ABSTRACT
Principles of Intestinal Parasitism The common parasites of the horse’s intestinal tract are widespread in their host population. The vast majority of horses harboring intestinal parasite infection suffer little or no adverse effects. So what are the determinants of parasite-associated disease? The most important is infection intensity. It is intuitive that horses with low numbers of parasites are unlikely to suffer any ill effects from their parasite burdens. A logical extension of this argument is that horses with high parasite burdens are most likely to suffer parasite-associated disease. This hypothesis is supported by numerous experimental studies, field investigations in horses and other host-parasite systems.1,2
A second general principle of intestinal parasitism is that not all horses are equally susceptible to infection. For reasons that are poorly understood, parasites tend to distribute in a nonrandom manner in their host population.1 Certain individuals in the population are “predisposed” to developing high levels of infection and others are resistant to infection. This results in an asymmetrical distribution: commonly 80% of the worms are present in 20% of the hosts. In other words, a few individuals harbor high numbers of parasites, but most have modest or low levels of infection. It is these heavily infected individuals that are at risk of parasite-associated disease. The concept of predisposition is further supported by observations that, after chemotherapeutic treatment of parasite infections, it is the same individuals that become heavily reinfected and harbor large worm burdens.3
