The World Health Organization 2030 goals for Taenia solium: Insights and perspectives from transmission dynamics modelling

Abstract

Taenia solium (TS), responsible for porcine cysticercosis, human taeniasis and (neuro)cysticercosis, was included in the World Health Organization neglected tropical disease (NTD) roadmap published in 2012. Targets set in this roadmap have not been met, but T. solium has been included in the consultation process for the new 2030 goals proposed for priority NTDs. Taenia solium transmission dynamics models can contribute to this process. A recent review has compared existing T. solium transmission models, identifying their similarities and differences in structure, parameterization and modelled intervention approaches. While a formal model comparison to investigate the impact of interventions is yet to be conducted, the models agree on the importance of coverage for intervention effectiveness and on the fact that human- and pig-focused interventions can be optimally combined. One of these models, cystiSim, an individual-based, stochastic model has been used to assess field-applicable interventions, some currently under evaluation in on-going trials in Zambia. The EPICYST, population-based, deterministic model has highlighted, based on simulating a generic sub-Saharan Africa setting, the higher efficacy (measured as the percentage of human cysticercosis cases prevented) of biomedical interventions (human and pig treatment and pig vaccination) compared to improved husbandry, sanitation, and meat inspection. Important questions remain regarding which strategies and combinations thereof provide sustainable solutions for severely resource-constrained endemic settings. Defining realistic timeframes to achieve feasible targets, and establishing suitable measures of effectiveness for these targets that can be quantified with current monitoring and evaluation tools, are current major barriers to identifying validated strategies. Taenia solium transmission models can support setting achievable 2030 goals; however, the refinement of these models is first required. Incorporating socio-economic elements, improved understanding of underlying biological processes, and consideration of spatial dynamics are key knowledge gaps that need addressing to support model development.

How modelling can help steer the course set by the World Health Organization 2021-2030 roadmap on neglected tropical diseases

“…Alternatively, the goals for schistosomiasis 11 , STH 8 , and onchocerciasis 13 seem achievable in some or most settings, depending on localised parameters like baseline prevalence, and already experienced duration of and adherence to mass drug administration (MDA) programmes. In the case of T. solium , a lack of internationally agreed goals for elimination or control curtails the ability to effectively model timelines; for example, the 2021-2030 NTD roadmap proposes the overall milestone of achieving “intensified control in hyperendemic areas”, without agreeing on technical definitions for T. solium endemicity levels, or defining measurable criteria for attaining “intensified” control 14 .…”

“…In addition to determining success or failure within the defined intervention time frames, modelling has provided insights into key factors of operational design like the treatment coverage necessary to reach goals in a given setting. Where it may not be possible, models can be used to test the efficacy of separate and combined chemotherapeutic 37 and non-pharmaceutical interventions 23 , 45 , 46 , including combined interventions that target multi-host systems for zoonotic NTDs 14 . Additionally, deciding the optimal timing 47 or frequency 48 , 49 of treatment, and knowing who to treat 50 , 51 are essential to the success of all interventions.…”

Validation of a spatial agent-based model for Taenia solium transmission (“CystiAgent”) against a large prospective trial of control strategies in northern Peru

“…Due, in part, to the high cost and lengthy time required to conduct prospective trials, optimal strategies for T. solium control have not yet been identified, and the 2020 targets for large-scale implementation have not been met. More recently, WHO called for transmission models to be deployed to optimize control strategies and reach agreed-upon targets for control [14], a goal that was recently reinforced in the WHO's 2030 goals for T. solium control [15].…”

“…Beyond a lack of validation, prior models have not incorporated certain features of T. solium transmission that may be important for model accuracy. First, no prior model has a spatially explicit structure, meaning that pigs and humans are assumed to mix homogenously to transmit the parasite, a limitation that was highlighted in a recent systematic review of available T. solium models [22], and in a report outlining key modeling improvements needed to meet the WHO 2030 goals [15]. Spatially clustered transmission patterns have been well-documented in endemic villages [23][24][25][26], and spatially targeted control strategies (e.g., "ring strategy") have shown success in field trials [27].…”

“…More recently, WHO called for transmission models to be deployed to optimize control strategies and reach agreed-upon targets for control [ 14 ], a goal that was recently reinforced in the WHO’s 2030 goals for T . solium control [ 15 ].…”