OSU Sheep Team

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

(Originally Published in Sheep Team Newsletter September 2007)

The single biggest threat to animal health and welfare on sheep and goat farms that use pastures as a significant part of lamb and ewe nutrition (and doe and kid nutrition) is internal parasite infection.  Although for the past 40 years producers have been able to minimize this threat with the routine use of highly effective dewormers that became available in the 1960s, it is now clear that on many farms most, if not all, of these dewormers are no longer highly effective as a result of the appearance of drug resistant worms.  As we learn more about how resistance to dewormers develops and the how the genes determining it are transferred, it has become clear that some of our older recommendations for dewormer use are no longer valid or perhaps even dangerous.

One of these, the “treat and move” strategy for lambs was discussed in an earlier article.  This approach involves sending treated lambs to a pasture that is free of, or nearly so, worm larvae.  The treated animals will stay relatively uninfected for several weeks, but this approach is a powerful force in selecting for drug resistant worms.  Treating all the lambs in a group and then moving to a safe pasture allows the survivors of treatment to enjoy a reproductive advantage.  In most cases, it is likely these survivors will be the ones carrying the resistance genes.  Their progeny will then develop on the new pasture with little or no competition from worms that do not have the resistance genes.  Depending on the season and weather, the immune status of the sheep, the stocking density, and length of time the new pasture is grazed, the resistant worms in those animals can build to significant numbers and create a pasture capable of making considerable change in the gene pool of the farm’s total worm population.

Another recommendation from the past that is now considered potentially dangerous is the routine treatment of ewes “in the jug” or at lambing time in the barn before turnout to pasture.  This practice can also select for drug resistant worms, especially Haemonchus contortus.  It is somewhat like the “treat and move” strategy in that survivors of treatment are more likely to be the resistant ones, and they are the ones that will serve to repopulate the next pasture with worm larvae.

In our climate where we often have some cold dry winters, Haemonchus contortus survives the winter primarily as larvae in a state of arrested development (sort of a suspended animation) embedded in the wall of the sheep’s fourth stomach, the abomasum.  It doesn’t survive as well over the winter on pastures as some other species of worms.  These arrested Haemonchus larvae resume development to the adult stage in the spring, especially around lambing time, and their eggs then seed the pastures for the next season’s life cycles.  This is the principal source of this worm for lambs, and because Haemonchus is a prolific egg layer, the buildup can become sufficient by June to cause severe parasitism and death even in ewes.

If all the ewes are treated in the barn before turnout, then most of the farm’s Haemonchus population is the survivors of the treatment in the ewes’ abomasum.  If the ewes then go to typical pastures where there are some other over wintered worm larvae, but few or no Haemonchus larvae, those surviving worms are the ones who repopulate the pastures with their larvae.  To the extent that these survivors of treatment in the barn carry resistance genes for dewormers, the farm’s worm population becomes shifted toward resistance.

Avoiding this problem requires implementing some strategy that will allow a few worms that have not had exposure to the dewormer to survive in order to produce eggs and larvae that will compete with those of the survivors of treatment.  As an alternative to deworming, ewes and lambs can be placed in a clean pasture and then moved to a second pasture before a severe buildup occurs.  This usually takes at least 4 weeks during the spring in our climate.  If additional clean pastures are available to move the ewes and nursing lambs to at the time a buildup is expected, this will break the cycle of buildup and protect both lambs and ewes.  However, many producers do not have this luxury and are somewhat unwilling to allow untreated ewes to go to pasture with their lambs because they are aware of the dangers.  Some strategies they could use that help avoid selection for drug resistant worms include treating only a portion of the ewes such as just the thin animals.  Some work in Europe and Australia suggests that leaving only 5-10% of the animals untreated may provide a large enough source of unselected worms to repopulate the pasture and still not heavily contaminate it.  This author is unaware of similar work in the USA but similar proportions may be sufficient.  Alternately, ewes could be moved to pasture and deworming delayed 4-7 days to allow some contamination to occur from unselected worms before treatment.

Another recommendation from the past that has recently come under scrutiny is the practice of deworming all the ewes during flushing or just before breeding.  Usually this is done in the late summer or early fall for winter lambing ewes.  The concern about his practice is as follows.

For most of the year, adult ewes have relatively high levels of acquired immunity to worms as a result of exposures during the grazing season. The exception is around lambing time and during lactation when this immunity is relaxed, or weakened, as we have discussed.  Normally, this immune response returns quickly after the lambs are weaned, and egg counts and worm burdens fall rather dramatically.  At breeding time, most ewes will have low worm burdens and low fecal egg counts because of their immune response.  New larvae acquired from grazing an infected pasture may not become egg-laying adults in ewes with this strong immune response which helps maintain this low worm burden.  Treatment of these ewes at this time tends to destroy the susceptible parasites leaving the resistant ones.  Because the ewes’ immune system may inhibit the establishment of new worms they ingest from pasture, the survivors of treatment will enjoy a prolonged period in which they are the principal producers of eggs that become infective larvae on the pasture.  This has the effect of selection for resistance on the farm worm population. Larvae developing from eggs deposited in late summer and fall are better able to survive over winter than older larvae, and they will be available when weather favorable for transmission begins again in the spring.

Well-fed ewes in relatively good body condition will probably not receive much of a benefit to a deworming at this time.  Therefore, some parasitologists now recommend that only thin ewes, immature ewes, or ewes with evidence of clinical disease be de-wormed at breeding time. Like the FAMACHA system, this is a targeted, or selective, deworming approach that allows some worms that have not been exposed to dewormers to survive in order to reduce selection pressures that lead to increased resistance.

It should be obvious from these two articles that if we are to effectively combat the increasing threat of drug resistant worms and prolong the effectiveness of those dewormers available to us, we must rethink how we use them in light of basic parasite biology and new knowledge of how resistance develops.  Sustainable parasite control will undoubtedly involve some use of dewormers for the foreseeable future, but we must be careful with them and integrate other management strategies in our control programs.