1. Department of Veterinary Sciences; University of Pisa; Via delle Piagge 2 56124 Pisa Italy
2. Experimental Zooprophylactic Institute of Lazio and Tuscany; Via Appia Nuova 1411; I-00178 Rome Italy
3. Animal Production Research Centre (PCM); Agriculture Research Council (CRA); Via Salaria, 31, I-00015 Monterotondo Rome Italy