For decades, New World Screwworm was considered a problem of the past in the United States. Thanks to an aggressive eradication program, it was eliminated from the U.S. in the 1960s.
Today, it’s back on the radar.
Recent detections in Mexico have prompted increased surveillance and prevention efforts along the southern border. While there are currently no widespread infestations in Texas, livestock owners should understand what New World Screwworm is, why it’s different from ordinary maggots, and how to protect their animals.
What Is New World Screwworm?
New World Screwworm is the larval stage of the New World screwworm fly (Cochliomyia hominivorax).
Unlike common blowflies that lay eggs on dead tissue, the female screwworm fly lays her eggs in open wounds or body openings. Once the eggs hatch, the larvae feed on living tissue, causing painful wounds that continue to grow larger as more larvae develop.
Without prompt treatment, these infestations can become life-threatening.
Which Animals Are at Risk?
Although cattle are most commonly affected, New World Screwworm can infest any warm-blooded animal, including:
- Cattle
- Horses
- Sheep and goats
- Swine
- Dogs and cats
- Wildlife
- In rare cases, people
Newborn animals are especially vulnerable because flies are attracted to healing navels, while any cut, scrape, surgical incision, or branding wound can provide an entry point.
Signs to Watch For
Early detection is critical.
Monitor your animals for:
- Wounds that become larger instead of healing
- Bloody or foul-smelling discharge
- Visible larvae (maggots) in a wound
- Excessive licking, biting, or irritation
- Swelling around an injury
- Animals that become depressed or stop eating
If you suspect New World Screwworm, contact your veterinarian immediately.
Prevention Is Your Best Defense
The best protection starts with good animal husbandry.
- Inspect animals regularly, especially during fly season.
- Clean and treat wounds promptly.
- Monitor newborn animals closely.
- Maintain an effective fly control program.
- Isolate affected animals until they can be examined.
Many veterinarians also recommend products containing ivermectin or doramectin as part of a prevention or treatment program when appropriate. Always follow your veterinarian’s recommendations and product label directions.
How Is It Being Controlled?
The good news is that New World Screwworm is closely monitored by animal health officials.
One of the most successful pest eradication programs ever developed involves releasing sterile male flies. When wild females mate with sterile males, no offspring are produced, helping suppress populations without harming beneficial insects or wildlife.
This strategy successfully eliminated New World Screwworm from the United States decades ago and continues to play an important role in preventing its spread today.
Stay Informed—Not Alarmed
New World Screwworm is a serious livestock pest, but it is not a reason to panic.
Knowing what to look for, practicing good wound management, and responding quickly to suspicious cases are the best ways to protect your animals. Staying informed allows livestock owners to recognize potential problems early and work with their veterinarian to keep animals healthy.
If you have questions about parasite prevention or livestock health products, we’re always happy to help point you in the right direction.
