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What is the New World Screwworm and What Should We Communicate to Horse Owners?

By Sally DeNotta, DVM, PhD

The New World screwworm (NWS) is a growing U.S. threat, and equine veterinarians are on the front line. The following is a primer on NWS. We encourage you to take advantage of the resources available through AAEP and USDA noted below to learn more.

NOT IF, BUT WHEN
New World screwworm is caused by NWS fly larvae (Cochliomyia hominivorax). This is not your typical “roadkill” maggot but a flesh-eating fly larva that feeds on healthy tissue. Screwworms plagued U.S. livestock until a landmark sterile-insect eradication program eliminated them in 1966. Today they remain endemic to parts of Central and South America and are inching northward. Recent outbreaks only 700 miles from our southern border have experts predicting a U.S. incursion by year’s end.

The NWS fly lays eggs in an open wound, gaining access through even the tiniest skin break—abrasions, lacerations or even insect bites—and hatch into maggots that burrow (screw) into healthy tissue. The larvae can affect any mammalian host, including people, so any animal is susceptible. A heavy infestation can kill the host; lighter infestations cause severe pain and discomfort, secondary infection and production losses. Fortunately, it is very treatable, but the potential economic impact is concerning. An outbreak would cost livestock producers millions, triggering trade restrictions and costly re-eradication efforts, so authorities are focused on preventing it from re-entering the United States.

In May, U.S. Secretary of Agriculture Brooke Rollins shut down the southern U.S. border to all cattle, horses and bison, and in mid-June unveiled an aggressive plan to mitigate U.S. entry of NWS. One component is another sterile-insect program, where male screwworm flies are irradiated and then strategically released into the wild to stop the lifecycle.

SOUTHERN BORDERS ON WATCH
The most likely arrival will be via wildlife that can freely cross the border. As such, livestock owners, horse owners and veterinarians will be on the front lines of recognizing and reporting NWS. Those in our southernmost border states should be the most vigilant, but that doesn’t mean NWS can’t spread northward on an infected animal in warmer seasons. This particular fly does not overwinter well, so it won’t take up permanent residence in cold climates. Visual identification is key.

• Any open wound that is not healing appropriately with a maggot infestation should be investigated. Unlike typical fly larvae (which eat only necrotic tissue), screwworms gorge on living flesh.
• Other clinical signs include extreme discomfort and agitation. The animal may exhibit headshaking, stomp, kick or try to bite the wound. A foul odor may also be present.

If you suspect NWS, collect larvae from the wound for identification and contact your state animal health official immediately.

WHAT SHOULD I TELL HORSE OWNERS?

• Be vigilant. Any wound—no matter how small—that harbors maggots warrants an immediate call to the veterinarian. Do not rely solely on fly spray or over-the-counter wound dressing.
• Understand the risk. Horses in southern border states are most at risk, but infected animals can be transported north.
• Act quickly. Early detection means straightforward treatment and minimal animal suffering. Mortality is rare if managed promptly.

If you or your client suspect NWS, contact your state animal health official immediately. Containment depends on timely reporting. Together, we can prevent the re-establishment of this devastating pest in the United States.

FOR MORE INFORMATION: 
USDA APHIS NWS Information
Equine Disease Communication Center
AAEP Webinar: NWS Update

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For more equine infectious disease information, visit the Merck Animal Health Equine page.