1. U.S. Department of Agriculture; Agricultural Research Service; Beltsville Agricultural Research Center; Animal Parasitic Diseases Laboratory; Building 1001 Beltsville Maryland 20705-2350 USA
2. Laboratorio de Inmunoparasitología; Facultad de Ciencias Veterinarias; Universidad Nacional de La Plata; Calle 60 y 118, 1900 La Plata Argentina
3. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET); Av. Rivadavia 1917 C1033AAJ Buenos Aires Argentina
4. Department of Anatomy and Physiology; Faculty of Veterinary Science; University of Pretoria; Onderstepoort South Africa
5. Institut für Epidemiologie; Friedrich-Loeffler-Institut; Bundesforschungsinstitut für Tiergesundheit; Federal Research Institute for Animal Health; Südufer 10 17493 Greifswald-Insel Riems Germany