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What to Know About New World Screwworms and Dogs
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With newly confirmed cases of the New World screwworm in the U.S., it’s not just the cattle industry that’s on high alert. Pet owners are wondering whether this flesh-eating parasite could harm them or their pets. Although current risk to people and dogs remains very low, infections can be serious and require immediate attention. Knowing what to watch for—and how to prevent exposure—can help protect your dog.
In This Article
What Is the New World Screwworm?
Despite the name, the New World screwworm (Cochliomyia hominivora) isn’t a worm. “It’s the larval stage of a fly, and it’s actually a parasitic flesh-eating fly,” Dr. Kate Elden, DVM and Chief Medical Officer at Dutch explains. “What makes it so dangerous is that these larvae feed on live tissue, not just dead tissue like most maggots we’re familiar with.”
Female screwworm flies are drawn to open wounds, even minor wounds. “The flies lay anywhere from 200 to 300 eggs right at the wound’s edge,” says Dr. Kelly Hood, DVM. “Within 12 hours, those eggs hatch into maggots that start actively feeding.” These larvae burrow into tissue using sharp mouth hooks, enlarging wounds and causing significant damage.
As the infestation progresses, the wound deepens, increasing pain and the risk of secondary bacterial infection. Dr. Elden explains that the destruction of healthy tissue is the primary concern, followed by the possibility of infection spreading throughout the body.
How Does the Screwworm Spread to Dogs?
Although livestock are most commonly affected, screwworms can infest dogs, cats, birds, and wildlife. “A wound is the entry point, and if your dog has one, they have a higher risk of infestation,” Dr. Hood says. Even a small scratch can attract a female fly.
Common entry points include:
- Open wounds
- Tick bites
- Scratches
- Hot spots
- Surgical incisions
- Newborn umbilicus (belly button)
- Skin folds or nasal folds
- Body openings such as eyes, ears, nose, mouth, and genitals
Young animals are more susceptible, while senior dogs may be at risk if wounds go unnoticed. Because screwworm flies resemble common houseflies, infestations can be difficult to spot early.
Human infections are rare. Dr. Elden explains that good hygiene and frequent wound care make infestation uncommon in people.
Signs of an Infestation
New World screwworm infestations are extremely painful. If your dog has a wound of any kind, Dr. Hood advises close monitoring. Is it healing or getting worse?
Any of the following signs warrant taking your dog to the vet immediately:
- A foul odor coming from a wound
- A wound that is growing larger
- Excessive drainage
- Swelling or irritation
- Visible movement of maggots
- Egg clusters in the wound
- Restlessness or distress
- Signs of pain
- Persistent licking or chewing at the area
Early intervention can significantly reduce complications.
Diagnosis of a Screwworm Infection
If you suspect your dog has been infected by the screwworm, it’s an emergency. The most telling sign of a screwworm infection is seeing the larvae directly in or around a wound.
“If we’re unsure about the larvae, we can collect them and submit them for species identification,” Dr. Hood says. “We also diagnose by reviewing travel history and what environment your dog has been in and whether they’ve been exposed to other animals, especially livestock.”
Veterinarians will remove visible larvae, clean the wound, and may use sedation in severe cases. Antibiotics are often prescribed if infection is present.
Because screwworm affects livestock and the food supply, it is a reportable disease in certain cases. Your veterinarian may recommend observation depending on the situation.
Treating Screwworm Infections
Treatment focuses on eliminating larvae, managing pain, and preventing infection. “In suspected cases of screwworm, Dr. Elden says the first line of treatment your vet will give is putting your dog on a monthly isoxazoline parasite preventative, if they’re not already on one.”
These oral medications help kill parasites and prevent further damage. Additional care may include fluids, pain relief, and wound management.
“Your vet will determine which medication is appropriate based on your dog’s specific situation,” Dr. Hood says.
“Screwworms are obligate parasites, meaning they have to get their food from live healthy tissue,” Dr. Elden says. “If they dig through enough tissue and cause a big enough wound that gets infected, that can cause an animal to be in a lot of pain.” In severe cases, infection can become systemic and life-threatening.
The U.S. Food and Drug Administration has authorized several products specifically for New World screwworms in dogs, including nitenpyram tablets (best known by brand names like Capstar or CapAction), Credelio (lotilaner) and NexGard (afoxolaner), Dr. Hood says.
How to Prevent an Infection
“The single best defense against the New World screwworm is keeping pets on routine, year-round prescription flea and tick preventatives like Credelio Quattro, Simparica Trio, and NexGard Plus,” Dr. Elden says. “Any dog that is on a parasite preventative that’s in the isoxazoline class is not going to get screwworm.” In addition to clearing screwworm infestations, these products prevent flea and tick bites and reduce scratching that can lead to wounds, which then attract female flies.
Routine care should include inspecting your dog frequently for wounds, keeping injuries clean, and limiting exposure to high-risk environments when needed. In areas closer to reported cases, additional protective measures may be recommended by your veterinarian.
Some owners may consider adding topical repellents to help deter flies, but it’s essential to use products safely and only as directed. Certain ingredients can be toxic to cats and should never be shared between pets.
“If you’re in or near an affected area, I’d be extra vigilant about limiting outdoor exposure and monitoring your dog’s skin, as well as any behavior changes,” Dr. Hood says. “As with many conditions, the earlier you detect, the better.”
Healing and Recovery in Dogs
As for recovery, it depends on the location and duration of the screwworm infection. “If it’s just started and the tissue damage is relatively mild, then it would look like a small wound healing,” Dr. Elden says. This can take weeks to heal. If the infestation is more severe or there’s a secondary infection, it’ll take a lot longer. Another delay with healing is how far down into the tissue the screwworm has gotten, which could even mean permanent damage.
What This Means for Dog Owners
There’s no need to panic. However, we should be prepared for more cases to pop up. “We have eradicated screwworm in the U.S. before in the 1960s,” Dr. Elden says. “We were able to get rid of it, but it takes time.”
As a dog owner, the most important thing you can do right now is to be vigilant about wounds, especially if you live in an affected area, and get your dog on a prescription parasite preventative.
Key Takeaways
- New World screwworm larvae feed on living tissue and can cause serious infections
- Dogs are most at risk when they have open wounds or irritated skin
- Early signs include odor, swelling, drainage, and visible maggots
- Immediate veterinary care is essential for suspected cases
- Year-round parasite preventatives are the best defense
Frequently Asked Questions
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What does a screwworm infection look like in a dog?
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It often appears as a worsening wound with swelling, drainage, a foul smell, or visible maggots. Dogs may also show pain or repeatedly lick the affected area.
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Is screwworm common in dogs?
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No. Cases in dogs are rare, and the current overall risk is low, though isolated cases have been reported in the U.S.
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Can humans get screwworm?
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It is possible but uncommon. As Dr. Hood notes, people can become infested through open wounds, though good hygiene makes this rare.
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How quickly does screwworm develop?
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Eggs can hatch within 12 hours, and larvae begin feeding immediately, which is why early detection and treatment are critical.
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What should I do if I suspect screwworm?
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Seek veterinary care immediately. Prompt treatment can prevent severe tissue damage and improve recovery outcomes.
This article is intended solely as general guidance, and does not constitute health or other professional advice. Individual situations and applicable laws vary by jurisdiction, and you are encouraged to obtain appropriate advice from qualified professionals in the applicable jurisdictions. We make no representations or warranties concerning any course of action taken by any person following or otherwise using the information offered or provided in this article, including any such information associated with and provided in connection with third-party products, and we will not be liable for any direct, indirect, consequential, special, exemplary or other damages that may result, including but not limited to economic loss, injury, illness or death.
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