They gain access to our bodies by using any of our natural orifices, or they create their own. They burrow and nibble their way to the most accommodating organ they can find, and then latch on with hooks, tentacles, or with just sheer determination. They ingratiate themselves in their human habitats, sometimes commandeering their hosts to behave in such a way as to facilitate their life cycles. We call them parasites, but they see themselves as survivors – masters of trickery and guile in perpetuating their species.
Parasites set out on a mission to cause just enough damage to incapacitate their benefactors, but falling just short of totally annihilating them. These creatures are too wise to pursue a course of mutually assured destruction; they have not survived the centuries by killing the goose that lays the golden eggs.
The global destruction inflicted by parasites far outstrips the combined harm of other neurological diseases – not that one could tell by reading the popular headlines of most neurology journals; by thriving on the weak and the deprived – the dregs of the earth who inhabit in the backwaters of the world -parasites have mastered the art of avoiding the inconvenient limelight. They know it is safer for them to avoid the curiosity of the headline writers, to evade the attention of health policy makers, and to keep away from social media chatter.
Parasites have had the upper hand over humanity for eons even though they are not faceless aliens or strange extraterrestrials. On the contrary, we have worked out their complex biology, and we know their intricate life cycles like the backs of our hands. We have charted their natural habitats, defined their intermediate hosts, and know their strengths and weaknesses. But, somehow, these cestodes and nematodes, these protozoa and trematodes, continue to evade our traps, perhaps even sniggering at our futile efforts to eradicate them.
The brain is a favoured organ for many parasites. They recognise that the highly nutritious and secluded environment of the nervous system provides succour for the teeming generations they treacherously unleash on their accommodating hosts every day. By disabling the brain, they limit the host’s capacity to strategically plan a counterattack. By hiding behind the blood-brain barrier, they tactically evade the body’s immune defences. Who are these most tenacious of ingenious invaders? In no specific order of perniciousness, here is our shortlist of the eight most parasitic infestations of the nervous system.
This infestation is caused by Plasmodium falciparum, a parasite which primarily infects red blood cells. Using a membrane protein called PfEMP1, it adheres to the walls of blood vessels thereby clogging the brain’s blood supply. It causes further damage by breaking down red cells, haemolysis, and by disrupting the blood-brain barrier. Check out more on cerebral malaria in the following neurochecklists:
- Cerebral malaria: pathology and clinical features
- Cerebral malaria: investigations
- Post-malaria neurological syndromes
The harbinger of neurotoxoplasmosis is Toxoplasma gondii. Humans acquire this by ingesting raw meat or drinking water contaminated by cat faeces. It may also be transmitted congenitally from mother to child. The parasite goes through different developmental phases which the biologists have labelled sporozoites, bradyzoites, and tachyzoites! See more in these neurochecklists:
PRIMARY AMOEBIC MENINGOENCEPHALITIS (PAM)
Naegleria fowleri, the causative parasite of PAM, is a thermophylic free-living amoeboflagellate. It accesses the nervous system through the nose, casually destroying the olfactory bulbs and orbitofrontal cortex in the process. The typical victim of N. Fowleri is someone who swims in freshwater or geothermal springs, or who ritually rinses the nose with neti pots. Read all about PAM in the following neurochecklists:
- Primary amoebic meningoencephalitis (PAM): features
- Primary amoebic meningoencephalitis (PAM): management
The twin agents of sleeping sickness are Trypanosoma brucei gambiense and rhodesiense. The are transmitted by the dreaded tsetse fly, and they cause two forms of the disease: the early hemolymphatic stage in blood and lymph nodes, and the later meningoencephalitic stage in the central nervous system. They leave a trail of destruction in their path, from a skin trypanosomal chancre, to the classical Winterbottom sign: rubbery lymph nodes in the neck. Much more on these two parasites, and their equally nefarious cousin Trypanosoma cruzi, in these neurochecklists:
- Sleeping sickness: central features
- Sleeping sickness: peripheral features
- Sleeping sickness: pathology
- Sleeping sickness: management
- Chagas disease: pathology
- Chagas disease: neurological features
- Chagas disease: peripheral features
- Chagas disease: management
There are three major species of schistosoma that bedevil humans, all with fancy names: mansoni, haematobium, and japonicum. Their intermediate hosts are fresh water snails in which they lay their eggs. These ova reach the nervous system either through arteries or via the Batson vertebral epidural venous plexus. More on these creatures in the following neurochecklists:
- Neuroschistosomiasis: pathology
- Neuroschistosomiasis: cranial features
- Neuroschistosomiasis: myeloradiculopathy
- Neuroschistosomiasis: management
The pig is the natural ally of Taenia solium, the pork tapeworm that causes the much feared neurocysticercosis. The disease is classified into parenchymal and extra-parenchymal forms, and these classically present with epilepsy and hydrocephalus respectively. The parasite has a striking appearance on brain imaging – multiple cysts containing a scolex visible to the trained eye. There is more to neurocysticercosis in these neurochecklists:
- Neurocysticercosis: parenchymal type
- Neurocysticercosis: extraparenchymal type
- Neurocysticercosis: management
The parasite Onchocerca volvulus is a filaria worm which is transmitted by the black fly species, Simulium. It is a burrowing worm which is found just under the skin, from where it releases loads of baby microfilaria daily. It is of interest to neurologists because it causes two forms of seizure disorder: onchocerciasis associated epilepsy (OAE) and nodding syndrome. It also causes Nakalanga syndrome which is associated with mental retardation. For more details, check out the following neurochecklists:
Last, but by no means the least, comes Echinococcus granulosus, the agent of cerebral echinococcosis or hydatid disease. Echinococcus eggs are transmitted through the faeces of canines such as dogs and wolves. The full blown disorder, hydatid disease, is a scourge of the nervous system which manifests as seizures, focal deficits, raised intracranial pressure, and cranial nerve palsies. Radiologists recognise hydatid disease by its tell-tale water lily sign on CT scans. Check out more in these neurochecklists:
And this is just the tip of an infernal iceberg of parasitic infestations. Check out the full list of parasites in this neurochecklist: