Ghosts of parasites past

Marco A. Vindas, Helene L. E. Midttun, Lauren E. Nadler, Romain Fontaine, Finn-Arne Weltzien, Øyvind Øverli, Ida B. Johansen

This is a plain language summary of a Functional Ecology research article which can be found here.

Have you ever considered the possibility that you are not entirely in control of your own mind and behaviour? Well, you should. Tiny parasites living inside human bodies, and other animals, have been shown to make small yet significant adjustments to personality, mood, and behaviour. For instance, some parasites will make you just a little bit more outgoing and sociable. Just enough to walk up and introduce yourself to the new cute guy that just moved in next door. You start dating, get married, kids come, and – voilà – all of a sudden, the size of your local population has changed by a little. Sounds all kind of crazy, but parasites are indeed considered important players in the ecological theatre, shaping population structure. They do this, in part, by changing how infected individuals behave. But the plot thickens. We have discovered that parasites can do these things without any longer being present in the body. They can just drop by to pay you a short visit and leave. Their short visit activates the body’s defence system which rapidly kicks them out. Yet, a ghost of this parasite has left a subtle impression.

Medaka fish change their behaviour after being exposed (but not infected) to a parasite, as if they are still reacting to the ghost of parasites past (credit: Anthony MRG Peltier) 

Observing Medaka, the common pet fish known as Japanese Rice fish, we have now documented lasting behavioural effects in hosts resisting infection. Despite repeated attempts to infect these fish, they resisted infection. Yet, the infection attempts resulted in the fish developing a more social personality and becoming more physically active. In consequence, important aspects of animal and human behaviour can in theory be dictated by known and unknown parasite encounters. For context, more than half of the species on earth are parasites, and humans are presently known to harbour some 400 out of these. But the question that arises now is not how many successfully infect us, but rather how many we simply encounter and defeat, but still mess with our minds.