The New World screwworm: Learn essential prevention tips, biosecurity measures, and how early detection safeguards agriculture.
No parasite is a pleasure to deal with in the equine world. Whether it is the common housefly or intestinal worms, these pests bring annoyance and pain to our horses. But just as the agricultural community feels confident in its management strategies, a devastating threat has reappeared on the radar: the New World screwworm.
Recent reports have put livestock owners on high alert. According to Florida Department of Agriculture and Consumer Services, “s crewworm larvae were detected on an imported horse from Argentina going through the routine and required exam at an approved United States Department of Agriculture import facility in South Florida. There has been no detection of screwworm outside of the quarantine facility. The affected horse was treated, and in an abundance of caution, all cohort horses at the facility were also examined and treated.”
Texas Agriculture Commissioner Sid Miller recently responded, “this serves as a stark reminder that while this pest was eradicated from the U.S. decades ago, the risk of reintroduction remains a constant threat to the American livestock industry.”
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What Are Screwworms?
The New World screwworm ( Cochliomyia hominivorax) is a parasitic fly that poses a significant danger to horses, cattle, and other warm-blooded animals. Unlike most fly larvae that consume dead or decaying tissue, screwworm maggots feed on living flesh. This behavior causes deep, serious wounds that can lead to systemic infection and, if left untreated, death.
The Screwworm Life Cycle
Screwworms thrive in tropical and subtropical regions. In warm, humid weather, their life cycle can be completed in as little as three weeks. Understanding this cycle is the first step in prevention:
- Adult Flies: These are larger than common houseflies and feature a distinctive metallic blue-green body with bright orange-red eyes.
- Eggs: Females lay up to 200 eggs on the edges of fresh, open wounds. These eggs hatch within 12 to 24 hours.
- Larvae (The Danger Phase): Once hatched, the larvae immediately burrow deep into healthy tissue. Over five to seven days, they undergo development stages that cause massive tissue destruction and extreme pain.
- Pupae: At maturity, the larvae exit the wound and fall to the ground. They burrow into the soil to pupate for seven to 14 days before emerging as adults to begin the cycle again.
Transmission and Risk Factors
Screwworms invade any break in a horse’s skin. Common egg-laying sites include:
- Cuts, scrapes, or insect bites.
- Surgical sites (such as castration wounds).
- Umbilical stumps on newborn foals.
- Moist body orifices.
Horses moved from or through endemic regions, such as Central and South America, are at the highest risk. As seen in the recent Florida case, international movement combined with inadequate quarantine can accidentally introduce this parasite to new areas. With USDA on high alert strict quarantines and examinations are in place and ongoing.
Symptoms to Watch For
Recognizing the signs early is critical. Look for:
- Wounds that do not heal or appear to be getting larger.
- A foul odor accompanied by abnormal discharge.
- Ragged wound edges.
- Visible maggots burrowing into healthy, pink tissue (unlike common blowflies).
- Lethargy and behavioral changes due to pain.
Treatment and Reporting
Immediate veterinary care is mandatory if screwworms are suspected. Because the U.S. is technically screwworm-free, veterinarians must report cases to state and federal authorities, such as the USDA, to trigger containment protocols.
Treatment involves the meticulous removal of every maggot, as any left behind will continue to feed. Wounds must be cleaned, debrided of damaged tissue, and treated with topical larvicides to prevent reinfestation.
Prevention and Global Security
Preventing a screwworm outbreak relies on strict government regulations and owner vigilance. The USDA Animal and Plant Health Inspection Service (APHIS) maintains a "sterile fly" barrier at the U.S.-Mexico border to prevent northward migration.
However, as Commissioner Miller noted, "Biosecurity is our first line of defense." Horse owners should maintain strict wound care protocols and report any suspicious maggot activity to their veterinarian immediately.
While the New World screwworm is not currently established in the United States, recent detections prove that we cannot be complacent. By remaining diligent and reporting early signs, we can protect our horses and the broader livestock community from this unwelcomed visitor.
