The detection of New World screwworm (NWS) in Texas and New Mexico this spring has renewed concerns about animal health and the movement of livestock throughout the United States.

Although the risk varies by region, all horse owners will want to understand what NWS is, how it spreads and how to recognize and prevent infestations. Staying informed about current regulations, transport requirements and prevention recommendations will help you protect your horse and navigate any changes as the situation develops.

What is New World Screwworm?

The New World screwworm (Cochliomyia hominivorax) is a fly native to the Americas. Once a devastating pest across the southern United States, the NWS caused billions of dollars in livestock losses before its eradication in 1966. Since then, the U.S. Department of Agriculture (USDA) and Panama’s Ministry of Agriculture Development have jointly operated an NWS sterile fly production facility that produces and releases sterile male flies to disrupt the fly’s life cycle.

In the wild, NWS larvae are far more likely to be noticed than the adult flies. Unlike most larvae (also called maggots) that horse owners may encounter, NWS larvae feed on living tissue rather than decaying organic matter or tissue. Female flies lay eggs in wounds or moist openings, and the larvae burrow head-first in a corkscrew-like pattern, preventing healing. Infestations can worsen and expand quickly, even becoming life-threatening.

Where to find them and what they do

Adult NWS flies are metallic blue-green and resemble many other blow flies commonly found in the United States. In fact, many metallic blow flies are beneficial insects that help decompose dead animals and organic matter and can even contribute to pollination, so accurate identification is important.

The NWS life cycle takes about 21 days, though it may speed up in warm weather and slow in cooler conditions. Female flies are attracted to the odors from infected wounds, moist skin, matted hair, urine or fecal contamination. They lay clusters of eggs on or near these sites, often around 200 at a time. Eggs can hatch within 10 to 12 hours, and larvae become visible within a few days. The larvae feed on living tissue for about five to seven days before dropping to the ground, burrowing into the soil and pupating (like a cocoon). Adult flies can emerge in as little as a week, allowing populations to build quickly under favorable conditions.

How infestations occur

Larvae are not spread from animal to animal. Instead, animals become infested when NWS female flies lay eggs on wounds, irritated skin or moist body openings. Infestations can begin in something as small as a tick bite, scrape, or the umbilical cord of a newborn foal, or in male horses, the sheath, or prepuce as well. Unlike many other flies, NWS does not always need an obvious wound to start an infestation. Females may also lay eggs around mucous membranes, including the eyes, nose, and mouth.

Early signs of NWS infestation include an unusual number of flies gathering around one spot, a wound that does not heal, or a sore that becomes larger, deeper or circular in appearance. Affected areas may have a foul odor, sometimes described as the smell of decay. Eggs may appear as small white, ribbon-like strips along the edge of the wound, and larvae may be visible within the tissue. Horses may also show signs of pain or distress, including head shaking, rubbing, depression, isolation from the herd, or reluctance to be handled.

Animals infested with New World screwworm require immediate veterin-ary attention. Treatment involves removing the larvae (maggots), thoroughly cleaning the wound cleaning and applying insecticides or larvicides to prevent further infestation.

What is the risk in your area?

Screwworm risk is closely tied to climate. New World screwworm flies develop most rapidly under warm, moderately humid conditions and can complete their life cycle in as little as two to three weeks when conditions are favorable. Because pupae develop in the soil and are sensitive to prolonged cold and freezing temperatures, permanent establishment of NWS is most likely in warmer regions of the South and Southwest. In some years, seasonal activity may expand northward during warmer months, particularly into other southern and Gulf Coast states.

In northern and some western states with freezing winters, NWS is less likely to establish permanent populations. However, horse owners outside higher-risk regions still need to be vigilant. Long-distance spread can occur through the transport of infested animals is an important pathway for introduction of NWS into new areas. If introduced during warm months, NWS infestations could persist locally until sustained freezing winter conditions eliminate introduced populations.

Moving targets: The role of transport

Following the detection of NWS in the United States this spring, the transport of horses is an increasing focal point in control efforts. The equestrian community is highly mobile, traveling frequently for competitions, sales, breeding and trail riding, creating opportunities for an animal with NWS to move long distances.

The USDA and state animal health officials are leading the response, working with veterinarians, diagnostic laboratories, entomologists, equine organizations and international partners to identify cases, contain infestations and communicate movement requirements. Depending on location and risk, response measures may include inspections, health certifications, quarantines, surveillance and movement restrictions for animals originating from or traveling through affected areas.

