1. Faculty of Veterinary Medicine, University of Forestry, Sofia, Bulgaria
2. National Diagnostic and Research Veterinary Medical Institute, Sofia, Bulgaria
3. Department of Occupational Medicine, Faculty of Public Health, Medical University, Sofia, Bulgaria
4. Department of Microbiology, Infectious and Parasitic Diseases, Faculty of Veterinary Medicine, Trakia University, Stara Zagora, Bulgaria
5. Unit of Medical Statistics and Molecular Epidemio¬logy, Universita Campus Bio-Medico di Roma, Rome, Italy
6. Department of Infectious Diseases, Military Medical Academy, Sofia, Bulgaria