1. Département des Productions Animales, Institut National de la Recherche Agronomique du Niger, BP 429, Niamey, Niger
2. Danish Centre for Experimental Parasitology, Department of Veterinary Pathobiology, The Royal Veterinary and Agricultural University, Dyrlægevej 100, 1870, Frederiksberg C, Denmark
3. Department of Animal Nutrition and Physiology, Danish Institute of Agricultural Sciences, Blichers Allé 1, Postbox 50, 8830, Tjele, Denmark