A quantitative post mortem analysis of urinary schistosomiasis in Egypt. II. Evolution and epidemiology

Further analysis of the data obtained from 190 unselected autopsies at the University of Cairo (Faculty of Medicine) hospitals reinforces our conclusion that a high prevalence of urinary schistosomiasis leads to infection intensity causing severe uropathy and mortality, both directly and by way of complications and sequelae. Based on histological study, two stages of urinary schistosomiasis must be considered in epidemiological work: "active disease", characterized by significant egg excretion; and "inactive disease", in which eggs are excreted rarely. The proportion between active and inactive cases is progressively reversed with advancing age, while mean tissue egg burdens rise, plateau, and ultimately decrease, most sharply beyond 50 years of age. A model of the progression of active disease has been derived from the relations of individual organ egg burdens to overall infection intensity, showing that both the onset and the termination of oviposition probably begin in the urinary bladder and spread centrifugally. Therefore, extravesical activity may persist longer than bladder activity. Severe uropathy and mortality occur at all stages of the disease and depend principally, but not exclusively, on egg burden, i.e., on infection intensity. Correlations of infection intensity with degree of uropathy show that severe disease is quantitatively separable from incidental disease by its tissue egg burdens and lesions. However, the factors determining death from urinary schistosomiasis are only partly understood. They include bilateral upper obstructive uropathy and, probably, focal egg concentrations leading to rapid obstruction, such as aberrantly high egg burdens in the left ureter relative to those in the bladder. Analysis of epidemiologically homogeneous population groups reveals close mutual relationships between the total frequency of infection (active plus inactive), the intensity of infection, and the frequency of severe uropathy. A statistical model predicts that any rise in frequency beyond a 30% threshold will result in a linear increase in the frequency of severe disease, whereas below that threshold the bulk of infections will be incidental. These insights, applicable only to pathological material, must be complemented by efforts to establish clinical and laboratory criteria defining the severity and stages of urinary schistosomiasis in living patients, and to examine their population dynamics, so that effects of therapeutic and preventive measures may be evalulated more precisely.