Elimination or Resurgence: Modelling Lymphatic Filariasis After Reaching the 1% Microfilaremia Prevalence Threshold

Abstract

The low prevalence levels associated with lymphatic filariasis elimination pose a challenge for effective disease surveillance. As more countries achieve the World Health Organization criteria for halting mass treatment and move on to surveillance, there is increasing reliance on the utility of transmission assessment surveys (TAS) to measure success. However, the long-term disease outcomes after passing TAS are largely untested. Using 3 well-established mathematical models, we show that low-level prevalence can be maintained for a long period after halting mass treatment and that true elimination (0% prevalence) is usually slow to achieve. The risk of resurgence after achieving current targets is low and is hard to predict using just current prevalence. Although resurgence is often quick (<5 years), it can still occur outside of the currently recommended postintervention surveillance period of 4–6 years. Our results highlight the need for ongoing and enhanced postintervention monitoring, beyond the scope of TAS, to ensure sustained success.

Defining a prevalence level to describe the elimination of Lymphatic Filariasis (LF) transmission and designing monitoring & evaluating (M&E) programmes post the cessation of mass drug administration (MDA)

“…The lack of predictive power at low monitoring thresholds for predicting bounce-back is consistent with the findings of Prada and colleagues, who found that single prevalence levels have poor specificity [15]. They propose a TAS methodology that detects the difference in prevalence between surveys, and show that it is able to provide greater specificity.…”

“…The first, by Davis and colleagues, examines the dependence of the chance of elimination on certain model parameters, using a simple model [14]. The second, a recent paper by Prada and colleagues modelling transmission of LF post MDA-cessation, has calculated the sensitivity and specificity of various monitoring thresholds [15]. We build on this analysis by considering three different baseline transmission settings, and by considering the predictive value of different monitoring thresholds, a statistic that accounts for the context in which the prevalence threshold is used [14].…”

Predicting the impact of disruptions in lymphatic filariasis elimination programmes due to the outbreak of coronavirus disease (COVID-19) and possible mitigation strategies

“…We have estimated the delays in LF elimination efforts due to the disruption caused by COVID-19, in two broadly defined settings, using three well-established mathematical models of lymphatic filariasis transmission 7,8 . Moreover, we explored how alternative enhanced strategies could be used to catch-up and recover from the disruption when programmes resume, and potentially accelerate progress towards the 2030 goals, if maintained longer term.…”

“…Strategies have been considered for accelerating progress towards the 2020 goals (now 2030) which could be used as mitigation strategies, such as biannual rounds of treatment or increased coverage [6][7][8] . Recent clinical trials and mathematical modelling studies have shown that the combination of all three drugs, commonly known as the triple drug (IDA), can substantially improve progress towards the goal 7,[9][10][11][12][13] .…”

“…A suite of three different mathematical models have been recently used to compare strategies to accelerate global elimination of LF 7 , as well as to assess the likelihood of resurgence after reaching the 1% mf prevalence threshold 8 . Those same models are used here to: 1) estimate the impact of MDA delays in the expected progress towards the 1% mf threshold, 2) establish mitigation strategies that are sufficient to catch-up the missed/delayed rounds, and 3) assess the acceleration provided from mitigation strategies that can be maintained longer term.…”