Immunity in human schistosomiasis mansoni. Regulation of protective immune mechanisms by IgM blocking antibodies.

After the demonstration of blocking antibodies during rat experimental schistosomiasis, the existence of such factors was investigated in human schistosomiasis. The depletion, in sera from S. mansoni-infected patients, of a given isotype (IgM) either by protein A-Sepharose (PAS) absorption or by fast protein liquid chromatography (FPLC) induced a significant increase in IgG-mediated killing of S. mansoni schistosomula by human eosinophils. Inhibition experiments showed that IgM-enriched fractions (PAS effluents) were able to inhibit eosinophil-dependent cytotoxicity mediated by IgG fractions (total sera or PAS eluates). Both IgG and IgM antibodies from infected human sera immunoprecipitated antigens of 30,000-40,000 Mr in the labeled detergent extracts of schistosomulum surface. The specificity of IgG and IgM for the 38,000 Mr antigen was suggested by competition experiments using two radiolabeled mAbs (IPLSm1, IPLSm3) directed against this antigen. Moreover, crossinhibition between IgG and IgM antibodies for the Mr 38,000 antigen could be directly demonstrated. The in vivo relevance of such IgM blocking antibodies in the context of human immunity to schistosomiasis was evaluated in two groups of children classified as resistant or susceptible to posttreatment reinfection. IgM antibodies specifically directed against the 38,000 Mr antigen were measured by a capture assay. The mean levels of IgM antibodies were significantly higher in the susceptible than in the resistant group both before and after treatment. These results are consistent with the idea that immunity to schistosomiasis could be attributable not only to the existence of antibodies with defined effector function, but also to the absence of blocking antibodies. The description of the existence in human schistosomiasis of antibody isotypes blocking the effector response against defined surface targets might lead to a new understanding of the mechanisms regulating immunity to reinfection against schistosomes and possibly other parasites.

Immunity to schistosomes: progress toward vaccine.

Among the major parasitic infections, schistosomiasis may be the most promising candidate for human vaccination. Information about mechanisms of immunity, gained mainly from experimental models but likely to be relevant to human infection, indicates a dynamic balance between protective and regulatory (blocking) mechanisms. Besides cell-mediated responses leading to macrophage activation, antibody-dependent cell-mediated cytotoxicity systems involving precise antibody isotypes and nonlymphoid cells (mononuclear phagocytes, eosinophils, and platelets) appear to be essential effectors of immune attack. The slow development of immunity in humans seems related to the production of antibodies that cross-react with schistosomulum surface antigen and block the binding of antibodies of the effector isotype. Schistosomes that survive in the bloodstream and produce chronic infections may evade the immune system as a result of intrinsic changes in membrane susceptibility and of transient expression of target antigens; at other stages of the parasite life cycle, cross-reactive molecules may be secreted that play an essential role in the induction of immunity. Several schistosome proteins have been characterized as candidates for vaccination. Among these, an antigen of 28 kilodaltons has been cloned and shown to be immunogenic in humans and protective in mice, rats, and baboons.

Hepatosplenomegaly associated with chronic malaria exposure: evidence for a pro-inflammatory mechanism exacerbated by schistosomiasis.

In sub-Saharan Africa, chronic hepatosplenomegaly, with palpable firm/hard organ consistency, is common, particularly among school-aged children. This morbidity can be caused by long-term exposure to malaria, or by Schistosoma mansoni, and it is exacerbated when these two occur together. Although immunological mechanisms probably underlie the pathogenic process, these mechanisms have not been identified, nor is it known whether the two parasites augment the same mechanisms or induce unrelated processes that nonetheless have additive or synergistic effects. Kenyan primary schoolchildren, living in a malaria/schistosomiasis co-transmission area, participated in cross-sectional parasitological and clinical studies in which circulating immune modulator levels were also measured. Plasma IL-12p70, sTNF-RII, IL-10 and IL-13 levels correlated with relative exposure to malaria, and with hepatosplenomegaly. Soluble-TNF-RII and IL-10 were higher in children infected with S. mansoni. Hepatosplenomegaly caused by chronic exposure to malaria was clearly associated with increased circulating levels of pro-inflammatory mediators, with higher levels of regulatory modulators, and with tissue repair cytokines, perhaps being required to control the inflammatory response. The higher levels of regulatory modulators amongst S. mansoni infected children, compared to those without detectable S. mansoni and malarial infections, but exposed to malaria, suggest that S. mansoni infection may augment the underlying inflammatory reaction.

