[1] Tufts Univ, Sch Vet Med, Div Infect Dis, North Grafton, MA 01536 USA
[2] Univ Roma La Sapienza, Ist Parassitol, I-00185 Rome, Italy
[3] Ist Super Sanita, Lab Biol Cellulare, I-00161 Rome, Italy
[4] Univ London Imperial Coll Sci Technol & Med, Dept Biol, London SW7 2BB, England
[5] Murdoch Univ, Div Vet & Biomed Sci, WHO, Collaborating Ctr Mol Epidemiol Infect Dis, Murdoch, WA 6150, Australia
[6] Columbia Univ, Ctr Environm Res Conservat, New York, NY 10032 USA