Are you prepared for New World screwworm (NWS)? Originally eradicated from the U.S. in 1966, NWS (Cochliomyia hominivorax) larvae (myiasis) presents a challenge many cattle producers have never experienced firsthand. The parasitic fly can affect livestock, wildlife, pets and even humans, and a modern outbreak is estimated to cost producers $4.3 billion per year.
Although not every operation faces the same level of risk, preparedness remains a shared responsibility. By understanding the risk, recognizing early signs and working with your local veterinarian on reporting and treatment protocols, you can help protect your herd and contribute to the long‑term resilience of the cattle industry.
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. One female fly may lay up to 3,000 eggs during her 10- to 30-day lifespan. The nose, mouth, genitalia and open wounds of warm-blooded mammals 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 can cause significant economic losses. A 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 46ºF.
Know the signs of NWS
To protect your herd and the industry, it is important to carefully monitor your property and animals for signs of infection – especially if you are located in a region where NWS is suspected.
Animals that have recently given birth, suffered an injury or had a surgical procedure, such as dehorning or branding, are most vulnerable. Newborn animals with fresh navels are also at risk. The following symptoms may call for further evaluation and treatment:
- Signs of pain and irritability
- Irritated or depressed behavior
- Headshaking
- Unhealing wounds that continue to grow
- Presence of maggots in wounds or other body openings
- Bloody discharge or the smell of decay
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?
If you suspect an NWS infestation, contact your veterinarian and/or state animal health officials immediately to report and develop a plan for further prevention and treatment. Visit online 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.
Build an NWS prevention plan
An effective NWS prevention plan relies on strong, proactive management to be successful. Integrated pest management (IPM) is an effective and environmentally sensitive approach to pest control that combines various management strategies. It relies on understanding the life cycles of pests and their interactions with the environment to minimize risks to people and the ecosystem. Consider the following IPM strategies when building a prevention plan:
- Education and training: Stay aware of NWS risk in your area and ensure you and your employees know how to identify NWS and the reporting steps.
- Monitoring: Regularly check animals for wounds and signs of NWS.
- Prevention: Facilities should be in good working order and not cause open wounds on cattle.
- Fly control: A solid fly control program for horn and face flies can also help deter NWS.
- Strategic treatments: Apply at castration, dehorning or when creating an open wound if in an NWS area.
Remember, female flies lay eggs around open wounds, so infestations are more likely following procedures like dehorning and castration. Newborn calves’ navels are also susceptible. Remain vigilant with management practices that could result in animal wounds.
Taking action against NWS
If left untreated, NWS can be fatal within seven to 14 days. Prevention and treatment options exist and are available. Your veterinarian can advise on the best protocol for your herd and area.
As the industry continues to prepare for NWS, it is important you remain vigilant and stay informed. Understanding the NWS life cycle and symptoms and working with your veterinarian to create a prevention and treatment plan, you can help the industry combat this threat.
References omitted but are available upon request by sending an email to the editor.
