J. H. Ouma

Immunity after treatment of human schistosomiasis mansoni. II. Identification of resistant individuals, and analysis of their immune responses.

Intensities of re-infection were monitored at three-monthly intervals after treatment of Schistosoma mansoni infections in a group of 119 Kenyan schoolchildren, whose levels of water contact were also observed. 22 children showed high reinfection intensities (greater than 100 eggs per gram of faeces) by 12 months after treatment, and were considered to be susceptible. Out of 70 children who showed low reinfection intensities during the same period (less than 30 eggs per gram), 35 showed high levels both of total water contact and of contact with sites containing infected snails. In these children, the relative lack of reinfection could not be attributed to a lack of exposure, and they were classified as resistant to reinfection. Comparison of the two groups, resistant and susceptible, revealed no difference in pretreatment intensities of infection. However, there was a marked difference in age, the mean age of the resistant group being two years greater than that of the susceptible group, within a restricted starting age range. These findings indicated that resistance was an acquired and age-dependent phenomenon, not obviously related to previous egg-induced pathology. Studies of immune responses revealed no clearcut correlate of resistance, but there were interesting differences between the two groups. Whereas anti-egg antigen responses declined after treatment to a greater extent in the resistant than in the susceptible group, antibodies mediating eosinophil-dependent killing of schistosomula rose markedly in both groups, strongly suggesting that the resistant children were being exposed to cercariae. Anti-adult worm antibodies rose sharply in both groups immediately after treatment, and thereafter declined to pretreatment levels. Although some individual children showed high levels of IgE anti-schistosomulum antibodies, there were no significant differences between the two groups. Since all children showed detectable levels of antibodies mediating eosinophil-dependent killing of schistosomula, the possibility was considered that such antibodies might be a necessary, but not a limiting, factor in immunity. Instead, the functional state of the effector cells mediating antibody-dependent killing might be limiting. Eosinophil levels, measured as an indirect estimate of eosinophil functional activity, did not differ between the two groups. There were, however, marked differences between different individuals in their capacity to produce eosinophil-stimulating monocyte mediators, and although this cannot yet be related to resistance, this aspect is worth further study.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult resistance to schistosomiasis mansoni: age-dependence of reinfection remains constant in communities with diverse exposure patterns.

In a fishing community on Lake Albert in Uganda the pattern of intensity of Schistosoma mansoni infection 6 months after treatment with praziquantel was found to be very similar to reinfection patterns seen in previously studied endemic communities: the profile peaks sharply at around the age of 10 years falling away rapidly to much lower levels in adults. This is in stark contrast to the patterns of water contact, which differ greatly between fishing and non-fishing communities. On Lake Albert, adults appear to be more heavily exposed than children. From these observations we conclude that adults are physiologically (perhaps immunologically) more resistant to infection after treatment than children.

Immunity in human schistosomiasis mansoni: prevention by blocking antibodies of the expression of immunity in young children

A total of 129 children were treated for Schistosoma mansoni infections, and followed for intensity of reinfection at 3-monthly intervals over a 21-month period. Blood samples were taken before treatment and at 5 weeks and 6, 12 and 18 months after treatment. This paper presents a statistical analysis of the relationship between various immune responses and subsequent reinfection. Responses analysed were: blood eosinophil levels; IgE antibodies against schistosomulum antigens; IgG antibodies mediating eosinophil-dependent killing of schistosomula; antibodies inhibiting the binding to schistosomulum antigens of two rat monoclonal antibodies that also recognize egg antigens; the levels of anti-adult worm and of anti-egg (total, IgM and IgG) antibodies; and IgM anti-schistosomulum antibodies. Results for each assay were well correlated for each of the five separate blood samples. None of the assays were predictive of resistance to reinfection, but susceptibility to reinfection was strongly correlated with results in the preceding blood samples for total anti-egg antibodies and the inhibition of binding of one of the two monoclonal antibodies. Further analysis also revealed a correlation between reinfection intensities and both IgM anti-schistosomulum antibodies and IgM and IgG anti-egg antibodies. These results are consistent with the hypothesis that early infections elicit the development, in response to egg antigens, of antibodies that block immune mechanisms directed against schistosomula. Blocking antibodies may be IgM, but might also be of an ineffective IgG isotype. The existence of such antibodies in young children would explain the slow development of immunity in the face of a range of detectable, potentially protective immune responses.

