As I am writing this, it is the second week of June and the high temperatures are predicted to be in the triple digits all week. The bad news is this will bring on heat stress for our sheep and goats. The good news is this will help with parasite problems that some of us have been dealing with.
Last month, I wrote about the biology of internal parasites and the natural defense system sheep and goats have. Then, I discussed how to use this information to defend your sheep and goats against them. In this article, I will focus on anthelmintics or dewormers and how to best use them to combat the barber pole worm (H. contortus).
Dewormers are a great tool to help combat parasites that affect sheep and goats. However, for many sheep and goat ranchers, these tools are not as effective as they used to be. Each time a dewormer is used a few parasites survive and reproduce. Overtime the population of parasites within your sheep and goat flock will become resistant to that dewormer. Here are ways of prolonging the effectiveness of dewormers:
Effective Dose: Read and follow label instructions for the proper dose according to animal body weight. Underdosing is a major cause of dewormer resistance. It is recommended to sort animals into groups of similar size and drench to the heaviest animal in the group. Also, deworming animals on an empty stomach may help increase effectiveness of the dewormer.
Worm Test: It is important to deworm animals at the right time. We don’t want to wait too long and let parasites get the upper hand for a couple of reasons. First, animals that become severely infected may die or require special care to fully recover. Second, high parasite loads in a herd/flock spread a lot of parasite eggs in the pasture, which increases the need for follow-up treatment. And finally, the higher the parasite load the higher the number of parasites that are not controlled by deworming, which increases resistance build-up. However, treating too often when parasites are not a problem is costly, time consuming, and increases parasite exposure to dewormers. The best method is to conduct a fecal egg count test on a flock or herd every couple weeks during the spring and summer. When the average fecal egg count rises above 500 eggs per gram, it is time to deworm the group or susceptible animals within the group, depending on management.
Drench Test: It is important to determine the efficacy a dewormer in your herd/flock. To do this, conduct a fecal egg count test on 10-15 animals at the time of deworming and conduct a follow up test on the same animals in 10 days. Dewormers should be 95 percent effective or higher. If a dewormer falls below 95 percent, it is advised to rotate to a new class of dewormer. Frequent or seasonal dewormer rotations should not be done. Also, if a dewormer is used until effectiveness falls below 50 percent, it reduces the chance of the dewormer working in subsequent years or in combination with other dewormers. In the US, we have 3 classes of dewormers labeled for sheep and/or goats. A chart is provided courtesy of Sheep 101 that has the different classes and their trade name products.
Refugia: Refugia is the term used when we intentionally allow some parasites to survive untreated. These parasites are susceptible to a drench and will breed with resistant parasites. There are numerous methods to accomplish this goal. A common method is the FAMACHA system. This system recommends to only deworm animals that are anemic, which is measured by an eye scoring system, and not treating animals that are not anemic. Another method is to run a group of animals that are not susceptible to parasitic infection and don’t require deworming, such as dry ewe lambs or yearlings, with animals that are susceptible and require deworming, such as lactating ewes. In contrast, treating the entire group and moving these animals to a clean pasture, creates a pasture of animals that are all carrying parasites that are largely resistant to the dewormer.
Combinations: Drenching with 2 or more classes of dewormers at the same time has shown to be an effective method. Two active ingredients provide an additive effect allowing fewer parasites to survive. It is not advised to mix drenches together but rather deworm with multiple products one at a time. In other countries, combination drenches are common in the marketplace but none have been approved in the US. Resistance development is likely to occur when dewormer combinations are used, so all the principles listed above still apply.
As a side note, we recently tested two dewormers on some lactating ewes in Sutton County. Product A was 85 percent effective, product B was 99.7 percent effective, and a combination of product A & B was 100 percent effective. We will no longer use Product A, except in combination with another dewormer.
In summary, parasite control is a complicated segment of flock/herd health. It is important to educate yourself on methods of parasite control and use fecal egg count testing to better understand your parasite management plan. Many sheep and goat ranchers will have effective but different parasite management plans. Sometimes, plans that work for your neighbor might not work for you. Find out what suites your land, livestock, and management best. In the coming months, I’ll write about natural products that are used to control parasites.
For more information or to provide feedback on this article or request topics for future article, contact me at firstname.lastname@example.org or 325-653-4576.