Anthelmintic resistance in ruminants: development, diagnostics, and procedures

Anthelmintic resistance of gastrointestinal nematodes in small ruminants, but also in cattle and horses, is now found worldwide. The reason for increasing anthelmintic resistance is, in particular, the extensive use of all the anthelmintic agents available on the market. A non targeted use leads to the selection of naturally occurring resistance genes within parasite populations. The most practical method for evaluating the efficacy of an anthelmintic is the fecal egg-count reduction test. To reduce the rate of anthelmintic resistance development, the available active substances must be applied less and in a targeted manner. When applying targeted (selective) treatment, part of the herd is left untreated. Therefore, it is necessary to identify the animals that require treatment for health or economic reasons. To decide on anthelmintic treatment, findings can be collected from single animals or from a group of animals in a herd. To determine which groups of animals are to be treated within a herd (targeted treatment), pooled fecal samples (cattle and small ruminants), serum pepsinogen concentration (cattle), or Ostertagia ostertagi antibodies in the bulk milk (cattle) can be analyzed. For individual animal (targeted selective) treatment, criteria including fecal egg count (cattle and small ruminants), conjuctival color as an indicator for infection with Haemonchus contortus (FAMACHA, small ruminants), body condition in adult animals (small ruminants), weight gain in juvenile animals (cattle and small ruminants), and the consistency of the feces (small ruminants) are used. These decision criteria can also be combined to enhance the informative value. Furthermore, an efficacy test of the anthelmintics used should be performed regularly at the beginning of the pasture season. During the pasture season, a low infection pressure should be maintained by pasture management strategies. The goal of sustainable parasite management is the reduction of anthelmintic treatment while maintaining the productivity and health of the animals and thus a longer effectiveness of the available drugs.