Transmission dynamics and economics of rabies control in dogs and humans in an African city

Abstract

Human rabies in developing countries can be prevented through interventions directed at dogs. Potential cost-savings for the public health sector of interventions aimed at animal-host reservoirs should be assessed. Available deterministic models of rabies transmission between dogs were extended to include dog-to-human rabies transmission. Model parameters were fitted to routine weekly rabid-dog and exposed-human cases reported in N'Djaména, the capital of Chad. The estimated transmission rates between dogs (beta(d)) were 0.0807 km2/(dogs x week) and between dogs and humans (beta(dh)) 0.0002 km2/(dogs x week). The effective reproductive ratio (R(e)) at the onset of our observations was estimated at 1.01, indicating low-level endemic stability of rabies transmission. Human rabies incidence depended critically on dog-related transmission parameters. We simulated the effects of mass dog vaccination and the culling of a percentage of the dog population on human rabies incidence. A single parenteral dog rabies-mass vaccination campaign achieving a coverage of least 70% appears to be sufficient to interrupt transmission of rabies to humans for at least 6 years. The cost-effectiveness of mass dog vaccination was compared to postexposure prophylaxis (PEP), which is the current practice in Chad. PEP does not reduce future human exposure. Its cost-effectiveness is estimated at US $46 per disability adjusted life-years averted. Cost-effectiveness for PEP, together with a dog-vaccination campaign, breaks even with cost-effectiveness of PEP alone after almost 5 years. Beyond a time-frame of 7 years, it appears to be more cost-effective to combine parenteral dog-vaccination campaigns with human PEP compared to human PEP alone.

Fig. 1.

Rabies transmission among dogs and from dogs to humans in N′Djaména (Chad). (A) Weekly incidence of rabid dogs per square kilometer. (B) Weekly incidence of exposed persons per square kilometer. Dots represent observed data points and the bold line their deterministic fit.

Fig. 2.

Simulation of interventions aimed at animal host reservoirs on the transmission of rabies: single mass-vaccination campaigns of dogs with coverage of 70% and 50%, respectively and culling campaigns of 5% and 10% of the dog population, respectively, during 2 consecutive years, compared to no intervention. (A) Transmission among dogs, number of rabid dogs per square kilometer. (B) Transmission from dogs to humans, number of exposed persons per square kilometer.

Fig. 3.

Sensitivity of the rabies transmission dynamic model to uncertain parameters (probability distributions are shown in Table S1). (A) Rabid dogs per square kilometer and (B) exposed persons per square kilometer with their respective 50% (gray dashed line) and 95% (gray dotted line) Monte Carlo uncertainty intervals of the sensitivity analysis for no intervention and 70% dog vaccination.

Fig. 4.

Cumulated and discounted costs of human PEP alone, with 95% uncertainty interval (black line with black dotted limits), and human PEP with dog vaccination with 95% uncertainty interval (gray line with gray dotted limits). Break-even points are numbered diamonds for sensitivity analysis with 3%, 5%, and 10% discount rates.

Fig. 5.

Average and discounted cost-effectiveness of human PEP alone with 95% uncertainty interval (black line with black dotted limits), and human PEP with dog vaccination with 95% uncertainty interval (gray line with gray dotted limits). Break-even points are numbered diamonds for sensitivity analysis with 3%, 5%, and 10% discount rates.