Katie Hampson, now a professor of Infectious Disease Ecology at the University of Glasgow, began collecting data for a study back in 2002, when she was just starting her PhD at Princeton University. She was interested in understanding how rabies – a deadly disease that infects humans primarily through a dog bite – spreads and persists among dogs in Tanzania in Africa.
Rabies is an example of an endemic disease: it is prevalent in some populations, especially in Africa and Asia, at low levels, even though it causes many deaths upon an outbreak. What allows lethal diseases like rabies to persist long-term at low prevalence? This question has hounded many scientists. Intrigued by the endemic nature of rabies and wanting to understand it better, Dr. Hampson established a contact tracing network to track the spread of rabies among the 50,000 dogs and 250,000 people in the Serengeti district of northern Tanzania.
With the help of collaborators and other people who later joined her lab, Dr. Hampton collected data about rabies infections in the area for over a period of nearly 15 years. The results and analysis of this massive project were recently published in Science, where the authors have reported that individual dog behaviour plays an important role in how rabies persists at low levels in the population.
Following the chains of transmission
“We started off with hospital records of patients who attended the clinic in the district that provided post-exposure vaccines,” Dr. Hampson said, going into the process behind the extensive contact tracing. “Magoto, my colleague, would know all the villages in the district, so we would then drive off to the right village and track down the patient.”
Matthias Magoto, a co-author in the study and a local resident and expert of the communities and landscape of northwest Tanzania, was working as a livestock field officer for the district government at the time. “We would then [interview] the bite patient to try and understand if the dog that bit the person or the child was really rabid or if it was healthy.”
Rabies, which causes nearly 60,000 human deaths a year worldwide, is caused by a virus called lyssavirus and is preventable by a vaccine. But if the vaccine isn’t administered in time, the virus can end up infecting the brain, leading to death in nearly 100% of those cases. Even though there is a variable incubation period after the bite, during which the dogs and humans show no symptoms, once the infection takes hold there is sadly no going back.
Rabies can be diagnosed in dogs rather easily based on clinical signs. If a dog that’s acting strangely bites other dogs or humans unprovoked, and in some cases dies after biting, it’s most likely to have been a rabid dog. As most dogs in the Serengeti district were owned by the different villagers – as opposed to the free-roaming stray dogs of India – the authors were able to trace the history of each bite and track down the dogs that were biting other dogs and humans. To make sure that they weren’t barking up the wrong tree, they also obtained brain samples from the dead dogs in most cases to confirm the virus’s presence.
“We followed the chains of transmission forwards and backwards in an as in-depth way as we possibly could,” said Dr. Hampson. “It was quite laborious, but incredibly interesting. We learned a huge amount about how rabid dogs behaved and how people behaved if they were bitten, if they managed to get the vaccines that they needed.”
It takes a bite
A way that rabies differs from typical infectious diseases is the extent to which it can truly spread in a population, something that the researchers also found in their data. Usually, infectious diseases like COVID-19 spread like wildfire and lead to many people getting rapidly infected, then recovering and developing an immunity against the virus. This is how we have been seeing the typical peaks and valleys of COVID infections over the last few years. But for rabies, the only way it can spread between dogs is by a bite. The researchers found that the spatial structure of the dog populations and the scale at which the virus spreads were very important. In most cases, the virus spread very locally as the dogs could only bite a few other dogs nearby.
By looking closely at how the virus affected dogs, the researchers were able to decipher how it wreaks a carefully controlled form of havoc in dog populations. As the virus attacks the central nervous system – which is a bit shielded from the immune system – there is, unfortunately, no natural immunity developed against the virus. But dogs that have been bitten and are in the non-symptomatic incubation phase could not get infected again, limiting the dogs that can get infected once the virus starts to spread.
Most infected dogs also eventually died or were killed once they showed symptoms and started biting, which also limited their ability to continue infecting. The researchers found that all these factors played a role in preventing the virus from spreading too much locally within a population.
But then why is it that rabies doesn’t just die out as more and more dogs get infected within a population?
‘Sparks from a forest fire’
The answer seemed to lie in the differences in how the dogs behaved once they got infected. Most of the dogs seemed to be biting around two other dogs in their immediate vicinity, but a few dogs bit a lot more, with one dog biting nearly 60 others. A small number of dogs also ran incredibly long distances, close to 15 km, away from their home after being infected, and ended up biting dogs very far away from their home. This variability was an important element when the researchers modelled their data – without which it would have been very difficult to study rabies outbreaks the way they happen. Simply taking the average of how many dogs were bitten would have led the researchers to miss the few “super-spreader” dogs that either bit many dogs or travelled vast distances.
In fact, by travelling long distances, some dogs ended up being a vehicle to carry the virus away from the local community, where one can slowly run out of susceptible dogs (as more and more get infected), to a new community where the virus has more room to thrive.
“It’s like sending out sparks from a forest fire. And all those little sparks are circulating in the landscape,” said Dr. Hampson.
Vaccine inequity redux
Keeping these different facets of how rabies seemed to circulate in mind, the researchers built computational models to dissect the nuts and bolts of rabies transmission in the population. By reconciling the movement and biting behaviour of rabid dogs along with the number of rabies cases they saw in their model, they realised that traditional infectious disease models don’t work well with predicting how rabies would spread. Rabies seemed to operate at a very local scale, with low rates of transmissibility – except in the few rare cases of super-spreader dogs that took the infection with them into new communities.
Another key finding of their study was that a lower density of dogs didn’t necessarily mean a drastically lower rate of transmission, implying that culling dogs to reduce their number wouldn’t help prevent the virus from spreading as much. Dr. Hampson firmly believes that large-scale dog vaccinations are the only effective way to break the chain of transmission and prevent rabies from spreading.
High-income countries have used vaccination drives to try and eliminate rabies in their dog populations, but sadly these life-saving vaccines are not equitably distributed around the world. The countries that now need these vaccines the most don’t have access to them because of their high cost.
India also has a high burden of rabies, with around 20,000 people dying every year of the disease. With 30-60 million stray dogs roaming the streets, much of the efforts in the country have focused on sterilising dogs over vaccinating them.
Culling for rabies a waste of time
Gowri Yale, a veterinarian with a PhD in rabies epidemiology, has been working to eliminate rabies in Goa with the help of large-scale surveillance and mass vaccination drives. Dr. Yale is a scientific advisor to Mission Rabies, a UK-based charity working with the Goan government to ensure at least 70% of the dogs in Goa are vaccinated. The efforts have led to a tremendous decrease in the number of rabid dogs in the state over the last few years.
But Goa doesn’t exist in isolation: it shares borders with densely populated Maharashtra and Karnataka, where the virus is still endemic. Dr. Yale and her colleagues have now begun advising vaccination efforts in Mumbai, Bengaluru, and Kerala as well.
“It’s a fantastic paper to promote mass dog-vaccination, which is the only way forward to eliminate canine rabies,” Dr. Yale said about the Science study. “If there is any discussion of culling dogs, their model shows that culling dogs is just a waste of time, money, and resources, and is also just not welfare friendly.”
Dogs and humans have evolved to live together over tens of thousands of years. After all, dogs are man’s best friend. Sadly, the deadly rabies virus causes these friends to turn into foes from time to time. But reducing dog numbers by killing them is ethically wrong and, now we know, scientifically ineffective. Dog vaccination is the only way to prevent the virus from spreading amongst dogs and a crucial way to also protect humans from the terrible fate of rabies.
Rohini Subrahmanyam is a freelance journalist.
