ON ESTIMATING THE BASIC REPRODUCTION NUMBER FOR SCHISTOSOMA-HAEMATOBIUM

Existing estimates of the basic reproduction number, R(o), for human s chistosomes are mostly in the range 1-4, implying that schistosomes sh ould be relatively easy to eliminate from endemic areas, which is cont rary to practical experience. An estimate of R(o) for a site in Zimbab we is obtained here using a mathematical model explicitly incorporatin g two features believed to be epidemiologically significant; age-depen dent exposure and acquired immunity. Parameter estimates are, as far a s possible, obtained independently, but the coefficients representing man-snail and snail-man transmission, as well as parameters representi ng effects of acquired immunity, must be estimated indirectly by fitti ng the model to field data. Heterogeneity in human exposure and contam ination is crudely incorporated by considering 'wormy' and non-wormy' fractions of the population. The results suggest R(o) to be in the ran ge 4-5 or more, higher than previous estimates and despite only modera te levels of infection at this site. It is shown that this estimate is sensitive to the form of the underlying model. The application of les s realistic models may lead to less reliable estimates of R(o).