Clinical parasitism in cattle has obvious effects such as anorexia, interference with digestion, anemia, bottle jaw, increased gut motility and disruption of fluid and mineral homeostasis (diarrhea) and a rough hair coat. But it’s the indirect, or subclinical effects of parasitism that can steal your profits, says veterinary parasitologist Bert Stromberg, PhD, of the University of Minnesota.

These indirect effects include suppression of immune responses, general and specific hyporesponsiveness, the initiation of immunopathology, and other physiologic responses that are adverse to productivity. In production terms, that can mean reduced weaning weights and reduced milk production in beef cows. Stromberg notes that in one study parasitism reduced milk production in beef cows just shy of 2 lbs. per nursing episode. That means fewer pounds of gain on calves.  

“Parasite removal benefits the host immune response,” Stromberg says. “Nematodes are very strong stimulators of immune system. Abomasal parasites such as Ostertagia exert the most profound effects on the immune system.” And that level of immunosuppression is directly related to the parasite burden.

There is no rapid and effective means to identify those animals at the highest risk for parasite-induced immunosuppression. Optimal parasite treatment and herd immunization programs need to be devised to minimize herd health problems and maximize vaccination efficacies, recommends Stromberg. “We must remove parasites before vaccination programs, or remove worms at the same time as vaccination. Vaccines don’t need the competition for the immune system.”