Characteristics of the study participants
A total of 930 participants were enrolled in the present study, of these 513 (55.2%) and 417 (44.8%) were female and male respectively. The mean age of the study participants was 23.3 ± 18.5 years. Of the study participants 87.2% reported to have never participated in a mass drug administration campaign against schistosomiasis. Only 12.8% reported to have participated at least once in the program. Table 1 shows the age and sex distribution of the study participants. All participants were from Ijinga Island, the majority of the adult participants reported to be involved in subsistence farming, livestock keeping and fishing.
Prevalence and intensity of infection of Schistosoma mansoni using Kato Katz technique
The overall prevalence of S. mansoni was 68.9% (95%CI: 65.8-71.8). The prevalence of S. mansoni among male individuals was 74.3% (95%CI: 69.9-78.3) and 64.5% (95%CI: 60.3-68.6) in female participants. This gender difference was significant (χ2=10.3523, P<0.001). In the age group of 6-15 years the prevalence of S. mansoni was 86.1% (95% CI: 82.4-89.1) and among the 16-25 years old the prevalence was 57.1% (95%CI: 47.9-65.8). Compared to the other age groups these 2 age groups of SAC, adolescents and young adults had a significantly higher prevalence of S. mansoni (χ2=105.61, P<0.001). Table 2 shows the prevalence of S. mansoni categorized by sex and age.
The overall geometrical mean eggs per gram of faeces (GMepg) was 85.7epg (95%CI: 77.5-94.8), with no sex differences (89.9GMepg for male versus 81.4GMepg for female, t=-0.2887, P=0.77). There was a significant difference in GMepg between the age groups 1-5 years (91,9GMepg, 95% CI:58.8-143.5) and 6 -15 years (108.6GMepg, 95% CI:95.2-124.0) (F-test=10.71, P<0.0001). The age group 6-15 years had the highest intensity of infection among all age groups. Table 3 shows the Geometrical mean eggs per gram of faeces of S. mansoni stratified by sex and age. Based on the WHO classification of intensity of infection, majority of the study participants had light (55.2%) to moderate (20.4%) intensity of infection. Only 12.9% of them had a heavy intensity of infection.
Prevalence of Schistosoma mansoni based on Circulating Cathodic Antigen tests
The laboratory results for the point-of-care Circulating Cathodic Antigen (POC-CCA) tests were available for 929 study participants. The overall prevalence of S. mansoni based on the POC-CCA test was 94.5% (95%CI: 92.8 – 95.8). The male individuals recorded the highest prevalence of S. mansoni compared to female individuals (96.2% versus 93.2%, χ2=3.9839, P=0.046). In relation to age groups, the age groups 6-15 years recorded the highest prevalence of infection compared to other age groups (98.1%, χ2=34.2723, P<0.001). Table 4 below shows the prevalence of S. mansoni infection based on POC-CCA test stratified by age and sex.
Prevalence of periportal fibrosis and associated factors
The overall prevalence of definite periportal fibrosis (PPF) (grade C-F) was 27.1% (95%CI: 24.3-30.1). 58,2% of the participants showed a normal liver pattern (grade A), 14,7% a grade B pattern, which was considered as not distinct for PPF according to Kaatano et al. (40) and King et al. (44). In relation to sex of the study participants, the prevalence of PPF was higher in male individuals than in female individuals (P<0.026). In general, the oldest age groups, 36-45 years and ≥ 46 years had a higher prevalence of PPF compared to other age groups (P<0.001). In relation to infection status, the findings indicate that individuals who had no S. mansoni detectable eggs in their stool samples recorded the highest prevalence of PPF (P<0.005). The findings indicated that there was no significant difference in PPF prevalence in relation to intensity of infection categories (P=0.42). Table 5 shows the prevalence of periportal fibrosis in relation to demographic characteristics of the study participants, infection status and intensity of infection.