Chemotherapy-based control of schistosomiasis haematobia. IV. Impact of repeated annual chemotherapy on prevalence and intensity of Schistosoma haematobium infection in an endemic area of Kenya.

To determine the effect of repeated, annual, age-targeted therapy on prevalence and intensity of Schistosoma haematobium infection in an endemic area, we treated all available, infected, school-age children (n = 2, 493) in the Msambweni area of Coast Province, Kenya with a randomized protocol of oral metrifonate (10 mg/kg for three doses each year) or praziquantel therapy (40 mg/kg as a single dose each year) for a period of one to three years. During 1984-1987, 1, 101 children completed three years of therapy, 550 received two years, and 842 received a single year. Annual followup revealed significant long-term suppression of S. haematobium infection in the targeted school-age population. Both cross-sectional analysis and study of individual outcomes suggested maximal suppression of infection after two years of therapy. Suppression lasted more than two years after cessation of treatment, and was associated with reduced community transmission (gauged by decreased prevalence among new study entrants and decreasing negative-to-positive conversion on annual parasitologic examinations). Comparison of metrifonate and praziquantel outcomes indicated greater suppression of infection and longer infection-free intervals for some subgroups given praziquantel. We conclude that annual population-based therapy targeted to schoolchildren has direct and indirect beneficial effects for endemic communities. In some specific situations, repeat therapy may not suppress transmission, and reduced drug efficacy may be observed after one to three years, suggesting the need for additional non-drug control measures in highly endemic villages.

Dynamics and control of Schistosoma haematobium transmission in Kenya: an overview of the Msambweni Project.

Ecologic modeling of Schistosoma transmission in endemic communities has suggested that antiparasite therapy targeted at the most heavily infected segment of the human population (i.e., school-age children) should have a significant impact on local parasite transmission. Our 1984-1991 experience with age-targeted therapy in the Msambweni area of Kenya has shown an overall decrease in area transmission within 1-2 years following initiation of annual treatment of school-age groups. Snail monitoring confirmed a continuing but variable reduction of vector infection rates. However, subgroup analysis showed significant differences in transmission suppression between more developed coastal villages with piped-water kiosks and villages with only limited access to safe water supplies. Villages without piped water were marked by higher initial prevalences of S. haematobium infection, greater prevalence among adults, longer and more frequent contact with high-risk water sources, and persistently high transmission despite compliance with parasitologic screening or drug therapy. We conclude that targeted therapy had a significant impact on S. haematobium transmission in some areas, but that more extensive or more prolonged coverage is necessary to reduce the rate of new infection in high-risk villages. Defining field-use algorithms, based on decision analysis of economic and ecologic parameters, should provide effective guidelines for selective versus mass treatment in expanded control areas.

Studies on schistosomiasis in western Kenya: I. Evidence for immune-facilitated excretion of schistosome eggs from patients with Schistosoma mansoni and human immunodeficiency virus coinfections.

Persons employed as vehicle washers in the town of Kisumu, Kenya are exposed for several hours each day to water in Lake Victoria that contains Schistosoma mansoni-infected Biomphalaria pherifferi snails. This results in a focus of high endemicity for schistosomiasis and these persons have very high concentrations of eggs in their feces (mean +/- SD = 1,469 +/- 1,581 eggs per gram [EPG] of feces). Fecal egg counts, but not circulating cathodic antigen (CCA) levels, in these schistosomiasis patients differed strikingly based on the patient's seropositivity for human immunodeficiency virus (HIV). Patients who were infected with S. mansoni and were seropositive for HIV had similar levels of CCA but excreted fewer eggs (643 +/- 622 EPG; n = 16) than individuals who were not seropositive for HIV infection (1,891 +/- 1,779 EPG; n = 37) (P = 0.009). Egg excretion ratios (EPG/CCA) of the seronegative group were also significantly higher than those of the seropositive group. Those in the seropositive group showed a significant correlative relationship between egg excretion ratios and CD4+ lymphocyte percentages. These observations are compatible with the hypothesis that schistosome eggs exit the human host through the requisite facilitation of functional immune responses, and that the efficacy of this process decreases in schistosomiasis patients co-infected with HIV as their peripheral blood CD4- cell levels decrease.

