Dose–Response Relationship Between Infection Intensity, Antibody Response, and Clinical Morbidity of Schistosoma Japonicum in an Endemic Focus in Napu Valley, Indonesia

Schistosoma japonicum transmission persists in localized endemic areas of Indonesia, including Napu Valley, Central Sulawesi, where low infection intensity complicates morbidity assessment. This study investigated dose–response associations between infection intensity, antibody response, and clinical morbidity among 132 farmers. Infection intensity was measured using the Kato–Katz method and expressed as eggs per gram of feces (EPG), while anti-Soluble Egg Antigen total IgG was assessed by ELISA. Infection intensity was transformed as ln (EPG + 1). Probit regression was used to assess IgG seropositivity and estimate the ED 50 , while ordinal and multinomial logistic regression evaluated clinical morbidity. Overall, 22.7% of participants had detectable egg excretion. Increasing ln (EPG + 1) was associated with a higher probability of anti-SEA total IgG seropositivity (β = 0.207, p = 0.045), with an estimated ED 50 of approximately 604 EPG. Increasing infection intensity was also associated with diarrhea (OR = 1.80; 95% CI: 1.37–2.36), bloody diarrhea (OR = 1.58; 1.10–2.26), and skin rash (OR = 1.92; 1.29–2.87). These findings demonstrate graded dose–response associations in a low-transmission setting; however, the ED 50 reflects a 50% predicted probability of anti-SEA total IgG seropositivity and should not be interpreted as a diagnostic threshold for active infection.