Livestock Management Considerations for New World Screwworm
Authors: Thomas B. Hairgrove1, Jacob W. Thorne2, Ron Gill3, Andy D. Herring4, Phillip Kaufman5, Sonja L. Swiger5
Introduction
The New World screwworm (NWS), Cochliomyia hominivorax, once eradicated from the U.S. in 1966, remains a threat due to potential reintroduction along the southern border.
These parasitic flies cause severe wounds by feeding on live tissue, leading to rapid health declines and even death in livestock. Proactive livestock management and seasonal planning are essential in reducing the risk of NWS infestation.
For more detailed background and biological information, refer to the full technical bulletin, “Rethinking Livestock Management to Consider Screwworm” (July 2025).
Understanding the Threat
- NWS flies lay eggs in wounds, including those in mucous membranes.
- Larvae burrow into live tissue, leading to a condition called myiasis.
- NWS typically does not transfer from infected animals to others, however larvae will feed on the same animal for 5-7 days before dropping off to complete their life cycle in soil.
- Spread occurs mainly through human transport of infected animals.
Seasonal Management Recommendations
NWS activity is temperature-dependent:
- High risk: 80°F+ with 30–70% relative humidity (life cycle in 2–3 weeks).
- Moderate risk: Temperate weather (life cycle in 3–4 weeks).
- Low risk: Below 59°F (life cycle in 2–3 months or less active).
To reduce exposure:
- Align breeding and birthing seasons with cooler months to avoid peak fly season.
- Avoid year-round exposure by defining breeding periods.
- Winter births may reduce risk and result in stronger post-weaning prices.
Management Practices to Adjust
Many common livestock procedures create wounds attractive to NWS flies. If these practices fall during high-risk months, increase observation and treatment.
Table 1. Schedule Production Practices to Reduce NWS Threat
| Low Fly Season (Cooler) | High Fly Season (Warmer) |
|---|---|
| Calving/lambing/kidding Castration Ear tagging or marking/branding Dehorning, tail docking Shearing, implant placement | Reconsider any elective wound-causing tasks Apply fly repellent at working facilities Increase frequency of livestock checks Promptly treat all wounds Cover surgical wounds if possible |
While some management practices can be confined to seasons with lower fly activity, it is not realistic that every operation can immediately reschedule management during these times. The important take-home message is that if flies are active and livestock have wounds of any size, diligence in monitoring for pests and utilizing best management practices to promote healing are critical.
Monitoring and Surveillance
- Regularly check animals after any procedure.
- Watch for odor, tissue damage or signs of myiasis.
- Be especially observant with fiber animals due to nicking during shearing.
A good practice for all livestock owners is to have a checklist of biosecurity measures.
Table 2. Livestock Producer Biosecurity Checklist
| Identify a regional veterinarian you can contact. | Surveillance: Be vigilant, stay alert and report abnormal or suspicious events. | ||
| Consider transportation schedules of animals relative to operation type. | Check the coats, ears and feet of livestock/guard dogs and companion animals. | ||
| Build a relationship with your AgriLife Extension county agent. | Build relationships with your neighbors for open conversations about suspicious events. | ||
| Use best management practices from quality assurance programs. | Observe animals regularly after castration or other surgical procedures, until completely healed. | ||
| Analyze your risks and develop a plan of action. | Monitor for ticks and other external parasites. | ||
| Monitor wildlife populations, hunting and recreational activities. | Keep squeeze chutes, alleys, etc. in good repair to prevent accidental wounds. |
Conclusion
Effective management of the New World screwworm risk requires aligning your livestock production practices with seasonal fly activity. When possible, schedule higher-risk procedures — such as castration, tagging or shearing — during cooler months when fly activity is lower. Maintain a regular surveillance routine to monitor animal health and wounds and respond quickly to any signs of infestation.
If you suspect New World screwworm in your livestock, do not wait. Contact the following authorities immediately:
- Texas Animal Health Commission (TAHC) for livestock and pets: 800-550-8242.
- Texas Parks and Wildlife Department (TPWD) for wildlife: 512-389-4505.
You should also contact your private veterinarian to assist with diagnosis and help coordinate sample collection and treatment. Prompt reporting and treatment are key to preventing the spread of NWS and minimizing long-term damage to the health of your herd and the livestock industry as a whole.
For complete management guidance and additional background, refer to the July 2025 technical bulletin: “Rethinking Livestock Management to Consider Screwworm.”
1Professor and Extension Veterinary Specialist, Department of Animal Science, College of Agriculture and Life Sciences, Texas A&M University, Texas A&M AgriLife
2Assistant Professor and Extension Sheep and Goat Specialist, Department of Animal Science, College of Agriculture and Life Sciences, Texas A&M University, Texas A&M AgriLife
3Professor and Extension Livestock Specialist, Department of Animal Science, College of Agriculture and Life Sciences, Texas A&M University, Texas A&M AgriLife
4Professor, Department of Animal Science, College of Agriculture and Life Sciences, Texas A&M University, Texas A&M AgriLife
5Professor, Department of Entomology, College of Agriculture and Life Sciences, Texas A&M University, Texas A&M AgriLife
