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How Do You Know Your Parasite Control Program Is Working?

June 19th, 2008 · 1 Comment

by William Shulaw, Extension Veterinarian

(Originally Published in Sheep Team Newsletter June 2004)

Seeing is believing, right? Most of us judge the success of our management or animal health programs by the results we observe. If we begin a vaccination program and no cases of the disease we are vaccinating for occurs, we may assume that the vaccine prevented disease. If we treat animals and they get better, we may assume we had the correct diagnosis or that the treatment was effective for whatever was wrong with the animal (or flock). But how trustworthy are our observations?

My Animal Science colleagues seem fond of a phrase that goes something like this: “You can’t manage what you don’t measure.” But measuring implies obtaining a measurement, usually a number, and record keeping with some analysis of the numbers to see if real change took place over some period of time. It also usually implies that there is some baseline measurement to compare to; perhaps for within flock comparison or an industry standard that becomes a flock goal. Taking measurements and recordkeeping aren’t always the most fun, but they are essential if we expect to efficiently manage and document the results of our work. A parasite control program is no exception to this.

Parasitologists and Extension veterinarians have been stressing monitoring the success of your parasite control program and the effectiveness of the dewormers you use for a long time. The only practical way of doing this is collecting samples of manure (fecal material) from sufficient numbers of animals and then performing quantitative fecal egg counts on them in order to estimate the parasite burden being applied to your pastures or to assess dewormer effectiveness. Recent observations this spring in a small flock illustrate the need for getting these measurements.

This flock owner has been collecting data on fecal egg counts, body condition scores, and FAMACHA score, on all the ewes weekly in an effort to document sustainable parasite control strategies that do not rely on regular deworming of all the sheep. The FAMACHA system uses a patented eye color chart to assist a producer in detecting anemia in the sheep. It allows one to de-worm only the most heavily parasitized animals thus leaving the worms in the remaining sheep unexposed to the dewormer. It is believed that selective deworming practices, such as this system, may prolong the useful life of dewormers. (more on this system later) The work began in very early March just before the ewes began lambing.

Fecal egg counts taken in the fall on the ewes were about 70 eggs per gram (epg) of feces, but just before lambing had risen to about 400 epg. This is probably a result of the maturing of worm larvae in arrested development in the abomasum and their subsequent production of eggs near the time of lambing. Parasitologists refer to this as the “periparturient rise” in egg counts and it is well documented in sheep. The ewes were not de-wormed at lambing time. By two weeks post lambing the egg counts on the ewes averaged about 1,000 epg with some as low as 50 epg and one as high as 3000 epg. This would be considered a typical response in lactating ewes.

By April 16, the egg counts averaged about 1,300 epg, some ewes had lost some weight, and the average body condition score was about 2.7; lambs were gaining weight as expected. At this point the decision was made to selectively de-worm those ewes with body condition scores less than three (five animals on the 0-5 scale) or FAMACHA scores of “3” or above (one animal scoring a “3”; scale 1-5) in the interest of minimizing pasture contamination. These six ewes were de-wormed with Valbazen, and they made up the majority of ewes with high fecal egg counts. The remaining eleven ewes were left untreated. Two weeks later, the post treatment egg counts of the treated ewes had actually RISEN by 28%, and the flock average was now 1800 epg with one of the treated ewes just under 6,000 epg. However, there were now no FAMACHA scores above “2” and body condition for the group, including the treated ewes, had generally improved and averaged “3”. By this time the ewes had been lactating about six weeks and had been receiving some corn and soybean meal which may explain the improvement in condition score. In fact, aside from the fecal egg count data, the overall appearance of the ewes and growth rates for the lambs were very good to excellent.

Although the numbers of animals sampled in the treated group was probably too small to definitively conclude that the worms were resistant to Valbazen (albendazole), it is certainly very suspicious. In keeping with the plan to de-worm selectively, the same ewes treated with Valbazen were treated with Ivomec Sheep Drench on May 7. At that time the average egg count for the treated ewes was 2,483 epg and their average body condition score was 2.8. Fecal egg counts for the untreated ewes averaged about 1,100 epg and the body condition score averaged 3.4. Fecal egg counts for samples from the ivermectin treated ewes taken 14 days later averaged 790 epg for an average reduction in egg count, calculated on an individual animal basis, of 72%. However, the egg counts for the untreated ewes now averaged 250 epg – a 79% reduction!! Furthermore, body condition scores for both the treated and untreated ewes now had improved to 3.1 and 3.5 respectively, and FAMACHA scores remained at just over an average of 1 the best score. In case you are wondering, the results at 7 days post treatment show the same trend, but I won’t confuse you with those numbers.

