Human Immunity toSchistosoma Mansoni: Observations on Mechanisms, and Implications for Control

Abstract

This review summarizes the personal experiences of the authors and their colleagues during ten years of field and laboratory studies on human immunity to Schistosoma mansoni infections. There is evidence for the very slow development with age of an acquired resistance to reinfection (demonstrable after chemotherapy of the primary infection) distinguishable from a lack of reinfection due to reduced exposure. The implications of this immunity for the design of chemotherapy programs targeted at infected schoolchildren are discussed. Observational studies on the mechanisms of immunity have demonstrated a marked helminthocidal capacity of eosinophils. Subsequent correlative studies have indicated a role of IgM and IgG2 "blocking" antibodies in maintaining the continued susceptibility of young children, and of IgE antibodies in mediating protection in older individuals. Some problems in studying human immunity, and the implications for vaccine development, are also discussed.

The impact of repeated treatment with praziquantel of schistosomiasis in children under six years of age living in an endemic area for Schistosoma haematobium infection

“…Information on schistosomiasis infection from a number of different geographical settings indicates that infection intensities and the development of immunological resistance to infection are related, in some way, to age (Butterworth et al 1984, 1985, 1992, Dessein et al 1992, Roberts et al 1993. Infection levels are normally highest in children and following chemotherapeutic cure, children are more rapidly reinfected and at higher rates than older children and adults , Butterworth et al 1992).…”

“…Infection levels are normally highest in children and following chemotherapeutic cure, children are more rapidly reinfected and at higher rates than older children and adults , Butterworth et al 1992). These higher rates of reinfection are in part related to higher levels of water contact by children, a feature characteristic of the majority of endemic settings.…”

Vaccination with antioxidant enzymes confers protective immunity against challenge infection with Schistosoma mansoni

“…Although immunity in schistosomiasis has been studied extensively in experimental models and in human populations (Butterworth et al 1992, Capron et al 1992, Dessein et al 1992, James 1992, Coulson 1997, immune correlates of resistance remain undefined. This is primarily because the various animal model systems and human studies have provided conflicting evidence regarding the immunological factors responsible for resistance.…”

“…It turns out that approximately 10% of those infected exhibit severe forms of disease. Although drug treatment is available (Butterworth et al 1992), often there are very high reinfection rates that make the treatment very costly especially in developing countries. With time, infection rates reach pre-treatment levels once drug treatment ceases (Gryseels et al 1994) and the risk of drug resistance development is eminent.…”

Age-related worm load and worm fecundity patterns in human populations, as indicated by schistosome circulating antigens

“…In this situation history of infection should thus not be related to age, in contrast to the situation in Maniema . Also, according to the current hypothesis in which resistance develops only slowly after several years of infection, acquired immunity should still be absent in Ndombo, while present in Maniema (Butterworth et al 1992, Gryseels 1994.…”