[1] UNIV LONDON IMPERIAL COLL SCI & TECHNOL, ASCOT SL5 7PY, BERKS, ENGLAND
[2] INST SALUD CARLOS 111, CNMVIS, SERV PARASITOL, E-28220 MAJADAHONDA, SPAIN
[3] PRINCE LEOPOLD INST TROP MED, B-2000 ANTWERP, BELGIUM
[4] UNIV LIVERPOOL, LIVERPOOL SCH TROP MED, LIVERPOOL L3 5QA, ENGLAND
[5] VIENNA TECH UNIV, MATH BIOL ABT, A-1040 VIENNA, AUSTRIA
[6] MRC, EXTERNAL STAFF, LONDON WC1E 6AS, ENGLAND
[7] UNIV PENN, SCH VET MED, PHILADELPHIA, PA 19104 USA