Treatment vs non-treatment of helminth infections in cattle: defining the threshold

Helminth infections are an important cause of lost productivity in livestoc k world-wide, often necessitating anthelmintic treatment. However, a large part of the anthelmintics are used indiscriminately because the parasite le vels are too low to justify treatment or because the treatments are not cor rectly programmed, resulting in undertreatment or overtreatment. The object ive of this paper is to discuss possible thresholds for anthelmintic treatm ent of some of the more important helminth infections in livestock, i.e. ga strointestinal nematodes, lungworms and liver fluke, to promote a more appr opriate use of anthelmintics. When possible, a distinction is made between therapeutic thresholds, production-based thresholds and preventive threshol ds. A "therapeutic threshold" is intended to identify (an) animal(s) with p arasite levels that necessitate immediate treatment. The therapeutic thresh old is basically the diagnosis of clinical disease, and can be determined r elatively easily for the parasites discussed in this paper. A "production-b ased threshold" is intended to measure the effects of sub-clinical parasiti sm on productivity parameters, such as weight gain and milk yield, before c linical disease occurs. Finally, a "preventive threshold" is meant to predi ct future infection levels, to enable the application of appropriate contro l measures. (C) 2001 Elsevier Science B.V. All rights reserved.