency Use Authorization (EUA) for an animal drug, Credelio, to combat this parasite. This regulatory action signals a concrete response to a confirmed, emerging threat.

The New World Screwworm (Cochliomyia hominivorax) is a flesh-eating parasite. It infests living tissue. This article provides a factual overview of the parasite's symptoms, a data-driven assessment of risk, and a detailed analysis of the newly authorized treatment, including its documented side effects.

The Parasite: New World Screwworm Biology

The New World Screwworm is a fly species whose scientific name, Cochliomyia hominivorax, means "man-eater." It is an obligate parasite of living warm-blooded animals. Its life cycle is the source of its destructiveness.

The Four-Stage Life Cycle

1. Attraction

A female fly is drawn to the warmth and moisture of any skin break on a dog. This includes:

• Surgical incisions (e.g., from spaying/neutering)
• Minor cuts and abrasions
• Tick bites and other insect wounds
• Skin infections or hot spots
• The navel of a newborn puppy

2. Egg-Laying

The fly deposits a mass of 100 to 300 eggs in a honeycomb-like pattern on the wound edges. These eggs hatch into larvae within 12 to 24 hours.

3. Infestation

The larvae burrow into healthy tissue. They use sharp mouth hooks to feed on living protein and bodily fluids, mechanically expanding the wound.

4. Maturation

After feeding for 5 to 7 days, the larvae exit the wound, fall to the ground, and pupate in the soil, developing into adult flies.

An untreated infestation leads to systematic tissue destruction, secondary bacterial infections, shock, and death.

Symptoms and Immediate Action

Early detection is critical. Dog owners should monitor any wound for these specific signs.

Behavioral Symptoms (Observe Your Dog)

• Focused, persistent licking or biting at a single area.
• Restlessness or an inability to settle.
• A sudden loss of appetite.

Clinical Symptoms (Examine the Wound)

• A wound that fails to heal and instead increases in size and depth.
• A distinct, foul odor emanating from the wound.
• A discharge that may be bloody, purulent, or serous.
• The presence of larvae. Screwworm larvae are:

1/4 to 1/2 inch long at maturity.

Pinkish or creamy-white in color.

Markedly segmented, giving a ribbed appearance.

Observable moving deep within the wound tract.

Systemic signs like profound lethargy, depression, or fever indicate an advanced, life-threatening condition.

Immediate Action Protocol

1. Contact a veterinarian or emergency clinic immediately. State your concern: "I suspect a screwworm infestation."

2. Do not apply ointments, oils, or other home remedies. These are ineffective against embedded larvae and delay professional treatment.

3. Prevent self-trauma. Place an Elizabethan collar (cone) on your dog if one is available.

4. Cover the wound loosely with a clean, dry bandage or cloth to prevent further egg-laying.

5. Do not attempt to remove the larvae. Extraction requires veterinary expertise, anesthesia, and specialized tools to ensure complete removal.

A Realistic Assessment of Risk

The threat of New World Screwworm is geographically defined. The USDA's Animal and Plant Health Inspection Service (APHIS) provides tracking data for this parasite.

Primary Risk Factors

Geography

The highest risk is for dogs residing in or traveling through specific regions. According to USDA data, this includes areas along the U.S.-Mexico border, particularly in South Texas, with ongoing surveillance in Southern New Mexico, Arizona, and California. The parasite's northward movement from South America since 2022 is documented.

Lifestyle

Dogs with outdoor access in these regions, especially working, hunting, or rural dogs, face higher exposure risk.

Wound Status

Any dog with an unmanaged or fresh wound in a risk area is vulnerable.

For dogs in the continental U.S. outside of these specific zones, the immediate risk remains low. However, the changing situation warrants awareness.

The Emergency Treatment – Credelio (Lotilaner)

In August 2025, the Secretary of Health and Human Services determined that New World Screwworm presents a significant potential for a public health emergency. This declaration enabled the FDA to issue an EUA for animal drugs.

The authorized drug is Credelio (lotilaner).

What is Credelio?

Credelio is an isoxazoline-class drug originally approved by the FDA in 2018 for the killing of fleas and ticks. Its safety profile is established from years of commercial use.

Mechanism of Action

Lotilaner works by selectively inhibiting GABA-gated and glutamate-gated chloride channels in the nervous systems of invertebrates. This inhibition leads to hyperexcitation, paralysis, and death of the parasite. For a screwworm infestation, Credelio kills the feeding larvae systemically.

Why Credelio Received the Authorization

The EUA process is product-specific. The drug's sponsor, Elanco, submitted the required data to the FDA demonstrating that Credelio is reasonable to believe effective for treating New World Screwworm in dogs. It is the only isoxazoline drug with this specific authorization.

A Transparent Analysis of Side Effects

All pharmaceuticals carry potential side effects. Informed consent is a cornerstone of veterinary medicine.

Documented Adverse Reactions

The FDA requires a class warning for isoxazolines, including Credelio, regarding potential neurologic adverse reactions. The specific events reported in dogs for this drug class include:

• Muscle tremors
• Ataxia (impaired coordination)
• Seizures

Incidence and Context

The FDA's cumulative data from post-approval reporting for isoxazolines indicates that these neurologic events are reported in a low percentage of dogs. The vast majority of dogs taking these drugs experience no adverse effects.

The risk of seizures is highest in dogs with a pre-existing history of seizure disorders.

Other, less serious reported side effects for Credelio include vomiting, diarrhea, lethargy, and decreased appetite.

The Prescription Requirement

Credelio is available only by prescription from a licensed veterinarian. This is a safety measure. A veterinarian must:

1. Confirm the diagnosis of a screwworm infestation.

2. Assess the dog's individual medical history, specifically screening for prior neurologic events.

3. Determine that the benefits of treatment outweigh the potential risks for that specific animal.

4. Instruct the owner on monitoring for potential adverse reactions post-administration.

Questions for Your Veterinarian

• “Given my dog's health history, is Credelio the appropriate treatment?”
• “What specific signs of a neurologic reaction should I watch for, and what is the emergency protocol if I observe them?”

Conclusion

The appropriate response to this information is preparedness, not panic.

A Tiered Action Plan

1. For Dog Owners in Risk Areas:

Conduct a daily, hands-on physical inspection of your dog. Feel for any skin lesions, scabs, or areas of tenderness.

Manage all wounds proactively. Clean even minor cuts and monitor them until fully healed.

Schedule a proactive consultation with your veterinarian to discuss local risks, prevention, and clinic protocols for suspected cases.

2. For All Dog Owners:

Be aware of parasite risks when traveling with your pet.

Understand that the authorization of Credelio provides veterinarians with a critical, evidence-based tool to address a severe and specific medical emergency.

Final Recommendation

Save your veterinarian's phone number and the address of the nearest emergency clinic in your contacts. Perform a full-body check on your dog today to establish a baseline of their skin health. Proactive awareness is the most effective defense.