Immunity and morbidity in Schistosoma mansoni infection: quantitative aspects.

Immunity to Schistosoma mansoni infection in humans can be studied most easily by monitoring serially the intensity of reinfection that occurs among individuals who have undergone chemotherapeutic cure, and whose levels of exposure to contaminated water is subsequently observed. Parallel studies can then be made of those immune responses that are correlated with an observed resistance to reinfection. This paper describes some of the difficulties associated with this approach, with particular reference to the authors' own studies in Kenya, and highlights a possible role of immunoglobulin E antibodies against adult worm antigens in mediating immunity.

Urinary tract morbidity in schistosomiasis haematobia: associations with age and intensity of infection in an endemic area of Coast Province, Kenya.

To gain better understanding of the natural history of Schistosoma haematobium associated disease, age- and intensity-related urinary tract morbidity were assessed in a cross-sectional study of Kilole (population 719) in Coast Province, Kenya. Overall prevalence of infection was 65% (39% light, 16% moderate, 9% heavy). Infection prevalence and mean intensity of infection were highest in the 5-14-year-old bracket for both sexes. Although significant intensity-associated increases in hematuria prevalence were noted for both children and adults in all infection categories, hematuria was more common in those less than 15 years of age. Children had a significant increase in the prevalence of dysuria at higher levels of infection, whereas adults did not. Radiographic study of a 1:9 random sample, stratified for age, revealed a greater prevalence of urinary tract granulomas in those less than 15 years. Subjects greater than 15 years of age had a greater frequency of hydronephrosis. Hydronephrosis, hydroureter, and bladder calcification were not associated with higher infection intensity. Among individuals with bladder calcification, a potential marker of cumulative inflammation, 87% had hydronephrosis or hydroureter, compared to a 40% prevalence among individuals without bladder calcification. These findings suggest that certain structural forms of urinary tract disease, such as hydronephrosis, progress during the course of untreated schistosomiasis haematobia despite age-related reductions in egg burden, whereas other forms of morbidity, such as hematuria, remain sensitive to the level of urinary egg excretion at the time of diagnosis.

Effect of mass chemotherapy and piped water on numbers of Schistosoma haematobium and prevalence in Bulinus globosus in Kwale, Kenya.

From June 1982 to May 1986 in a small village in Kwale, Kenya, we studied seasonal fluctuations in populations of Bulinus globosus, prevalence of Schistosoma haematobium infection in this snail, and effects of chemotherapy and piped water supply on infection rate of snails. In the perennially-flowing Pemba River, relatively small numbers of snails were collected; they were found only during the hot dry season (December to March). In a tributary stream, the Kadingo River, whose flow ceased at the end of both the cool and hot dry seasons, snail numbers peaked at the end of the cool dry season (October to November) and at the beginning of the hot dry season (January). Large numbers of infected snails were found in the Kadingo River from November to January (short rainy season and beginning of dry season). Selective mass chemotherapy with metrifonate and provision of piped water were begun in February and March 1984. These control measures achieved a significant reduction in the infection rate of snails (P less than 0.001); the annual infection rate for the 2 years before treatment was 9.3% and 13.1%, and for the 2 years after treatment was 3.5% and 3.4%.

Dose-finding study for praziquantel therapy of Schistosoma haematobium in Coast Province, Kenya.

To assess the efficacy of low dose praziquantel regimens in comparison with standard 40 mg/kg dosing in the treatment of urinary schistosomiasis, a random allocation dose-finding trial was performed in children and adults from a Schistosoma haematobium endemic region in Coast Province, Kenya. Following an initial screening, 280 individuals with greater than or equal to 50 eggs/10 ml urine were randomly assigned to receive either 10, 20, 30, or 40 mg/kg of the drug in a single oral dose. Two to three months later, cure rates of 26%, 68%, 78%, and 84% were found for the 10, 20, 30, and 40 mg/kg doses, respectively. The results of 10 mg/kg oral dosing were significantly worse than for all other doses in terms of cure rate and of post-treatment prevalence of morbidity. The 40 mg/kg dosing resulted in a significantly higher cure rate than the 20 mg/kg doses; nevertheless, there was no significant difference between 20 mg/kg and 40 mg/kg doses in terms of mean post-treatment intensity of infection or post-treatment prevalence of hematuria or proteinuria. For large-scale control programs, oral 20 mg/kg praziquantel therapy for urinary schistosomiasis may prove as effective as the standard oral 40 mg/kg dosing for control of infection-associated morbidity and reduction of parasite transmission.

