A Follow-Up on Strategic Parasite Control in Sheep and Goats

By Dr. Robert S. Rew

I was so impressed by Sally Colby's article, "Moving Toward Strategic Parasite Control in Sheep and Goats," in the November 2012 issue of Farming, I felt compelled to follow it up. I have been involved with cattle, sheep and goat parasites for the last 37 years as a scientist in basic research, development and technical support for parasite control products. In that time, I have found several basic principles that guide my approach to parasite control.


There are several species of internal roundworms in sheep and goats. The most pathogenic is Haemonchus contortus, the barber's pole worm, which causes major blood loss. However, there are certain species in the lung and small and large intestines that can cause significant damage as well.

Probably next in line for pathogenicity are the flatworms Fasciola hepatica, the common liver fluke, and Fascioloides magna, the deer fluke. These flatworms can be transmitted where the correct aquatic snail exists, there is some standing water (even a deep hoofprint filled with water), and there are deer. They cause blood loss and liver fibrosis. A single Fascioloides in a sheep liver will kill it in a few months.

Worms are diagnosed by fecal egg counts and/or hematocrits. Fecal egg counts are a qualitative technique, not a quantitative technique. There is not a direct correlation between egg numbers and worm numbers, because the immune system may downregulate egg counts; there may be non-laying larvae in the animal, and each species of parasite has a different number of eggs laid per day (10 to 1,000 per female per day). Therefore, fecal egg count reduction tests are also only qualitative techniques and thus give limited information.


It is important to differentiate basic issues of the hosts. Cattle are immune to internal parasites within nine months of exposure. Most sheep and goat breeds do not have an effective immune system for internal parasite control, except for poor doers like native Florida sheep.

Cattle and sheep are grazers; goats are browsers. The result of this is that the enzymes in the livers of cattle and sheep that detoxify chemicals do it slowly; however, goats have very active liver enzymes. Therefore, goats need to be given a dose per pound that is one-third higher than that for sheep and cattle. We have traditionally given goats the same dose as sheep, but to get the same efficacy we need to give goats the higher dose.

There are individual sheep and goats that have effective immune systems against internal parasites, and that trait may be passed on to their offspring (but not always). Because the most pathogenic internal roundworm species in sheep and goats is a bloodsucker (H. contortus), we can see the individuals that are immune as measured by their eye color (red versus white) after exposure to this parasite. This is the basis for the FAMACHA test. Of course, if the parasite challenge is too severe, all of the individuals may succumb to blood loss as the immune system is overwhelmed.


There are three major classes of chemicals to control internal parasites: levamisole, benzimidazoles (thiabendazole, albendazole, fenbendazole) and macrocyclic lactones (ivermectin, doramectin, moxidectin).

There are certain efficacy differences between classes. Levamisole acts for one day and kills only adult stages of the internal parasites. Benzimidazoles act for two to three days and kill both adult and some larval stages of the internal parasites. Albendazole is also effective against liver flukes. Macrocylic lactones act for 20 to 30 days and kill both adult and all larval stages of both internal and external parasites, but not flukes.

Choice of product and timing of treatment should be designed for your individual herd or flock with the help of an animal health professional. Designing a rotational program or using these product classes in combination should be discussed with your veterinarian. The judicious use of these chemicals can significantly delay the appearance of resistance to the drug or address resistance that is already present.


Any time that a deworming product is put into a sheep or goat grazing system, selection for resistance begins. This presupposes that a gene or genes are present in the worm population that can impede the binding or transport of the product or enhance the metabolism of the product. This implies that the fewer times a product is used, the slower clinically detectable resistance to the drug will develop.

Refugia was thoroughly discussed in the November 2012 article. My addition would be to use the FAMACHA test to reduce the frequency of dosing and the percentage of individuals that need to be dosed as a major part of my refugia program. In addition, stocking density, pasture rotation, increased dietary protein, etc., need to be considered in discussions of refugia with your veterinarian.

Simple management techniques used in developing nations may also be included. Reducing the time of grazing by a flock by housing the sheep or goats for several hours in a raised house with a slatted floor can significantly reduce parasite challenge. Grazing goats on brush instead of pastures will also significantly reduce parasite problems.

Finally, overall good health of the animal allows it to reach its maximum capacity to resist the pathology of a parasitic infection. Viral infections reduce immunity to parasites, and the opposite is also true - internal parasite infections reduce the ability to resist viral infections or allow proper viral vaccine activity.

My final principle is that individual properties, individual management techniques, individual flock genetics, individual rainfalls, individual pasture types, individual supplement feeding programs, etc., make maximized management decisions highly difficult using broad-brush principles. Therefore, these principles that I have discussed need to be uniquely adjusted to fit your specific system. I would enjoy visiting some individual properties in my area to go into detailed analyses.

Dr. Robert S. Rew, M.S., Sc.D., is president of Rewsearch Consulting in West Chester, Pa.