Are there breeds that are more resistant to internal parasites?


Yes. Some sheep/goat breeds are natually more resistant to internal parasites (worms). Raising or crossing with one of these breeds should result in fewer problems with internal parasites. If you live in a climate where parasites are especially problematic (warm, moist), it would be wise to choose a more resistant breed. Unfortunately, none of the wool or dairy breeds have resistance to worms.

Hair sheep
Caribbean-derived hair sheep are more resistant to internal parasites than wooled breeds. They trace their roots to North Africa. They are more resistant because they evolved in a tropical climate with greater natural exposure to parasites. These breeds include St. Croix and Barbados Blackbelly. Composites of these breeds (hair x wool) are also considered more resistant.

Resistance of the composite breeds is usually intermediate between the Caribbean-derived hair breeds and conventional wooled breeds. Of the composites, only the Katahdin has been documented as having improved resistance to parasites compared to conventional wooled breeds. Other hair sheep composites include the Royal White and St. Augustine, both Dorper x St. Croix crosses. There is also an Americanized version of the Barbados Blackbelly (American Blackbelly). Some of the "exotic" hair sheep breeds are Blackbelly crosses.

While the Dorper is a hair sheep, it is not considered to be more resistant to parasites. It originated in a dry climate in South Africa. The Dorper has many desirable characteristics, but parasite resistance is not one of them. The resistance of the St. Croix is well documented. It is the most resistant breed in the US. It is being used as a model to study parasite resistance at the "basic" scientific level. Scientists are looking for the genes that control parasite resistance, in hopes that those genes could be tranferred to other breeds.

Wooled breeds
Sheep breeds native to the southeastern US are considered to be more resistant to internal parasites than the wooled breeds which originated in England, Europe, and New Zealand. These native breeds evolved as a result of natural selection in a humid sub-tropical environment. Prior to the second world war much of Florida was open range. These breeds include the Gulf Coast (or Louisiana) Native and the Florida Cracker (or Florida Native). They are medium wool sheep with a variety of physical attributes.

Considering other wooled breeds, there is evidence that the Texel has some resistance to internal parasites. The Texel is a popular terminal sire breed. It is a heavy muscled medium wool breed that originated in the Netherlands. Lambs sired by Texel rams show improved parasite resistance. This makes the Texel a good choice to sire grassfed market lambs.

It's possible that there are other more resistant breeds of sheep, especially among rare breeds that have not been changed due to artificial selection. It's also possible that someone has a population of a "susceptible" breed that has developed resistance to parasites, due to a survival of the fittest scenario. However, just because animals don't need dewormed doesn't mean they are resistant. They may be resilient (only) and/or lack exposure to have developed immunity (resistance).

There is less documentation (proof) about parasite resistance in goat breeds. For several years, Tennessee State University did research comparing different meat goat breeds and their crosses: Boer, Spanish, Kiko, Myotonic, and Savanna. They determined the Myotonic to have the lowest fecal egg counts. Kiko and Spanish were more resistant than Boers. The Boer was most susceptible to parasites.

The Boer is native to South Africa and was bred in a dry climate. Similar to the Dorper, the Boer has many desirable characteristics, but parasite resistance is not one of them. The Kiko was developed in New Zealand, a moist climate. The breed was selected for hardiness and productivity and seems to have adapted well to US climates. Myotonic and Spanish goats are both indigenous to the US; they evolved under the climatic conditions here and are considered the hardiest of the US meat goat breeds. It makes sense that the Myotonic would be the most resistent breed, since it probably developed in Tennessee, a humid climate.

The dairy goat breeds are considered highly susceptible to internal parasites. So is the Angora goat. African dwarf goats (Pygmy and Nigerian Dwarfs) may have some natural resistance (to parasites), but it is likely that the resistance would be quite variable (maybe even lost) in US animals, due to the conditions in which these goats are raised (a much more pampered environment than Africa).

Resistance to protozoan parasites (e.g., Eimeria spp.) is a different story. While there is evidence that some breeds and individuals may be more resistant to coccidia infections and that heritabilities may be similar to worms, much less research has been done. It is not known if simultaneous selection for resistance to both worms and coccidia is possible or if there is even a favorable genetic correlation. In fact, it has been theorized that animals that are more resistant to extracellular parasites (worms) may be more susceptible to intracellular parasites (coccidia). Fortunately, coccidia is less of a problem with pastured animals and is manageable under intensive production environments.


Additional resources

Research publications of Richard Browning, Tennessee State University
Parasite Immunology Lab at West Virginia University