Identification of snails infected with schistosomes by ELISA employing monoclonal antibodies: Schistosoma mansoni in laboratory snails (Biomphalaria glabrata) and in field snails (Biomphalaria pfeifferi) from Kenya.

An enzyme-linked immunosorbent assay (ELISA) employing monoclonal antibodies was used for detecting Schistosoma mansoni antigens in hemolymph of laboratory snails (Biomphalaria glabrata) in Kenya. Infected laboratory snails shedding cercariae were differentially identified by ELISA from uninfected snails with 100% sensitivity and specificity. Prepatent infections were detected by ELISA from 2 weeks after exposure to miracidia. Thus, ELISA revealed infection 3 weeks before maximal patency was reached (5-6 weeks post-exposure). Infected field snails (B. pfeifferi) shedding cercariae were differentially identified by ELISA, with 100% sensitivity and specificity, from uninfected field snails and from snails naturally infected with other trematodes (echinostomes and strigeids). Prepatent infections with S. mansoni were readily identified by ELISA in field snails. A case is demonstrated where infection rate, as determined by shedding test alone, was 9.8%, whereas the combined figure of prepatent and patent infection rates was 22.9%

Urine circulating soluble egg antigen in relation to egg counts, hematuria, and urinary tract pathology before and after treatment in children infected with Schistosoma haematobium in Kenya.

A cohort of 117 school children infected with Schistosoma haematobium was followed-up after therapy with praziquantel (0, 2, 4, 6, 12, and 18 months) and various infection and morbidity parameters (egg counts, hematuria, soluble egg antigen [SEA] in urine, and ultrasonography-detectable pathology) were quantified. At the onset of the study, 97% of the children were positive for S. haematobium with a geometric mean egg count of 45.7 eggs/10 ml of urine. Eighty-one percent of the children were positive for SEA in urine with a geometric mean SEA concentration of 218.8 ng/ml of urine. Ninety-two percent and 56% of the children were microhematuria positive and macrohematuria positive, respectively. Two months after treatment, all infection and morbidity indicators had significantly decreased. Reinfection after treatment as determined by detection of eggs in urine was observed by four months post-treatment while the other parameters remained low. The clearance of SEA was slower than that of egg counts while pathology resolved at an even slower pace. Levels of SEA and egg output showed similar correlations with ultrasound detectable pathology; these correlations were better than the correlation between hematuria and pathology.

Long-term suppression of adult bladder morbidity and severe hydronephrosis following selective population chemotherapy for Schistosoma haematobium.

Repeated selective population chemotherapy of school age children reduces infection and morbidity associated with Schistosoma haematobium infection. To examine the long-term effect of this treatment on susceptibility to re-infection and late disease, a cohort of Kenyans (n = 194) were re-examined for infection and urinary tract morbidity 7-13 years after they underwent annual ultrasonography and treatment for an average of 5 years beginning in 1984 as children. Controls were previously untreated age-matched individuals residing in the same or adjacent villages. The overall prevalence and intensity of infection were equivalent between the 2 groups. In contrast, the prevalence of bladder wall pathology was 11-fold lower in previously treated (1.5%) versus untreated subjects (17%). Severe hydronephrosis was completely reversed. These data demonstrate that treatment significantly reduced urinary tract morbidity despite re-infection, and suggest that the important risk factors for urinary tract morbidity in adulthood are cumulative intensity and duration of infection during early adolescence.

Low-level Plasmodium falciparum transmission and the incidence of severe malaria infections on the Kenyan coast.

