When we first got into sheep, in 2006, it was recommended to (de)worm sheep at regular intervals then move those sheep to a “clean” field (one that sheep had not been on in at least a couple weeks). There are 3 classes of parasite drugs, each targeting certain types of parasites, and it was also suggested to use a medication from a different class at each interval.
From research and from many farms dealing with parasites that are now resistant to certain anthelmintic drugs, recommendations have changed over the past few years. Shepherds are now advised to test sheep for parasites and (de)worm after sheep have been in an area for at least a few days so that any parasites that aren’t killed by the medication and are expelled from the animal’s body will have a better chance of breeding with “normal” parasites instead of other resistant parasites. Testing can determine what parasites are present and, therefore, what class of drug (if any) is needed.
With this new info and advice about parasite resistance to worming medications, I have been doing fecal tests every 3-4 months on my flock to see if I need to treat them. If I notice certain sheep are acting off I will do individual fecal tests on those sheep. My ewes were last wormed in Jan 2017 and my boys were wormed in the summer of 2017.
If there are obvious signs like pale eyelids (indicative of anemia), I worm with cydectin right away instead of waiting for a fecal because barber pole worms can become a fatal problem very quickly. In this case, I would run a fecal 1-2 weeks after treating to see if a second treatment is needed.
I have noticed that some of our lambs have issues with parasites over their first summer and autumn, and need multiple treatments. This doesn’t necessarily indicate that they will always be susceptible to a “normal” parasite load. I’ve noted that most of those same sheep will not need worming their 2nd year. Once they are bigger they seem to be more resistant to parasites.
Parasites can be an issue to sheep that we bring to the farm or that we sell. The stress of moving to a new place can trigger worms to become more active. Each farm also has different populations of parasites and it can take time for the sheep’s body to adjust to the population of the new farm. Because of this, I recommend checking new sheep more often – doing a fecal once a month or so and checking eyelids weekly.
Questions? Leave a comment below and I will answer as best I can. I urge you to do your own research, though. The Maryland Small Ruminant Page is a great resource run by Susan Schoenian, Sheep & Goat Specialist for University of Maryland Extension. The Ohio State University Sheep Team runs a sheep blog that is very informative. If you don’t have a local vet who is knowledgeable with sheep, Pipestone Veterinary Service can be reached by email or phone for sheep questions. You can also Google topics and find many other resources. If you don’t have a local vet who can run fecal tests, you can send samples to MidAmerica Agricultural Research.