Before traveling with horses, review NWS control requirements in your home state as well as your destination, while also staying abreast of USDA updates and directives from state animal health officials. In addition, follow any guidance provided by veterinarians and equine organizations and other competition or breed associations.

Because animal health regulations can change rapidly during an outbreak, requirements for interstate movement, competitions, sales, imports and exports may be updated with little notice. Owners planning international travel or horse sales will want to pay particular attention to changing import and export requirements, as trading partners may implement additional restrictions in response to new detections.

Of utmost importance is avoiding transporting any horse with a suspicious wound or possible NWS infestation. Keeping that horse in place and contacting a veterinarian or state animal health official immediately is one of the most important steps owners can take to prevent a local case from becoming a wider outbreak.

Defense strategy: 7 things horse owners can do

Unfortunately, standard fly control measures aren’t enough to protect against NWS. Most fly control programs for equine facilities target nuisance pests such as house flies, stable flies, and mosquitoes with manure and water management, biological control with parasitoids and feed through control, among other options. These actions are still important, but reducing nuisance flies does not eliminate the risk of NWS. A female screwworm doesn’t develop in manure or organic waste, so these standard fly management methods are not sufficient to eliminate screwworm risk. There are however steps that you can take to protect your horse and others in your area from the NWS threat.

1. Inspect horses at least daily.

Check horses at least once a day, paying special attention to the ears, sheath, umbilicus of newborn foals, tick bites, other wounds and mucous membranes. Even abrasions in hooves can be susceptible. Early detection is critical.

2. Manage wounds promptly.

Clean and protect all wounds immediately, no matter how minor they appear. Monitor healing closely and contact your veterinarian if a wound worsens, develops a foul odor, attracts unusual numbers of flies, or contains eggs or larvae.

3. Use chemical controls correctly.

Although some pesticides are effective against NWS, many equine fly repellents are not labeled specifically for screwworm, so data is limited on how they will work. Use all products according to label directions—do not increase application rates or frequency. Consult your veterinarian and current USDA guidance for recommendations (see the “Related Reading” page 00) on the best products to use.

4. Continue fly and tick prevention methods.

Maintain your regular fly and tick management program, including fly masks, boots, sheets, repellents, and routine tick checks. Apply topical repellents with a mitt, cloth, or brush to improve contact with the hair coat and overall coverage.

These steps can help reduce fly contact and tick bites, but they are not a substitute for frequent wound inspections and prompt wound care. Don’t rely on garlic for fly or screwworm prevention. Research has not demonstrated that feeding garlic effectively repels flies in horses, and excessive consumption can cause adverse health effects, including Heinz body anemia. Likewise, oral ivermectin dewormers is not a screwworm prevention tool. While ivermectin is effective against many internal parasites, there is currently no evidence that routine oral ivermectin deworming prevents screwworm infestations in horses.

5. Consider the risks associated with veterinary procedures.

Because screwworm flies are attracted to wounds, take extra precautions around surgical sites, castration wounds, and other procedures that break the skin. In higher-risk areas, schedule elective procedures during cooler months when fly activity is lower.

6. Stay informed about regulations.

Movement restrictions and animal health requirements may change as the situation evolves. Stay informed about current USDA and state animal health regulations, especially before transporting horses, attending events, purchasing animals, or crossing state lines. Stay connected with your equine societies and state department of agriculture to stay informed about the latest updates.

7. Report suspected cases immediately

If you have questions about screwworm risk or prevention, con- tact your veterinarian, state animal health officials, or your local Extension service. These sources can provide the most current recommendations and regulatory information. If you suspect NWS, do not move the horse and contact your veterinarian and state animal health officials immediately. Treatment typically requires wound cleaning, removal of larvae and use of approved products under veterinary and regulatory guidance.

The potential return of the New World Screwworm to North America is a serious agricultural issue, but it is not a reason to panic. The United States has vastly superior surveillance, electronic traceability and response infrastructure today compared to decades past. By practicing diligent wound vigilance, monitoring horses regularly and understanding the risks of travel, we can all play a crucial role in protecting our animals and the broader equine industry.

Go to the EQUUS Screwworm Tracker for the latest on New World screwworm situation in the United States.

Our expert

Erika T. Machtinger, PhD, is an Associate Professor of Veterinary Entomology at Pennsylvania State University, Penn State Extension Vector-Borne Disease Team Lead, and Director of the VectorED Network. Her research and Extension programs focus on ticks, flies, and other arthropod pests affecting animals and public health. A lifelong equestrian and active competitor in dressage and eventing, she combines scientific expertise with practical experience to advance vector-borne disease prevention and animal health.