The transmission of Plasmodium falciparum was studied in relation to the incidence of severe malaria infections at Sokoke and Kilifi town, Kilifi District, Kenya. Intensive mosquito sampling during a one-year period yielded Anopheles gambiae s.l., An. funestus, and An. coustani. Anopheles gambiae s.l. was the predominant vector, comprising 87.9% and 97.9% of the total anophelines collected in Sokoke and Kilifi town, respectively. The proportion of An. gambiae s.l. with P. falciparum sporozoite infections was 4.1% (20 of 491) in Sokoke and 2.2% (3 of 138) in Kilifi town; no infections were detected in An. funestus or in An. coustani. Entomologic inoculation rates indicated that residents were exposed to only 8.0 infective bites per year in Sokoke and 1.5 in Kilifi town. Transmission was detected during only six months in Sokoke and three months in Kilifi town despite low-level, year-round vector activity. The yearly incidence of severe P. falciparum infections in children, 1-4 years of age was 24.1 per 1,000 in Sokoke and 4.2 per 1,000 in Kilifi town. Monthly patterns of transmission corresponded closely with the incidence of severe infections. At these sites on the coast of Kenya, the spatial and temporal incidence of severe malaria infections is associated with low-level P. falciparum transmission by vector populations.

Schistosomiasis haematobia in coast province Kenya. Relationship between egg output and morbidity.

Several studies of schistosomiasis haematobia in Africa have revealed a correlation between intensity of infection as measured by urine egg counts and severity of disease as determined by intravenous pyelography. The present study consisted of a survey of 390 school children in the coastal area of Kenya involving a single egg count, and intravenous pyelograms in a stratified random sample of 69 children; the results showed a greater prevalence of urinary tract disease in those with higher intensities of infection. This survey was then followed by a more detailed study in which nine consecutive daily egg counts were done on 121 children; 17 of these children, subdivided into three groups with different intensities in infection, were given intravenous pyelograms. The results were similar in the 11 children with minimal and moderate counts (averaging, respectively, less than 1 egg and 167 eggs/10 ml urine daily), with approximately 30% having bladder or renal abnormalities. In comparison, all of the six children with heavy counts (averaging 1,288 eggs/10 ml urine daily) had bladder lesions and five of them had renal lesions.

Studies on schistosomiasis in western Kenya: II. Efficacy of praziquantel for treatment of schistosomiasis in persons coinfected with human immunodeficiency virus-1.

Praziquantel is the drug of choice for schistosomiasis chemotherapy. Although the exact mechanism of how praziquantel kills schistosomes remains poorly understood, the immune response of the host is an important factor in drug efficacy. It is thus possible that disease states of humans that lead to immunodeficiencies, such as infection with human immunodeficiency virus-1 (HIV-1), may render praziquantel less effective in treating schistosomiasis. To test this hypothesis, persons with high levels of Schistosoma mansoni infection who were or were not also infected with HIV-1 were treated with a standard regimen of praziquantel and monitored by quantitative fecal examination and plasma circulating cathodic antigen. Both groups responded to praziquantel therapy equally and individuals with low percentages (< 20%) of CD4+ T cells did not differ from individuals with higher CD4 cell percentages. These data demonstrate that persons with HIV-1 infection can be treated effectively for schistosomiasis with praziquantel.

Predisposition to urinary tract epithelial metaplasia in Schistosoma haematobium infection.

Although there is strong epidemiologic evidence linking Schistosoma haematobium infection with carcinoma of the bladder, the utility of cytologic screening for urinary tract cancer has not been critically evaluated in S. haematobium-endemic populations. The present cross-sectional study examined urine cytology findings among 1,014 residents (ages 1 to 91) of the S. haematobium-endemic Msambweni area of Coast Province, Kenya. Among 705 evaluable cytology specimens, prevalence of inflammation (39%), hyperkeratosis (30%), metaplasia (33%), and frank atypia (0.4%) was notably higher than in previously studied, non-endemic populations. Overall, S. haematobium infection was strongly associated with increased risk for cytologic abnormality (> 2.8-fold relative risk of metaplasia or hyperkeratosis; P < 0.001). Age-group analysis confirmed parallel increases in metaplasia and S. haematobium infection prevalence early in life (from age I to 15 for both boys and girls). However, above age 20, metaplasia prevalence persisted at 33-45% prevalence despite a decline in infection prevalence and intensity. Prevalence of advanced (moderate or severe) metaplasia showed two age-related peaks: the first at 10-14 years of age (at the time of peak infection), and the second among subjects > or = 60 years old. No cancers were detected in the study population either on cytology or on follow-up ultrasound examination. These data suggest an age-dependent progression of cellular abnormalities in the urinary epithelium that is associated with chronic S. haematobium infection, which becomes independent of concurrent infection intensity as subjects grow older. Implications for cancer screening are discussed.

