Growing parasitic resistance to anthelmintics threatens the usefulness of these drugs in controlling the parasites in our domestic livestock and ruminant populations. This problem is already reaching critical levels among goats and sheep. Although research is underway there are no new classes of drugs at this time.
The present widespread shoot-first-and-ask-questions-later approach to the administration of anthelmintics to our livestock and domesticated ruminants is accelerating the process of drug resistance in the populations of parasites that afflict them. Old habits die hard, but a new culture needs to emerge in our use of these drugs.
Historically, the immune systems of most grazing animals are sufficient to keep their parasitic load at a manageable level. Free range animals shed parasite eggs in their feces, but opportunities for transmission are relatively low because the eggs are distributed over a large territory, rather than concentrated in relatively small pastures and paddocks. Domestication of grazing animals is one of the hallmarks of our path from small hunter-gather groups to large civilizations and urban populations sustained by agriculture and animal husbandry on an industrial scale. With the confinement and concentration of animals the equilibrium between parasite and host has begun to tilt in the parasites’ favor. Worm egg counts are increasing in livestock and domestic ruminant fecal specimens. We need to intervene with drugs, but we need to be selective with regard to which animals receive anthelmintics.
The immune systems of most domesticated herd animals are sufficiently robust to keep their parasitic load in check with little or no help from drugs. Most, but not all. It is this vulnerable few who need our intervention with drugs. If not treated they become hosts to huge numbers of parasites and the primary source of transmission as they shed the large numbers of eggs in their feces.
No drug is one hundred percent effective. A parasite elimination rate of 97% is generally considered quite good; rates of 90 to 95 percent are acceptable with some parasites. The relatively small numbers of parasites that survive exposure to anthelmintics do so because anomalies in their DNA protect them. They pass their DNA on to their offspring, who are also protected. It is, in effect, a selection process that over time will undermines the efficacy of drugs. This is a cost associated with the use of these drugs, and we accept it when treating an animal is truly in need of our help. To extend treatment beyond this group to virtually the entire population of a domestic species and to do so repeatedly according to a regimen that has become part of our animal husbandry culture, unnecessarily hastens that process.
The McMaster Testing Method utilizing the advanced worm slide designed by FEC Source enables the identification of animals truly in need on treatment with anthelmintics, the animals whose immune systems are unable to keep their parasitic load under control. We have refined our worm slide to make it easier and more efficient to use. It is an effective and economical tool that enables the responsible use of our most effective protection against the parasites that afflict our livestock and domestic ruminant populations.