Imagine that pesky tabby cat has been pooing in your backyard again. Unbeknown to you, it has transferred some of the parasite spores it was carrying onto your herb garden. Unintentionally, while preparing a tasty salad, you forget to rinse your hands and infect yourself with the Toxoplasma gondii spores. For months you display no symptoms, then after six months you are driving your auto more aggressively, taking opportunities in road junctions and generally filled with more road rage as you angrily gesticulate with fellow drivers. Could all this be linked to that tasty salad?
T. gondii is a fascinating protozoan parasite which, like many similar organisms, needs to move between several different host species in order to fully develop and reproduction. As such, it appears to have evolved clever methods to stimulate transmission between hosts more likely. For instance, analyses have found that once rats intermediate hosts are infected they display less caution towards cats the final stage hosts and so the parasite is more likely to be passed on.
An increasing number of studies indicate humans known to be infected with these parasites could be more susceptible to schizophrenia, bipolar disorder, aggression and even increased suicide. Study have even suggested you are two to three times more likely to have a car crash if your blood tests positive for the parasite. This is particularly striking when it has been predicted that 30% -5 0 %~ ATAGEND of the worldwide population may carry the parasite.
Not so cute when you know what theyre carrying. Shutterstock
Chicken or egg ?
Very often criticisms of these studies come down to a chicken and egg topic. Correlation doesnt necessary mean causality. Are those aggressive, fast-driving people or those with behavioural conditions more likely to catch the parasites, or does the parasite cause these behavioural traits? Many of the studies were done retrospectively rather than looking at people behaviour before and after they became infected with the parasites. So for now, we cant was sure whether your road rage truly was linked to your salad.
What we do know is that there are plenty of examples in wildlife where parasites can manipulate the sex, growth, maturation, habitat and behaviour of their hosts. Hair worms, for instance, complete their lifecycle in a river or stream and appear to making such a hosts crickets attracted to water.
The effects of the parasite dont be brought to an end, either. The hapless crickets can provide fish with alternative solutions food source to their usual diet of aquatic invertebrates and, for parts of the year, can form a substantial part of their diet. So manipulating parasites can be important to preserving healthy ecosystems.
Some ant species infected by trematode flukes are manipulated in a way that builds them cling to the tops of blades of grass, which means theyre more likely to be feed by sheep. This enables the fluke to complete its life cycle in the sheep.
Chestburster. mardeltaxa/ Flickr, CC BY-NC-SA
A type of barnacle parasite known as a rhizocephalan, which fees its crab host from the inside out, is known to feminise its male hosts by castrating them. Scientists have suggested they are then more likely to look after the parasite sac that explodes through their abdomens, much like a female would tend to her eggs.
Switching on genes
Through advances in molecular biology, we are increasingly working out how these parasites can change behaviour by altering gene expres the route genes can be turned on or off. For example, work in our laboratory at the University of Portsmouth is trying to uncover the mechanism that enables a newly discovered species of trematode parasite making such a shrimp-like( amphipods) hosts more attracted to the light.
Trematodes: little blighters. Josef Reischig/ Wikimedia Commons, CC BY-SA
These amphipods would prefer to be hiding under seaweed on our shores, escaping their bird predators as the tide recedes. By chemically mapping the brains of infected shrimp, scientists have discovered that parasites somehow altered the shrimps’ serotonin, a mood neurotransmitter discovered throughout the animal kingdom. Our recent analyses have indicated that infected shrimps have subtle modifications to their serotonin receptors and the enzymes that render serotonin.
Other surveys have shown amphipods hosting similar parasites are over 20 times more likely to be eaten compared to non-infected specimens. Again, this highlights the often-overlooked importance of brain-bending parasites in the natural order of food webs.
We often think we must have discovered all the species possible in well-studied locations such as the UK, but many fascinating new manipulating parasites are yet to be discovered on our doorsteps. Our knowledge of how these brain-bending parasites interact with human species will no doubt develop more strongly over the next decade.
Alex Ford, Reader in Biology, University of Portsmouth
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