In the wealthy world, rabies is mostly just a metaphor, or a horror from our grandparents' time. In Africa and Asia, however, it kills almost 60,000 people a year. A mass vaccination campaign for dogs has proven that it is possible to slash these rates cheaply, even while many dogs stay unprotected.
We have been able to eliminate smallpox and soon, we hope, polio, because these are diseases restricted to humans. It's much harder to entirely stop a virus that can persist in wild animals. However, few people catch rabies by being bitten by a raccoon or mongoose. Instead, the main danger occurs when our best friend becomes our worst enemy. If a dog catches rabies from a wild reservoir, it can spread it through the canine population, particularly in places where stray dogs are common.
In 2012 and 2013 vaccination campaigns were run in N’Djaména, the capital of Chad. Vaccinating every dog in the city would be a Herculean task, but it was hoped that if enough animals could be protected then herd, or in this case pack, resistance, would reduce the transmission of the disease to negligible levels.
Professor Jakob Zinsstag of the University of Basel tracked the trial, along with a large team of Swiss and Chadian scientists. They found that even though the campaign only reached an estimated 71 percent of the city's dogs each year, that figure was enough to cut the incidence of rabies by 95 percent for dogs, and 99.8 percent for humans in 2014.
Herd resistance refers to the fact that, for any infectious disease, transmission rates drop when some of the potential subjects become immune. If enough of the population is protected, epidemics cannot take hold. Anti-vaccinators ignore or dismiss the concept because it interferes with their view of vaccination as a purely individual choice. In reality, it is only because of the herd resistance provided by a largely vaccinated community that not being vaccinated is anything close to safe.
The level of vaccination required to produce strong herd resistance varies greatly with circumstance. “Reaching sufficient coverage to interrupt dog rabies virus transmission and prevent reintroduction requires an in-depth understanding of dog ecology, dog-human interactions, and the social and cultural determinants of vaccine acceptability, as well as the effective deployment of vaccines,” Zinstag wrote in Science Translational Medicine. The N’Djaména trial provided an opportunity to establish what is needed on a more widespread basis.
Tragically, despite the initial success, rabies reasserted itself more quickly than the researchers had anticipated, probably because they had underestimated rabies rates in surrounding areas. Nevertheless, the work not only shows the potential for cheaply reducing rates of this terrible disease but also adds to what we know about control of animal-borne infectious diseases.