Grazing trials were conducted on contaminated pastures by scientists at the Agricultural Research Service in Beltsville, Maryland. St. Croix lambs developed an almost complete resistance to the parasite after four to six weeks of exposure. Dorset lambs, a common wool breed, were grazed under the same conditions and required treatment for parasite infection. The St. Croix had over 99% fewer worms in the abomasum (fourth stomach) than the Dorset, and passed only 0.5% as many parasite eggs in the feces.

The resistance is accompanied by a high number of certain immune cells called globule leucocytes, in the abomasum of the St. Croix. Although the exact method is not known, these cells might function to prevent the parasite from attacking the stomach lining, or expel the parasites from the abomasum.

When the St. Croix are crossbred with sheep of a lesser tolerance to parasites, offspring lose most of the genetic resistance. Understanding the immune process could lead to the transfer of resistance through breeding or vaccination. This can potentially benefit the bovine industry, where losses due to stomach parasites total $300 million annually. Additional research is needed.