1. Farm Animal Clinic; School of Veterinary Medicine; Aristotle University of Thessaloniki 54124; Thessaloniki Greece
2. Department of Medicine; Faculty of Veterinary Science; University of Thessaly; 43100 Karditsa Greece
3. Clinical Trial Unit; Intervet SPAH, P. O. Box 31 5830 AA Boxmeer The Netherlands
4. Department of Obstetrics; Reproduction and Herd Health; Faculty of Veterinary Medicine; University of Ghent; Salisburylaan, 133 B-9820 Merelbeke Belgium