1. Department of Mathematical Sciences and Technology, Agricultural University of Norway, PO Box 5003, 1432 Ås, Norway
2. School of Civil and Environmental Engineering, The University of New South Wales, UNSW-Sydney, 2052 Australia and Department of Parasitology, Mycology and Water Microbiology, Swedish Institute for Infectious Disease Control, SE-171 82 Solna, Sweden
3. School of Civil and Environmental Engineering, The University of New South Wales, UNSW-Sydney, 2052 Australia