The constant risk posed by GI parasites in grazing cattle, the development of AR in bovine nematodes and the continuous pressure to reduce the use of synthetic chemicals in food animals require integrated approaches that combine the field monitoring of AR and the use of novel control methods to safeguard animal health and productivity in a sustainable manner. In the present investigations, a range of field, controlled and laboratory studies were performed to advance our knowledge on the efficacy of the most common anthelmintic drug used in Danish cattle, the potential use of forage chicory as a complementary control of GI nematodes and the role of SL in the anthelmintic activity of chicory.
Field studies (Manuscript I) revealed a reduced efficacy of injectable IVM to control GI nematodes in FSG calves in three of six Danish farms investigated. Cooperia oncophora
was the main species surviving IVM treatment in all farms, but O. ostertagi was also identified post-treatment in one farm with confirmed reduced treatment efficacy. Yet, due to the low number of farms enrolled in the FECRT, this study may not represent the general situation in Denmark. Moreover, the presence of IVM-resistant nematodes suggested by the FECRT in three farms with reduced treatment efficacy should preferably be confirmed by CET. High FEC aggregation markedly affects the performance of the statistical methods used to estimate drug efficacy and results provided by such methods for highly over- dispersed datasets and with low sample size (n ≤ 10) should be interpreted with caution and standardised guidelines for the design and evaluation of FECRT in cattle are pressingly needed. More than 98% of all anthelmintic treatments performed in the six farms investigated during the 11 years prior to our FECRT were ML, mostly IVM pour-on products. Accordingly, preliminary studies revealed that ML were the predominant anthelmintic drugs in Danish cattle production during 2010 to 2014 (unpublished data). The reduced efficacy of IVM detected in this study and the widespread use of ML in Danish cattle, suggest that farmers and their advisors should be aware of potential lack of efficacy and preferably closely monitor the efficacy of their anthelmintic treatments. Also other initiatives should be implemented in order to slow-down the selection of resistance to IVM and other ML e.g., use of other drug groups or targeted selective treatments with the still effective compounds. In addition, novel parasite control methods are required to reduce the reliance on anthelmintics while decreasing parasite burdens in grazing cattle.
Two independent controlled experiments (Manuscript II) demonstrated that calves fed with ≥ 70% of chicory DM in the diet had significant reductions in the adult worm burden of the pathogenic O. ostertagi in the abomasum (60 – 66% reduction), while C. oncophora in the small intestine was unaffected. Forage chicory (fresh and silage) was readily consumed by calves and the reduced burden of O. ostertagi in these animals was also
associated with a decrease in FEC from Day 22 p.i. onwards, which suggests that dietary chicory selectively affects the adult worms but not the preceding larval stages. Stabled animals fed chicory silage had higher weight gains compared with controls. In contrast calves allocated to a pure chicory pasture in its second year showed lower growth rates due to poor regrowth and low nutritional quality of the chicory crop. Therefore, grazing of calves on pure chicory under Danish conditions seems to require careful planning and monitoring to secure acceptable production gains. Sesquiterpene lactones were identified only in fresh chicory and silage and these PSM may contribute to the observed anthelmintic effects. In addition, the anthelmintic effects of chicory were preserved despite ensiling, and this may be a practical way to conserve and use chicory in farms, independent of season and pasture availability. Pelleting may be another option, which, however, was not tested in the current studies. Furthermore, chicory is well-suited for cultivation in Denmark and other Northern temperate climates and thus show promise as anti-Ostertagia feed for use in cattle that may reduce infection levels and the need of drug treatments.
The observed in vivo effects of forage chicory against O. ostertagi were confirmed by
in vitro studies. These studies (Manuscript III), demonstrated direct anthelmintic effects of SL-containing extracts from two chicory cultivars against O. ostertagi L1 and adults, but not towards L3. Interestingly, chicory extracts from the tested cultivars showed distinctly different potencies and SL profiles; the more potent extract isolated from chicory Spadona had a significantly higher concentration of only one compound, DI-8-DOL. Whether the increased activity of Spadona extract is related only with the higher concentration of DI-8- DOL or to the synergistic effect of all SL in chicory Spadona is not clear. In addition, other molecules with anti-parasitic activity and not detected by chromatographic analyses (i.e. due to poor absorption of UV light) may have been present in the chicory extracts. Moreover, the differences in potency between chicory cultivars in vitro should be confirmed
in vivo. In this context, our results can help to target the responsible compound(s) and to select cultivars with higher anthelmintic activity for cattle and potentially other herbivores.