Egg excretion patterns as criterion for the determination of prevalence and intensity of infection in male and female individuals in different age groups in a schistosome endemic area in the Limpopo Province
Urinary schistosomiasis is a disease caused by the worm parasite Schistosoma haematobium. Approximately 3 million people of whom the majority are children, staying in the sub-tropical areas of the Limpopo, Mpumalanga and Kwa-Zulu-Natal provinces, are infected with this parasite. Although the initial pathology of heavy infections is limited to blood in the urine, long term infections may lead to, amongst others, hydronephrosis and cancer of the squamous epithelial tissue of the bladder.
Investigations that aim to collect information regarding the general epidemiology of schistosomiasis in a speciﬁc area, as well as the prevalence and intensity of infection, are primarily based on the presence of schistosome eggs in the urine. The results of these studies which are mainly done by making use of a single urine sample collected between 10:00 and 14:00 are often used as criteria to express the degree of morbidity, the prevalence and intensity of infection, worm load burden, as well as the presence of genital lesions and urinary tract disease in females. This is, however, only possible if the number of eggs excreted at a speciﬁc time is constant. Although there are serological techniques available to diagnose urinary schistosomiasis, parasitological methods are in spite of the above mentioned imperfections, still preferred for large scale epidemiological studies because of its cost effectiveness. It is however, of paramount importance to collect more information regarding the patterns of egg excretion as it occurs in males, as well as females in different age groups.
This study was done in the Mamitwa Village in the Limpopo Province. Urine samples were collected at 8:30, 10:30, 12:30, 14:30 and 16:30 from schistosome positive males and females in the 4-9, 10-14 and 15-22 age groups. This was done for 6 consecutive days in the 4-9 and 10-14 year age groups and for four consecutive days in the 15-22 year age group. The eggs in each sample were separated from the urine by means of ﬁltration and ﬁxed with 4% formaldehyde after which they were counted. The number of eggs were expressed as the number per 10ml urine and categorized as not infected (no eggs per 10ml), lightly infected (1-50 eggs per 10ml), mildly infected (51-150 eggs per 10 ml) and heavily infected (>150 eggs per 10ml). The results were expressed as percentage per time of collection and statistically analysed by making use of Anova- and Tukey tests.
Although there was signiﬁcant variation in the results when the different age groups in both the male and females were compared, a few constant tendencies were evident. In spite of the fact that only schistosome positive individuals were selected for this investi-gation and that a number of urine samples were collected from each individual over a number of days, false negatives were found at each collection period for all the age groups in both sexes, a phenomenon that was also true with regard to the intensity of infection. The number of false negatives was signiﬁcantly lower for the females in the 4-9 year age group at the 12:30 collection period. This was also true for the females in the 10-14 year age group between 10:30 and 12:30 but no signiﬁcant results could be found at the remaining groups. When comparing the percentages of infection in all the intensity categories for every collection period, urine collected from 10:30 onwards revealed the most true positive cases, while this was the case from 8:30 at the 15-22 year age groups.
The fact that the 15-22 age group had the most false positives was expected because it corresponds with the decrease in intensity, as well as the prevalence of infection with an increase in age. The peak in the intensity of infection found in the 10-14 year age groups is probably due to numerous contacts with schistosomiasis infested water at this stage. The most mildly infected females in the 4-9 year age group were found between 10:30 and 14:30 while the most heavily infected individuals in this group were found between 12:30 and 14:30. No peaks were recorded for the males in this age group. With regard to the 10-14 year age group, a peak in the mildly infected females was evident between 10:30 and 14:30 and between 12:30 and 16:30 for the heavily infected category. A peak in infection for the heavily infected category for males in this age group was found between 10:30 and 16:30. No such peak was found in the females in the 15-22 year age group while there was only a peak in the heavily infected category in the males of this age group.
From this study it can be concluded that in spite of numerous urine collections from infected individuals 100% prevalence could not be found in any of the groups during any of the collection periods. It was further evident that the general accepted time of a peak in excretion between 12:00 and 14:00 is not necessarily the best to establish the prevalence of infection in a community, especially when such an investigation also takes the intensity of infection into account. It is further evident that more urine samples at different times should be collected in order to increase the accuracy of the results.