Sunday, 22 February 2009

Why do the cysts in neurocystcercosis end up in the brain? Accident or design?

Life cycles can be complicated things.  One of my pathology lecturers used to reassure us that he wouldn't test us on the life cycle of some of the parasitic worms, because every time he tried to work them out, he ended up with carnivorous sheep!

I think some of the difficulty stems from not being able to distinguish the woods from the trees. Fundamentaly, Taenia solium, like certain other parasitic worms (helminths), needs an intermediate host before it can settle down in a definitive host.  

A definitive host is where the worm goes to retire in its old age.  Therefore, it is in the definitive host that it exists in its adult form.  It must also replicate (like all evolutionary organisms), and so it must be producing eggs.  It makes sense that T. solium therefore sits in the human intestine, shedding eggs for as long as it can. 

The intermediate host is a temporary vehicle that the helminth needs in order to prepare for its glorious days in the definitive host.  It therefore exists in this vehicle in something other than adult form.  It must also stand a reasonable chance of getting to the definitive host.  In this case, T. solium tries to burrow its way into the nice juicy meat of the intermediate host, hoping to be eaten by the definitive host.  If that occurs, the infectious cysts in the skeletal muscle of the poor dead intermediate host get awoken, and try to find their way into the intestine, ultimately to stay there as mature adults. Thus, the cycle is completed.  Got it?

In the case of T. solium, the definitive host is one of us humans, while the intermediate host is a pig.  That's what is supposed to happen, at least.  This all goes a bit haywire on occasion, if we happen to ingest one of the infectious eggs (made from an adult tapeworm sitting in our, or somebody else's, intestine).  Put another way, things go belly up if we accidentally act as the intermediate host, instead of as the definitive one.  

Under the belief that they have been successfully ingested by a pig (which is false, or at least only metaphorically true), the eggs go into cyst-making mode.  They thus wait expectantly to be eaten by the definitive host, a day that will never come.  They can wait for years on end, and are so successful at hiding from the immune system that they almost never evoke a response until they die of old age.

Oh, and why do they go to the brain (or eye) in  humans, and not the skeletal muscle like they are programmed to?  Simple: they are programmed to go to the skeletal muscle of a pig, and being in the wrong host messes up their plans.  Making cysts in the brain or eye is a mistake (not many of us are cannibals).  It is a mistake they make because they think they are in the correct intermediate host (i.e. a pig), rather than in a human.

In the brain they most commonly cause seizures and headaches.  Here's a MRI of a brain infected with neurocysticercosis.  The think to look for is the ring-shaped lesion towards the bottom, which is causing all the problems.

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