I've been hearing for several years that the miracle dewormers, the endectocides, are losing their effectiveness on cattle parasites, but I was seeing no significant data.

Last week all that changed when Merck Animal Health released data from its long-term Fecal Egg Count Reduction Test (FECRT) database.

More than 24,000 samples were evenly divided between pre- and post-treatment, with a 14-day treatment period in the middle. They came from 24 states and more than 250 participating veterinary clinics.

The results show the endectocides, as a group, have dropped into the average range of 51% to 57% effective. Individual product rankings go as low as 21% for pour-on products and 39% for injectables. The endectocides are also known as macrocyclic lactones.

Dr. Harold Newcomb, Merck technical services veterinarian, says this decline in efficacy is surely because these deworming agents have been so "used and abused," including the very likely and common producer practice of underdosing. But this is also the natural outcome of using anything that attacks a "pest" organism. It is a normal part of the natural war that goes on constantly between all organisms.

Merck's data says the other most-used compounds, the benzimidazoles, remain very effective with average efficacy ratings of nearly 99%. No data was offered for the imidazothiazoles.

Merck and its panel of advisors are suggesting a combination deworming application in the future that includes two or more classes of deworming products to be certain few resistant parasites escape and to keep efficacy ratings well above the recommended 90% efficacy threshhold. Using its own products as an example, such as Safe-Guard or Panacur in addition to its own endectocides, provided an average of 99.1% deworming efficacy, Merck says.

Newcomb also suggests producers and their veterinarians should consider DNA testing of internal parasite populations to help target the right products for the species present. This should include a minimum of 20 samples to overcome variations in parasite life cycles and shedding.