A model framework to estimate impact and cost of genetics-based sterile insect methods for dengue vector control

Abstract

Vector-borne diseases impose enormous health and economic burdens and additional methods to control vector populations are clearly needed. The Sterile Insect Technique (SIT) has been successful against agricultural pests, but is not in large-scale use for suppressing or eliminating mosquito populations. Genetic RIDL technology (Release of Insects carrying a Dominant Lethal) is a proposed modification that involves releasing insects that are homozygous for a repressible dominant lethal genetic construct rather than being sterilized by irradiation, and could potentially overcome some technical difficulties with the conventional SIT technology. Using the arboviral disease dengue as an example, we combine vector population dynamics and epidemiological models to explore the effect of a program of RIDL releases on disease transmission. We use these to derive a preliminary estimate of the potential cost-effectiveness of vector control by applying estimates of the costs of SIT. We predict that this genetic control strategy could eliminate dengue rapidly from a human community, and at lower expense (approximately US$ 2~30 per case averted) than the direct and indirect costs of disease (mean US$ 86-190 per case of dengue). The theoretical framework has wider potential use; by appropriately adapting or replacing each component of the framework (entomological, epidemiological, vector control bio-economics and health economics), it could be applied to other vector-borne diseases or vector control strategies and extended to include other health interventions.

Figure 1. Overview of model components.

Figure 2. Vector and epidemiological dynamics, with release ratio 10∶1.

Release ratio C: 10. Over time (A) total number of vectors, (B) total number of infectious hosts (primary or secondary infections), by serotype (1: solid line, 2: dashed), (C) total number of hosts recovered from secondary infection (solid) or susceptible to either or both serotypes (dashed). Default parameter values (Table 2), with initial conditions host population N ₀: 2 million and primary infections I ₁: 1, I ₂: 2. The release ratio is sufficiently high (), that the vector and virus are eliminated. Over subsequent years, immunity is lost from the host population.

Figure 3. Vector and epidemiological dynamics, with release ratio 1∶1.

Release ratio C: 1. Over time (A) total number of vectors, (B & C) total number of infectious hosts (primary or secondary infections), by serotype (1: solid line, 2: dashed); Default parameter values (Table 2), with initial conditions host population N ₀: 2 million and primary infections I ₁: 1, I ₂: 2. With this low release ratio (), the vector population is reduced but remains above the transmission threshold vector abundance (); panel (C) is on different scales (note the much longer time period) and shows that the disease returns after initial suppression and persists in the longer term.

Figure 4. Importance of different parameter values to cost per dengue case averted.

Relative change in cost per dengue case averted as a result of increasing each parameter value, one at a time, by 5%, for 5 year (black) or 10 year (white) release program. This is shown as “standard elasticity”, i.e. the relative change in the cost per case averted divided by the 5% relative change in each parameter value. Default parameter values (Table 2), initial host population N ₀: 2 million, and release ratio C: 10. ¹ C was increased by 5% only in the calculations of cost, with the effective release ratio kept at 10 in the epidemiological model, to represent losses during delivery of engineered males. ²We also tested a 5% increase in the mosquito mortality rate for males only, which affects the numbers to be released and hence the program costs.