Elucidating the causal mechanisms of Taenia solium transmission in humans, pigs and the environment: A global systematic review

BACKGROUND: Taenia solium cysticercosis/taeniasis disease complex is a multifaceted neglected tropical zoonosis. Previous reviews on T. solium cysticercosis/taeniasis risk factors have been limited by geographic region and/or host, making it difficult to understand the complex web of causes underlying infection at the various stages of its life cycle. Our objective was to elucidate the causal mechanisms involved in the transmission of T. solium to all hosts through a systematic review of the literature.

METHODOLOGY/PRINCIPAL FINDINGS: We conducted a systematic literature search about the epidemiology of T. solium infection published before May 2020 (OpenScience protocol https://doi.org/10.17605/OSF.IO/U64K3). We searched PubMed, Web of Science, and CABAbstracts, and reference lists of systematic reviews identified in our searches for relevant studies meeting the inclusion criteria: i) describing a risk factor-outcome association related to T. solium infection, and ii) published in English, French, Spanish, or Portuguese. Qualitative or quantitative epidemiologic associations were extracted and evaluated according to a modified version of the Quality Assessment Tool for Quantitative Studies. Our review is reported according the PRISMA guidelines. We extracted 876 associations from 159 studies. Risk factors of human cysticercosis are well-studied, while taeniasis is least-studied (10% of associations). The evidence suggests that male gender is a risk factor for human cysticercosis while female sex is a risk factor for porcine cysticercosis. The direction of association did not differ according to the quality of the studies across most studied risk factors.

CONCLUSIONS/SIGNIFICANCE: Our rigorous systematic review is the first to consider risk factors of human and porcine T. solium disease simultaneously, allowing a synthesis of information from both hosts in order to elucidate causal mechanisms for risk factors of cysticercosis. Our work also highlights areas of T. solium epidemiology which are understudied, such as the causal mechanisms of taeniasis.