Modelling canine rabies elimination in India through mass dog vaccination
Creators: name: Radhakrishnan, Sreejith
Publisher: name: Imperial College London
Year: 2022

description: The ‘Zero by 30’ campaign aims to globally eliminate dog-mediated human rabies deaths by 2030. Theoretical and empirical studies have shown that annual mass rabies vaccination (MRV) campaigns that vaccinate at least 70% of the dog population in an area can effectively control canine rabies outbreaks and eventually eliminate it. Achieving such coverages in free-ranging dog (FRD) populations, the main source of human infections in rabies-endemic regions, can be a major challenge where most FRDs are unowned and so not easily accessible for vaccination. Despite bearing the largest burden of human rabies deaths globally, few studies have explored the population characteristics of FRDs in India in the context of rabies elimination, particularly accessibility for vaccination. Similarly, there are limited studies of dog ownership practices (DOP) relevant to rabies control in India. We conducted a longitudinal field study over 16 months in a cohort of unowned dogs (UDs), semi-owned dogs (SODs) and ODs at an urban (human population of 240991 individuals) and a semi-urban (25861 individuals) site each in Kerala, south India. The study gathered data on dog population characteristics, DOP and pre- and post-vaccination rabies virus neutralizing antibody (RVNA) dynamics. In round 1 (R1) pre-vaccination blood samples were collected from all dogs, after which they were vaccinated against rabies, collared and microchipped where necessary and released. Data on demographic characteristics (sex, age, body condition etc.) and DOP were also collected. As many dogs as possible from this cohort were recaptured at approximately ~30 days (R2), ~150 – 180 days (R3) and ~365 days (R4) after first capture to collect post-vaccination blood samples. All serum samples were tested to assess post-vaccination RVNA titre dynamics and rates of decline. These data were used to parameterise an age-structured deterministic compartmental Susceptible-Exposed-Infectious-Vaccinated (SEIV) model incorporating assumptions about accessibility for vaccination. The model was used to assess the impacts of varying various demographic, immunological and MRV campaign parameters on prospects of rabies elimination within 20 years of campaign implementation. In R1, 577 dogs across all ownership categories were captured. Only 12% of FRDs were owned, with about 60% of ODs in R1 being free-ranging. Only 29% of ODs were vaccinated against rabies. Approximately 26% (95% CIs: 22 – 31%) of all dogs sampled in R1 had RVNA titres ≥ 0.23 IU/ml. Mixed-effects logistic regression models found higher recapture probabilities for sterilised dogs and lower probabilities for UDs, dogs from the semi-urban site and those with pre-vaccination RVNA titres ≤ 0.5 IU/ml or no detectable titres. Over 80% of dogs recaptured in R2 had titres &gt; 0.5 IU/ml, irrespective of age or vaccination history. Mixed-effects linear regression models identified significant associations between post-vaccination RVNA titres and age at vaccination, sterilisation status and RVNA titre levels in R1. Titres were estimated to drop below 0.5 IU/ml approximately 200 days (95% CI: 167 – 256 days) after achieving post-vaccination peak levels. However, titres declined at a faster rate for ODs and completely/partially confined dogs compared to dogs without owners and completely FRDs. We also found evidence suggesting the occurrence of non-lethal rabies infections in FRDs. The SEIV model indicated that as accessibility for vaccination increased, rabies elimination was possible in a wider range of scenarios within shorter timeframes, generally within 10 years of implementation of vaccination campaigns, and required lower vaccination coverages. Where ≤ 20% of dogs were accessible, campaigns needed to consistently vaccinate &gt; 95% of dogs for &gt; 20 years to eliminate rabies. Rabies elimination was possible in most scenarios, typically with annual campaigns, even with &lt; 70% effective vaccination coverages in the total dog population. The model also highlighted the complex interplay of demographic factors and disease transmission, with high birth rates resulting in higher rabies cases, irrespective of juvenile mortality or adult lifespan. Mass rabies vaccination continues to be the most effective rabies control method; however, the implementation and frequency of MRV campaigns must account for varying accessibility of FRD populations and consider variations in demography and immunological dynamics. Rabies control in India will require a multi-pronged approach incorporating more responsible dog ownership, access to veterinary care, effective MRV and dog population and waste management, while ensuring the use of properly stored, high-quality vaccines and where necessary, the use of alternative vaccination methods such as oral vaccines to access as many dogs as possible.