Parasites Lost

escargot 7

Image courtesy of Virginie’s illustrator, Doobla

It’s noon somewhere in the Brazilian rainforest. A biologist is relentlessly inspecting the soil and the leaves located around an ant colony. Finally, by lifting a leaf, she finally finds what she was seeking: a mouldy ant attached to the main vein in the leaf by its mandible. “There is no doubt – it is indeed a zombie ant specimen!”, thinks the biologist. Meanwhile in Texas another biologist is standing nine feet above a pool that contains fish caught in the sea. He is measuring the time during which a fish swims on the surface of the study area. One particular fish has spent eight out of 15 minutes swimming on the surface of the pool. He thinks: “We’ve got one!”  – but what are these scientists searching for?

Behaviour dictated by parasites

These two scientists are working on a relatively new branch of science that studies the changes in behaviour and appearance of organisms caused by the parasites living in them. The parasites are looking to move from an organism (the host) to another – using very radical techniques. These mind manipulators can disguise their hosts to look like food for predators, or even push them to the brink of suicide. Blimey! The parasites ‘use’ different organisms during their lives just as we change housing in our own lifetimes. Outside of their hosts, these little creatures cannot survive, nor can they move to their next “home.” But by controlling the mind and appearance of their host, the parasites can use them as a vehicle to get to where they want to be, and as discreetly as possible.

Poor little ants…

Lets take a look at an example of these parasites in action. In a field in the early morning, while grazing sheep start munching their breakfast, an ant climbs up a blade of grass and waits for a gruesome fate. Desperate, or rejected by its family*? No, this ant is a victim of Dicrocoelium dendriticum, more commonly known as “brain worm”. Without the presence of this parasite, an ant would never venture atop a blade of grass, as it might be eaten by a sheep. But that is exactly what the brain worm wants, because its next host has to be a ruminant. In order to reach this new destination, Dicrocoelium settles in the brain of the ant and forces it to climb to the top of a blade of grass and wait to be gobbled up by a dozy sheep. In this way, Dicrocoelium continues to survive at the expense of its initial unfortunate host.

Another example of ant mind manipulation is that carried out by another species – a fungus… The mouldy ant discovered by our biologist in Brazil is actually the victim of Ophiocordyceps fungus, which forces the ant to go under a leaf near the ground, steadfastly remain there – and die. These insects are called “zombie ants”. Upon the death of the ant, the fungus inside its body cavity grows out of the shell and disperses its spores on the ground below it; these spores can then infect other ants who venture nearby.

Continuing with our ant theme yet another ant can be seen keeping his abdomen aloft – usually black, he now harbours an orange-red colour so he looks like a small berry. This is the result of some deadly work by the parasite Myrmeconema neotropicum. Its goal once inside the ant is to get to its next host, a bird. To achieve this, the parasite changes the appearance and behaviour of the ant. The ants holds its orange-red abdomen in the air like the tail of a cat. The ant then becomes an obvious source of prey for berry-eating birds. Game over for the ant, but one big step for Myrmeconema!

Even snails and grasshoppers…

Similar examples are described in other species such as grasshoppers and snails. Spinochordodes tellinii infects the brain of the grasshopper and induces the hapless hopper to drown itself, because the next parasite stage occurs in water. When the time comes, Spinochordodes forces its host to dive into the nearest source of water and drown. Think about this parasite next time you see dead insects in a swimming pool. Nice.

Another tasty example: Leucochloridium paradoxum. This parasite infects snails. It produces a yellow-green mass of larvae which move in the antenna of its host. The snail’s antenna now looks like an appetizing yellow and green caterpillar. Yummy. The snail is then forced to move during the day, becoming more visible to birds, which, you guessed it! – are the next host of Leucochloridium.

