Schistosomiasis in the Republic of São Tomé and Principe: characterization of Schistosoma intercalatum

Prevalence of intestinal parasitic infections among school children in capital areas of the Democratic Republic of São Tomé and Príncipe, West Africa

BACKGROUND

Although the Democratic Republic of Sao Tome and Principe (DRSTP) has undertaken school children-based deworming programs against intestinal parasitic infections (IPIs) using a single dose of mebendazole annually since 2005, it remains unclear as to the outcome to date. The present study intends to investigate the recent IPIs status among school children living in capital areas of the DRSTP.

METHODS

A total of 252 school children (121 boys and 131 girls) of grades 4 and 5 from 4 primary schools located in the capital areas participated in the present study and their fresh fecal specimens were examined for the presence of any parasites using the merthiolate-iodine-formaldehyde concentration method as conducted.

RESULTS

The overall prevalence of IPIs was 64.7% (163/ 252). No significant gender difference in prevalence between boys (67.8%) and girls (61.8%) was found (p = 0.3). The majority of school children were infected with a single species of parasite (55.8%). Altogether, 12 different intestinal parasite species were identified in DRSTP school children, of which 9 species were pathogenic and the remaining 3 were non-pathogenic.

CONCLUSION

Improving the detection method, sanitation facilities and personal hygiene as well as utilizing combined drugs are all important measures to greatly reduce IPIs in DRSTP school children.

Natural Interactions between S. haematobium and S. guineensis in the Republic of Benin

Schistosomiasis is a parasitic disease which affects millions of people around the world, particularly in Africa. In this continent, different species are able to interbreed, like Schistosoma haematobium and Schistosoma guineensis, two schistosome species infecting humans. The Republic of Benin is known to harbor S. haematobium, but its geographical situation in between Nigeria, Mali, and Burkina Faso, where S. guineensis was found, raises the question about the possible presence of S. haematobium/S. guineensis hybrids in this country. We conducted morphological analyses on schistosome eggs and molecular analyses on schistosome larvae (high resolution melting (HRM) analysis and gene sequencing) in order to detect any natural interaction between these two species of schistosomes. The morphological results showed the presence of three egg morphotypes (S. haematobium, S. guineensis, and intermediate). Three genotypes were detected by ITS2 rDNA HRM analysis: S. haematobium, S. guineensis, and hybrid, and their percentages confirmed the results of the morphological analysis. However, sequencing of the CO1 mtDNA gene showed that all the samples from Benin belonged to S. haematobium. Our results provide the first evidence of introgression of S. guineensis genes in S. haematobium in Benin.

Application of single strand conformational polymorphism (SSCP) analysis with fluorescent primers for differentiation of Schistosoma haematobium group species

To assess the utility of single-stranded conformational polymorphism (SSCP) analysis for the differentiation of schistosomes, using methods adapted for a Perkin Elmer ABI Prism 377 automated sequencer, 3 isolates of Schistosoma haematobium, 2 of S. intercalatum and single isolates of S. curassoni and S. bovis were selected for study. Two fluorescently labelled, double-stranded polymerase chain reaction products, amplified from the mitochondrial cytochrome oxidase subunit 1 (CO1) gene and the nuclear ribosomal second internal transcribed spacer (ITS2), were generated from single male and female worms. Changes in electrophoretic mobility of fragments within an SSCP profile revealed variation at individual, isolate and species levels. The mutational basis between representative SSCP profiles was confirmed by direct sequencing, demonstrating that single point substitutions were detectable. SSCP analysis has considerable potential as an alternative molecular method of identification and characterization of schistosomes. More broadly, fluorescence-based SSCP analysis is applicable to almost any gene target from any species of parasite and is a powerful molecular tool for genetic profiling.

Epidemiological and genetic observations on human schistosomiasis in Kinshasa, Zaire

A survey for Schistosoma intercalatum conducted in Kinshasa, Zaire, in September 1994 revealed a prevalence of 3.6% (n = 167). Three isolates of schistosomes were made by exposing Bulinus wrighti to miracidia hatched from eggs collected from 2 infected children. Characterization of the isolates by biochemical (isoenzymes of phosphoglucomutase), molecular (restriction fragment length polymorphism and randomly amplified polymorphic deoxyribonucleic acid analysis) and morphological (egg measurements) techniques confirmed the existence of an autochthonous transmission focus of S. intercalatum in Kinshasa. The study also provided evidence of the occurrence of natural hybridization between S. intercalatum and S. haematobium. No potential snail host for either species was found in the 2 rivers examined. Apart from Bu. globosus from Zambia and Bu. wrighti, snail infection experiments showed an incompatible relationship between the parasite isolates and snails belonging to the Bu. forskalii group, the Bu. iruncatus/Bu. tropicus complex, and the Bu. africanus group.

Competitive exclusion in human schistosomes: the restricted distribution of Schistosoma intercalatum

A study on the immune and mating interactions between Schistosoma intercalatum and S. mansoni has been carried out. The immunologically mediated interactions and mating patterns observed between the two species demonstrate that S. mansoni is competitively dominant over S. intercalatum. When S. mansoni females are absent, S. mansoni males are more competitive than S. intercalatum males at pairing with S. intercalatum females. The use of a mathematical model shows how the abundance of S. mansoni worms per host could dramatically affect the mating probability of S. intercalatum females with their homologous males. It appears that the competitive exclusion between these two species of schistosomes may probably be an important factor restricting the distribution of S. intercalatum in Africa.

Morphometric variability of Schistosoma intercalatum eggs: a diagnostic dilemma

Variability of Schistosoma intercalatum eggs in shape and size, and their similarity to those of S. haematobium presented a problem of species identification when egg morphology was the diagnostic criterion used in a study of human schistosomiasis conducted on São Tomé and Principe. More than 2500 egg measurements were obtained by light micoscopy to gather data relating to size variability of S. intercalatum eggs, to evaluate whether factors such as age of host, sex of host and intensity of infection are correlated with variability, and the data were compared with previously published measurements on different isolates and strains of S. intercalatum: the range in length (104-203 microns) embraces most of the measurements reported in other studies of S. intercalatum eggs. There was no correlation either between age and sex of the host, or intensity of infection with variability of egg size. Comparison between measurements of the eggs of S. haematobium, S. intercalatum and S. bovis eggs are presented.

Schistosomiasis in the Republic of São Tomé and Principe: human studies

The only schistosome species found in stool specimens in the local population of the republic of São Tomé is Schistosoma intercalatum. An initial survey of schoolchildren showed an overall prevalence of 10.9%, with some schools reaching 29%. No S. haematobium egg was found in 782 urine specimens from the local population, although some were seen in the urine of Angolan soldiers stationed near the capital city. One village in the endemic area, San Marçal, had an S. intercalatum prevalence of 43%, with 14 persons > 40 years of age harbouring severe infections. The transmission area is restricted to the north-east of the main island, where 5 foci apparently account for most of the infections. Seven cases recorded from Principe may be explained by the fact that the children were attending school at São Tomé. Women carrying out domestic activities are more at risk of contracting the infection because of longer periods of water contact than men. The morbidity produced by the infection is restricted to splenomegaly and blood in the stools. High prevalences have been found of Ascaris lumbricoides and Trichuris trichiura, and hookworm and Stronglyloides stercoralis were also observed. Praziquantel was well tolerated and appears to be a good tool for control purposes, although reinfection in the transmission area apparently occurs rapidly. Control strategies based on chemotherapy should take into account an older age group as well as the schoolchildren. Focal mollusciciding and the introduction of washing facilities may also have a role to play in control. The possible recent introduction of the infection to the island is discussed.