New World Screwworm in Dogs: Signs and Prevention

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, especially with the first confirmed case of screwworm in a dog living in New Mexico. Although this case appears to be isolated, officials are encouraging area residents to be vigilant and monitor their animals for signs of this parasite.

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According to the U.S. Department of Agriculture (USDA), the screwworm presents a serious threat to livestock and wildlife, but the current risk to people and pets is very low. That said, Dr. Kate Elden, DVM and Chief Medical Officer at Dutch notes that it’s still a concern. An infection is serious, and it can become painful and potentially life-threatening. Read on to learn about the New World screwworm, how it spreads to dogs, signs to watch for, and options for treating and preventing this parasite in dogs.

What Is the New World Screwworm?

Despite the name, the New World screwworm (Cochliomyia hominivora), isn’t a worm like the ones you find in soil. “It’s the larval stage of a fly, and it’s actually a parasitic flesh-eating fly,” Dr. Elden 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 and scratches. “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.” The screwworm has sharp mouth hooks which allow it to burrow or screw into a wound and feed on healthy tissue.

As maggots continue to hatch, the wound can become larger and deeper, causing pain and increasing the chances of a secondary infection. Dr. Elden explains that the main problem is that healthy skin, tissue, and muscle are destroyed. The next issue is getting a potential bacterial infection when tissue is exposed to air, dirt, and the maggots’ saliva. This secondary infection can affect the whole body and cause even more tissue damage.

How Does the Screwworm Spread to Dogs?

Although livestock are at greatest risk of infestation, the screwworm can spread to dogs, cats, birds, and wildlife. Dr. Elden explains that it’s very rare for humans to get the screwworm and that it’s treatable. The bigger threat is to the food supply and cattle industry. Dr. Hood adds that people can be infested in the same way as animals if, for example, they have an open wound, spend a lot of time outdoors, or travel or live in an area where screwworm flies are active. Luckily, because of how often humans practice personal hygiene, it’s also rare to be infested.

So how can dogs become infected? “A wound is the entry point, and if your dog has one, they have a higher risk of infestation,” Dr. Hood says. Even just a small scratch is enough to attract a female fly. Other possible entry sites include:

• Open wounds
• Tick bite
• Scratch
• Hot spot
• Surgical incision
• Newborn umbilicus (belly button)
• Raw nasal folds or skin folds
• Body openings such as the eyes, ears, nose, mouth, and genitals

Some age groups are more at risk than others. “Any warm-blooded animal can be susceptible, but newborns and young animals tend to have a higher infestation rate,” she says. Dr. Elden adds that senior dogs are at risk if hot spots or wounds go unnoticed. Since screwworm flies look like a common housefly, pet owners may not notice the difference.

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:

• Wound giving off a foul odor
• Wound getting bigger
• Wound draining a lot
• Swelling or irritation
• Movement of maggots in the wound
• Egg masses in the wound
• Your dog is restless or distressed
• Your dog is in pain
• Your dog is excessively chewing or licking the site

Diagnosis of a Screwworm Infection

If you suspect your dog has been infected by the screwworm, it’s an emergency. Dr. Elden emphasizes that this requires immediate veterinary attention. 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.”

Your vet will remove as many of the maggots as they can and clean the wound. “For severe cases, this could require sedation,” Dr. Hood says. The vet may also prescribe an antibiotic if the wound looks like it’s becoming infected.

Another reason for collecting larvae for laboratory testing is that the screwworm affects the food chain, making it a reportable disease. At this time, livestock and wildlife with suspected screwworm cases should be reported to the USDA. Pets should be brought to the vet immediately, but no reporting is required as of yet. Because of these requirements, it’s possible that your vet may suggest keeping your dog for observation.

Treating Screwworm Infections

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 isoxazoline medications are once-a-month prescription oral medications that come in chewable tablets. They’re designed to expel and kill the worms. Depending on the severity of their condition, your dog may need fluids or pain medication.

The U.S. Food and Drug Administration has authorized several products specifically for New World screwworms in dogs, including Credelio (lotilaner) and NexGard (afoxolaner), Dr. Hood says. Talk to your vet about preventatives to decide which is right for your dog.

“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.” Eventually, if there are enough larvae causing extensive damage, this can progress to a systemic infection and even become fatal.

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.

“The main thing is making sure that pets are on these medications and fast, especially if they’re in a border state with Mexico, close to it, or have any travel history to Mexico,” she says.

For pet dogs, the risk is low. If your dog is at risk, it can be helpful to add a topical treatment to your dog’s year-round flea prevention plan for extra protection. If you live in Texas, New Mexico, Arizona, or Southern California, Dr. Elden recommends the oral monthly preventatives that will kill a flea or tick if they land on and bite an animal. Consult your vet about your region’s risk level if you have concerns. “A second layer to repel flies from landing on a dog would be to put a spot-on treatment like permethrin, which is toxic to cats,” she says. Other options include K9 Advantix and Vectra 3D.

However, these parasite preventatives come with an important warning. Never use your dog’s flea and tick products on your cat, as some of these are toxic to cats. Another caution concerns where you seek treatment. Not all online veterinary clinics can ship prescriptions, and some emergency veterinary clinics do not prescribe preventatives. Accordingly, it’s best to confirm product availability and shipping options.

“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.” She recommends checking your dog regularly for cuts or injuries, keeping any wounds clean, and contacting your vet if you notice any concerning signs.

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.

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