Each year, nearly 60,000 people around the world are dying of rabies, a deadly virus most often transmitted to humans through animal bites. On 99% of these deaths are due to bites from domestic dogs. Rabies is invariably fatal once symptoms develop, so it is vital that the person receives treatment when exposed.
Treatment consists of a series of vaccinations called post-exposure prophylaxis. These are very effective in preventing rabies when given quickly. But sometimes people don’t seek treatment because they are unaware of the risk of rabies. Even when they know treatment is urgent, some may still find it difficult to access it due to its high cost and often limited availability.
Instead of relying solely on post-exposure prophylaxis, an alternative strategy is to focus interventions on animal populations responsible for maintaining the virus and transmitting it to humans. Vaccination of domestic dogs has been shown to be a successful and profitable means of preventing human rabies. But it is still not practiced routinely in the countries most affected by rabies.
Although this is mainly due to lack of investment, concerns are often expressed that wildlife may play a role in maintaining rabies transmission and that dog vaccination may therefore be ineffective. . This is of particular concern in wildlife-rich areas of sub-Saharan Africa, for example in the Serengeti ecosystem where rabid wild carnivores including hyenas and mongooses have led to human deaths from rabies.
Domestic dogs have been shown to be the only species needed to maintain rabies in most of Africa. It means that dog vaccination should control the disease in all species. But in parts of Namibia and South Africa, rabies is believed to be maintained independently in wildlife such as jackals and bat-eared foxes.
The goal of our study was to assess the impact of dog vaccination on rabies in south-eastern Tanzania, where no vaccination had previously been practiced. The study took place from January 2011 to July 2019 in rural Lindi and Mtwara. Five rounds of domestic dog vaccination campaigns took place between 2011 and 2016, each covering more than 2,000 villages. These regions contain many areas of suitable wildlife habitat including forest reserves, plantations and the Selous Game Reserve. Regions were selected for vaccination so that the potential impact of wildlife on rabies elimination could be assessed.
Contact tracing to discover cases of rabies
Our objective was to collect detailed data on the rabies situation in order to understand the impacts of these vaccination campaigns. But official rabies death figures are often underreporting the true burden of disease as most people die of rabies at home and are not counted in the statistics. To combat this issue, we used health facility data to guide extensive contact tracing.
Hospital records of patients bitten by animals were used to identify people potentially exposed to rabies. We then tracked down and interviewed these people to determine the details of the bite, including the species involved and whether or not the biting animal was likely to have been rabid. During contact tracing, other bite victims and owners of rabid animals were identified and traced. So, in addition to collecting valuable data, we educated people about the risks of rabies and the importance of seeking treatment.
Somewhat unexpectedly, we found that over 40% of the animal rabies cases we detected were jackals. This is highly unusual given that domestic dogs typically account for the vast majority of cases. We also found evidence of chains of rabies transmission in jackals and of frequent cross-species transmission – that is, transmission from dogs to jackals and vice versa.
During the period of widespread dog vaccination, we saw substantial declines in animal rabies cases and human exposures to rabies throughout the study area. In 2011, we recorded 218 potential human exposures to rabies and 18 deaths. This figure fell to just 15 exposures in 2017 and a single death in 2016 and 2019.
Despite the high level of wildlife involvement, vaccination of domestic dogs alone appears to reduce the risk of rabies in all species. In 2017, only 12 cases of rabies in dogs and 7 in jackals were reported, compared to 77 and 74 respectively during the first year of the study. After mass vaccination of dogs was stopped in early 2017, canine rabies cases started to increase in some districts. We suspect this may be due to waning immunity in the dog population.
Why is this important?
These regions have unusually high proportions of wild rabies. But our study still found that vaccinating domestic dogs reduced the number of rabies cases in all animal species, which also significantly reduced the risk of rabies for humans.
The importance of sustained annual vaccinations of dogs is evidenced by the observed increase in canine rabies after the end of canine vaccination campaigns.
If we have to prevent people from dying unnecessarily of this preventable disease, it is essential to continue investments in the vaccination of domestic dogs and the presence of rabies in wild animals should not be considered as an obstacle to the implementation of these programs.