Worms & Goat Immune System

A Strong Immune System & Worm Infection

A goat's major defense mechanism against worm infection is the immune system. When infectious agents enter the body, the immune system reacts through a series of pathways that mobilize various components (antibodies, killer cells, etc.) that then attack and kill the invaders. These com-ponents act on the larval stages in the mucosa and the adults. How strong the immune response depends on several factors.
The immune system matures with age; therefore, young animals are relatively susceptible to infection and become more resistant with age. So, young animals usually harbor the heaviest infection levels and suffer the most severe consequences. Adult animals have developed stronger immunity and harbor fewer worms. One way infection level is measured is by quantifying the number of eggs being passed in the feces. Relatively high and low fecal egg counts are usually seen in young and adult animals, respectively. Young animals are more subject to clinical disease where signs of infection (diarrhea, rough hair coat, anemia, weight loss, bottle jaw, etc.) are seen. In older animals, infection usually be-comes more subclinical where the only subtle sign may be weight loss. However, nutrition and/or stress can alter a host’s immune competence. Under poor nutrition and stressful conditions, the immune system loses some effectiveness, cannot respond adequately, and, no matter what the age of the animal, the effects of infection can become worse (see Best Management Practices to Control Internal Parasites in Small Ruminants Fact Sheet: Nutritional effects on parasites, Steve Hart, October, 2018).
Of particular note in small ruminants is a phenomenon called the periparturient rise (PPR) in fecal egg output (see Best Management Practices to Control Internal Parasites in Small Ruminants Fact Sheet: The periparturient egg rise, Joan Burke, April 2020). This occurs at or around parturition and extends through most of the lactation period. Because parturition and lactation are stressful conditions, the dam’s immune system is compromised. Furthermore, nutrients are partitioned preferentially to support mammary and fetal development and then lactation, which also decreases the animal’s ability to generate an effective immune response to worm infection. Providing a high protein diet will help partition-ing to support immune function.
Breeding and selecting for genetic resistance to infection is also possible, as within breed and between breed resistance has been demonstrated. The mechanism of resistance is predicated on improved immune response, thus fewer worms (see Best Management Practices to Control Internal Parasites in Small Ruminants Fact Sheet: Genetic selection, using cross breeding and estimated breeding values, Scott Bowdridge and Andrew Weaver, February 2019; On farm se-lection for resistance to parasites, Jim Morgan, January 2021).
The prepatent period (time between ingestion of infective larvae to egg laying adult females) of most worms is about 3 weeks, but this period can be extended (by a few months) for worms that have the capability to enter a period of delayed or arrested development called hypobiosis. This occurs during the season of the year when the environmental conditions are unfavorable for development and survival of the free-living larval stages. In warm climates, this happens either during summer or winter depending on the worm. In colder climates, all worms capable of hypobiosis will arrest in the winter.