科研之友
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摘要
Grazing cattle worldwide are exposed to gastrointestinal parasites (GIN), which affect their well-being, health and production. An increasing number of drug resistant GIN have been reported over the last years, particularly in countries with a warm and dry climate. Heat and drought stress not only have direct effects on cattle health, but also indirect effects, for example on fodder production and parasites of cattle (Gauly et al., 2013). Rising temperatures and drought-stress also shorten the lifespan of the infectious third stage GIN larvae (L3). In Germany, Cooperia oncophora and Ostertagia ostertagi are the most important GINs of cattle. They spend most of their life cycle outside the host on the pasture and are therefore exposed to environmental conditions. Two experiments investigated the possible impact of climate change in Germany on the behavior and survival of L3 of C. oncophora. The results confirmed that higher temperatures and long lasting drought stress have negative effects on the survival and fitness of L3 (Knapp-Lawitzke et al., 2014; Knapp-Lawitzke et al., 2016).
Although this sounds to be a positive effect, it has considerable negative impact regarding the development of drug resistance. These consequences can be illustrated using the refugium model. The refugium is the fraction of a parasite's population not exposed to the drug. For GIN, this corresponds to eggs and larvae on pasture and parasitic stages in non treated animals. A reduced refugium was shown to increase anthelmintic resistance (van Wyk, 2001; Wolstenholme et al., 2004). Different climate models predict warmer and dryer conditions during the grazing season (Jacob, 2012). This could lead to a diminished refugium of L3 on the pasture, potentially accelerating the development of resistance. The speed with which anthelmintic resistance can arise on pastures under present weather conditions (2013) was shown in a field experiment (Knapp-Lawitzke et al., 2015). In this experiment, four groups of calves successively grazed on a pasture, which was previously contaminated by a group of calves infected with a partially drug-resistant O. ostertagi population. Each group was treated with increasing but subtherapeutic benzimidazole doses. Pyrosequencing demonstrated an increase of genes conferring resistance in the parasites from 15-63 % before, to 55-88 % after treatment. This effect could not be corroborated by fecal egg count reduction test (FECRT). The FECRT is mainly used in the field to detect drug-resistance but it is not accurate enough for the early-stage detection of drug-resistance and in cases of mixed infection with susceptible and resistant GIN species. The use of frequent and accidentally subtherapeutic treatments may occur in the field. In combination with climate change and a reduced refugium, this could lead to serious problems with anthelmintic resistance. Therefore, there is a need for the development, evaluation and application of novel methods to control GIN infections
- 出版日期2017-4
