When people have had their stools examined and are told that they have Blastocystis, most of them will not have a clue about what that is. And eventually they'll be told that it's a parasite. A parasite? As in tapeworm? Ok it's not. But then what? As in malaria? Oh... ok, I see... So it's....? Huh? As in ... what???
Now, which are those parasites in and on your body, and what in fact makes a parasite? Depends on who you ask. For parasitologists and public health/clinical microbiologists, a parasite means something along the lines of a eukaryotic organism (i.e. not a bacterium and not a virus) that is not a fungus and that is capable of living and maybe even multiplying on or inside another organism. Some organisms are considered somewhere in between parasites and fungi, such as microsporidia and Pneumocystis. But whether an organism is a fungus or a parasite is not important in most cases. You will also sometimes see that 'parasite' is used as a term meant to cover living organisms causing disease, and in this sense the term may include for instance bacteria and viruses; for instance. A lot of research deals with 'host-parasite' relationships, evolution of virulence and tolerance in parasites and hosts, respectively; also here, bacteria may be referred to as parasites.
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| Mosquitos are practically parasites that may transmit other parasites. Source (eyeweed on Flickr). |
The word
'parasite' stems from Greek, and means something like 'eating beside' or 'eating at someone else's table'. Parasitism is a non-mutual symbiotic relationship where one organism (the parasite) benefits at the expense of another (the host).
People like me
usually divide parasites (sensu stricto) into protozoa (single-celled)
and helminths (multi-cellular; worms). Effectively, this should be protists and
helminths, since not all single-celled parasitic eukaryotes are
protozoa. Please note that most protists and helminths (the nematode
fraction) are free-living, - but some have adapted a parasitic life
style and very effectively so.
So, when we're told by doctors that we are in fact hosting parasites, - how do we react? I guess some of us will be quite alarmed: Creatures eating defenseless hosts from within, castrating them and turning them into zombies come to our minds, for instance
Sacculina,
Dicrocoelium, and
Leucochloridium,
just to mention a few ones (if you're not familiar with these ones, I
suggest you look them up - you will hardly believe what they are capable
of doing, and despite the horrifying subtlety and cold-bloodedness with which these creatures operate, one can
hardly help marveling on how cunningly evolution makes way for some organisms'
ability to exploit others). Other parasites are known to cause less spectacular phenotypic changes while having huge consequences for human health
and disease: Malaria continues to be a significant cause of morbidity
and mortality in many larger regions, and recently, diarrhoea caused by
species of
Cryptospordium was recognised as one of the
most significant health issues in infants and toddlers in
select sentinel areas sub-Saharan Africa and South Asia.
Some parasites, however, are commensals (ie. they just sit there with a more or less neutral outcome) or even beneficial to the host; for instance, there's
evidence of ciliates assisting herbivores in metabolising cellulose. So while, these organisms from one point of view are parasites, the hole symbiotic relationship between these protozoa and herbivores may be seen as mutualistic. Maybe this particular relationship started out as 'parasitsm' but developed into 'mutualism'? There may be a lot more examples of this. Animals usually host various types of parasites, and humans probably used to host a much larger zoo of parasites than many of us do today; what is the public health significance of the recent and rapid 'defaunation' of humans in certain parts of the world?
At least technically,
Blastocystis is also a parasite: Sitting in the colon, it lives on food delivered by its host, and thereby it certainly eats at someone else's table. Moreover, the parasite is probably not capable of completing its life cycle without a host. But what does it do apart from eating? Does it do us any good just like the ciliates in the herbivores?
Blastocystis has co-evolved with humans (and other host species) and maybe humans have learned to exploit
Blastocystis so that it's not only
Blastocystis exploiting us? Does
Blastocystis compete with other organisms in the gut? Does it secrete substances that impact other organisms including the host, and if so, in what way? What's its impact on the immune system? Etc.
I guess the take-home message here is that 'parasite' is just a word, - a name for something, and there are examples of parasitism turning into mutualism. Not all parasites induce disease, and parasites are not always organisms that should be sought eradicated.
Literature:
