[1] Univ Hohenheim, Fachgebiet Parasitol, D-7000 Stuttgart, Germany
[2] Univ Tubingen, Inst Zoophysiol, AG Parasitol, D-72074 Tubingen, Germany
[3] Univ Liverpool, Liverpool Sch Trop Med, Liverpool L3 5QA, Merseyside, England
[4] Univ Liverpool, Fac Vet Med, Liverpool L69 3BX, Merseyside, England