Evaluating the effectiveness of localized control strategies to curtail chikungunya

Abstract

Chikungunya, a re-emerging arbovirus transmitted to humans by Aedes aegypti and Ae. albopictus mosquitoes, causes debilitating disease characterized by an acute febrile phase and chronic joint pain. Chikungunya has recently spread to the island of St. Martin and subsequently throughout the Americas. The disease is now affecting 42 countries and territories throughout the Americas. While chikungunya is mainly a tropical disease, the recent introduction and subsequent spread of Ae. albopictus into temperate regions has increased the threat of chikungunya outbreaks beyond the tropics. Given that there are currently no vaccines or treatments for chikungunya, vector control remains the primary measure to curtail transmission. To investigate the effectiveness of a containment strategy that combines disease surveillance, localized vector control and transmission reduction measures, we developed a model of chikungunya transmission dynamics within a large residential neighborhood, explicitly accounting for human and mosquito movement. Our findings indicate that prompt targeted vector control efforts combined with measures to reduce transmission from symptomatic cases to mosquitoes may be highly effective approaches for controlling outbreaks of chikungunya, provided that sufficient detection of chikungunya cases can be achieved.

Figure 1. Model diagram.

The model consisted of humans (H) and mosquitoes (V). Humans could be susceptible (S_H), exposed (E_H), pre-symptomatically infected (Î_H), asymptomatically infected (A_H), symptomatically infected (I_H), quarantined (Q_H), or recovered (R_H). Mosquitoes could be susceptible (S_V), exposed (E_V), or infected (I_V).

Figure 2. Model validation.

We compare the time series of chikungunya cases generated by our model for R₀ =4 against empirical data of chikungunya cases reported in Dominica during the 2013–2014 outbreak. The solid line represents the average realization of our model while the histogram bars represent the empirical data from Dominica.

Figure 3. Simulated spatiotemporal dynamics of chikungunya cases.

Four snapshots of spatial distribution of cases are shown at time t = 0, 70, 150, 200 days following the index case.

Figure 4. Effect of vector control for reducing the attack ratio of chikungunya.

Average attack ratio of chikungunya for ranges of vector control efficacies and disease surveillance sensitivities for different R₀ values (A,D) R₀ = 2, (B,E) R₀ = 4, (D,F) R₀ = 6. We compared (A–D) Aedes albopictus and (D–F) Aedes aegypti as disease transmission vectors.

Figure 5. Effect of combined vector control and transmission reduction measures for reducing attack ratio of chikungunya.

Average attack ratio of chikungunya for ranges of vector control efficacies, disease surveillance sensitivities, and efficacies of transmission reduction measures for symptomatic individuals for different R₀ values (A,D) R₀ = 2, (B,E) R₀ = 4, (D,F) R₀ = 6. We compared (A–D) Aedes albopictus and (D–F) Aedes aegypti as disease transmission vectors.

Figure 6. Relationship between Basic reproductive number (R₀) and the Reproductive number under control (R_c) for different efficacies of control implementation.

Here, we assumed equal efficacies for cases detection, perifocal vector control, and disease reduction measures for symptomatic individuals. We compared (A) Aedes albopictus and (B) Aedes aegypti as vector species.

Figure 7. Impact of the efficacy of case detection on the chikungunya attack ratio when combined with efficacious vector control and transmission reduction around symptomatic individuals.

The efficacies of vector control and transmission reduction measures were varied from 70 to 90%. We defined the effectiveness of controlling the outbreak as the probability that an index case will produce fewer than two secondary cases. The attack ratio was computed when an index case produced at least two secondary cases. We compared timing of intervention initiation with a one-week versus two-week delay from detection of the index case.