Stack Dewormers

Thinking Has Changed About Dewormer Programs

Victoria G Myers
By  Victoria G. Myers , Progressive Farmer Senior Editor
Efficacy rates for traditional dewormer programs are a growing challenge. There are solutions. (DTN/Progressive Farmer photo by Becky Mills)

Cattle dewormer programs with less than 90% efficacy, are likely doing more harm than good. Programs should show that much of a reduction in fecal egg counts (FEC) 14 days post-treatment. If they don't they are contributing to parasite resistance.

A new efficacy summary from a national FEC reduction database looking at field efficacy, shows many of today's deworming programs are coming in well below that 90% level.

The average producer may never notice there's a problem, says MidAmerica Ag Research veterinary parasitologist, Don Bliss.

Bliss stresses operations are being robbed every day through parasites' negative impacts on feed intake, average daily gain, milk production, conception rates and immune responses to vaccines and disease.

He notes the truest readings on FEC reduction tests (FECRT) are found when samples are taken from cattle 6 months to 2 years of age. MidAmerica Ag Research uses FECRT data from Merck Animal Health. The company maintains the world's largest FECRT database to monitor efficacy of anthelmintics, and to track how those might change and shift over time.


New data, which averages performance numbers from 2009 through 2018, shows efficacy of pour-on endectocide dewormers, as a category, at 51.3%. This means collection and analysis revealed about half of the eggs found in pre-treatment fecal samples still existed 14 days post treatment. Efficacy percentages by product show the following: Ivomec at 20.8%; Ivermectin at 45.9%, Dectomax at 66%; Cydectin at 72.2% and Eprinex at 22.9%.


Endectocide injectables were comparable in efficacy to pour-ons in this latest report. As a whole, the category had an efficacy rate of 57.4%. By product, rates were: Ivomec at 44.8%; Ivomec Plus at 46.6%, Dectomax at 78%, Cydectin at 83.6%, Ivermectin at 39.4% and LongRange at 64%.


How can producers get to that 90%-plus efficacy level? These control levels are consistently found through the concurrent use of two classes of anthelmintics.

Looking at Safe-Guard and Panacur formulations, overall efficacy was 98.7%. Here is how that breaks down: Safe-Guard Suspension Paste at 98.7%; Panacur Suspension at 98.8%; Safe-Guard Feed at 99.7%; Safe-Guard 1.96% at 98.1%; Safe-Guard Mineral at 95.9% and Safe-Guard Liquid at 95.8%.

The best efficacy percentages in the study were seen with a Safe-Guard or Panacur formulation plus various endectocides. All combinations averaged an efficacy rate of 99% or better. No combination hit the 100% mark.


Harold Newcomb, technical services veterinarian with Merck Animal Health, says combination treatments will help delay the onset of resistance, and keep each class of dewormer working longer than if it were used repeatedly or in a rotation.

He recommends concurrent use of two or more classes of anthelmintics. There are three classes available in the U.S. These include Benzimidazoles (Safe-Guard and Panacur), Endectocides or macrocyclic lactones (Ivermectins) and Imidazothiazoles (Levamisole).

Newcomb stresses it's important to use a full dose of dewormer, based on actual weight. Under-estimating weights and using less than a full dose speeds up selection for resistance.

These steps can add to the cost of a dewormer program, but Newcomb cautions producers that without a good program they may be losing as much as 30 pounds in weaning weights and seeing 10% to 12% lower conception rates in cow herds.

He advises doing a FECRT at time of deworming, and if efficacy is below 90% get in touch with your herd veterinarian to make adjustments.

"We all look at the cost of things," says Newcomb. "But when we look at the cost of implementing an effective deworming program, versus the cost of not doing it, you just have to know deworming pays. It may be the most profitable thing a cattle producer can do. It is the basis of any herd health program. If you get this wrong, you won't get anything else right."


Victoria Myers