Immunity after treatment of human schistosomiasis mansoni. I. Study design, pretreatment observations and the results of treatment

This paper describes the design of a study on immunity to reinfection after treatment of children with Schistosoma mansoni infections, the initial observations on transmission that led to the selection of the study population, the effects of treatment, and the results of immunological tests carried out before and at five weeks after treatment. Iietune village in Machakos District, Kenya, was selected on the basis of high prevalence and intensities of infection in a small preliminary survey, a stable population living in a small area amenable to detailed study, and a lack of previous intervention in the area. Subsequent observations over a pretreatment period of one year confirmed that prevalence and intensities of infection among children attending the local primary school were high. This was associated with extensive contact of members of the community with water-bodies shown to contain large numbers of infected snails. Analysis of pretreatment intensities of infection and water contact patterns in the schoolchildren allowed the selection of 129 children showing a broad scatter between: (a) high intensity, low water contact, and predicted to be non-immune, and (b) low intensity, high water contact, and predicted to be immune. These children were treated with oxamniquine, 30 mg/kg in divided doses. Five weeks after treatment, 70% of children showed apparent complete cure, and the over-all reduction in geometric mean egg output was 98.9%. Since these children represented only a small proportion of the whole community, there was no obvious reduction in transmission, as reflected by snail infection rates, during the following five-month period. Thus, we are in a position to determine whether successfully treated children do or do not become reinfected in a high transmission environment in which it will be possible to make direct estimates of exposure. Immunological tests carried out immediately before treatment were consistent with a pattern of high exposure leading to the early expression of immune responses in most infected children. Eosinophil levels were elevated in 61% of the children, all of whom showed detectable levels of antibodies against adult worm and egg antigens, as measured by ELISA. In addition, all patients showed antibodies capable of mediating eosinophil-dependent killing of schistosomula. At five weeks after treatment, eosinophil counts and anti-adult worm antibody levels had risen, whereas anti-egg antibodies remained grossly unchanged. The wide variation in the levels of responses shown by different individuals will allow us to test whether such responses are associated with resistance to reinfection during the follow-up period.

A statistical approach to schistosome population dynamics and estimation of the life-span of Schistosoma mansoni in man

Dynamic models which predict changes in the intensity of schistosome infection with host age are fitted to pre-intervention Schistosoma mansoni data from Kenya. Age-specific post-treatment-reinfection data are used to estimate the force of infection, thus enabling investigation of the rate of worm death. An empirical and statistical approach is taken to the model fitting: where possible, distributional properties and function relationships are obtained from the data rather than assumed from theory. Attempts are made to remove known sources of bias. Maximum likelihood techniques, employed to allow for error in both the pre-intervention and reinfection data, yield confidence intervals for the worm life-span (CI95% = 5.7-10.5 years) and demonstrate that the worm death rate is unlikely to vary with host age. The possibilities and limitations of fitting dynamic models to data are discussed. We conclude that a detailed, quantitative approach will be necessary if progress is to be made with the interpretation of epidemiological data and the models intended to describe them.

Comparison of different chemotherapy strategies against Schistosoma mansoni in Machakos District, Kenya : effects on human infection and morbidity

A comparison was made of the long-term impact of different methods of administration of chemotherapy (oxamniquine, 30 mg/kg in divided doses; or praziquantel, 40 mg/kg) on prevalence and intensity of Schistosoma mansoni infection in four areas in Kangundo Location, Machakos District, Kenya. In Area A, treatment was offered in October 1983 and again in April 1985 to all infected individuals. In Area H, treatment was offered in April 1985 to individuals excreting greater than or equal to 100 eggs per gram (epg) of faeces. In Area S, treatment was offered in April 1985 to all infected school children, within the framework of the primary schools. In the witness area, Area W, treatment was given in April 1985, for ethical reasons, to a small number of individuals excreting greater than or equal to 800 epg. Prevalence and intensities of infection were subsequently monitored at yearly intervals for three complete post-treatment years. In the Area S schools, clinical examination was also carried out at yearly intervals. Treatment of all infected individuals on two occasions (Area A) was the most effective and long-lasting way of reducing prevalence and intensity of infection. In this area, however, some earlier interventions had been carried out and pre-treatment intensities were lower than in the other areas. Treatment only of infected schoolchildren (Area S) also had a marked and prolonged effect, comparable to or better than treatment of individuals with heavy infections (Area H). Treatment of infected schoolchildren also caused a persistent reduction in the prevalence of hepatomegaly, and there was suggestive evidence from intensities of infection in community stool surveys (but not from incidence rates) of an effect on transmission. In all study areas, reinfection was most rapid and most intense among children. These findings are discussed in the light of theoretical considerations and of results from other studies, both on schistosomiasis and on intestinal helminths. We conclude that, in areas of low morbidity such as Kangundo, chemotherapy of schoolchildren only, at intervals of up to 3 years, is a satisfactory way of producing a long-term reduction in both intensity of infection and morbidity.

