New World screwworm flies lay eggs in open wounds, and their larvae eat living tissue. The best protection for pets and livestock is to check animals often, treat and cover wounds quickly, and report any suspected case at once.
Key takeaways
- New World screwworm is a parasitic fly whose larvae feed on the living flesh of warm-blooded animals, entering through wounds or body openings.
- Ars Technica reports that the USDA has identified 74 animal cases in the United States since the flies arrived in June, and that they have now reached a third state.
- Dogs and cats can be infested as well as cattle and other livestock, so pet owners in affected areas need to check their animals closely.
- A wound that smells bad, keeps getting bigger or has moving larvae in it needs a vet straight away, and should not be treated at home.
- Screwworm is a reportable animal disease in the United States, so suspected cases should go to a vet or the state animal health authorities.
What is happening with screwworm in the United States?
The New World screwworm (Cochliomyia hominivorax) is a fly native to the Americas. What makes it dangerous is its larvae. Most fly maggots feed on dead tissue. Screwworm larvae burrow into living flesh. A female fly lays her eggs on the edge of a wound, or in a moist body opening such as the nose, ears or genital area. When the larvae hatch, they feed inwards and make the wound deeper and wider.
Ars Technica reports that the US Department of Agriculture (USDA) has identified 74 animal cases since the flies entered the country in June, and that the infestation has now spread to a third state. The same report says pets are among the animals affected, including dogs and a cat attacked in a single week. The report as summarised here does not say which states are affected or give a breakdown of species beyond those pets.
Why is it in the news now?
There are two reasons. First, the fly has spread from one state to a third within a few months. That suggests it is becoming established rather than appearing as a one-off incursion. Second, the cases now include household pets. Earlier coverage of screwworm focused mainly on the cattle industry. Infested dogs and cats bring the risk into ordinary homes, and many owners have never heard of the parasite.
The 74-case figure comes from the USDA, as reported by Ars Technica. Totals like this change as surveillance continues, so readers should check current figures with official sources rather than relying on any one number.
What background does a newcomer need?
Screwworm used to be a major livestock pest across the southern United States. Over several decades it was eliminated using the sterile insect technique. Large numbers of male flies are bred, sterilised with radiation and released. Wild females that mate with them lay eggs that do not hatch. Female screwworms usually mate only once, so repeated releases push the population down until it collapses.
After the pest was eradicated in the United States, the same method was used to push it south through Mexico and Central America. A permanent barrier of sterile fly releases was kept up near the border between Panama and Colombia. In recent years the fly has moved north again through Central America and into Mexico. This led to US restrictions on livestock imports and to plans to increase sterile fly production. The details of the current US incursion, including exactly how the flies arrived, are not set out in the source material.
The life cycle is short. Eggs can hatch within about a day, and larvae feed for several days before dropping to the ground to pupate. That is why delay matters. A small wound can become a serious injury within days.
Who is affected, and how?
Livestock keepers. Cattle, sheep, goats, pigs and horses are all at risk. Common entry points include branding, castration, dehorning and tagging wounds, tick bites, and the navels of newborn animals. Infestations cause pain and weight loss. Animals can die if no one notices.
Pet owners. Dogs and cats can get screwworm through small cuts, bite wounds, surgical incisions, skin infections, or matted and soiled fur near body openings. Animals that spend time outdoors, older or less mobile pets, and those recovering from surgery may be more exposed.
Wildlife. Deer and other wild mammals can be infested. Wildlife can also carry the fly into new areas and make it harder to control.
People. Human infestations are possible but uncommon. They usually involve an open wound or poor access to care. Anyone who finds larvae in a wound of their own should see a doctor promptly.
Where do informed people disagree?
Experts agree that screwworm is serious. They disagree more about the best response and how urgent it is. One debate is about border policy. Restricting livestock movements from affected countries protects US herds but disrupts trade, and opinions differ on when restrictions should be tightened or eased. A second is about capacity. The sterile insect technique works, but only if enough sterile flies are produced. People argue over how quickly new production can be brought online and whether current supply is enough.
There is also a difference in tone. Some veterinarians stress that a well-tested method exists and that infested animals usually recover if treated early. Others warn that each new state shows containment is falling behind. It is not yet known which view the coming months will support.
