Parasite resistance

I’ve been writing a bit about parasites this summer because it has been such a bad year for them in this region. We had little winter-kill of parasites last year because of very mild weather, and the heat and humidity since have been such that it has created a “perfect storm” for the worst of these parasites to proliferate and infest the flock.

In the course of my research this summer on reigning in these out-of-control parasites, I called my friend and Iowa State University Extension Sheep Specialist Dan Morrical to discuss various control options. As part of our conversation, he explained that he would not suggest deworming for barber pole worm until fecal egg counts (FECs) of that parasite variety exceeded 300 or so. Keep in mind that by counting the number of eggs in a one gram sample, a fecal egg count will roughly reflect the number of parasites producing eggs within the sheep – and anything over 300 or so eggs is considered “high.” When he said this, he made me curious as to exactly what our flock’s FECs were for that particular nematode – but our conversation soon continued in other directions.

As we talked, I explained that one of our goals over the years had been to produce sheep that were resistant to internal parasites, hopefully reducing the need for dewormers overall. Our progress in this area had not been so evident this year, since our flock has required more deworming than usual, but that was true of all flocks in this region. It has been a bad year for sheep and parasites. I knew that the strength of our sheep against these internal intruders would be told in their symptoms and their FECs. The goal is to produce sheep that can carry a heavy worm load and not display symptoms – or better yet, to resist against the worms and not carry them in the first place, regardless of how bad the fields might be.

I do know we have made progress on this front over the years – we use less dewormer now than ever before. Every once in a while, I will hear from people who have purchased our sheep for their flock, and they tell me that although others in their flock are suffering from bottle jaw or anemia (both symptoms of heavy parasite loads), our sheep generally go about their business without issue. The feedback has been all positive – but again, I have to ask myself what the numbers look like. It is the FECs that will tell the full story, and few people go through the trouble of getting these. Have we really made progress? The way to know is to make these tests in a year like this one – when nearly every flock in the region is having trouble with parasites.

Our own testing of grazing ewe lambs will begin in the next few weeks, now that breeding season has ended. These are the sheep that will have the highest loads: the youngest flock members who have been grazing the longest on our fields (the ram lambs have been on hay for two months). My focus will now turn to these young ewes as they come in from our pastures for the winter. Yet the other day, I got a bit of verification from a good friend. She had to pull two of her ewe lambs from the flock – one because she was quite anemic and the other to keep her company in the barn as they worked to eliminate the anemia. Both of these ewe lambs trace back 100% to our lines – their sire and dams all come from our flock. As we will do in a few weeks for our own ewe lambs, this friend gathered up fecal samples to see what kind of parasite loads these girls carried and sent them off to the lab. This test was interesting to me because she was sampling two very different girls – one that did have symptoms of parasites and another of the same age that did not. I couldn’t help but wonder how heavy their loads would be, since even the girl with anemia had been grazing heavily infested pastures for the entire summer with little relief from dewormers since weaning in May.

When the results came back, I was astounded. The lamb with the anemia had a FEC for nematodes (which include the nasty barber pole worm) of 29,000 – quite a bit above that 300 egg threshhold! Even the lamb with no symptoms at all had an FEC of 26,000 – and she was doing just fine! Honestly, this information was mind-boggling to me! It is obvious that our program to develop parasite resistance in our sheep is working.

It will not be long before we do our own testing here – both before and after deworming. Yet, with egg counts so high (and I have no reason to believe that ours will be any lower than those of this particular farm), even a dewormer that has an incredibly good kill rate of 98% will result in an FEC of nearly 600 eggs AFTER deworming (2% of 29,000 is 580) – still much higher than the 300 egg threshhold Dan and I discussed on the phone. It will take some work to eliminate the parasites attacking our flock – not only within our sheep, but also on our fields next spring. Because of these very high numbers, a simple dose of dewormer will not do the trick – it will take some careful planning to get levels back down where they have been in past years and where they belong.

This has all given me a lot to think about and figure out. I don’t have the solution yet – it is still evolving – but I am working on putting it all together. It is a work in progress and I will let you know when the plan comes together. Stay tuned.

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