From an analysis of the distributions of measures of transmission rates among hosts, we identify an empirical relationship suggesting that, typically, 20% of the host population contributes at least 80% of the net transmission potential, as measured by the basic reproduction number, R 0 . This is an example of a statistical pattern known as the 20͞80 rule. The rule applies to a variety of disease systems, including vector-borne parasites and sexually transmitted pathogens. The rule implies that control programs targeted at the ''core'' 20% group are potentially highly effective and, conversely, that programs that fail to reach all of this group will be much less effective than expected in reducing levels of infection in the population as a whole.

No abstract

Women with genital schistosomiasis had an almost three-fold risk of having HIV in this rural Zimbabwean community. Prospective studies are needed to confirm the association.

Up to 75% of women with urinary schistosomiasis have Schistosoma haematobium ova in the genitals. This study aimed to describe the prevalence of gynecologic S. haematobium infection and to differentiate the disease from sexually transmitted infections (STIs). Gynecologic and laboratory investigations for S. haematobium and STIs were performed in 527 women between the ages of 20 and 49 in rural Zimbabwe. Genital homogenous yellow and/or grainy sandy patches, the commonest type of genital pathology, were identified in 243 (46%) women. Grainy sandy patches were significantly associated with S. haematobium ova only. Genital S. haematobium ova was also significantly associated with homogenous yellow sandy patches, mucosal bleeding, and abnormal blood vessels. The presence of ova was not a predictor for ulcers, papillomata, leukoplakia, polyps, or cell atypia. Mucosal sandy patches seem to be pathognomonic for S. haematobium infection in the female genitals. Coexistence of ova and other lesions may not be causal.

Antibody responses to Schistosoma haematobium of 280 Zimbabweans were studied in two areas of differing infection levels. 133 of the subjects came from a low infection area with a prevalence of 33.8% and geometric mean infection intensity of 0.8 eggs per 10ml of urine, while 147 of the subjects came from a high infection area with a prevalence of 62.7% and geometric mean intensity of 3.2 eggs per 10 ml of urine. Both the age-prevalence and age-intensity curves exhibited a peak shift. IgA, IgE, IgG, IgG1, IgG2, IgG3, IgG4, and IgM antibody levels against soluble egg antigen (SEA) and whole worm homogenate (WWH) were assayed by ELISA. Females produced significantly more anti-SEA IgG1, IgG4, IgM, anti-WWH IgE and IgG1. People from the high infection area produced significantly more anti-SEA IgE, IgG3 and anti-WWH IgG3 and significantly lower levels of anti-SEA IgA, IgM and anti-WWH IgM when the effects of infection intensity, sex and age had been allowed for. The age profiles of anti-SEA IgA, IgG and anti-WWH IgA and IgM reflected current levels of infection while anti-WWH IgG1, IgG2 and anti-SEA IgG1 evolved more slowly with age, and anti-WWH IgE rose with age in both areas. Some antibody responses, anti-SEA/WWH IgM, anti-SEA IgG1 and possibly anti-SEA/ WWH IgA showed different age profiles in the two areas, with changes in antibody levels occurring at a younger age in the high infection area suggesting that immune responses are evolving more rapidly in residents of this area. This result clearly demonstrates that antibody levels are not a function of age alone.

Treatment of 41 Schistosoma haematobium-infected children, 5-16 years old, with the drug praziquantel induced a switch from a predominantly IgA-specific antibody response to a predominantly IgG1 response within 12 weeks. A cross-sectional survey suggests that the same switch occurs naturally, but over several years, as children age (n = 251). The switch may be driven by alterations in cytokine levels in response to the release of antigens by dead or damaged parasites. Adults are more resistant to schistosome infection than children, and the switch to an "adult" response suggests that praziquantel treatment may have an immunizing effect, with benefits extending beyond a transient reduction in levels of infection.

Antenatal multimicronutrient supplementation may be one strategy to increase birth size.

Genital schistosomiasis may be a differential diagnosis to the STDs in women who have been exposed to fresh water in endemic areas. Because of the chronic nature of the disease in adults, we suggest to pay special attention to the prevention of morbidity.