Dicrocoelium is a stand out example of a parasite altering host behaviour when viewed alongside other similar parasites due to the level of sophistication in the control wielded by the parasite (for background info on Dicrocoelium click here). In the most basic of examples of parasite induced altered behaviour there isn’t so much mind control as there is physical handicapping. Take for instance the trematode parasite Curtuteria australis which burrows into clams. Once burrowed the trematodes form cysts in the foot (the part that sticks out of the clam shell) of the clam which, with enough accumulation, can start to impede the clam’s ability to dig in the sand.
This inability to dig efficiently makes them more vulnerable to one of their main predators, birds. Which, you guessed it, are hosts for the clam dwelling parasites. This is considered a basic form of manipulation because the case could be made that this is coincidental and not actually a deliberate act of manipulation that is being perpetrated by the parasite, it could be viewed as a by-product. If one takes the stance that these parasites have evolved to specifically effect or control the host in a way that is of direct benefit to the parasite by increased transmission or fitness for example, then we must include any systems where morbidity due to infection could lead to increased predation as parasite altered behaviour.
Dicrocoelium shows higher levels of manipulation because it is not merely impeding function or health but rather; infection leads to the ant hosts abandoning any semblance of normal ant behaviour for a time and acting in ways that uninfected ants would never do. There are a number of systems in which the manipulation shows this level of sophistication but I believe Dicrocoelium still has them beat. The first of these systems happens to be one of the most famous, the Cordyceps fungus.
This fungal parasite infects ants with airborne spores that float on the wind and land on the host. There are many species of insects each with its own arthropod host, insects and spiders mainly. The fungus then grows into the host brain and as it does it seems to make them go mad. Rightfully so too, I mean there’s only fungus growing in its brain. The hosts start to not only twitch and move erratically but they also seem compelled to head up, climbing plants until the fungus has grown so large that it kills the host and bursts out of its head. Hopefully by this time the ant has climbed up to a good spot where the spores have the greatest chance of being carried farther on the wind. This is an amazing sight and there are lots of cool videos and research on these killer mushrooms but in my mind it still doesn’t quite have the level of precision that Dicrocoelium or my next example exhibit.
The manipulation of the behaviour by the Cordyceps is clear but it is one way. Once the infection is in the behaviour is changed and the effects increase with time culminating in death, unlike Dicrocoelium which does not kill its host. Furthermore, this could be another example of impediment meaning the weird behaviour could merely be a side effect of the fungus destroying the ant brain. As I’ve discussed in a previous article on Toxoplasma, figuring out the difference can be difficult.
The last example I want to bring up before I get to Dicrocoelium shows a highly evolved level of host manipulation but it too is one way. This last example is also different because it is an example from a parasitoid. What’s this? A parasitoid is an organism, many species of wasps actually, whose offspring obligatorily live off a host and which always causes death. These hosts tend to be eaten alive. The Emerald Cockroach Wasp is just one of these organisms and also has lots of cool videos on YouTube.
This wasp has evolved to out duel cockroaches and plant two deadly and precise stings to the brain of the roach. These stings inject an amazing bit of venom that quickly acts on the cockroach nervous system, the first one freezing the roach for a small amount of time so the second sting can be made. The second sting takes away the cockroach’s ability to make its own decisions making it completely at the whim of the wasp who then leads the roach, like a dog on a leash back to its lair where it will become the main course for the hungry wasp larvae. The wasp lays the eggs on the roach, who just sits there, not moving, unable to regain control of its mind and body for up to TWO WEEKS while the eggs hatch and start eating it alive until there is nothing left. This is one bad wasp.
There is one big reason for me though, as to why Dicrocoelium has all these others beat. Dicrocoelium shows so such a high level of control in that it can somehow both take and restore control of the host; and it does so daily. The infected ants climb up the plants and clamp on with their jaws remaining there, still alive, until they are accidentally eaten by grazing mammals, in which they grow to adults and reproduce. The thing is though; they only do this when it’s cool out. In field observations from our lab the maximum temperature is around 19oC.
The ants can be witnessed clinging to plants in the early morning and in the evening but during the hot parts of the day they are nowhere to be found. There is the odd one that apparently doesn’t get the message and ends up dying on the plant, but these aren’t that common. This switching on and off, of the behaviour, shows a level of control unlike any of the other systems and is likely the way in which the parasite perfectly balances the payoff of increasing its exposure to the next host, with the cost of host death by exposure to heat and other elements. It is sacrificing short term prolonged exposure, staying out all day, with intermittent exposures, going out when the host is less likely to die, that can then go on indefinitely.
There is also less evidence for the control to be a side effect of damage or poor health as the host is never killed by the parasite. In fact it’s in the best interest of the parasite to keep this host alive. This raises some interesting questions too about what the ants’ behaviour is like when it’s not clinging to plants. Does it revert back to normal, meaning the parasite no longer has control and that the host is at risk of dying during its day to day ant life; which could be significant, ants naturally lead dangerous lives. Or does the parasite retain control and compel the ants to return to the nest maybe, where they could be fed and protected, ready to go back out only to try and pass along their parasitic friends to unsuspecting hosts. This would be an enormous level of control exhibited by this parasite if the latter turned out to be true, adding to an already impressive resume which also boasts the potential ability to detect temperature cues. Let’s not forget that one, they seemingly know when the right time to stop and start their clinging is. This is why the Dicrocoelium parasite is the finest example of host mind control and truly worthy of the Zombie Ants title.