Parasite Management By: Guest Author Richard Ehrhardt Ph.D.

I have been working in the area of parasite management in the North Central Midwest for the past 5 yrs and as a producer/academic in upstate New York before that.   I agree that parasite management is fairly simple if you do not graze lactating ewes and lambs.  Large flocks (500-2500) that I work with in the North who have abandoned grazing ewes and lambs and changed their production system to graze only dry ewes have far fewer problems with parasite infection.  This is because dry ewes have strong immunity to parasites whereas lactating ewes and especially lambs do not.   This is a profitable mode of production for some of these producers but not all.  We also have flocks in this region that are profitable but have an entirely different mode of production that minimize input costs (buildings, labor, equipment) and keep lambs and ewes on pasture as a back bone of their production system..  What we have learned is that both modes of production can be profitable if carefully managed and like anything, unprofitable if poorly managed.

I also agree that parasite management programs need to be specifically tailored to individual farms and should involve a DVM to be successful and properly and legally executed.  Each farm has a unique resource base, feeding system (grazing or otherwise), management system, etc. so a one-size-fits-all approach is obviously not going to work.   There are however some basic guiding principles and facts to understand in developing these programs.

  1. Drug resistance in gastrointestinal nematode  populations infecting sheep is extremely common and widespread in the Northern USA as is in other parts of the country and World.   We have documented resistance to every chemical class in large flocks in Michigan and reports from large flocks in Ontario are the same (northern regions but perhaps more humid than the western Midwest region).  To be more specific ivermectin has the same efficacy as does treating with water in many flocks in the North.  Each farm has a unique pattern of resistance so ivermectin resistance although common, is not an absolute for all just as for the rest of the products within chemical classes.   This work has involved careful and meticulous use of validated methods of measuring resistance.
  2. It is widely accepted concept that failure to maintain a refugia population is a major factor in the acceleration of drug resistance.  This concept is nearly uniformly accepted is by DVMs, veterinary parasitologists  and animal management experts from throughout the world.  There is however considerable debate on the best and most feasible way to do this. Frequent treatment  and treating all animals when there is no untreated parasite  population in sheep OR pasture is clearly accepted as a major means by which resistance accelerates.   FAMACHA is one way to maintain refugia and may be appropriate for some systems.  I agree that it is not a solution for all farms because of feasibility issues and challenges in early detection in lactating ewes and lambs.  However it is a valid system for some and should not be summarily discarded.  All flocks need a refugia maintenance plan regardless of production system and these should be developed by DVMs and health and management experts on a farm by farm basis.  Refugia maintenance is a principle embraced by progressive crop farmers as well to combat herbicide resistance so this approach is not unique to livestock industries.

Hopefully, I have persuaded you that education and research in this area is very important and that this work needs to involve successful practitioners like Dr. Kennedy who have undeniable experience in sheep production medicine. Without this connection, any potential impact will be compromised.



Richard Ehrhardt  Ph.D.
Small Ruminant Specialist
Michigan State University Extension

About Ask-a-Vet Sheep

Veterinary services, procedures, biologicals, and drugs mentioned in this publication represent the personal opinions and clinical observations of the contributing author. They are in no way intended to be interpreted as recommendations without the consent of the producer’s own practicing Veterinarian. We strongly urge that producers establish a patient-client-veterinarian relationship that allows extra-label use when there are no drugs approved for treatment or if approved drugs are not effective. This procedure allows veterinarians to go beyond label directions when “prudent use” is necessary. The limited availability of drugs and biologics in this country is a major factor in restricting the growth of the sheep industry and allowing producers to compete in the world market place.
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