Vaccination of domestic dogs reduces rabies in the wild. Why it matters

Each year, nearly 60,000 people around the world die from rabies, a deadly virus most often transmitted to humans through animal bites. More than 99% of these deaths are due to bites from domestic dogs. Rabies is invariably fatal once symptoms develop, so it is essential that a person receive 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 aren’t aware of the risk of rabies. Even when they know treatment is urgent, some may still have difficulty accessing 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 the animal populations responsible for maintaining the virus and transmitting it to humans. Vaccination of domestic dogs has proven to be an effective and cost-effective way to prevent 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 vaccination of dogs may therefore be ineffective. . This is of particular concern in areas rich in wildlife in sub-Saharan Africa, for example in the Serengeti ecosystem where rabid wild carnivores, including hyenas and mongooses, have resulted in human deaths from rabies.

Domestic dogs have been shown to be the only species needed to maintain rabies in most of Africa. This means that dog vaccination should control the disease in all species. But in parts of Namibia and South Africa, it is believed that rabies is maintained independently in wildlife like jackals and bat-eared foxes.

Rabies vaccination.
Katie Hampson

The aim of our study was to assess the impact of dog vaccination on rabies in south-eastern Tanzania, where no vaccination had been performed previously. 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 can be assessed.

Contact tracing to discover cases of rabies

Our goal was to collect detailed data on the rabies situation to understand the impacts of these vaccination campaigns. But official figures for rabies deaths often underestimate the true burden of the disease, as most people die from rabies at home and are not counted in the statistics. To combat this problem, we used data from healthcare facilities to guide an in-depth contact tracing.

Hospital records of patients bitten by animals were used to identify those 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 rabies. During contact tracing, other bite victims and rabid pet owners 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.

Quite unexpectedly, we found that over 40% of the animal rabies cases we detected were jackals. This is very unusual given that domestic dogs usually make up the vast majority of cases. We also found evidence of chains of rabies transmission within jackals and frequent transmission between species – i.e. transmission from dogs to jackals and vice versa.

During the period of generalized dog vaccination, we observed a substantial decrease in animal rabies cases and human exposure to rabies throughout the study area. In 2011, we recorded 218 potential human exposures to rabies and 18 deaths. That figure fell to just 15 exhibitions in 2017 and to just one death in 2016 and 2019.

Researchers take a sample of a jackal's head surrounded by trees and soil.
Collecting brain tissue on the head of a rabid jackal.
Eliud Kissinger

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 were reported in dogs and 7 in jackals, compared to 77 and 74 respectively in the first year of the study. After the end of the mass vaccination of dogs in early 2017, cases of canine rabies started to increase in some districts. We believe this may be due to decreased immunity in the dog population.

Why is this important?

These regions have unusually high proportions of savage rabies. But our study still found that vaccinating domestic dogs reduced the number of rabies cases in all animal species – it also significantly reduced the risk of rabies in humans.

The importance of sustained annual dog vaccinations is underscored by the observed increase in canine rabies after the end of canine vaccination campaigns.

If we are to prevent people from dying unnecessarily from this preventable disease, it is essential that there is a continued investment in the vaccination of domestic dogs and the presence of rabies in wildlife should not be seen as a barrier to the implementation of these programs.

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