With New World Screwworm threatening the Texas border, veterinarians and cattle producers have questions. Here are evidence-based answers to the three most common.
1. How Long Does Ivermectin Protect Cattle from Screwworm?
Short answer: 16-20 days of residual protection after a single subcutaneous dose.
A key 1987 study published in the Journal of Economic Entomology established that ivermectin at 200 mcg/kg provides effective protection against screwworm larvae establishment for approximately 16-20 days post-injection. This residual effect is why the FDA’s Emergency Use Authorization targets high-risk windows—birth, castration, and wound occurrence—rather than continuous dosing.
Clinical implication: If a calf is treated at birth, a second dose may be warranted at castration if performed more than 2-3 weeks later, particularly in high-risk border areas.
2. Can I Give Ivermectin to Newborn Calves for Screwworm Prevention?
Short answer: Yes. The FDA EUA specifically authorizes IVOMEC Injection for calves within 24 hours of birth.
Newborns are among the highest-risk animals for screwworm infestation. The umbilical stump provides an ideal entry point for female screwworm flies seeking wound tissue for egg deposition.
Dosing for newborns:
- Standard 200 mcg/kg (1 mL per 110 lbs body weight)
- Subcutaneous injection
- Administer within 24 hours of birth
Safety note: Ivermectin at labeled doses has a wide safety margin in cattle. The EUA was issued after FDA determined that benefits outweigh risks in the current emergency context.
What the EUA does NOT cover:
- Female dairy cattle producing milk for human consumption
- Veal calves
- Off-label use in other species (do not extrapolate to dogs—breed-specific toxicity risks exist)
3. What’s the Difference Between FDA EUA and Conditional Approval for Screwworm Drugs?
Three FDA pathways are now active for screwworm products. Here’s how they differ:
| Product | Pathway | Use | Key Difference |
|---|---|---|---|
| IVOMEC Injection (ivermectin 1%) | Emergency Use Authorization (Feb 2026) | Prevention only | OTC, fastest to market, temporary authorization |
| DECTOMAX-CA1 (doramectin) | Conditional Approval (Oct 2025) | Prevention AND treatment | Requires continued data submission, 5-year conditional period |
| EXZOLT-CA1 (fluralaner pour-on) | Conditional Approval (Dec 2025) | Prevention AND treatment | Pour-on formulation, different withdrawal period |
EUA vs. Conditional Approval:
-
EUA (IVOMEC): Issued during a declared animal health emergency. Authorizes use of an existing approved product for a new indication without full efficacy data. Temporary—ends when emergency declaration is revoked.
-
Conditional Approval (DECTOMAX-CA1, EXZOLT-CA1): A 5-year pathway allowing marketing while the sponsor continues collecting efficacy data. More permanent than EUA but requires annual renewal and ongoing studies.
Practical takeaway: IVOMEC is currently the only OTC injectable option. DECTOMAX-CA1 and EXZOLT-CA1 offer both prevention and treatment but may have different availability and veterinary oversight requirements depending on your state.
Key Resources
- USDA Screwworm Program: screwworm.gov
- Report suspicious cases: 1-866-536-7593
- FDA EUA fact sheet: fda.gov/media/190966/download
Related Posts
- FDA Authorizes Emergency Screwworm Prevention for Texas Cattle
- New World Screwworm: Texas Border Alert and What Veterinarians Should Know
Updated February 2026. For current drug availability and protocols, consult USDA APHIS and your State Veterinarian.