R-CALF USA Urges Approval of Ivermectin Feed-Through to Prevent Screwworm Outbreak

June 9, 2025

In a formal letter to the U.S. Food and Drug Administration’s Center for Veterinary Medicine (CVM), R-CALF USA’s Animal Health Committee called for swift approval to add ivermectin as an oral feed-through in livestock and wildlife feeds as a precaution against the potential introduction of New World Screwworm (NWS).

The request comes amid growing concerns over the possible northward spread of NWS into the United States. The urgency was amplified after a recent false report claimed NWS had been detected in Missouri, causing a downturn in cattle futures markets.

R-CALF USA expressed support for using feed-grade ivermectin as a proactive and proven measure to protect both domestic herds and wildlife in the southern border regions, particularly near Mexico. The organization noted that this control method has been successfully used to combat the spread of cattle fever tick.

“The potential impact on the United States livestock markets would be immensely negative if an actual case of New World Screw Worm was identified in the U.S.A.,” wrote R-CALF USA Animal Health Committee Chair R. M. Thornsberry, D.V.M. “Until an adequate response with gamma ray irradiation of male screw worm flies can be implemented, the feeding of ivermectin is a very real defense."

Thornsberry added, “R-CALF USA urges the CVM to approve this vital control mechanism without delay to safeguard the U.S. livestock industry from a resurgence of this historically devastating pest.”

Source: R-CALF USA