OSU Sheep Team

By William Shulaw, DVM, Extension Veterinarian

(Originally Published in Sheep Team Newsletter September 2004)

In our last electronic newsletter article I wrote about using several approaches to monitor sheep and make some decisions for selective deworming.  One if these is the use of fecal egg counting techniques.  What do we mean by fecal egg counts (FECs)? Most techniques to examine feces (manure) for parasite eggs involve mixing the sample with a concentrated sugar or salt solution that has a relatively high specific gravity.  This lets the worm eggs float to the top of the liquid where they are collected on a glass or plastic slide and then examined under a microscope.  The heavier material tends to sink more so one can look for the eggs on the top of the liquid.  However, as opposed to a simple flotation procedure that only allows one to see the different kinds of eggs present, FECs provide an estimate of the number of eggs present in a specific quantity of manure.  They are usually expressed as the number of “eggs per gram” (epg) of feces.  Therefore they are termed quantitative.  Although simple flatations can give us some information, the most valuable information comes from quantitative egg counts.

The most common method of determining FECs for sheep and goats is the McMaster technique.  Although there are several variations of how this is done, the basic method uses a weighed fecal sample, a known dilution in the flotation solution, and a specialized counting slide to count the eggs.  (Pictures of the slides and technique are in the ASI Sheep Production Handbook)  After the slide’s chambers have been filled with the manure suspension in flotation solution, the eggs are counted under a grid that defines a known volume of the suspension.  Usually the area under two grids is counted and the results averaged and multiplied by a dilution factor.  Because the number of grams of feces and their dilution is known, the result gives you an estimate of the number of eggs in a specific amount of manure eggs per gram (epg) of feces.  McMaster counts are not harder to do than simple flotations, and the equipment is relatively inexpensive and reusable.  Many veterinarians in Ohio are trained to do them, and some currently offer this service.  Most methods require at least two grams of manure, and usually four grams are used as this amount provides a more accurate estimate.  This means you need to provide your veterinarian with about a tablespoonful of manure for a proper exam.  One pellet is not enough.

How many samples are enough?

Research and observations over the last 40-50 years consistently show that egg counts, as well as worm burdens, from a group of individuals are not distributed across a typical bell shaped curve.  Usually only a few individuals have very high counts, and even when severe parasitic disease is present in a group, there frequently are animals in that group with very low FECs.  For example, in a relatively recent investigation of ivermectin resistance where lambs were actually dying from parasitism, the average FEC for a group of 46 lambs was 3800 epg of feces.  However, two animals had FECs of zero and the lowest twenty one were each less than 1000 epg.  The top five animals had counts of 13,800; 20,050; 23,950; 25,000; and 29,250 epg.  If you wanted a reliable estimate of the average count for the group, would samples from three animals, or even five, be enough?  Not likely.  Generally speaking, you need samples from about 15 animals to get a reliable estimate of the group average.  For those of you reading this that are statistically inclined, you are thinking “Yes, but using a simple numeric average for populations like this is flawed!”  You are correct, however, for reasons I can’t really get into in this article, a simple group average is the accepted measure used by parasitologists the world over.

What can fecal egg counts tell us?

Like most measurements in biology, FECs are a snapshot in time.  They may remain rather stable over time, or they may explode to very high levels in one to two weeks; especially during summer weather conditions favorable to Haemonchus contortus.  On the farm I described in our last article, the weekly FECs performed for the group of March-born lambs maintained continuously on pasture averaged 0; 42; 89; 1,050; and 1,950 epg from May 14 through June 11.  This dramatic change was the result of the lambs’ gradual increase in consumption of infective larvae as they consumed more forage; the buildup of worm larvae on the pasture as a result of the prolific egg producer, Haemonchus, becoming the predominant worm species; and the fact that the parasite life cycle from egg-to-egg takes about 21 days for completion under ideal conditions.  It is also characteristic of what happens on many Ohio pastures in a typical summer.  Therefore, a single egg count for a group of lambs or ewes taken out of context with other information is not predictive of what is going to happen nor is it a good measure of the worm burden the animals are carrying.  Nevertheless, FECs do give us some information about what is happening at the time the samples are taken.  The two best uses for FECs are to monitor the rate of pasture contamination and to determine whether drug resistance is present in the worms on the farm.

Monitoring pasture contamination.

Monitoring the rate of pasture contamination can be a tool the producer can use in making decisions such as when to move animals from a pasture to avoid a buildup that may lead to a dangerous situation, or it might be used to assess how much contamination is occurring in order to make decisions about future use of the pasture during that grazing season.  For example, if pastures used for lambing out ewes in April and May have received a relatively heavy egg burden, it may be wise not to graze them with lambs later in the summer.  They may be safe for dry ewes and could be used by an unrelated species.  Monitoring contamination rate can help make that decision.

Routine monitoring of FECs following the deworming of groups or selected individuals can also provide information as to whether deworming was successful.  In our experience, if the average FEC of a group is much above 100-150 epg 10-14 days following a deworming, either the drug was not as effective as it should be or the egg count was very high when the animals were treated.  This should be a red flag to signal that further information about dewormer effectiveness is needed.  If one were to see an average FEC of above 2000 epg on samples collected thirty days after deworming a group of lambs, it may indicate that the pasture they have been grazing is pretty heavily contaminated.  This is not an unusual observation when non-persistent dewormers such as Valbazen, Tramisol, or Ivomec Sheep Drench have been used and the lambs continue to graze a contaminated pasture.

Determining whether drug resistance is present.

The most readily available method to determine the effectiveness of a dewormer is by using FECs.  The most common approach used for sheep and goats is to collect about 15 samples from animals at the time they are treated, determine the group FEC, and then collect samples again from those animals 10-14 days later and again determine the FEC.  If the drug is working as we would like it to, there should be at least a 95% reduction in the average FEC for the post-treatment samples.  It is best to sample the same animals both times, but if 15-20 animals from the group are used, it may not always be necessary.

An alternate approach uses an untreated control group of animals.  In this approach, the test group of 15 or more animals is treated with a dewormer, and then 10-14 days later, FECs are determined on samples from the animals in the test group and for a similar group of untreated animals.  As in the other method, we are looking for at least a 95% reduction in average FEC in the treated group compared with the control animals.  This method accounts for variation in the groups that might not be attributable to the dewormer; such as we described in the last newsletter.  It also has the additional advantage of requiring considerably fewer total samples if several drugs are being tested at the same time because both pre- and post-treatment egg counts are not required and several test groups can be compared to the control group.  You do have to know, or expect, that average egg counts will be above at least 250 epg in the control group for valid comparisons.  Lambs or ewes can be used with either approach but don’t mix the two in a test.

I am often asked when to perform resistance testing.  This is a good question, and there is no single correct answer I am sure.  However, I usually suggest to our producers that if resources are limited, testing should be done in mid to late summer.  My rationale for this is:  At the present time Haemonchus contortus is the most important worm we have to deal with here in Ohio.  Although there are several common species of worms in sheep and goats which produce similar-looking eggs that can’t be readily distinguished from Haemonchus under the microscope, it is usually safe to assume that by July, at least 90-95% of the eggs of this type will be Haemonchus.  Therefore, resistance testing here in Ohio in mid to late summer will give us a good idea of what dewormers will do against this very important worm species.  Haemonchus season begins earlier in the year in the South, and it may not be the most important worm in the more arid regions of the West, so producers in these areas have to adjust their approach to their conditions.

Most parasitologists today recommend conducting resistance testing at least every two years, and testing for resistance does require significant work and expense.  However, not knowing whether the dewormer you are using is effective can be more than expensive.  It can be disastrous.