On Wednesday I wrote about our recent losses of lambs to internal parasites. I know that this is a nasty topic, but in the sheep world, understanding parasites is critical to flock survival. I’ve written about this topic more than once this summer because this has been an unusual year, with the parasites worse than I’ve ever seen them. The usual winter kill didn’t happen because of mild temperatures in what should have been our coldest months; and that mild winter was followed by a hot, humid, and rainy summer. All of these are ideal conditions for internal parasites to bloom on the fields.
Many years ago, we took part in a parasite study run by the University of Wisconsin. It was a win-win situation: the experts gained information about how resistant the parasites might be to common dewormers, and we producers learned more about the parasite profile on our properties. A large part of the study involved the fecal egg count (FEC), a way of quantifying how severely parasitized a sheep might be by counting shed eggs in a particular sample size of manure.
As a result of this study, I learned a lot about my sheep and the parasites they carry:
1) The oldest sheep are generally the most resistant to internal parasites. If you added together the FEC of all of the ewes aged three and older, the total was approximately equivalent to a single ewe in the one- to two-year group. If you added together all of the FECs of the one- to two-year group, it was approximately equal to a single ewe lamb’s FEC. The worst hit by parasites are the youngest of the flock. At the time we participated, the recommendation was to not deworm the three year and older group, and to only selectively deworm the one- to two-year-olds.
2) Sheep of any age can temporarily resist infestation by these parasites if the sheep are healthy and strong — but they may eventually succumb as a result of external stressors. For example, only a few weeks after a surprise visit and big scare by a neighbor’s dog, we had a high percentage of our lamb flock suddenly show signs of internal parasites. It takes only a bit of stress from another source to drop resistance and allow the parasites to take hold. We have seen this time and again: when a sheep is on pasture and becomes stressed for whatever reason, they are likely to soon show signs of internal parasites. We therefore keep a close eye on any sheep who we know have had issues. For example, when trimming hooves last weekend, Ivy had some hoof problems that required extra attention and left her limping for a couple of days. Although Ivy is seven years old — supposedly in the “safe” range — her injured hoof makes her much more susceptible to a bloom of internal parasites. As a result, I’m watching her closely.
3) Not all parasites cause the same symptoms. We have three different groups of parasites in our fields. Most common are the hookworms and roundworms that cause scouring (diarrhea) when present in quantity. We see these basically all summer, with a usual break in midsummer when things get hot and dry and the grass stops growing. We also have Haemonchus contortus or “barber pole worm,” which causes anemia — generally without scouring. This parasite is most common in our fields in the early summer, and mostly hits the young lambs either just before or just after weaning. That same summer dryness and heat that sets back the first group of parasites usually also sets this one back too. We’ve never had a death from Haemonchus during the grazing season after June — until this year. Finally, there are tapeworms, which seem to be most common here in May. All of the sheep dewormers will kill the first two groups, but only one of the three classes of dewormers will kill tapeworms. Yet I’ve found that most of the lambs will eliminate tapeworms on their own without treatment in about 4-6 weeks, so instead of immediate treatment, we simply wait. If after that time period I’m still seeing evidence of tapeworms, I deworm only those who need it with the one dewormer that works against tapeworms.
4) Trying to control internal parasites through frequent deworming is counterproductive. A dewormer will kill only a percentage of the parasites that are present. Those that survive are then more resistant to that dewormer the next time it is needed. Frequent deworming over time decreases the efficacy of the dewormer to the point that it is no longer useful. The key is to use dewormers sparingly and to never deworm the entire flock, leaving a group of less resistant parasites in the bellies of those who were not dewormed. Those less resistant parasites will breed the more resistant, reducing the overall resistance of the parasites in the fields. I often tell new shepherds that they are breeding both sheep and internal parasites; the key is to produce the strongest sheep and the weakest parasites.
Because there are only three classes of dewormers available to US flocks, once resistance is developed to all three, there is nothing left to do and nowhere to go for help with parasites. As a result, we have to start trying to deal with the issue of parasites now, using dewormers carefully and essentially as a last resort. Although this year has been the perfect storm for parasites, a hard freeze is just around the corner, bringing the end of this grazing (and parasite) year.