Black and white steer standing in a pen looking at the camera

Stay informed about New World screwworm

By Deana Hardee, DVM, Technical Services Manager, Merck Animal Health

The U.S. cattle industry remains on edge as the threat of New World screwworm (NWS) moves toward the U.S.-Mexico border. Since NWS was eradicated from the U.S. in 1966, many cattle producers have never experienced the parasite’s effects.

The U.S. Department of Agriculture (USDA) released comprehensive steps to combat the spread of NWS, but it’s up to cattle producers to educate themselves on how to protect their herds if NWS crosses the border.

Image courtesy of APHIS

Understanding NWS life cycle

Female flies lay eggs in a shingle-like pattern around the edges of wounds or near mucosal openings on an animal’s body. The nose, mouth, genitalia and open wounds are prime targets. Even a wound as small as a tick bite can attract a female NWS fly.

After eggs hatch 12 to 24 hours later, NWS larvae (maggots) attack their host. Larvae feed on the animal’s live tissue, burrowing in a distinctive screw-shaped pattern that gives the parasite its name. An NWS infestation is painful for the animal and causes significant economic losses. Severe infestation can be fatal.

After feeding on the live animal for five to seven days, NWS larvae drop off the host and burrow into the ground to pupate. NWS flies emerge from the pupal stage in as few as six to eight days, or up to 54 days, and the cycle begins anew.

NWS flies prefer hot, humid environments. Cooler temperatures slow the activity of this devastating pest. NWS pupae cannot survive soil temperatures that drop consistently below 46º F.

Know the signs of NWS

Cattle producers in regions where NWS is suspected should carefully monitor their property and animals.

The following cattle symptoms may call for further evaluation and treatment¹:

Irritated or depressed behavior
Head shaking
Presence of maggots in wound(s)
Bloody discharge or a foul odor from wound(s)

NWS larvae can burrow deep into living flesh. A small wound may have a large pocket of larvae lurking beneath the skin’s surface. Other species of maggots may be visible on top.

Suspect NWS in your cattle?

Cattle producers who suspect NWS infestation should contact their veterinarian and/or state animal health officials immediately to report and develop a plan for prevention and treatment. Visit http://www.screwworm.gov/ for more information on reporting.

Carefully clean the wounds of affected cattle and remove all visible larvae. Consider an antiseptic to prevent secondary infections. Your veterinarian may prescribe an approved treatment to eliminate any remaining larvae.

Protect your cattle against NWS

EXZOLT® CATTLE-CA1 (fluralaner topical solution) from Merck Animal Health is an innovative new tool to protect cattle against NWS. With a novel mode of action and no known resistance, EXZOLT CATTLE-CA1 delivers fast-acting protection and is the first and only ectoparasiticide conditionally approved by the FDA for both NWS and cattle fever tick in cattle. The prescription product is administered as a single-use pour-on. It circulates in the bloodstream, breaking the parasites’ life cycle to defend cattle against infestations.

Management is a key part of your NWS prevention plan. Remember, female flies lay eggs around open wounds, so infestations are more likely following procedures like dehorning and castration.¹ Newborn calves’ navels are also susceptible. Be strategic about management practices that could result in animal wounds. Integrated pest management combines common sense practices to manage the fly. Considerations for NWS include the life cycle, seasonality, wound creation/management, facility upkeep, chemical or management fly control methods, biological fly control methods (sterile insect technique) and much more.

For more information, consult with your veterinarian or visit EXZOLTCATTLE-CA1.com.

References

U.S. Department of Agriculture. Animal and Plant Health Inspection Service. New World Screwworm. December 2025. https://www.aphis.usda.gov/livestock-poultry-disease/cattle/ticks/screwworm
U.S. Department of Agriculture. Animal and Plant Health Inspection Service. New World Screwworm: Be aware and prepare (Information for veterinarians). April 2025. https://www.aphis.usda.gov/sites/default/files/factsheet nws-private-veterinarians.pdf

IMPORTANT SAFETY INFORMATION

Conditionally approved by FDA pending a full demonstration of effectiveness under application number 141-617.
It is a violation of Federal law to use this product other than as directed in the labeling.
Not for use in humans. Keep out of reach of children. Accidental exposure may cause skin and eye irritation. Highly flammable. Keep away from heat, sparks, open flame or other sources of ignition. Use only for the prevention and treatment of infestations caused by New World screwworm (Cochliomyia hominivorax) larvae (myiasis) and treatment and control of cattle fever tick (Rhipicephalus microplus) in beef cattle 2 months of age and older and replacement dairy heifers less than 20 months of age. Not for use in bulls intended for breeding 1 year of age and older, dairy calves, and veal calves. Cattle must not be slaughtered for human consumption within 98 days of treatment. If cattle are continuously exposed to temperatures at or above 60° F after product administration, then cattle may be slaughtered for human consumption 44 days after treatment. Violative residues may result if cattle are exposed to temperatures below 60° F after administration and are slaughtered at 44 days. For complete safety information and product dosing instructions, refer to the product label.