My research interests branch into a few areas, but my main focus is on behaviour-altering parasites. These are parasites which alter their host’s behaviour in a way that benefits the parasite’s survival and reproduction, often utterly controlling their behaviour and turning them into ‘zombies’. What I’m often asked when I discuss it is about humans, and thankfully modern humans seem free from these parasites. There are parasites which do alter our behaviour, but it appears incidental and of no benefit to their reproduction (through us at least). The closest we have is the exceptionally lethal group of viruses causing rabies, and thinking about them can help us understand how these parasites may work in other species.
Rabies is fascinating for many reasons (I’m fascinated by morbid things, okay), so let’s start with when the animal is infected. In the beginning, the virus hides in our peripheral nervous system, where it hijacks the mechanisms of the neurons there to take it up through them towards the brain. While this is a complex method of reproduction, it has its advantages – it helps it avoid an immune response, both by how thoroughly it hides in the cell and that much of the nervous system is what’s called immune privileged. This means that its path through our body is stealthy and that it can reproduce and wreak all kinds of havoc largely unchallenged or even unnoticed. The virus hides within neurons and their normal processes, reproducing and then moving to the next cell – crawling its way towards the ultimate target of the brain. While stealthy in terms of the immune system, this process is not kind to the neurons it travels through, as they are not built for the transportation the virus demands of them. This causes the peripheral neuropathy (loss of sensation or muscle control) that marks rabies infection, as the devastated neurons can no longer send messages to or from your brain.
Once the virus reaches the brain, it produces the more popularly known effects of aggression and irritability, but it’s important to keep the earlier stages in mind – as a whole, it forms a honed method for the virus to reproduce and spread. The aggression serves a purpose, though – as mentioned above, the virus must get into the nervous system to avoid the immune system and getting into neurons from another part of the body is difficult at the best of times – it’s much easier to just put it in there directly. Of course, there are things like skin and other tissue in the way, but one way to get past all of that and go directly into the nervous system is via biting, hence the aggression. The brain isn’t exposed in an animal’s mouth, though, so the virus reproduces a bunch in the salivary glands, accumulating virus capsules in the mouth – each capable of infecting a new host. For good measure, while in the nervous system, the virus will induce an intense physical and mental aversion to drinking water, which leads to another historical term for the disease – hydrophobia. This, combined with increased salivation, leads to an undiluted and high concentration of the virus in saliva – together with increased aggression, it seems like the perfect combination to transmit that virus right into a victim’s nervous system for the cycle to begin again.
While we are not rid of rabies globally (it can infect virtually any mammal, making it difficult to eliminate), it is rare in humans, and there is a vaccine. The vaccine was developed by Louis Pasteur of dairy fame, and was originally a daily injection over ~2 weeks, so give some thanks to modern developments when your arm is sore after your Covid vaccines over the next few weeks.