Dewormer Products Available for Virginia Cattle

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Dr. Dee Whittier, Extension Veterinarian, Beef Cattle VA-MD Regional College of Veterinary Medicine

The Food and Drug Administration announced July 30 the approval of a new dewormer product that will soon be marketed in the United States by Pfizer, Inc. The new product contains doramectin, an avermectin related to ivermectin which has been available for nearly 15 years in the US as Ivomec". Ivomec" has been the only dewormer in its class available in the US but there are several other products that are available worldwide.

The table below (see printed copy) lists dewormer products commonly used in Virginia along with some important characteristics of each. The table notes that doramectin, like ivermectin, has very good activity in killing most of the internal parasites or worms that affect cattle in the US. In addition it kills sucking lice and cattle warbles or grubs.

Doramectin also has what is termed a "residual effect" that is greater than any product currently on the market in the US. The term "residual effect" refers to the action that a dewormer may continue to exert after it has been given. The dewormers that were available initially had very limited length of effect. Most of these products , even though they did a great job of killing parasites that the cattle had on board, were gone from the system in a day or two. This meant that cattle that returned to contaminated pastures soon became re-infected. This means that in the sequential spring deworming program that we have recommended for yearling cattle these dewormers had to be given every three weeks for three times (0,3,6) in order to clean pastures and allow for optimum season-long freedom from worm damage. The three-week interval comes from the three week period that the worms have for development from consumed larvae to egg-laying adults. With Ivomec" the program could be revised to allow for a 5 week interval between treatments because the two week residual effect could be added to the three week development time. This has made the Ivomec" program into the 0,5 program requiring only two treatments.

The Dectomax" product is expected to have a label claim for 3 to 5 weeks of residual effect. What changes will this provide in recommended treatment programs? Some aspects are obvious while others will probably require some research in our area. The interval between recommended treatments for yearling cattle will certainly be greater at 6 to 8 weeks. Will a single treatment provide season-long control? I think this is an unanswered question that will require research in Virginia. Will the Dectomax" product show some advantage in performance over other products for mid-season treatment of spring born calves? Again, research will be needed.

These unanswered questions should encourage us to continue finding the best ways to use this new product and the other excellent products available in the most economically sound ways to provide the best profits from effective deworming programs.