Immunity after treatment of human schistosomiasis mansoni. III. Long-term effects of treatment and retreatment

Abstract

Group mean Schistosoma mansoni reinfection patterns are presented for 2 years after treatment with oxamniquine in 1981 of over 100 9- to 16-year-old Kenyan schoolchildren, and for one year after retreatment in 1983 with either oxamniquine or praziquantel when most (nearly 700) infected people in the whole community were treated. Quality control confirmed comparable Kato egg counts throughout the study. Continuing transmission after 1981 raised prevalence to nearly its original level within 6 months, but intensity remained suppressed throughout the 2 year follow-up and very few children reacquired heavy infections (greater than 400 eggs/g). Age and sex had significant effects: reinfection diminished with age, especially among boys--a pattern not apparently attributable to differential water contact. Children with heavy pretreatment infections tended to develop heavy reinfections but this trend was not statistically significant on a group basis, nor were similar trends during the period of less pronounced transmission following the 1983 community treatment. Oxamniquine was equally effective in children receiving it in both 1981 and 1983, and the efficacy of praziquantel resembled that of oxamniquine. In this area of Kenya, repeated chemotherapy will be needed to contain transmission, probably annually or biennially, unless supplemented with other, effective control measures. These findings confirm the beneficial effects of treating even a limited segment of a community at intervals of a year or more without necessarily stopping transmission. They are also compatible with recent findings on potential immune mechanisms in man.

Observed Reductions in Schistosoma mansoni Transmission from Large-Scale Administration of Praziquantel in Uganda: A Mathematical Modelling Study

“…Parasite intensity estimates were made prior to treatment each year, so the change in the FOI (expressed as a ratio of the FOI at baseline) is calculated as an average of the rising value across the previous year. The reductions in the FOI seen after each round were assumed to be equal across ages, as studies have shown that the infection profile generally returns to the same pattern following reinfection [36].…”

The development of an age structured model for schistosomiasis transmission dynamics and control and its validation forSchistosoma mansoni

“…The most often quoted values are 1-4 for the basic reproductive number and 4 years for the worm lifespan [1] and these values are used here. However, Woolhouse, Hasibeder and Chandiwana (unpublished observations) calculated higher values (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16) for the basic reproductive number by taking into account acquired immunity and heterogeneity in exposure and contamination. Also, recent analysis of the worm lifespan suggests a longer lifespan of approximately 8 years (Fulford and colleagues, unpublished observations).…”

“…Note that these are purely descriptive functions and do not model processes of birth and death. The data used in this [15] and Sturrock and colleagues [16]. See text for other details of programmes.…”

“…A number of children had previously been treated in 1981 and the pretreatment intensity had to be obtained from an earlier publication [16]. This child targeted treatment had to be simulated before also simulating the mass treatments in 1983 and 1985.…”

The role of the scientific research in the control of schistosomiasis in endemic areas

“…Another aspect extremely significant is that in the reinfection after the treatment generally the individuals present reduction of the number of eggs in the stool (Kloetzel 1963, Prata et al 1980, Sturrock et al 1987.…”