Modeling the Interruption of the Transmission of Soil-Transmitted Helminths Infections in Kenya: Modeling Deworming, Water, and Sanitation Impacts

Abstract

Kenya, just like other countries with endemic soil-transmitted helminths (STH), has conducted regular mass drug administration (MDA) program for the last 5 years among school aged children as a way to reduce STH infections burden in the country. However, the point of interruption of transmission of these infections still remains unclear. In this study, we developed and analyzed an age structured mathematical model to predict the elimination period (i.e., time taken to interrupt STH transmission) of these infections in Kenya. The study utilized a deterministic age structured model of the STH population dynamics under a regular treatment program. The model was applied to three main age groups: pre-school age children (2-4 years), school age children (5-14 years), and adult populations (≥15 years) and compared the impact of two interventions on worm burden and elimination period. The model-simulated results were compared with the 5 year field data from the Kenyan deworming program for all the three types of STH (Ascaris lumbricoides, Trichuris trichiura, and hookworm). The model demonstrated that the reduction of worm burden and elimination period depended heavily on four parameter groups; drug efficacy, number of treatment rounds, MDA and water, sanitation and hygiene (WASH) coverage. The analysis showed that for STH infections to be eliminated using MDA alone in a short time period, 3-monthly MDA plan is desired. However, complementation of MDA with WASH at an optimal (95%) coverage level was most effective. These results are important to the Kenyan STH control program as it will guide the recently launched Breaking Transmission Strategy.

Keywords: Kenya; deworming; mathematical modeling; soil-transmitted helminths; water sanitation and hygiene.

Figure 1

Conceptual framework showing a three age-structured model.

Figure 2

Trend of the hosts mean number of eggs per gram (epg) as observed from the 5-year national deworming program in Kenya. (A) Ascaris lumbricoides. (B) Hookworm. (C) Trichuris trichiura.

Figure 3

Prevalence trend as observed from the 5-year national deworming program in Kenya. (A) Ascaris lumbricoides. (B) Hookworm. (C) Trichuris trichiura.

Figure 4

Model 1 solution for Ascaris lumbricoides: Here, no intervention was assumed.

Figure 5

Model 2 solution for Ascaris lumbricoides: We assumed various MDA plans as indicated in (A–C) i.e., τ = 1.0 for yearly plan, τ = 0.5 for 6-monthly plan, and τ = 0.25 for 3-monthly plan. We assumed treatment coverage of 75% for each host group and drug efficacy (h) of 80%. These assumptions followed the current WHO and NSBD guidelines (13).

Figure 6

Model 3 solution for Ascaris lumbricoides: We assumed annual MDA plan with various WASH levels as indicated in (A–E) i.e., ϕ = 0 for no WASH, ϕ = 0.35 for 35% WASH, ϕ = 0.55 for 55% WASH, ϕ = 0.75 for 75% WASH, and ϕ = 0.95 for 95% WASH. Further, we assumed treatment coverage of 75% for each host group and drug efficacy (h) of 80%. These assumptions followed the current WHO and NSBD guidelines (13).

Figure 7

Model 1 solution for hookworm: Here, no intervention was assumed.

Figure 8

Model 2 solution for hookworm: We assumed various MDA plans as indicated in (A–C) i.e., τ = 1.0 for yearly plan, τ = 0.5 for 6-monthly plan, and τ = 0.25 for 3-monthly plan. We assumed treatment coverage of 75% for each host group and drug efficacy (h) of 95%. These assumptions followed the current WHO and NSBD guidelines (13).

Figure 9

Model 3 solution for hookworm: We assumed annual MDA plan with various WASH levels as indicated in (A–E) i.e., ϕ = 0 for no WASH, ϕ = 0.35 for 35% WASH, ϕ = 0.55 for 55% WASH, ϕ = 0.75 for 75% WASH, and ϕ = 0.95 for 95% WASH. Further, we assumed treatment coverage of 75% for each host group and drug efficacy (h) of 95%. These assumptions followed the current WHO and NSBD guidelines (13).

Figure 10

Model 1 solution for Trichuris trichiura: Here, no intervention was assumed.

Figure 11

Model 2 solution for Trichuris trichiura: We assumed various MDA plans as indicated in (A–C) i.e., τ = 1.0 for yearly plan, τ = 0.5 for 6-monthly plan, and τ = 0.25 for 3-monthly plan. Additionally, for each MDA plan we varied the drug efficacy (h) as either 65% or 95%. We assumed treatment coverage of 75% for each host group. These assumptions followed the current WHO and NSBD guidelines (13).

Figure 12

Model 3 solution for Trichuris trichiura: We assumed annual MDA plan with various WASH levels as indicated in (A–E) i.e., ϕ = 0 for no WASH, ϕ = 0.35 for 35% WASH, ϕ = 0.55 for 55% WASH, ϕ = 0.75 for 75% WASH, and ϕ = 0.95 for 95% WASH. Further, we assumed treatment coverage of 75% for each host group and drug efficacy (h) of 65%. These assumptions followed the current WHO and NSBD guidelines (13).

Figure 13

Showing the impact of MDA and WASH interventions parameters on R₀ assuming an annual MDA plan. The parameters evaluated were drug efficacy (blue line), proportion of individuals treated (red line) and WASH coverage (green line). The values of all the parameters ranged from 0 to 100% (or 0 to 1.0).

Figure 14

Showing the impact of MDA and WASH interventions parameters on R₀ assuming a bi-annual MDA plan. The parameters evaluated were drug efficacy (blue line), proportion of individuals treated (red line) and WASH coverage (green line). The values of all the parameters ranged from 0 to 100% (or 0 to 1.0).

Figure 15

Showing the impact of MDA and WASH interventions parameters on R₀ assuming a tri-annual MDA plan. The parameters evaluated were drug efficacy (blue line), proportion of individuals treated (red line) and WASH coverage (green line). The values of all the parameters ranged from 0 to 100% (or 0 to 1.0).