Relationships between Plasmodium falciparum transmission by vector populations and the incidence of severe disease at nine sites on the Kenyan coast.

The transmission of Plasmodium falciparum was studied in relation to the incidence of severe malaria infections at nine sites in the Kilifi District in Kenya. Intensive mosquito sampling during a one-year period yielded Anopheles gambiae s. l., An. funestus, An. coustani, An. squamosus, An. nili, and An. pharoensis. Anopheles gambiae s.l. was the predominant vector, comprising 98.4% of the total anophelines collected. Overall, 3.5% of 2,868 An. gambiae s.l. collected indoors and 0.8% of 261 collected outdoors contained P. falciparum sporozoites. Transmission was detected during 10 months, with peak periods from June to August and December to January. In eight of the nine sites, entomologic inoculation rates (EIRs) averaged only four infective bites per year (range 0-18); an annual EIR of 60 was measured for the site with the highest intensity of transmission. The incidence of severe malaria infections, ranging from 8.6 to 38.1 per 1,000 children (0-4 years), was not associated with EIRs. At these sites on the coast of Kenya, a high incidence of severe disease occurs under conditions of very low levels of transmission by vector populations. With respect to conventional approaches for vector control in Africa, decreases in transmission, even to levels barely detectable by standard approaches, may not yield corresponding long-term reductions in the incidence of severe disease.

Detection and quantification of soluble egg antigen in urine of Schistosoma haematobium-infected children from Kenya.

While research on alternative diagnostic and morbidity markers for infection with Schistosoma haematobium has been going on for a long time, egg counts continue to be used as the gold standard, and infection intensity is thought to reflect the severity of the disease. However, this relationship is not always clear and fluctuation in egg output makes it difficult to classify prevalence correctly. The use of circulating adult worm antigen detection as an alternative diagnostic technique has been applied with varying success. However, this is a measure of worm burden and does not reflect the tissue egg load(s). In the present study we have used an assay that detects soluble egg antigen (SEA) in urine of S. haematobium-infected children, and we have evaluated the applicability of the assay as a diagnostic and morbidity indicator. To evaluate this assay, we have studied a group of 470 children from two schools (Tsunguni and Kibaokiche) in the Coast province of Kenya; 84.8% and 77% were egg-positive while the percentage positive as determined by the SEA-ELISA were 78.8% and 76.2% in Tsunguni and Kibaokiche, respectively. In both schools, SEA levels in urine of S. haematobium-infected children significantly correlated with egg counts (Pearson's r=0.73, P < 0.0001) and with hematuria (Spearman's r=0.65, P < 0.0001). In addition, urinary tract pathology as determined by ultrasound significantly correlated with the SEA levels in urine (Spearman's r=0.3, P < 0.001). The SEA-ELISA compared well with microhematuria within egg count classes and with egg counts within hematuria classes.

Age-dependent reduction of schistosome fecundity in Schistosoma haematobium but not Schistosoma mansoni infections in humans.

Understanding the dynamics of schistosome infections is problematic because direct measurements of worm burden are not possible. Hitherto, the relative intensity of infection has been estimated by the number of parasite eggs excreted. Egg excretion is assumed to have a consistent relationship with worm burden with duration of infection. We have tested this assumption in Schistosoma mansoni- and S. haematobium-infected populations by looking at the relationships between a circulating parasite antigen, egg excretion level, host age, and parasite density. The study was carried out in two populations because experimental models suggested that S. haematobium but not S. mansoni suffers immune-mediated reduction of fecundity. The results were consistent with this observation, showing that S. mansoni egg output remains stable irrespective of host age or infection intensity while S. haematobium has a substantially reduced egg production with host age. This information is fundamental to understanding the immunology and epidemiology of human schistosomiasis and thus practical approaches to disease control.