Types of Toxin
You will often hear of different types of poison. They can be classified by the source or by how they work. For example a neurotoxin targets and damages the nervous system. A lot of these terms come from Greek, as the majority of medical terminology does. So let’s take a look at a few of these. Starting with neurotoxins.
- Heavy Metal Neurotoxins
- Naturally Occurring Neurotoxins
- Clostridial Bacteria Toxins
- Animal Produced Toxins
- Final Notes
- Ivermectin
- Important Note
Neurotoxins
As I said, neurotoxins target the nervous system. They damage nervous tissue or interfere with the function of the nerves, brain or spinal cord. This can lead to serious symptoms such as seizures, paralysis, tetany (lockjaw) and permanent brain damage. Death can occur from the effect on the brain or paralysis of the diaphragm leading to asphyxiation. That means dying from lack of oxygen as the muscles controlling breathing stop working. They are pretty nasty. The other problem, is they are everywhere. Let’s look at some specific examples.
Heavy Metal Neurotoxins
Lead
Lead is a heavy metal, the chemical symbol is Pb. It has been used in all kinds of things from paint to make-up. Currently, it is in batteries, such as car batteries, and in radiation protection. Because it is a soft metal, it is used in protective aprons in radiology. Soil and water contamination is a source too. Lead poisoning is cumulative, that means levels of lead build up in the body over a long time. Symptoms include fatigue, vomiting, constipation, delayed development (in children), hearing loss, seizures and pica. Pica is a condition where the patient eats non-food items like soil or stones. However, it is treatable. EDTA is a chelating agent, that binds the lead so the body can clear it.
Mercury
Another metal, the chemical symbol is Hg. Mercury is unique in that it is the only metal that is liquid at room temperature. ‘Quicksilver’ is another name for the metal. Vapour containing mercury, ingestion and even absorption through the skin, are all ways that mercury toxicity occur. Environmental mercury accumulates in fish and other aquatic animals, and thus can get into the food chain. Glass thermometers containing mercury are rarely used now with the advent of digital ones, as these are much safer than the mercury ones.
Symptoms of mercury poisoning include paraesthesia (itching/burning pain), skin discolouration and peeling, sweating, tachycardia (fast heart rate), hypertension (high blood pressure) and behavioural changes. Remember the Mad Hatter of Alice in Wonderland? Or the phrase ‘mad as a hatter’? These come from mercury poisoning in milliners and other exposure to mercury compounds. Compounds containing mercury were used in felt hat making, and the vapours would affect the hat makers. They would show changes in behaviour like excitability, insomnia, irritability and delirium. Tremors would also develop and the neurological damage led to loss of senses in some cases.
Arsenic
Another heavy metal, with the chemical symbol As. It is a naturally occurring metal often in combination with sulphur. Alloys of arsenic with lead are in car batteries and ammunition. Arsenic compounds are used in pesticides, and it does contaminate groundwater in some places. It is bioaccumulative, like lead, it can build up inside the body. High doses cause acute toxicity, presenting symptoms related to the gut as well as signs of brain swelling and seizures. A characteristic pale band on the fingernails indicates chronic exposure, but is not always present. Arsenic may be carcinogenic too.
Naturally Occurring Neurotoxins
Ethanol
Yes, I mean what you think I mean. Alcohol, which many people regularly consume, is a poison. It’s fine in moderation, but excessive consumption can be deadly. Drinking far too much at once can lead to coma, brain damage and death. Chronic overexposure, such as people suffering from alcoholism, causes liver damage. Cirrhosis is the result and this end-stage liver disease will kill if not treated. So people do willingly ingest a poison. I’m not saying alcohol is always bad, I used to drink until I became unwell, but remember! The dose makes the poison. So go easy on your liver sometimes.
Ammonia
Ammonia (NH)3 is a naturally occurring compound that is a byproduct of the nitrogen cycle and microbial activity. Some of the bacteria in the gut produce it during digestion. Breakdown of urea produces ammonia, which happens with urine saturation. Ammonia has an acrid and unpleasant smell, which you will recognise anywhere. It’s so unpleasant because ammonia is an irritant to the eyes, skin and respiratory system. It is sometimes used in household cleaners and should NEVER be mixed with bleach! EVER! Combining bleach and ammonia will create chloramine gas which is extremely toxic and dangerous.
But we’re talking about neurotoxicity right? Well, as I mentioned, bacteria in the gut produce ammonia as part of normal digestion. A healthy individual can handle this small amount as the liver will break ammonia down into urea which is excreted by the kidneys. If there is an issue with the liver, severe impairment or an issue with the hepatic portal vein, ammonia will build up in the bloodstream. This results in inflammation and can progress into siezures, coma or death. This is known as ‘hepatic encephalopathy’. Hepatic means liver-related, and encephalo- means the brain. The suffix -pathy means disease.
