1. Department of Animal Sciences, University of Wisconsin, Madison, Wisconsin 53706
2. Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences, N-1432 Ås, Norway
3. Scienze Entomologiche, Fitopatologiche, Microbiologiche Agrarie e Zootecniche, Universitá degli Studi di Palermo, 90128 Palermo, Italy and
4. Istituto Zooprofilattico Sperimentale della Sicilia “A. Mirri,” 90129 Palermo, Italy