On the use of age-intensity data to detect immunity to parasitic infections, with special reference to Schistosoma mansoni in Kenya

We consider two phenomena, related to the host age-intensity profiles of parasitic infections, which have been suggested to be indicative of acquired immunity: (i) a lower age of peak intensity among more intensely infected hosts; and (ii) a decline with age in the dispersion of the distribution of parasites between hosts. We demonstrate that these phenomena occur among Kenyan schoolchildren infected with Schistosoma mansoni, although the magnitude of both is small. We also examine the mathematical models underlying these predictions and conclude that both phenomena are possible in the absence of acquired immunity or, indeed, in the absence of any density-dependent effect. In our opinion, insufficient attention has been focused upon mathematical models, describing the null hypothesis, i.e. density-independent models. In particular, we regard the usual assumptions made for the two stochastic components of these models, describing the heterogeneity between hosts and the probabilistic nature of infection and death of parasites, as too rigid and unrealistic. We demonstrate that deviation from these assumptions undermines the qualitative distinctions between models which describe acquired immunity or density dependence and those which are density-independent.

Frequent Umbilical Cord—Blood and Maternal-Blood Infections with Plasmodium falciparum, P. malariae, and P. ovale in Kenya

The prevalence of malaria infection in 102 paired maternal-blood and umbilical cord-blood samples was assessed by microscopy and polymerase chain reaction (PCR) in a holoendemic area in Kenya. Plasmodium falciparum single-species infection was detected in maternal peripheral blood (3.4%), whereas microscopy indicated that no Plasmodium species were in cord blood. In contrast, maternal-blood samples showed a PCR prevalence of 48% for P. falciparum, 25% for P. malariae, and 24% for P. ovale, and cord-blood samples showed a PCR prevalence of 32%, 23%, and 21%, respectively. Although mothers with mixed-species infections were more likely to have offspring infected with mixed species, the specific malaria species were discordant in paired maternal- and cord-blood samples. Triple-species infections were observed in 11 cord- and maternal-blood samples at a 5.5-fold greater frequency than expected. These findings indicate that Plasmodium species infections in cord blood are common, occur at lower densities, and may be acquired before parturition.

Cytokine Responses to Plasmodium falciparum Liver-Stage Antigen 1 Vary in Rainy and Dry Seasons in Highland Kenya

ABSTRACT Seasonal epidemics of malaria occur in highland areas of western Kenya where transmission intensity varies according to rainfall. This study describes the seasonal changes in cytokine responses toPlasmodium falciparum liver-stage antigen 1 (LSA-1) by children (≤17 years old) and adults (≥18 years old) living in such a highland area. Fourteen- to 24-mer peptides corresponding to the N- and C-terminal nonrepeat regions of LSA-1 stimulated production of interleukin-5 (IL-5), interleukin-10 (IL-10), gamma interferon (IFN-γ), and tumor necrosis factor alpha (TNF-α) by peripheral blood mononuclear cells (PBMC) from 17 to 73% of individuals in both age groups in both seasons. IL-10 and TNF-α responses were more frequent during the high-transmission, rainy season than during the low-transmission, dry season (73 and 67% versus 17 and 25% response rates, respectively). In contrast, there was no seasonal change in the proportion of LSA-1-driven IFN-γ and IL-5 responses. Children produced less IFN-γ than adults, but IL-5, IL-10, and TNF-α levels were similar for both age groups. Depletion of CD8+ cells from PBMC decreased IFN-γ but increased IL-10 production. Individuals with LSA-1-stimulated IL-10 responses in the dry season were less likely to become reinfected in the subsequent rainy season than those without IL-10 responses (25% versus 49%;P = 0.083). These data support the notion that maintenance of LSA-1-driven IL-10 and TNF-α responses requires repeated and sustained exposure to liver-stage P. falciparum. In contrast, IFN-γ responses increase slowly with age but persist once acquired. CD8+ T cells are the major source of IFN-γ but may suppress production or secretion of IL-10.

Evidence for predisposition of individual patients to reinfection with Schistosoma mansoni after treatment

Statistical analysis of the relationship between intensities of infection before treatment and during reinfection after treatment in a sample of 119 Kenyan schoolchildren demonstrated a positive association, indicating that the individuals differed consistently in their tendency to become infected. This association was stronger in young children but the trend was detectable in older individuals. Possible reasons for this variation and for its apparently greater influence in younger age groups are discussed.