There are three key types of parasites and one type of prion that I wish to address here. The types of parasites I will be looking at here are Toxoplasma gondii, Tenia solium and Nagleria sp. and the type of prion will be CJD.
Types of Parasites:
- Type of Parasite: Tapeworm
- Method of transmission: Faecal-oral
Taenia solium is also known as pork tapeworm, and is a contributing factor in some cases of epilepsy, as well as muscle pain. Cases of epilepsy associated with this tapeworm are caused by neurocysticerosis, where cysts of Taenia solium lodge themselves in the brain after they pass through the intestinal wall and up towards the brain through the bloodstream. These cysts are stubborn little critters: they can survive within the brain
for years on end and can appear on medical scans like visible blotches, as seen in the picture alongside. Ironically, however, it is not the cysts that trigger the seizures: rather, it is the body’s own inflammatory response in order to help the immune system fight off the cysts which triggers the seizures in areas where the cysts are located. If the cysts end up in the motor areas of the brain, the same inflammatory response can also trigger muscle pain.
- Type of Parasite: Protozoa
- Method of transmission: Ingestion of raw food, blood transfusions, organ transplants, through the placenta
Toxoplasma gondii is one of the most common parasites to infect humans, and this particular parasite is also responsible for the disease known as toxoplasmosis. Despite the fact that humans are one of the hosts that can carry this parasite, we are not the ideal (or the definitive host): this parasite prefers cats, as it is only in their intestines where sexual reproduction takes place. In humans, this parasite can only makes identical copies (or reproduce asexually). Toxoplasma gondii has four separate stages of life: tachyzoites, bradyzoites,
sporozoites and merozoites: the first three occur in the human body, but the last only takes place in cats. In healthy adults, it is generally asymptomatic, and at worst, a flu-like illness occurs in the acute stage (the immune system then successfully gets rid of it). However in both children and those with weakened immune responses, this parasite can cause serious or even fatal illness, as toxoplasmosis takes hold. The picture alongside shows toxoplasmosis within the brain: the parasites are shown as abscesses/ white lesions (located around the central regions of the brain). These lesions often have dead tissue within the centre, as the parasite kills the tissue it lodges itself into. Toxoplasmosis triggers encephalitis, and can also increase the likelihood of other (psychiatric) diseases occurring within the brain. In rare cases, it can also trigger the production of skin lesions: this condition is known as cutaneous toxoplasmosis.
- Type of Parasite: Amoeba
- Method of transmission: Breathing in of amoebae from swimming
If you watched the early seasons of House, you probably would’ve heard of Naegleria from the double episode involving a cop and then Dr. Foreman getting sick: the cop ultimately dies and Dr Foreman ends up in a coma when the other doctors perform a brain biopsy to diagnose the infectious agent. Nagleria lurks in dirty water, and it enters the brain when it is inhaled through the nose. It is a particularly nasty parasite, as it causes encephalitis (brain swelling) which is both necrotizing (destroys brain tissue) and kills quickly. It is often known around medical circles as the ‘brain-eating amoeba’.
The route in which Naegleria enters the brain is quite complex. It first attaches to the olfactory nerve and then the olfactory bulbs, and the amoeba feeds on these areas, causing cellular death and bleeding. It then migrates deeper into the brain and consumes it. Nageleria can occur in three separate forms: a cyst stage, a trophozoite stage and a flagellate stage, which I’ll talk about in more detail in Intro to Parasites: the cyst stage is primarily for protection and the other two are reproductive stages.
Type of Prion
CJD (Creutzfeldt-Jakob disease)
- Method of transmission: Ingestion of food contaminated by prions (which I’ll discuss in Intro to Prions)
You may have heard on the news in the last few years about BSE (bovine spongiform encephalopathy) or mad cow disease in which numerous cattle in Britain were euthanized as a result: their bodies had to be incinerated and a massive quarantine was established. CJD is basically the human version of this particular disease. Luckily for us, it is quite rare: however for those that do have it, it is unfortunately always fatal with an average prognosis of 7 months to live. As the picture shows alongside, it tends to cause spongiform encephalopathy: that’s just a fancy way of saying that the brain is being transformed from a semi-solid structure into a sponge-like structure, causing it to literally shrink and somewhat collapse at the same time. CJD also tends to cause dementia in patients that have it: it is subtle at first, with some degree of memory loss and changes in memory and behaviour, however the condition is rapidly progressive: it only gets worse with time.