Revisiting density dependence in human schistosomiasis using sibship reconstruction

Schistosomiasis is a devastating neglected tropical disease caused by trematode parasites of the genus Schistosoma. Although earmarked for control and elimination by the World Health Organization, schistosomiasis remains a major public health concern, endemic in 54 countries and affecting approximately 240 million people worldwide. Moreover, many fundamental aspects of schistosome population biology remain unresolved, impeding the design, optimisation and evaluation of intervention strategies targeting control and elimination.

A key unknown is whether and to what extent schistosome populations are regulated by density-dependent population processes. In dioecious helminth infections, density-dependent fecundity describes the reduction in egg production by female worms in high worm burden within-host environments. For human schistosomiasis, unlike some intestinal worms, investigating density-dependent fecundity is hampered by the inaccessibility of adult worms within hosts, due to the intravascular location of the parasite.

Current understanding of this fundamental population process is limited to data collected from two autopsy studies conducted over 40 years ago, with subsequent analyses having reached conflicting conclusions on the operation of density- dependent egg production.