The relationship between expelled eggs, morbidity and age: implications for current Schistosoma mansoni elimination policies

Direct morbidity assessments are rarely included in monitoring and evaluation of Schistosoma mansoni control programmes. Instead, the number of eggs-per-gram (EPG) of faeces is used as a proxy for morbidity. The World Health Organization targets schistosomiasis elimination as a public health problem (EPHP) by 2030, defined as <1% heavy infections (≥400 EPG for intestinal schistosomiasis). However, recent findings challenge this link between infection intensity and morbidity.

Prevalence and intensities of S. mansoni infection were diagnosed by Kato-Katz and point-of-care circulating cathodic antigen tests in 287 individuals, 3-74 years-old, from Bugoto, Uganda. Ultrasound examinations following the Niamey protocol characterised metrics of morbidity: periportal fibrosis (PPF), portal vein dilation (PVD) and left parasternal line (PSL) enlargement. Additional morbidity markers included anaemia and self-reported symptoms. Malaria status was determined using rapid diagnostic testing. Logistic regression models elucidated potential predictors of morbidity.

PPF, PVD, PSL and anaemia prevalence were 9%, 34%, 33% and 13% respectively. School-aged children (SAC) had the highest infection intensity, but pre-school-aged children (PSAC) were significantly more likely to have PVD, PSL and anaemia than other age groups. Current S. mansoni infection predicted only self-reported symptoms of blood in stool and rash. As infection intensity increased, so did the likelihood of anaemia and fibrosis, but this was significant only at levels much higher than the 400 EPG threshold. Current malaria infection, measured with rapid diagnostic testing, was associated with PVD and anaemia.

Our findings add to growing evidence against using ≥400 EPG as a proxy for schistosomiasis morbidity within control programs, urging a revaluation of targets. The age-related distribution of morbidities observed, with a notable burden in PSAC, highlights a critical need to elucidate the impact of less-specific morbidities on host health and its interplay with current and past infections with S. mansoni and other parasites.

Author Summary

Schistosomiasis is a parasitic disease that affects millions of people worldwide, especially in sub-Saharan Africa. Monitoring and control programs typically rely on counting parasite eggs in stool as a proxy to measure disease severity, but recent evidence questions whether this method accurately reflects the real health impact of the disease.

Here we examined 287 individuals in Bugoto, Uganda, aged 3 to 74, using both stool and blood tests to detect Schistosoma mansoni infection. We also used ultrasound to assess liver damage and other health complications caused by the parasite and used rapid testing of participants blood to diagnose malaria. Surprisingly, we found that even at very low S. mansoni infection levels, young children had significant liver-related complications and anaemia. We also found that only very high egg counts were associated with severe health problems such as liver fibrosis. Additionally, current malaria infection was strongly linked to anaemia and liver abnormalities, complicating the picture further.

Our findings suggest that the current World Health Organization thresholds for defining disease severity does not provide an accurate picture of the true burden of schistosomiasis, particularly in young children and in regions of co-endemicity with malaria. This research highlights the urgent need for new methods to assess the impact of the disease.