Discipline: infection; Key words: helminths, anthelmintic resistance, phytotherapy, bioactive forages, vaccines, grazing management.
An elicitation exercise was conducted to collect and identify pressing questions concerning the study of helminths in livestock, to help guide research priorities. The authors cited below are the primary global experts who compiled the report. Whereas this presentation does not cover research results as usual, I consider it important to share their ideas with the reader as a main focus of our R & D is in control of parasites and because prominent researchers in the group are South African or South African of origin.
Plant-based Control: With the increasing emergence of anthelmintic resistance (AR) in helminths of livestock, alternative options are in demand, especially for the integrated control of gastro-intestinal nematodes (GINs). Plants and plant secondary metabolites (PSMs) appear to be a promising option. Different PSMs (e.g., tannins) have shown antiparasitic effects when used as nutraceuticals or in phytotherapy. Two hypotheses have been invoked to explain the anthelmintic properties of PSMs: pharmacological-like effects through disturbance of the parasite life cycle, or indirect effects on the host immune response. In both cases, more studies are needed to identify the mechanisms of action of PSMs and their effect on helminth populations, including those with high levels of AR, as well as the potential role of PSMs in managing helminths other than GINs. Feeding ‘bioactive forages’ can also improve nutrition and performance, and reduce GHG emissions, quite apart from any impacts on helminths. The interactions between different PSMs and between PSMs and anthelmintics remain largely unexplored, and contrasting results have been described. The development of refined methods to assess the anthelmintic potential of plant compounds is needed. Some practicalities around the use of PSMs on farms also need to be addressed, such as regulation of mode of distribution, level of inclusion in feed, and potential residues in animal products.
Other Alternative Control Methods: The objective of integrated parasite management (IPM) is to limit the level of parasitism below acceptable limits while delaying the emergence of drug resistance. This aim has motivated the search for and refined use of PSMs as well as other alternatives to commercial chemical anthelmintics, including vaccines, host resistance, and grazing management. Good pasture management is one of the major means to limit the intake of infective larvae by animals, for example, by the use of parasite-free fields, pasture rotations, and rotation of grazing animals, taking into account the seasonal dynamics of helminth transmission. Manipulation of environmental conditions that play a role in the development of intermediate stages may also be a form of alternative control. For example, grazing away from wet pasture, where feasible, markedly lowers the risk of Fasciola hepatica infection, due to lower exposure to infection near intermediate snail host habitats. Free-living stages of GINs may also be targeted directly, for instance through application of urea or other nitrogen-based fertilizers to pasture. Certain bioactive forages, for example, chicory, are also thought to hamper the development of free-living stages, either by reducing the fitness of eggs excreted from hosts grazing on the forage, or because the physicochemical properties of the forage reduce larval availability on herbage. Biological control based on nematode-trapping fungi (Duddingtonia flagrans, Arthrobotrys musiformis) or entomopathogenic bacteria can also reduce the number of free-living stages on pasture and the level of host infections; results from mechanical stressors, such as a diatomaceous earth, are less promising. Refined understanding of the mechanisms of action of these non-chemotherapeutic alternative control methods, and how they might be applied to manage helminth populations on farms, provides potentially fruitful avenues for further research.
E. R. Morgan, N-A. A. Aziz, A. Blanchard, J. Charlier, C. Charvet, E. Claerebout, P. Geldhof, A. W. Greer, H. Hertzberg, J. Hodgkinson, J. Höglund, H. Hoste, R. M. Kaplan, M. Martínez Valladares, S. Mitchell, H.W. Ploeger, L. Rinaldi, G. von Samson-Himmelstjerna, S. Sotiraki, M. Schnyder, P. Skuce, D. Bartley, F. Kenyon, S. M. Thamsborg, H. R. Vineer, T. de Waal, A. R. Williams, J. A. van Wyk, & J. Vercruysse, 2019. Trends in Parasitology. https://doi.org/10.1016/j.pt.2018.10.006 20 pages.