Clostridial Bacteria Neurotoxins
Botulinum Toxin
Another one many will be familiar with. You may know it as ‘Botox’. It has both clinical and cosmetic applications and small doses are generally safe when properly administered. It causes flaccid paralysis, so the muscles relax. Botulinum toxin can cause disease, known as ‘botulism’. Not the most creative, I know, but it means you know what it is. The toxin is produced by a bacteria called Clostridium botulinum, part of a family of pathogenic bacteria. Improperly canned or stored food, unwashed produce and infected wounds are all sources of botulism and can make someone very ill.
Symptoms of botulism include fatigue, weakness and vertigo. You may see facial weakness, difficulty swallowing (dysphagia), breathing difficulties and paralysis. Gastrointestinal symptoms like vomiting and diarrhoea are also common. Still 5-10% of cases are fatal, so it needs to be taken seriously. A lot of the symptoms are consistent with other neurological conditions like stroke or myaesthenia gravis. Treatment includes antitoxin and possibly ventilation support if the breathing muscles are affected.
Tetanus or ‘Lockjaw’
Staying in the same family, tetanus is another clostridial disease. Caused by Clostridium tetani, a bacteria that is found everywhere including soil, saliva, dust and manure. Usually it is caused by an infected wound, where the bacteria sit and produce their toxin. Tetanus results in muscle spasms, often starting at the jaw, hence the name ‘lockjaw’. It can progress to the rest of the body and affect the muscles used for breathing. This kind of paralysis is neurotoxic but death is often due to asphyxiation as the paralysis prevents breathing.
Treatment includes antitoxin and supportive care. Vaccines are readily available. Tetanus affects other species too, such as horses. Horses are especially vulnerable to tetanus, so here in the UK they are vaccinated against it. It is rarely seen in other animals like dogs. Even with treatment, 10% of cases are fatal.
Animal Produced Neurotoxins
Tetrodotoxin
Fish, in the order Tetraodontiformes produce tetrodotoxin. Noteably, the puffer fish. The toxin is in the skin, liver, gonads and intestines of the fish. It is a fatal poison when ingested, causing damage to the gastrointestinal tract and the nervous system. Symptoms include paraesthesia, muscle weakness, nausea and vomiting. The Simpsons made this one famous, where Homer goes to a sushi restaurant and orders the fugu.
Pufferfish is a popular delicacy in east Asia. Fugu (pufferfish in Japanese) sashimi is popular in Japan. Chefs need specialist training to serve pufferfish. The toxin causes paralysis, affecting the diaphragm and causing asphyxiation. People have died from this, hence the heavy regulations around serving pufferfish.
Final Notes on Neurotoxins
This is just a few examples, but these are the most common and well-known neurotoxins. The main causes of death are brain damage or paralysis of the muscles involved in breathing, leading to asphyxiation. Some of them have treatment options, Tetanus antitoxin exists, and some don’t. Some of them have beneficial uses too, because the dose makes the poison.
As for common ones in animals, metaldehyde is one I have personally seen and treated in a dog. It was horrible. Metaldehyde is in slug pellets, a molluscicide commonly used by gardeners to keep the slimy pests at bay. Dogs will eat the slug pellets. It can lead to muscle tremors, hyperaesthesia (very sensitive to pain and touch), anxiety and ataxia (poor coordination). Eventually it will progress to convulsions and seizures. There’s no antidote. You can attempt to control the seizures but sadly the case I saw didn’t make it.
Ivermectin
Ivermectin is an antiparasitic drug, used in farm animals to control both external and internal parasites. It is effective parasite control in some pet species. The drug targets the nerve and muscle cells of invertebrates. Ivermectin is administered orally, by injection or topically onto the skin. It cannot normally cross the blood-brain barrier in mammals, so it has no effect on the nervous system. Except there are some dogs, commonly collie breeds who have a mutation in the MDR1 gene that means ivermectin can cross the blood-brain barrier and cause neurological symptoms. It is also severely toxic to chelonians: tortoises and turtles.
During the Covid-19 pandemic, some people tried ivermectin as a treatment. There is no evidence of its efficacy so do not use it. Many people took it without prescription and led to a shortage of the drug for animal use. As always, please do not take animal medicines off label without a prescription from a qualified doctor. Finally, do not give animals any medications without advice and a prescription from a qualified vet. Be careful!
Important Note
Remember that this is here to provide information only and is not a substitute for medical advice. If you are concerned about a person or an animal, please contact a local qualified person for help. In the USA, call Poison Control. In the UK contact NPIS and your local emergency hospital/vet. The Veterinary Poison Information Service has an advice line for pet owners here.