INFECTION wirn SCHISTOSOMES

The proportion of ail cancers represented by urinary bladder cancer varies greatly wi thin Africa and the Middle East, and the ratio of male to female frequency of occurrence is nearly as variable (Parkin, 1986). ln Egyt, the proportion of bladder cancers among aIl cancers in men is twce that in Zambia, four times that in Zimbabwe and 10 times that in Algeria. Very few formai assessments of the correlation between bladder cancer incidence and the prevalence of S. haematobium have been done, but there are many informai descriptions of geographical correspondence between the areas affected by the two diseases. Most of the early c1inical descriptions of urinary bladder cancer in cOl1nection wIth evidence of schistosomiasis come from the Nile Delta, where there are few unexposed populations and no population-based incidence data (see section 2.2.1); however, in countries with less universal exposure, observations have been made on the geographical relationship between exposure to S. haematobium and bladder cancer occurrence. The common geographical pattern of occurrence of S. haematobium and bladder cancer has been noted by investigators in almost ail endemic African countries (Table 2). ln addition to the link between the risk of a subpopulation for a haematobium schistosomiasis and the risk of the same population for urinary bladder cancer, a slightly more direct link has been noted; the proportion of bladder cancers that are squamous histologically in the population of a country is related to the proportion of cancerous bladder specimens from that population which contain evidence of past schistosomal infection in the form of eggs or egg remnants (Lucas, 1982a). This has been noted even within countries; in Iraq, for example, 36.1 % of bladder cancer cases from the north are squamous-ceIl tumours and 4.9% have evidence of S. haematobium, whereas in the south, where S. haematobium is