1. Agroscope Liebefeld-Posieux Research Station ALP, Tioleyre 4, 1725 Posieux, Switzerland
2. ETH Zurich, Institute of Agricultural Sciences, Zurich, Switzerland
3. Institute of Parasitology, Vetsuisse Faculty, University of Zurich, 8057 Zurich, Switzerland
4. Chemistry and Biochemistry Laboratory, School of Agriculture, Policy, and Development, University of Reading, 1 Earley Gate, P.O. Box 236, Reading RG6 6AT, United Kingdom; and
5. Section of Epidemiology, Vetsuisse Faculty, University of Zurich, 8057 Zurich, Switzerland