Mice and fish too…

Have you ever seen a mouse or a rat running about near your cat? Crazy, yes, and also victim to a parasite! Toxoplasma gondii is a tiny parasite that lives in rodents and must end its life cycle in a cat. Same pattern as before! Toxoplasma lodges in the brain of its host and changes the rodent behaviour by making it less fearful, even adventurous. Some researchers have even observed an attraction to cat odour exhibited by rodents infected with Toxoplasma.

To finish this incomplete and incredible list of parasitically modified organisms, may I re-introduce our fish-gazing biologist in Texas? When infected with the parasite Euhaplorchis californiensis, the fish starts to swim closer to the surface and becomes the prey of seabirds that, you guessed, are Euhaplorchis’ next choice of residence.  Our biologist above his pool was measuring the time a fish spent on the surface., He thereafter checked these fish to see if they were carrying parasites. This experiment allows scientists to observe that fish which swam for the longest time period near the water’s surface were those who had more Euhaplorchis californiensis hiding within their brains.

The chicken, or the egg?

But how did this perilous partnership start? Let’s consider a scenario in which the relationship developed from one party evolving mechanisms to better influence the other. In this case does the parasite control the host via the chemicals it produces? Or is it the specific positioning in the brain that causes the strange behaviour?  Alternatively could the relationship have begun with the parasite travelling to the brain not to control the mind but instead to simply escape the host immune system? With the parasite developing techniques to control its host at a later stage? In other words, did the parasite find a way to control his shelter first, or is this relationship a defence mechanism-turned-weapon? These are issues approached by scientists around the world.

rodeo (colo3)

Are you being taken for a ride? (Image courtesy of Virginie’s illustrator, Doobla)

…and what about mankind?

Man is not spared – according to scientists, humans also suffer from parasitic mind control. The best known example of human takeover is that by Toxoplasma gondii. We have already mentioned this tiny being, which normally infects preferably rodents and cats and can accidentally infect humans. Toxoplasmosis is acquired via consumption of raw or undercooked meat previously contaminated with the parasite. Similarly, poorly rinsed vegetables and fruits can harbour the parasite. There is some indication that you can also catch it from the faeces of an infected cat, so step away from the cat litter! Symptoms related to Toxoplasma infection are difficult to discern from those of flu. About a third of the world population is suspected to be infected. If a pregnant woman is infected, perhaps by handling cat litter, Toxoplasma can be transmitted to her baby across the placenta – the consequences of this are severe, even resulting in stillbirth. This parasite forms a cyst (about 1/100 the size of a pea) in the brain of man; apparently it changes the behaviour of the human host just as it does in rodents.

This is the theory of Dr. Jaroslav Flegr, world-renowned for his work on Toxoplasma in humans. Many studies have confirmed the involvement of Toxoplasma in behaviour changes. Humans and mice both suffer the ravages of this tenant by exhibiting longer reaction times and different behaviours when faced with new stimuli – so says Dr. Flegr. However, he says, the situation is more complicated in humans because it seems that infected men and women act inversely as a result of chronic stress. Changes in behaviour are subtle  and not directly noticeable. To date, this eminent researcher works on the effects of Toxoplasma on sexual behaviour of men. It’s actually possible to contract the parasite during sex. According to Flegr’s results, the parasite increases its chances of being transmitted from one host to another by increasing the sexual activity of humans via a burst of testosterone. Against all expectations, there is no treatment to remove Toxoplasma from its host, with Dr Flegr adding that future studies should address this issue as well as the prevention of infection.

The animal world has experienced the full destructive power of certain parasites – such as those causing malaria or gastrointestinal nematodes. However aside from obvious disease-causing effects it is clear that parasites also influence their hosts’ will, or even their survival instinct. To achieve their purposes, these little entities can change mind and appearance of their host and make them palatable for their future habitat. Do they do so on purpose? – we don’t know. Man is also a parasitic victim; the work of many scientists, including Dr. Flegr (and myself) still promises us many surprises.

She just loves parasites

She just loves parasites

This article is by Virginie Barrère, who runs a bilingual (!) parasitology blog with some great illustrations by Doobla.

*The anthropomorphic ant.

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