This is a bit of a departure from the way I usually talk about parasites. They do have a bit of an image problem, coming mainly from their reputation for killing people, animals and plants. Normally I try to avoid talking up their more deadly attributes, but an acquaintance casually dismissed infections with parasites in humans as if they simply won’t happen. I thought, ‘this won’t do at all’, so here’s part 1 of 4 snapshots of just how many parasites may be out to get you.
Before we start, I want to make a few things clear. First, this is not an exhaustive list, just what I can think of off the top of my head, with some details from my trusty Foundations of Parasitology text and supplemented with stats and pictures from the internet. Therefore there’s every chance that I’ve neglected to include something. Second, the likelihood of infection with any given parasite varies hugely and is dependent on a whole range of factors that I’m not even going to get into. As I said, this is a snapshot.
Plasmodium spp. – the most famous protozoan parasite. Several species cause malaria in humans, and millions of people (mostly children) die every year from the disease. Present in tropical and subtropical areas, vector-borne (mosquitoes).
Entamoeba histolytica – an amoeba that will give you dysentery and you might die. Present mainly in areas of poor sanitation. Faecal-oral transmission.
Naegleria fowleri – a water-borne amoeba that will infect your brain, via your nasal mucous membranes. Usually fatal, but infective amoebas are usually in low prevalence in water. There is currently an outbreak in Pakistan that has killed 10 people.
Leishmania spp. – vector-borne (phlebotomine sandflies) protozoans, causing visceral or cutaneous leishmaniasis. Can be fatal. Present in tropical and subtropical areas.
Trypanosoma spp. – vector-borne (tsetse flies in Africa, triatomine bugs in S. America) protozoans, causing sleeping sickness in Africa and Chagas disease in S. America. Can be fatal, especially in acute phase of infection. Chronic infection also occurs. Present in tropical, subtropical Africa and South America.
Toxoplasma gondii – coccidian parasite of cats. Humans get infected as intermediate hosts (regular intermediate hosts are rodents). Not likely to kill an immunocompetent human, but will possibly make you feel a bit crappy. More important is the potential danger for foetuses, with spontaneous abortions and stillbirths associated with toxoplasmosis of mothers (not just human ones either). Cosmopolitan distribution, zoonotic.
Cryptosporidium parvum – coccidian parasite with common transmission being faecal-oral, or occasionally water-borne. Will cause diarrhoea, and can kill severely immunocompromised people. Cosmopolitan distribution, can be zoonotic.
Giardia lamblia (sometimes called duodenalis): highly infective, usually via faecal-oral route. Not generally fatal, but with symptoms including diarrhoea and bloating, likely to be highly uncomfortable at the very least. Cosmopolitan distribution, can be zoonotic.
There are many more parasitic protozoa that can potentially infect humans. In developed areas with high standards of hygiene (theoretically), the risk of infection with a protozoan is fairly low. But even in these areas, pathogens like Toxoplasma, Giardia and Cryptosporidium can lurk close by.
Stay tuned for Part 2 – Platyhelminths.