If you've been following recent animal health news, you've probably heard growing concerns about New World Screwworm (NWS). Once eradicated from the United States, this destructive parasite has re-emerged in parts of Mexico in recent months, and has now been detected in New Mexico and Texas.
While cattle often receive the most attention in discussions about screwworm, horses are highly susceptible to infestation and can suffer severe tissue damage if infections go unnoticed.
Understanding what New World Screwworm is, how it spreads, and how to protect your horses is critical for every owner, trainer, breeder, and equine professional.
What Is New World Screwworm?
New World Screwworm is the larval stage of the screwworm fly (Cochliomyia hominivorax).
Unlike common maggots that feed on dead tissue, screwworm larvae feed on living flesh. Female flies are attracted to open wounds, where they deposit clusters of eggs. After hatching, the larvae burrow into healthy tissue, causing painful and rapidly expanding wounds.
Without treatment, infestations can become severe and may even be life-threatening.
Why Horses Are at Risk
Horses are naturally prone to the types of wounds that attract screwworm flies.
Common entry points include:
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Cuts and lacerations
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Wire injuries
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Hoof abscess drainage sites
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Surgical incisions
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Castration wounds
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Branding or identification wounds
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Fly bite reactions
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Tick bite sites
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Umbilical areas in newborn foals
Even the smallest wound can attract a female screwworm fly.
Because horses often spend time outdoors during peak fly activity, they may be exposed before owners realize a wound exists.
Signs of New World Screwworm Infestation
Early detection is essential.
Watch for:
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Wounds that suddenly enlarge
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Excessive drainage or bleeding
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Foul odor from a wound
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Visible maggots within tissue
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Intense irritation or pain
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Horses biting, rubbing, or kicking at a wound
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Swelling around the affected area
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Depression, lethargy, or reduced appetite
One hallmark of screwworm infestation is that the wound continues to worsen rather than heal.
Larvae in a wound are cause for immediate action; do not assume they are common barn fly or horse fly maggots. It’s difficult to distinguish between NWS and common maggots; particularly in the early stages of infestation.
Why Screwworm Is So Dangerous
A single female screwworm fly can lay hundreds of eggs at one time.
As larvae feed, they create larger wounds that attract additional flies, leading to repeated infestations. The resulting tissue destruction can cause:
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Severe pain
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Secondary bacterial infections
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Weight loss
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Lameness
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Reduced performance
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Permanent tissue damage
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Death in severe cases
Recognizing infestation and getting treatment for affected animals is essential to help prevent serious long-term effects.
What to Do If You Suspect Screwworm
If you suspect an infestation:
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Isolate the horse if possible.
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Contact your veterinarian immediately.
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Do not attempt to aggressively remove larvae without veterinary guidance.
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Follow all treatment recommendations.
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Monitor nearby horses carefully.
Because New World Screwworm is a reportable animal disease, veterinarians may work with state or federal animal health officials if a case is suspected.
Prevention: Your Best Defense
The most effective protection is preventing flies from accessing wounds in the first place.
Inspect Horses Daily
Perform a thorough visual inspection every day, especially during fly season.
Pay close attention to:
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Legs and lower limbs
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Hooves
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Belly and sheath areas
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Under the mane
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Tail head
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Surgical or healing wounds
Treat Wounds Immediately
Clean and manage even minor injuries promptly.
Keep wounds clean and protected according to your veterinarian's recommendations.
Maintain Fly Control
A comprehensive fly-control program can help reduce overall fly burden around horses. Common flies often create entry wounds for the screwworm fly, making it all the more important to reduce their populations as much as possible.
Consider:
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Fly sprays
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Fly masks
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Fly sheets
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Fly predators
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Manure management
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Elimination of standing water
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Environmental sanitation
Support Overall Health
Healthy horses generally recover from injuries more efficiently and are better equipped to handle environmental stressors.
A balanced nutrition program that supports digestive health, hydration, and overall wellness can help maintain normal immune function and recovery.
What About Feed-Through Fly Control?
Many livestock operations utilize feed-through insect growth regulators to help reduce fly populations in manure. While these products can play an important role in integrated pest management programs, horse owners should work with their veterinarian to determine which fly-control strategies are appropriate for their specific operation.
No fly-control program should replace routine wound inspection and prompt treatment.
Staying Vigilant
The reappearance of New World Screwworm serves as an important reminder that wound management is more than basic horse care. It's a critical component of disease prevention.
By performing regular inspections, treating wounds promptly, maintaining effective fly control, and supporting overall health, horse owners can help protect their horses from one of the most destructive parasites in the Western Hemisphere.
When it comes to screwworm, early detection can make all the difference.
