[1] Univ Pavia, Dipartimento Biol & Biotecnol, I-27100 Pavia, Italy
[2] Ctr Agri Ambiente G Nicoli, Bologna, Italy
[3] Univ London Imperial Coll Sci Technol & Med, Div Mol & Cell Biol, London, England
[4] Univ Perugia, Dipartimento Med Sperimentale, I-06100 Perugia, Italy
[5] Ist Zooprofilatt Sperimentale Lombardia & Emilia, Brescia, Italy
[6] Yale Univ, Dept Epidemiol & Publ Hlth, Sch Publ Hlth, New Haven, CT 06520 USA