Protect Your Dairy From New World Screwworm

By Dr. Jamie Jonker

The New World screwworm (NWS), scientifically named Cochliomyia hominivoraxis, is a dangerous pest that can affect livestock and any warm-blooded animal, even humans. This parasitic fly lays its eggs in open wounds, and then the young larvae, often called maggots, feed on the host animal’s living tissue. Recently, NWS has had a resurgence in parts of Central America and Mexico. The United States has taken measures to prevent the spread of screwworm to its borders, but livestock producers should be vigilant about monitoring for this rare threat.

Why is New World screwworm so dangerous?

Many types of flies and maggots feed on dead animal tissue, but the New World screwworm is especially dangerous because NWS maggots use sharp mouth hooks to eat the living tissue of the host. Because of this, untreated wounds will continue to grow and will attract more flies and maggots until the animal is essentially eaten alive.

NWS myiasis, or infestation, occurs when the NWS flies are attracted to open wounds of any kind; animals that have recently given birth, suffered an injury, or had a surgical procedure (such as dehorning or branding) are most at risk. Untreated umbilical cords of newborn animals and foot lesions are commonly infested sites, and even extremely small wounds, like tick bites, are enough to attract NWS flies. Once the flies lay eggs and the larvae hatch, the maggot infestation is extremely painful for the host animal and can be hard to treat. A single infected animal in the herd provides a source for many more flies to infect other animals.