I have mentioned many times recently that our sheep have been fighting quite a load of parasites because of this year’s weather. I am sure that people who don’t have experience with sheep are thinking something like, “Why don’t you just kill them off?” Well, to understand why this is the case, you need to understand more about how things got this way in the first place.

All sheep have parasites. Internal parasites suck blood causing anemia, others cause damage to the lining of the intestines, and yet others cause a combination of these, but all sheep have some numbers of internal parasites if they have ever been on pasture. External parasites are a whole different issue than internal parasites and are fairly easy to both get rid of and also to keep out – basically, don’t bring them in on sheep, and kill them if they do come, sanitizing all of the indoor areas. External parasites don’t and can’t survive except if on a sheep – get them off the sheep and in a short period of time, they are dead. Not so with internal parasites – they lead a good portion of their lives near the ground and in the fields, and are picked up by grazing sheep. These are the tougher ones to deal with.

Since sheep learn what to eat in a pasture setting from their mothers and flock-mates, we consider it an important life lesson for lambs to graze with the flock, so keeping them away from the parasites in spring is not an option. From these early grazing experiences, theylambs learn what is and is not edible – what is or is not poisonous – when turned out into a field. Grazing with the flock is a critical part of becoming a sheep and learning to survive – and it also allows us to monitor which lambs do better against these internal parasites than others. If they are going to come across them – and it is virtually guaranteed that they will – it is best to breed and select for sheep that can fend off some level of parasites on their own.

The result of allowing them onto pasture with the flock, however, is that they pick up a load of parasites from the time they are very young. From studying our sheep and the parasites that call our sheep home, we have learned that many of our adults eventually develop some resistance to most of our internal parasites; even though they graze the same fields as our lambs, they shed very few parasite eggs, telling us that they carry a very light parasite load. The lambs, however, are not so lucky. It takes years to develop this resistance, and the lambs get hit hard by the parasites that are out in the fields – even in a good year. It is the lambs that we watch the most carefully for the negative effects of a heavy parasite burden and most often assist in trying to get rid of their issues.

There are dewormers that we can use to kill the parasites when they begin to severely impact one of the flock’s sheep. The shepherd then has a decision to make: deworm only the one, or deworm the whole flock? The problem we now have is that in the past, many shepherds have dewormed the entire flock, figuring that if one has had an issue, the whole lot likely have some level of parasites – let’s kill ’em all off!

I can understand this sentiment. You look at lambs who are struggling to put on weight, or who have such a bad case of diarrhea (scours, in sheep parlance) that not only the area under the tail, but also both back legs are soaked and filthy with liquid manure. You want to help, and there is this drug…

But dewormers should be our last line of defense – over the years, I believe that they have been too heavily used. The issue boils down to this… Say a sheep – lamb or adult – is carrying in its digestive system 1000 parasites and is anemic, struggling to gain weight. You give that sheep – and the entire flock – a dose of dewormer, and that dewormer is super effective, killing 99% of the parasites it encounters (no dewormer is 100% effective). That means that you will have 10 parasites left in that one sick sheep. Those ten parasites are obviously somewhat resistant to that dewormer since they didn’t die, and they continue to plague the sheep – but since there are only ten, the sheep bounces back and seems to do fine – you think the problem is solved and all the parasites are dead – but they obviously are not.

Meantime, those ten resistant worms shed eggs into your fields, and eventually, those eggs find their way into your flock again as new parasites – new parasites that are now a bit more resistant to that dewormer than the ones that died. They end up inside the flock of sheep and breed with the other more-resistant parasites because that’s what the sheep of the rest of the flock had also dropped into the field. You now have a bunch of parasites that are more resistant to your dewormer. The next time you find yourself in the same situation – sometimes as little as four to six weeks later, you use your dewormer again and unbeknownst to you, this time, it only kills 97%, and your poor sick sheep who again had 1000 parasites is now left with 30 parasites who spread even more resistant parasite eggs into your field.

As time and deworming goes on, the parasites in the field become more and more resistant to that dewormer – much like we are finding bacteria is becoming resistant to our antibiotics for humans. Repeated exposure brings on increasing resistance, and eventually those parasites don’t die in sufficient levels after deworming. At that point, your dewormer doesn’t work and your vet will likely suggest that you switch dewormers. Now, of the three families of dewormers, you only have two that you can use in your fields – and one of those cannot be used when the sheep are pregnant – which covers at least five months of each year.

Our options are limited, and the best option is to avoid the whole resistant parasites scenario – but no matter how careful you are, you can still find yourself and your flock dealing with this problem to some degree. We will discuss this more this next week, as it is a problem that will not be going away anytime soon.


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