Population dynamics of owned, free-roaming dogs : implications for rabies control

Abstract

BACKGROUND : Rabies is a serious yet neglected public health threat in resource-limited communities in Africa, where the virus is maintained in populations of owned, free-roaming domestic dogs. Rabies elimination can be achieved through the mass vaccination of dogs, but maintaining the critical threshold of vaccination coverage for herd immunity in these populations is hampered by their rapid turnover. Knowledge of the population dynamics of free-roaming dog populations can inform effective planning and implementation of mass dog vaccination campaigns to control rabies. METHODOLOGY/PRINCIPAL FINDINGS : We implemented a health and demographic surveillance system in dogs that monitored the entire owned dog population within a defined geographic area in a community in Mpumalanga Province, South Africa. We quantified demographic rates over a 24-month period, from 1st January 2012 through 1st January 2014, and assessed their implications for rabies control by simulating the decline in vaccination coverage over time. During this period, the population declined by 10%. Annual population growth rates were +18.6% in 2012 and -24.5% in 2013. Crude annual birth rates (per 1,000 dog-years of observation) were 451 in 2012 and 313 in 2013. Crude annual death rates were 406 in 2012 and 568 in 2013. Females suffered a significantly higher mortality rate in 2013 than males (mortality rate ratio [MRR] = 1.54, 95% CI = 1.28–1.85). In the age class 0–3 months, the mortality rate of dogs vaccinated against rabies was significantly lower than that of unvaccinated dogs (2012: MRR = 0.11, 95% CI = 0.05–0.21; 2013: MRR = 0.31, 95% CI = 0.11–0.69). The results of the simulation showed that achieving a 70% vaccination coverage during annual campaigns would maintain coverage above the critical threshold for at least 12 months. CONCLUSIONS AND SIGNIFICANCE : Our findings provide an evidence base for the World Health Organization’s empirically-derived target of 70% vaccination coverage during annual campaigns. Achieving this will be effective even in highly dynamic populations with extremely high growth rates and rapid turnover. This increases confidence in the feasibility of dog rabies elimination in Africa through mass vaccination.

AUTHOR SUMMARY : Rabies is a deadly disease caused by a virus that in Africa is maintained in populations of owned, free-roaming domestic dogs. Rabies can be controlled by mass vaccination, by ensuring that a certain proportion of the dog population is immune to the disease. Maintaining this proportion of immune animals creates herd immunity, reducing the spread of disease even among non-immune individuals, eventually leading to its elimination from the population. Maintaining herd immunity to rabies in free-roaming dog populations can be challenging, particularly in communities that lack regular access to veterinary services. In these communities, mass vaccination is usually implemented in annual campaigns, of relatively short duration. Between campaigns, the proportion of immune individuals in the population declines, often dropping below the critical threshold as vaccinated dogs die and susceptible dogs enter the population through birth or migration. We measured these rates of birth, death and migration in a typical population of free-roaming dogs in South Africa, and showed that vaccinating 70% of the population during annual campaigns would be sufficient to maintain herd immunity to rabies in the period between campaigns. This is achievable even in populations that have high turnover and are growing rapidly—the most challenging circumstances to maintaining herd immunity. These findings increase confidence in the feasibility of eliminating dog rabies from Africa through mass vaccination.