Sheep & Goats

 DETERMINING  WORM  RESISTANCE
TO  DEWORMERS

 By:  Dr. William Shulaw, Extension Veterinarian, The Ohio State University

Although it is essential for sheep and goat producers to develop complementary strategies for sustainable parasite control, most will need to use chemical dewormers, at least occasionally or for selected animals, for the foreseeable future.  When dewormers are used, it is crucial that they actually work with a high degree of effectiveness if the control strategy is to be successful.  How do you know the product you used was effective?

There are currently only two ways to determine if the dewormer you wish to use is effective.  The first is the fecal egg count reduction test (FECRT).  This approach estimates the ability of a drug to reduce egg counts in feces compared to a control group.  Past articles in the Sheep Team Newsletter have described this approach (June 2004, http://knox-cms.ag.ohio-state.edu/agriculture/livestock/sheep/sheep-team-newsletter/sheep-team-newsletter-default ) and another excellent resource on the technique can be found at http://www.dpi.nsw.gov.au/agriculture/vetmanual/specimens-by-disease-syndrome/diseases_of_livestock/anthelmintic_resistance .  This method requires 15-20 animals per group for each dewormer tested; including the untreated control group.  Therefore, in small flocks perhaps only one chemical class per grazing season can be tested.  A quantitative egg counting method, like the McMaster method, must be used, and the fecal egg count at the time of treatment must average at least 200-300 eggs per gram of feces for it to be valid.  It is important that the 15-20 animal group size is used because of the wide variation in egg counts typically seen across a group of animals.  For example, in a project we are working on this summer, in one group of 20 lambs fecal egg counts ranged from a low of 600 eggs per gram to 18,000 eggs per gram.  Lambs or ewes can be used for a FECRT, but only one or the other and not mixed groups.  Fecal samples are collected 12-14 days after treatment unless ivermectin or moxidectin is being evaluated where 15-16 days is more appropriate.  The FECRT can be performed by many veterinarians, and the equipment needed is not difficult to obtain or expensive.  If a dewormer is still highly effective on a farm, we expect that the egg count reduction in the treated group will be 95% compared to the untreated control group.

The main drawback to the FECRT is that by the time you can detect developing resistance to a dewormer, the proportion of resistant worms in the total worm population is relatively high, and continued use of the product in traditional ways may result in a rapid increase in the resistant proportion to the point where the drug is virtually useless.  If that point has not already been reached, it will take very selective and careful continued use to maintain a practical level of effectiveness. 

A second way to detect resistance to dewormers is something called the larval development assay.  In this assay, multiple drug classes can be evaluated at one time, and their effectiveness is estimated by determining how readily worm eggs develop to the third stage infective larvae in the presence of a series of increasing concentrations of dewormer.  This is usually done in a plastic plate with multiple small cavities containing nutrients for larval development and the dewormer classes to be tested.  A significant advantage of the technique for the producer is that a single composite sample of fecal material from only 10-15 representative animals can be sent to the laboratory for testing.  Small flock owners can get information about all three chemical classes of dewormers with one set of samples.  Results are available in about 2 weeks

The larval development assay can detect developing resistance in the worm population at an earlier stage than can the FECRT.  This can give a producer a “heads up” that a dewormer must be used very carefully if he or she expects to be able to continue using it effectively.  It can also be used as a monitoring tool to detect changes in resistance patterns over time.  As with the FECRT, the average egg count for the sample sent for the assay must be high enough for the laboratory to harvest enough eggs to put in the plastic plate.  For the most accurate results, samples should not be sent from animals that have been recently treated with a dewormer.

Presently, the only larval development assay available in the United States is the DrenchRite^® assay which is conducted in Dr. Ray Kaplan’s laboratory at the University of Georgia.  The assay requires considerable time and technical expertise and therefore, must be scheduled in advance.  Samples cannot be stored and must be collected and promptly shipped by overnight courier; however, this is not difficult.  There are specific instructions for collecting and packaging the sample, and they are also easy to do.  The contact for arranging a DrenchRite^® assay is Ms. Sue Howell, Department of Infectious Diseases, Room 2212, College of Veterinary Medicine, University of Georgia, Athens, GA 30602; voice: (706)-542-0742.  Additional information on the assay can be found at http://www.scsrpc.org/ under the “Smart Drenching” link. 

Currently, parasitologists recommend testing for dewormer effectiveness about every two years.  Testing will involve some cost, but if you are one of the unfortunate producers whose options have become very limited, it may help avert a costly disaster or a season of very poor performance.  It may signal a need for you to make some major management changes in your sheep or goat operation.