At bivariate analysis, being a male, belonging to age groups 16-25 years, 26-35 years, 36-45 years and ≥ 46 years were associated with periportal fibrosis (Table 6). On multivariable analysis the following variables were associated with periportal fibrosis (Table 6):
a) being male (aRR=1.08, 95%CI: 1.02-1.16, P<0.01),
b) belonging to the age group 16-25 years (aRR=1.23, 95%CI: 1.05-1.44, P<0.01),
c) belonging to the age group 26-35 years (aRR=1.42, 95%CI: 1.21-1.67, P<0.001),
d) belonging to the age group 36-45 years (aRR=1.56, 95%CI: 1.31-1.84, P<0.001)
e) and belonging to the age group ≥46 years (aRR=1.64, 95%CI: 1.41-1.92, P<0.001)
Left liver lobe hepatomegaly, splenomegaly and their associated factors or predictors
Data on left liver lobe measurements were available for 909 study participants. Overall, 51.9% (472/909, 95%CI: 48.7-55.2) of the study participants had enlarged and grossly enlarged left liver lobe. Of these, 79% (373/472) and 21% (99/472) had enlarged and grossly enlarged left liver lobe respectively. In relation to sex, female study participants had higher prevalence of enlarged (59.5% versus 40.5%) and grossly enlarged left liver lobe (62.6% versus 37.4%, χ2=10.0110, P<0.01). In the age groups, the age group 6 -15 years recorded the highest prevalence of enlarged (49.6%) and grossly enlarged (36.4%) left liver lobe compared to other age groups (χ2=29.1165, P<0.001). No statistical difference in prevalence of enlarged and grossly enlarged left liver lobe was observed in relation to S. mansoni infection status (P=0.82) and intensity of infection categories (P=0.47).
Splenomegaly was detected in 31.2% (278/892, 95%CI: 28.2-34.3) of the study participants whose data were available. Of these participants, 212/278 (76.3%) and 66/278 (23.7%) had moderately to marked enlarged splenomegaly. Male individuals recorded a higher prevalence of moderately (27.8%) and markedly enlarged spleen (9.5%) than the female individuals (χ2=13.3959, P<0.001). Similarly, there was a significance difference in prevalence of moderately and markedly enlarged spleen in relation to S. mansoni infection status. Individuals who had S. mansoni detectable eggs in their stool samples had a higher prevalence of moderately (26.1%) and markedly enlarged spleen (7.7%) (χ2=6.6121, P<0.037).
Hepatosplenomegaly (enlargement of both spleen and liver) was detected in 55.8% (155/895) of the study participants. Other morbidities detected were ascites (0.2%), collateral veins (1.7%) and gall bladder wall thickening (9.7%).
On linear regression model using height adjusted ultrasound measurements of the left liver lobe as a continuous outcome variable for individuals who had detectable S. mansoni eggs in stool samples, the findings demonstrated that, the ultrasound adjusted left liver lobe measurements were significantly associated with being female (P<0.03), being in the age groups 1-5 years (P<0.013), 6-15 years (P<0.04) and S. mansoni intensity of infection (P<0.034) (Table 7).
On bivariate analysis, factors associated with splenomegaly were mainly being male, belong to the age groups 16-25 years and 26-35 years and having detectable S. mansoni eggs in stool samples. Similarly, in the multivariable analysis, being male (aRR=1.15, 95%CI: 1.06-1.24, P<0.001), belong to the age group 16-25 years (aRR=1.27, 95%CI: 1.05-1.54, P<0.02) and age group 26-35 years (aRR=1.32, 95%CI: 1.08-1.61, P<0.01) (Table 8).
Comparison of the height adjusted mean deviations of organs in relation to infection with S. mansoni
The mean deviations of ultrasound measurements of left liver lobe, spleen and portal vein were compared between individuals who were infected or uninfected with S. mansoni based on Kato Katz technique. Overall, the height-adjusted ultrasound measurements of the left liver lobe (t=-0.7280, P=0.46) did not differ between the two groups. There was a significant difference in height-adjusted ultrasound measurements of the spleen (t=-2.0616, P<0.04) and portal vein (t=6.2970, P<0.001) between individuals who were infected or not with S. mansoni. For the height-adjusted length of the spleen, individuals who were infected showed an enlargement of the spleen compared to uninfected individuals. For the portal vein, individuals who were infected had more dilated (increased diameter) portal vein diameter compared to uninfected individuals.