So what would you conclude from these observations? If we just had the overall appearance of the ewes and lambs to judge our management plan by, we would probably think the first Valbazen deworming was a treatment success when, in fact, fecal egg counts in the treated ewes actually continued to rise. Without the fecal egg count data from the ivermectin treatment, we probably would conclude that it worked too. After all, two weeks post-ivermectin treatment, ewe body condition was excellent and the lambs were looking fine. However, a mere 72% reduction in fecal egg count in the treatment group would normally suggest, if we had more animals in the treatment group, that significant resistance to ivermectin was present in the worm population on this farm. Statistically speaking, there is no real difference in the reductions in fecal egg counts of the ivermectin-treated versus the untreated ewes; but the fact that the untreated and treated ewes had similar reductions in count makes one wonder if the ivermectin had any effect at all.

So what lessons are to be learned from this experience? Perhaps most importantly, without objective data to measure results, appearances can be very deceiving. The gradual improvement in body condition and FAMACHA scores might be interpreted as successful results of the deworming(s). However, the reductions in body condition in the ewes are pretty much expected as a result of good milk production; and improvement in body condition would be expected as milk production decreases, if enough energy is available. Furthermore, the lack of obvious reductions in fecal egg shedding after deworming certainly suggests to us the possibility that drug resistance in the worms may be present in the flock.

We would normally like to have at least 15 animals in a treatment group to assess the effectiveness of deworming in reducing fecal egg count. And we want to see at least a 95% reduction in egg count, after a 10-14 day period has elapsed, to conclude that a drug is effective and that significant resistance is not present. Obviously we did not have this many samples in this flock; however, we did have some data that is not often collected when people try to measure drug effectiveness an untreated control group. Many times we get into a situation where we try to assess the effectiveness of one or more drugs where severe disease caused by parasitism is already present, and an untreated control group is not practical. But because fecal egg counts in a flock are dynamic, an untreated control group is always advisable for comparison purposes when the health of the animals is not at stake. In this case, fecal egg counts for the untreated animals changed similarly to the treated animals following Valbazen treatment (>25% rise) and Ivomec treatment (>70% fall) suggesting that neither drug was very effective. The rise and fall that was observed is most likely explained by the classic periparturient rise and fall of egg counts that typically occurs over a period of about 8 weeks. This rise in egg counts in mature ewes is attributed to the relaxation of their immunity to parasites during lactation.

Monitoring egg shedding patterns and the effectiveness of dewormers can only be done using sufficient sample numbers and QUANTITATIVE egg counting methods. Usually the most appropriate method is a variation of the McMaster technique which uses a weighed fecal sample, a known dilution in a flotation solution, and a specialized counting slide to count the eggs. The result is usually expressed in eggs per gram of feces. Simple flotation methods and results expressed in +, ++, +++ etc. are not very informative and don’t lend themselves to calculations. McMaster counts are not harder to do, and the equipment is inexpensive. Many veterinarians in Ohio are trained to do them, and some currently offer this service. It is our intention to more definitively determine the resistance status of the worms in this flock later this summer.

The hard part of this project is just beginning. Fecal egg counts for the ewes are now averaging about 200 epg and their body condition could be described as “chubby”. The lambs have been weaned and have not yet been treated. The first eggs in the lambs began appearing in their feces when they were about six weeks old. They won’t develop much of an immune response to internal parasites until they are about 6 months old. The goal is to continue to use selective deworming practices and pasture management strategies to control parasitism and prolong the useful life expectancy of the dewormers we have available. Fortunately, we have some tools to measure what we are managing. Do you?

Tags: Parasites

1 response so far ↓

  • 1 Determining Worm Resistance To Dewormers // Jun 20, 2008 at 2:52 pm

    […] group.  Past articles in the Sheep Team Newsletter have described this approach (June 2004, “How do you know your parasite control program is working? ) and another excellent resource on the technique can be found at “Anthelmintic […]

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