This article has been published previously in a 2006 edition of On Dit (the University of Adelaide student magazine), but like the media bastards they are, they edited and gutted it to nothing. Here’s the full article as intended. Also, I did write it as an undergrad so please forgive any mistakes, factual, grammatical or otherwise.
Holy stinking faeces, Batman!
You might wonder why you need to get a tetanus shot whenever you impale yourself with a stick or rub manure into your newly-acquired open wound. It’s because ~100 000 people die every year from tetanus every year, you dirty coprophiliac! OK, that’s misleading; most deaths are newly-born babies due to unsanitary birthing conditions, but even healthy adults can still die if left untreated.
Tetanus is caused by the toxin released by Clostridium tetani, an obligate anaerobic (i.e. it cannot live in the presence of oxygen) bacterium. Four disease types exist: local, which only affects the muscles around the site of infection and is generally non-fatal; cephalic, an uncommon version that causes damage to the brain cells after a head trauma; neonatal; and general, the focus of this article.
C. tetani lives naturally in soil, dirt the gastro-intestinal tract of many animals (including ourselves) and the faeces of such animals. Transmission can occur when the skin is pierced with an infected object, but most cases occur when the umbilical cord of unvaccinated neonates are cut with dirty equipment. Also, an African custom of rubbing animal dung on the umbilical cord after birth helped spread it around. You cannot catch tetanus from someone who is infected with it.
Due to its ability to form spores, it can survive in the environment for years on end. However, once in anaerobic environments (such as a deep cut), it comes out of its hibernation and proliferates. After infection, there is generally an incubation period of eight days, but can range from three days to three weeks. During this time, C. tetani is producing its toxin, tetanospasmin, which moves from the infection site to the central nervous system.
Tetanospasmin is an enzyme that cuts up a specific protein. This is not unusual in itself; our stomachs are filled with them (e.g. trypsin and pepsin) to help us digest proteins, our blood-clotting mechanism is dependent on them, many hormonal systems depend on them, etc. What’s special about tetanospasmin is the specific protein it cleaves: synaptobrevin. This protein enables the release of certain neurotransmitters, more specifically inhibitory neurotransmitters. Don’t worry if this doesn’t make sense at the moment, keep reading.
A short detour through neuromuscular physiology
When you move any muscle (even involuntarily), there’s a whole lot of unseen business that goes on. We’ll work backwards from the muscle being stretched. To contract one muscle fibre, you fire off one nerve. Easy as that. That one nerve originates from the spinal cord. Here’s where it gets a little trickier (though massively simplified).
In the spinal cord, for each muscle nerve, there’s a bunch of other nerves leading from other places: nerves from the brain to dictate conscious or unconscious contraction, and from reflex sensors (such as one from the knee tendon, in the case of the knee-jerk reflex). Some of these nerves say “WOO! FIRE OFF!” (excitatory nerves), others say “NO, DON’T, STUPID!” (inhibitory nerves) depending on where they come from. Whether a particular muscle nerve fires off or not depends who wins the shouting match.
This system allows you to stop yourself doing stupid things due to reflexes. For example, say something falls out of your hand towards the floor. Your reflex sensors from your hand and eyes notice this and say to your arm “Go catch it”. Then your brain recognises the falling object as a knife. Past experience says you should not attempt to catch knives, so that part of your brain sends the message “Stop trying to catch it!” to that same nerve. Conscious brain messages are generally louder than the others, so that arm muscle nerve stops firing and thus you stop trying to catch the knife.
Such complex behaviour out of such a simple system is one of the reasons why biology is so sexy.
Now for something completely on topic
So if you have synaptobrevin cut up and not working, the inhibitory nerves aren’t able to say anything. Whenever there is any small stimulus to contract a muscle, the patient is unable to stop it.
This, combined with the fact that tetanospasmin affects the shortest nerves first, explains the early symptoms of lock-jaw (technically known as trismus), spasms and risus sardonicus, so-called ironic smile caused by the forced contraction of the muscles around the lips, neck and face.
Left untreated, the victim’s back, leg and arm muscles contract tightly and spasm. Muscles can contract so hard that they fracture the arm or leg bones, even break backs. Death eventually occurs after the breathing muscles uncontrollably spasm.
Apparently it’s pronounced “No-bul” not “Nob-blee”
Treatment comes in the form of an anti-serum. The technique for the production of antiserum was invented by Emil von Behring in 1901 and earned him a Nobel Prize. It ended up saving countless people during WWI and laid down the foundation for a whole new way of treatment of toxins and venoms.
First, you get a whole bunch of toxin and inactivate it (usually using formaldehyde). The end product is no longer toxic and is called a toxoid. The toxoid is injected into an animal, say a horse or sheep, which will then produce antibodies binding to it. The serum (a term for the blood minus the blood cells and clotting factors) of the animal is then taken and can be injected into a human with tetanus. Since the body doesn’t have to produce its own antibodies, the toxin begins to be neutralised almost immediately.
This is called passive immunity; compared to usual vaccinations, where the human body itself produces the antibodies, is called active immunity. Human antibodies are now used for passive immunity instead of animal ones to decrease allergic reactions.
Also, antibiotics are given to kill the bacteria themselves. In late stages, muscle relaxants and anaesthetics are used to decrease the severity of the spasms.
Prevention lies in vaccination of everyone at risk with the toxoid and not washing cuts with poop. The vaccination wears off in 5 – 10 years, so it’s important to get booster shots.
Of course, I’m not a doctor and you shouldn’t be taking this article for medical advice, so… whatever, do what you want.