All About Rabies!
First posted on Tumblr on 9/5/21.
Let’s start with a little bit of Diagnosis Detectives! This case is taken from Virusphere by Frank Ryan, an excellent book that I highly recommend. A healthy 73 year old Canadian man presented to hospital with a headache and fever. This swiftly progressed to muscle spasms, excessive salivation, aggression, anxiety and a host of other neurological symptoms. He slowly lost all his mental faculties. After questioning, his family said he’d been bitten by a bat a few months ago but they hadn’t treated the bite as it didn’t seem serious. By this point doctors had a vague idea of what must be going on, but no treatment worked. The man fell into a coma and, when medical intervention was removed after two months, he died. As you may have guessed by now, the man had rabies.
Rabies is caused by some members of the genus Lyssavirus such as the Australian bat lyssavirus and the imaginatively named rabies virus. These viruses are fairly average-sized and have a bullet-shaped capsid. They also have an RNA genome which is pretty cool.
The natural host of the lyssavirus is bats. Bats are often asymptomatic carriers of rabies. They are actually asymptomatic carriers of a lot of things - bats host the highest number of viruses of any living thing.
That’s not necessarily a bad thing. Even humans have natural viruses that live in us constantly, just like we have a bacterial microbiome. Our virome, as it is called, has been shown to have an effect on both the health of us, the host, and also the microbiome (thanks bacteriophages!). But I’m not here to fangirl about bacteriophages. That can wait for another time. My point is, lyssaviruses usually live decent, non-rabies-causing ‘lives’ inside bats. That is, until they cross the species barrier in a process called zoonosis. Yes, that’s how covid came about too. And, in fact, all the recently emerged viral diseases, such as the original SARS and HIV.
Lyssaviruses’ host cells of choice are nerve cells, and they are also found in the saliva which aids in transmission.
Almost all rabies cases are transmitted by a bite from an infected animal, such as a bat, dog or fox. No human-to-human bite cases have ever been recorded. All human-to-human transmissions are the result of organ transplants. Which, uh, kinda sucks.
The lyssaviruses’ mode of operation, once they’re inside a non-host organism (which can include any warm-blooded animal, and some strains can even infect reptiles) is to go straight for the brain. They enter the nervous system and slowly work their way up the brain stem into the cerebral cortex and meninges. This process usually takes between 1 and 3 months, though it can take as little as 4 days and as much as 6 years, depending on where the bite is and how much virus was transmitted.
Once the virus reaches the brain, it causes inflammation which cause the initial symptoms of fever and headaches. From there brain damage sets in, causing all those lovely neurological symptoms that can range from paralysis to hallucinations to changes in behaviour.
Interestingly, this is an example of a pathogen changing a host’s behaviour to facilitate transmission. The virus makes the host more aggressive and impulsive by damaging its cerebral cortex, so it’s more likely to bite something or someone and pass on the virus. Smart! Or, well, evolutionarily beneficial.
You may also have heard of rabies causing hydrophobia. This happens in about 80% of cases and can be so awful that even the mention of drinking can cause horrible throat spasms. If you want to upset yourself, there’s a lovely video on the Wikipedia article for rabies of an infected man trying to drink water. It’s fascinating, and heartbreaking. Anyway, this makes the victim less likely to want to swallow, meaning their mouth is always full of virus-filled saliva for when they actually do bite someone.
After symptoms have set in, death is rapid - 2 to 10 days after the first fever, though as seen with the example at the start, this can last longer with the right interventions. Nevertheless, the victim will die. Only 14 people have survived rabies after symptoms set in.
The only way to treat rabies is to get vaccinated within 10 days of a possible infected bite. You can also get vaccinated beforehand but due to the cost of the vaccine only people with high chances of catching it are given it. The high cost of the vaccine also means it is hard to find in developing countries. This is especially awful because those countries usually have high populations of bats, stray dogs or both, which can cause huge risks to the human population. Rabies is classed as a neglected tropical disease.
And on that lovely note, thanks for reading my massive infodump on one of the best-known viruses! I think its one of those viruses that really does deserve its terrifying reputation. Best of luck to anyone who does end up exposed. Luckily, the vaccine is 100% effective if given in good time!