What should pet owners and livestock keepers do in practice?
Check animals daily. In or near an affected area, look over pets and livestock every day, paying close attention to wounds and body openings. Warning signs include:
- a wound that gets bigger instead of healing;
- a strong, unpleasant smell;
- bloody or brown discharge;
- visible larvae, or movement inside a wound;
- an animal licking, scratching or shaking its head more than usual, or seeming restless and in pain.
Treat wounds promptly. Clean cuts and scrapes, and ask a vet about suitable wound sprays or dressings. Keep surgical sites covered and checked. Groom long-haired pets so fur near the tail and rear stays clean and dry.
Time routine procedures carefully. Livestock keepers may be advised to put off procedures that create wounds, such as castration or dehorning, while fly activity is high. Local veterinary or extension advice should guide this.
Do not treat a suspected infestation yourself. Pulling out a few larvae can leave others deep in the tissue. A vet can remove all of them, treat the wound and collect samples for identification. Confirming the species matters because other flies cause less dangerous maggot infestations.
Report suspected cases. Screwworm is a notifiable disease. Contact a vet, or the state veterinarian’s office or USDA animal health officials. Reports help authorities decide where to target sterile fly releases and movement controls.
Check animals before moving them. Before taking pets or livestock out of an affected area, inspect them for wounds and follow any official movement rules.
What should readers watch for next?
Key indicators are whether the case count keeps rising, whether more states report infestations, and whether sterile fly releases expand inside the United States. Announcements about new fly production facilities, changes to livestock import rules and any new treatment approvals will also be worth following. Seasonal patterns matter too. Fly activity usually drops in cold weather, but how far that will slow the spread this time is not known. The USDA and state animal health authorities are the most reliable sources for current maps and guidance.
Frequently asked questions
What is a New World screwworm?
The New World screwworm is a fly native to the Americas whose larvae feed on the living tissue of warm-blooded animals. Females lay eggs on wounds or moist body openings. The hatched larvae burrow in and enlarge the wound. Without treatment, infestations can cause severe injury and death. The pest was once eradicated from the United States using sterile male fly releases.
Can my dog or cat get screwworm?
Yes. Dogs and cats can be infested through cuts, bites, surgical wounds, skin infections or soiled fur near body openings. Ars Technica reports that dogs and a cat were among the animals attacked in the current US outbreak. Owners in affected areas should check their pets daily and contact a vet promptly if they see a wound that is getting worse or smells bad.
What does a screwworm infestation look like?
Typical signs are a wound that keeps enlarging, a strong foul smell, bloody or brownish discharge, and larvae visible or moving inside the wound. The animal may lick or scratch the area, shake its head or seem distressed. Infested wounds often look deeper than expected, because larvae feed inwards in groups. Any of these signs needs urgent veterinary attention.
Should I remove maggots from my pet’s wound myself?
No. Removing some larvae at home can leave others deep in the tissue, and the wound may need cleaning, pain relief and medication only a vet can provide. A vet can also take samples so the species can be confirmed. That matters because screwworm is a reportable disease and other fly species cause less serious infestations. Keep the animal calm and seek care quickly.
Can humans catch screwworm?
Human infestations can happen but are uncommon. They usually involve an open wound, particularly where hygiene or access to medical care is limited. The parasite does not pass from animals to people by touch. Flies lay eggs directly on a person’s wound. Anyone who finds larvae in a wound should see a doctor promptly and mention any possible exposure.
How do I report a suspected screwworm case?
In the United States, contact your vet first. They can examine the animal and alert the authorities. You can also contact your state veterinarian’s office or USDA animal health officials directly. Screwworm is a notifiable animal disease, so early reports help officials track the spread, target sterile fly releases and decide on any restrictions on moving animals.
Sources and further reading
- Ars Technica: health reporting on the spread of screwworm to a third US state and on infestations in pets.
- United States Department of Agriculture, Animal and Plant Health Inspection Service: official case updates and guidance on screwworm.
- State veterinarian offices: local reporting requirements and rules on moving animals.
- Veterinary and agricultural extension services: practical advice on wound care and fly control for pets and livestock.
Surfaced from the rss:arstechnica signal “parasitic fly outbreak spreading”. AI-assisted draft, editorially reviewed.
