How do parasites recognise their hosts?

Thanks Kate. Parasites are organisms that live in or on other organisms. These freeloaders are dependent on their hosts for their survival, causing harm to benefit themselves, offering nothing in return. That’s why they have to come up with all sorts of tricks to evade the immune systems of their hosts, developed over generations of co-evolution.

You’re right to point out that often they can go on some extremely complicated journeys to reach what’s known as their primary or definitive hosts, where they will become adults and reproduce. Here to tell us about some of them is Professor of Parasitology at the University of Cambridge, Catherine Merrick. She spoke to James Tytko…

Catherine - There are at least two primary sets of cues that parasites rely on to know that it is in the right place. Firstly, the general environment: reacting to changes in temperature and pH and so on.

For example, many food and water-borne parasites have a resistant cyst form that survives in soil or water, shielding them from damage. These cysts only emerge and replicate when chemical signals from the host’s gut - like the low pH of stomach acid or bile salts in the small intestine - prompt the parasite to activate.

Notable examples of this include Cryptosporidium, which caused a major diarrheal outbreak in Devon this summer, and Toxoplasma, one of the most common parasites, which is spread through cysts found in cat feces.

Temperature is another common signal. The reproductive cells of malaria parasites transform and rapidly mate when the temperature drops from the human body’s typical 37°C to the 20°C inside a mosquito.

James - This is what we’re competing with in the biological arms race against malaria. Research has also shown that malaria parasites will sync up with our body clocks to help their development, another part of their ingenious life cycle. What about the second set of cues parasites use, Catherine?

Catherine - The second cues involve a more specific interaction with the host itself. Toxoplasma, which I mentioned earlier, is also interesting here. It will emerge from its cysts in the gut of any warm-blooded animal. However, if it detects that it is inside a cat, it reacts differently. In the cat's gut, the parasite undergoes sexual reproduction and produces new cysts, which are then excreted in the feces and can infect others. In other hosts, such as humans, Toxoplasma behaves differently: it burrows through the gut wall, remaining dormant in body tissues until a cat consumes the host.

The exact mechanism that enables the parasite to distinguish a cat’s gut remains unknown, but what’s interesting is that we see a similar fussiness in malaria parasites. For instance, malaria parasites that infect mice cannot invade human blood cells, and vice versa. This specificity is due to the interaction between proteins on the surface of red blood cells and those on the parasite: without the precise match, the parasite cannot invade. Such specific interactions are of great interest, particularly as potential targets for vaccines.

James - Indeed. The World Health Organization recently approved a new malaria vaccine that works by injecting malaria proteins into the bloodstream, along with an adjuvant to kickstart the body’s immune system. This process leads the body to develop antibodies so it’s ready for next time it comes across this specific protein,

Thanks so much to Catherine Merrick, Professor of Parasitology at the University of Cambridge for helping with that one.