OSU Sheep Team

By William Shulaw, Extension Veterinarian, The Ohio State University

(Originally Published in Sheep Team Newsletter June 2007)

Recently an article on the detection of glyphosate resistance in giant ragweed in a neighboring state was sent by email to many of us in OSU Extension.  Glyphosate is the active ingredient of Roundup and several other products now on the market.  It has been, and remains, an extremely useful herbicide for farmers, gardeners, and homeowners.  The article discussed how resistance to a chemical like this develops and ways to both manage resistant weeds and to reduce the potential for resistance occurring on farms.  This was the most recent of several articles I have read on glyphosate-resistant weeds, and they have reminded me of the similarities with development of worms that are resistant to anthelmintics (dewormers) in sheep and goats.  Perhaps quotes from this article can help us understand a little about parasite biology and prevention and management of resistance:

*       ”Glyphosate-resistant giant ragweed was found in one county last year, but as of this spring, resistant varieties are now located in at least 10 counties.  The rapid-growing weed has the ability to significantly reduce yields, and because varieties resistant to glyphosate are becoming more common, it’s going to become increasingly difficult to manage for growers.” Anthelmintic resistant worms are a global problem and are being increasingly diagnosed in US sheep and goat operations.

*       ”One of the reasons why Roundup Ready technology is so popular is because when glyphosate – the active ingredient in Roundup – was developed, it was very affective against giant ragweed and the only economical tool for managing ALS-resistant giant ragweed,” he said. “Now that the weed is developing resistance to glyphosate, we currently have very few options to deal with it.” The first really modern chemical dewormer class to be introduced was thiabendazole in the early 1960s.  By 1964 the first reports of resistant worms in sheep had already been published.  Then came levamisole in the early 1970s and resistant worms were reported by 1979.  Ivermectin was introduced to the world in 1981 and reports of resistance were already being published by 1988.  The ivermectin-containing product approved for sheep was introduced in the US in the early 1990s and reports of resistance in goats in Texas were presented within a year indicating that widespread use of ivermectin-containing products approved for other species had probably created this situation.  These new products were much more effective than phenothiazine, and the older, more dangerous compounds like nicotine, and producers welcomed them because they made the job of controlling internal parasites simpler and cost effective.  Routine use also allowed the maximum effects of genetics and nutrition to be achieved.  For years, producers had a new chemical class each decade to replace the older ones that had become less effective.

*       ”No new herbicide modes of action have been introduced in the last 25 years and there are no new ones in the pipeline. ” Parasitologists are telling us that the situation for sheep and goats is essentially the same.  We can no longer rely heavily on chemical dewormers for control of parasitism.

*       ”We suggest using at least two tactics or herbicides for dealing with the most troublesome weeds”  “Reliance on one chemical helps weeds develop resistance and also lessens the odds that your weed-management efforts will be effective for the entire season.” Frequent exposure to the same chemical is the most common reason for development of resistant worms.  This puts selection pressure on the worm population and favors survival of worms that may be genetically resistant.  Although the proportion of genetically resistant worms in a flock may be very small compared to the overall worm population, it can be selected for just as we select for certain production traits.  Parasitologists are now questioning whether strategies for rotating among the classes of dewormers are beneficial, but there is NO question that the frequent use of dewormers, especially when they are administered to ALL animals, is a very powerful selection force for resistance.  The practice of routinely deworming all animals at monthly intervals is a recipe for selecting resistant worms.

*       ”This summer a multidisciplinary team of researchers and Extension specialists will try experiments in some 20 to 30 counties to determine best practices in weed management.  The strategies will include use of different herbicides, sprayer application factors, and learning more about the genetics of resistance.” Although resistance genetics in worm species are incompletely understood, some researchers believe that ivermectin resistance in Haemonchus contortus, possibly the most important worm we have to worry about in Ohio, is a dominant trait.  Practically speaking, this means that not only is a female worm with one copy of the gene resistant, but more importantly, half of the 3000-5000 eggs she produces daily will produce resistant progeny no matter the genotype of the male worm she mates with.  If she is homozygous for the trait (has two copies of the gene) all her progeny will be resistant.  The situation appears to be different for levamisole, where resistance in H. contortus is a believed to be a recessive trait.  If this is correct, two copies of the gene are needed for a worm to be clinically resistant to levamisole.  This may explain why levamisole resistance appears to be less common in sheep and goat flocks than ivermectin resistance; it is harder to select for.

Regarding management and prevention of glyphosate-resistant ragweed, the article highlights some recommendations that also have similarities with recommendations for management of resistant worms.

“In the meantime, some good rules of thumb regarding glyphosate use for weed control include:”

*       ”Applying the correct rate of glyphosate based on weed size.” Another important factor in selecting for resistant worms is under dosing dewormers.  Under dosing frequently happens when animals are dosed according to an average weight for the group.  Effectively this under doses half the animals.  Dewormers should be dosed according to an accurate body weight if possible.  Ivermectin drench and the benzimidazoles (Valbazen in sheep and Safeguard in goats) can probably be given safely when dosed for the heaviest animal in the group, provided there aren’t wide extremes, but levamisole should be more accurately dosed as there is a more narrow safety margin.  Other ways dewormers may be under dosed include, incorrectly calculating the volume to use; incorrectly diluting products that must be mixed with water (levamisole drenches); and using equipment that is not calibrated, or faulty, such that the correct amount isn’t delivered.  Equipment should be checked every time it is used to be sure that it is delivering the correct amount of product.  This can be easily done by checking the delivered amount with an old plastic syringe with the plunger removed.

*         “Starting with a clean field.” Strive to use pastures that have low levels of worm larvae for animals that are at greatest risk of severe parasitism such as lactating females and weaned lambs.  It is especially helpful to turn lactating ewes into a clean pasture after the lambs are about two weeks old.  They are the main source of worm larvae for their lambs, and the periparturient rise in egg counts in ewes lasts about 4-8 weeks after lambing.  Moving to a clean pasture when the lambs are relatively small helps reduce exposure to both.  Moving weaned lambs to clean pastures such as spring seeded annuals, hayfield re-growth, or pastures previously grazed by cattle, provides them some protection against heavy challenge for at least three weeks and possibly 8-9 weeks depending on their age and previous exposure.  However, it is NOT a good idea to de-worm ALL the animals and then move directly to a clean pasture because that approach takes only survivors of the treatment to the clean pasture and fosters selection for resistance.  Selectively deworming individual animals using the FAMACHA technique, or deworming only thin animals, allows animals harboring small numbers of non-selected worms to seed the new pasture and reduces the rate at which resistance develops.  Alternatively, deworming could be done a few days after the move which would allow pasture seeding with worm larvae that had not had the chemical exposure.

*         “Just because a weed does not seem to be affected by glyphosate does not mean that it’s resistant.” If deworming did not appear successful, logical questions to be asked are: Was the dewormer properly diluted and administered?;  Was the equipment properly calibrated to deliver the correct dose?  In addition, use of a dewormer when the pasture is heavily contaminated may lead to the conclusion that the drug was ineffective when the real reason was that the animals become heavily re-infected shortly after dosing.  Under such pasture conditions, it is not uncommon for losses to continue in spite of effective treatment.  In these cases, the animals are only helped by moving them off the heavily contaminated pasture.

*         “There are also some weeds that have a natural or inherent tolerance to glyphosate, such as velvetleaf.” If you are seeing worms in the feces, they are possibly tapeworm segments.  Tapeworms are not affected by ivermectin and levamisole but fortunately, they are not a significant source of loss for most sheep producers.  The important worms like Haemonchus are not visible in the manure.

The weed article continues, “If weeds continue to crop up, there are many factors that producers can check in order to determine if resistance is a problem in their field.  Those include:”

*         “Noting whether rain could have washed the herbicide away prior to plants absorbing it.” Relevant questions for the sheep producer might include: Did the sheep spit it out?  This is especially applicable if a bolus form of dewormer was used, but careful administration of drenches is also important to be sure the animal got the full dose.  Was the dewormer properly administered?  Current recommendations for drenches are to be sure to place the dewormer in the back of the mouth as this helps insure that the drug goes into the rumen.  If it is deposited in the forward portion of the mouth, it is possible that much of the drug will end up in the abomasum and the drug will be less effective than it should be.

*        “Determine if only one species seems unaffected but all other [weed] species in a field are controlled.” It is critical that producers KNOW that the dewormer they are using is effective to avoid disasters.  Currently the only ways to do that are by conducting a fecal egg count reduction test with their veterinarian or by utilizing the services of the University of Georgia’s veterinary parasitology laboratory which performs a larval development assay to test for dewormer resistance http://scsrpc.org/SCSRPC/Files/Files/D’Rite%20Scsrpc11-05.pdf .  They should realize that if these tests are conducted in mid-summer, Haemonchus egg output may completely overshadow that of other worms making it difficult to determine whether the other worm species are also resistant.  This may not be critical for many flocks whose main problem is with the blood feeding Haemonchus, but could be relevant in some parts of the US.

*         “Finding out if other farmers in the area are experiencing similar problems.” Whether or not your neighbor has drug resistant parasites in his sheep is not important for you find out unless you bought your sheep (or goats) from your neighbor!  Resistance genes for dewormers are widespread in parasites in sheep and goat populations although in many flocks their prevalence has not yet reached the critical level where dewormers are completely ineffective.  However, purchase of new animals is a common way to introduce resistance genes.  If producers are purchasing replacement animals, they should isolate them in a barn or dry lot where worm eggs from resistant worms cannot contaminate pastures where the home flock will graze.  Ideally, fecal samples for quantitative egg counts should be taken at the beginning of a quarantine period, and the new arrivals should be dewormed with at least two of the three chemical classes of dewormers.  Fourteen days later, fecal egg counts should again be done to determine whether resistance might be present and whether significant egg shedding is still occurring before these animals are allowed to graze the pastures.

The article also refers to the use of soil-applied residual herbicides to help in the control of glyphosate-resistant ragweed.  Unfortunately, there are currently no treatments farmers can apply to soil or pasture that have been shown to control parasites.  However, research has demonstrated the feasibility of using a special type of fungus which traps and kills worm larvae as they hatch from the egg.  An Australian company has expressed interest in commercially marketing such a product but it is not yet available.

Sustainable parasite control, like weed control, requires knowledge of biologic principles and an integrated approach using several strategies.  For some producers, this may seem too complicated or overwhelming.  It doesnt have to be.  Most producers can use more than one strategy and reduce their reliance on chemical dewormers.  Consult your veterinarian or members of the Sheep Team to discuss possible options you might use.