Anthelmintic

[6] From the 1920s to the 1970s, halogenated hydrocarbons (such as chloroform, carbon tetrachloride, tetrachloroethylene and hexachloroethane) were used in a string of continually more efficacious anthelmintics, until their underlying host toxicity was revealed.

[13] This is a major threat to the sustainability of modern ruminant livestock production, resulting in reduced productivity, compromised animal health and welfare,[12] and increased greenhouse gas emissions through increased parasitism and farm inputs.

Results in sheep and goats since 2010 reveal an average prevalence of resistance to benzimidazoles of 86%, moxidectin 52%, and levamisole 48%.

In cattle, prevalence of anthelminthic resistance varied between anthelmintic classes from 0–100% (benzimidazoles and macrocyclic lactones), 0–17% (levamisole) and 0–73% (moxidectin), and both Cooperia and Ostertagia survived treatment.

Reasons for this include the fact that cattle receive anthelminthic drugs less frequently than sheep, and the different nature of their faecal pats that could leave different numbers of resistant infective larvae on the pasture.

From the 1950s to the 1980s, new classes of effective and inexpensive anthelmintics were made available every decade, leading to excessive use throughout agriculture and disincentivizing alternative anti-nematodal strategies.

[17] Due to the problem of anthelmintic resistance, research into alternatives is continuing, including in the field of rational drug design.