By Dr. William Shulaw, Extension Veterinarian, The Ohio State University
(Originally Published in Sheep Team Newsletter September 2007)
Peak worm transmission season has been with us for a few weeks already, and some shepherds in Ohio have already experienced episodes of clinical disease and poor growth caused by worm burdens. For most people, the blood-sucking worm, Haemonchus contortus, or the “barber pole worm” causes this loss. Although the FAMACHA system can be very effective in controlling this worm, it must be properly and regularly applied, and it is extremely critical that the drug being used to treat the individual animals selected be effective.
Because this worm is a very prolific egg layer, worm larvae burdens on pasture can build to dangerous levels very quickly and persist for several weeks. For this reason, and perhaps unfortunate past experiences with severe parasitism, many shepherds have approached parasite control by routine treatment of grazing lambs, and sometimes ewes, with dewormers given every twenty-one to twenty-eight days. If the drug is an effective one, this will usually prevent severe parasitism and may allow for maximal growth. However, it is a prescription for selecting worms that are resistant to the drugs used.
There are other recommendations from the past that are still being advised and used but which, in light of newer information about parasite biology and development of drug resistance, should be abandoned. One of these recommendations is the so-called “treat and move” strategy. This technique is especially useful for young growing lambs, and its effectiveness was demonstrated by research here in Ohio in the early 1980s. It involves treating all the lambs in a group and then moving them to a “safe” or “clean” pasture which is defined as one with no, or very low numbers of, worm larvae on it. This can be a hayfield that has been harvested and allowed to regrow, a pasture that has had cattle on it earlier in the grazing season, or one that hasn’t yet been grazed by sheep. Lambs with very low worm burdens that are placed on pastures that have no worm larvae on them will remain relatively uninfected for several weeks to several months. This is an ideal situation for the lambs and for the shepherd, but unfortunately, we now know that it too can be a powerful force for selecting for drug resistant worms.
It works like this: No dewormer is truly 100% effective and some worms survive treatment. In addition, we now know that genes for drug resistance exist in the important worm species in virtually all domestic sheep populations across the world. The proportions of worms carrying these resistance genes vary from flock-to-flock; but they are there and we can select for them. 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.
Current strategies to prevent this from happening and to help maintain the long-term effectiveness of available dewormers revolve around the concept of retaining a portion of the worms that do not have the resistance genes and allowing them to reproduce preferentially. This is a difficult concept to “sell” to many sheep farmers; especially those who have enjoyed the benefits of modern dewormers these past 40 years or so. However, it is quite clear that we are approaching a time when we will no longer have that luxury.
The first of these strategies involves a targeted approach to treatment such that only the animals that are shedding the highest numbers of eggs and which may be the most burdened by worms are treated. The FAMACHA scoring system does this using a patented color chart that reflects the amount of red blood cell loss in an animal. It is specific to Haemonchus contortus only, but is quite useful here in Ohio. When this system is used, typically only 20-40% of animals will need treatment; unless they are already experiencing severe parasitism before the planned move. This allows the untreated animals, which are usually carrying relatively low worm burdens, to repopulate the next pasture with worm genetics that are similar to those before treatment. (for more information on FAMACHA see http://sheep.osu.edu/category/parasites/) Another approach is the so-called “delay the move after the dose” strategy. This allows the treated animals to become lightly re-infected before going to the clean pasture. This helps ensure that contamination of the new pasture will occur with larvae from worms that have not had drug selection pressure put upon them. This may be especially useful if signs of parasitism, such as anemia or bottle jaw, have already appeared in lambs before moving to the clean pasture. The number of days to graze the infected pasture before moving depends on how heavily infected it is and the relative susceptibility of the animals (lambs versus less susceptible non-lactating ewes), but in general 4-7 days of grazing offers a useful compromise. It is important to note that deworming with moxidectin will not allow this strategy to work because of its persistent activity in killing incoming larvae from pasture that can be as long as 35 days.and articles from past issues of the Sheep Team Newsletter at
A variation of this strategy is to “move then dose.” This means grazing the new pasture a few days before deworming to allow some contamination to occur. Less information is available to recommend the length of time to graze before treatment and it will depend on the level of egg shedding when the animals are moved. Animals with severe parasitism caused by Haemonchus can be shedding tremendous numbers of eggs so the pasture can be come contaminated relatively quickly, and they may suffer more stress from the move. In most cases it should be safe to wait a week before treatment if the animals are apparently healthy.
Other strategies researchers have studied include using weight, weight change over time, or body condition score as a determinant of which animals need deworming. Details for using these strategies may become clearer with time, and they may be most useful for parasite infections that are not predominantly caused by Haemonchus. Because Haemonchus infections can build to dangerous levels quickly and animals (especially lambs) can die when they are still in good body condition, weight or weight change may not be reliable in areas where this parasite is the predominant one.
With the rather widespread occurrence of drug resistant worms across the world, and the specter of having no new chemical classes available in the foreseeable future, parasitologists and forage specialists are working diligently to devise new strategies that do not rely heavily on chemical dewormers to assist sheep producers in achieving sustainable control of parasitism. Please consider how you might integrate some of this newer information into your management plan. You can contact your veterinarian or a member of the OSU Extension Sheep Team for more information.