• Question: why can parasites be dangerous and what type of parasites can be dangerous and cannot be dangerous?

    Asked by #Asia_ali to Mark, Franco, Claire on 14 Jun 2016. This question was also asked by ?.
    • Photo: Franco Falcone

      Franco Falcone answered on 14 Jun 2016:

      Hi @ #Asia_ali

      there are some very dangerous diseases caused by parasites (malaria, sleeping sickness, amoebiasis) and some parasites cause a lot of sickness without really killing you. On the other hand, some parasites may go unnoticed, that is you have them, but you don’t even know. And some parasites like headlice are unpleasant but not dangerous.
      So it’s the whole spectrum, from pinworms on one end to sleeping sickness on the other end. But whether you get sick or not also depends on additional factors, such as your immune system, nutritional status, and the parasite load, in other words if you are healthy and well fed and have few parasites, you will cope better than someone who is undernourished and has a high parasite load!

    • Photo: Mark Booth

      Mark Booth answered on 14 Jun 2016:

      Hi Asia

      Many parasites are dangerous to their hosts because of the way they exploit the hosts. Sometimes it may not be deliberate but as a consequence of evolution. It has never been fully understood why malaria kills so many people whereas many worm parasites do not. Some worm parasites cause more damage with an increasing burden of infection – a few worms cause very little damage but lots of worms can be very harmful. Some parasites – including malaria, become less harmful the more often you are infected due to people becoming partially immune. I say partial because it is not like getting measles where you are immune for life. After getting malaria the immunity may not last very long. Some worm parasites like hookworm can live for many years and we don’t seem to become immune even after years of exposure.
      Some parasite like the trypanosomes are dangerous because they attack the nervous system and the drugs to treat them are not very effective

    • Photo: Claire Bourke

      Claire Bourke answered on 16 Jun 2016:

      Hi Asia,
      I guess to answer your question I started thinking about what ‘dangerous’ could mean; in the parasite world different species use different tactics so if you mean dangerous as in deadly, parasites such as malaria are among the most dangerous because they cause huge numbers of deaths. The World Health Organisation estimated that there were around 438 000 deaths due to malaria in 2015 alone, which is a real tragedy and certainly makes malaria a very dangerous parasite in that sense.

      Another way to think about ‘danger’ is in terms of how much disease and disability they cause (epidemiologists cause this ‘morbidity’) – so, even if a parasite doesn’t kill, it might make you very sick or a little bit sick but for a very long time (i.e. high morbidity). This is where worms might be considered very dangerous because although they kill fewer people than malaria, they infect millions of people world wide and can live for a very long time inside someone (decades in come cases), causing a whole lifetime of illness.

      As Franco and Mark say, in addition to dangerous parasites, some have very little effect on the health of the person that they infect and can ‘freeload’ off that person for a long time, without being dangerous.

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