New World Screwworm: Information for Pet Owners

Laurie Anne Walden, DVM

Adult New World screwworm fly. Photo by US Department of Agriculture.

New World screwworm causes serious and sometimes fatal disease in animals infested with this parasite. The animals most often affected are livestock (especially cattle), horses, wildlife, and dogs, although all mammals and birds can be infested. The disease is rare in humans.
 
New cases have recently been identified in Texas and New Mexico, decades after the parasite was eradicated from the United States. The overall risk to animals and people in the United States is still very low. No cases have been found on the East Coast as of the date of this post.
 
What Screwworm Is
New World screwworm is not a worm. It is the larvae (maggots) of the fly Cochliomyia hominivorax. The flies lay eggs on an animal, usually in open wounds or around natural body openings. The larvae that hatch from the eggs feed on the animal’s tissue, causing painful and potentially life-threatening wounds.
 
Other kinds of flies also lay eggs that hatch into maggots and feed on animal tissue. The difference between New World screwworm and the others is that New World screwworm feeds on living tissue. Other maggots feed on dead and decomposing tissue, performing nature’s cleanup work.
 
How New World Screwworm Spreads
This is not a contagious disease; an infested animal doesn’t spread the parasite directly to another animal. Animals become infested when C hominivorax flies lay eggs on their body. This parasite does not contaminate the food supply. New World screwworm spreads geographically when the flies migrate or when the eggs or larvae are carried to new areas.
 
New World screwworm flies live in warm, humid regions. These flies were once found throughout the southern United States, Mexico, and Central America. They were eradicated in these areas by international collaboration to prevent the movement of infested animals and to introduce sterile flies that prevent fly reproduction. The flies still live in some Caribbean islands and parts of South America, and they have been moving northward over the past few years.
 
Signs of Screwworm Infestation

• Maggots or masses of small white eggs in wounds, skin folds, or body openings like nose, ears, genitalia, and navel of newborn animals
• Wounds with a foul odor of decay
• Wounds that become larger or deeper over time
• Signs of pain, such as loss of appetite, loss of energy, or avoiding contact with people or other animals

 
Treatment
New World screwworm is a reportable disease, meaning that veterinarians must report suspected cases so the disease can be tracked. Treatment involves removing all visible eggs and larvae, treating the bacterial infection associated with the wound, and treating the animal’s general illness (which can be serious in animals with severe infestations).
 
For dogs and cats, some prescription antiparasite products are very effective for treating New World screwworm infestation. The US Food and Drug Administration has issued either conditional approval or emergency use authorization for three Credelio products (containing lotilaner) and two Nexgard products (containing afoxolaner) for New World screwworm treatment in dogs and cats. Other products are likely to be effective and might also receive authorization later on.
 
What Pet Owners Should Do

• The risk to pet dogs, cats, ferrets, birds, and other warm-blooded animals is currently very low in North Carolina.
• Be aware of the possible risk, especially if your pets have recently traveled to areas affected by New World screwworm.
• Keep your pets on the parasite prevention product that your veterinarian recommends all year round.
• Check your pets regularly for wounds.
• If you think your pet might have maggots or fly eggs, contact your veterinarian before you try to treat the infestation yourself. If it’s New World screwworm, it must be reported to state and federal agencies.

 
For Updates and More Information

• Nationwide: screwworm.gov (US Animal and Plant Health Inspection Service)
• North Carolina: https://cvm.ncsu.edu/news/new-world-screwworm-in-the-u-s-what-it-means-for-north-carolina/ (NC State University)

 
Image source: https://www.aphis.usda.gov/livestock-poultry-disease/cattle/ticks/screwworm/new-world-screwworm-photo-gallery

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