How a parasite take control of an ant's brain
A number of parasites show a remarkable ability to control the behaviour of their hosts for their own benefit. The fluke Dicrocoelium forces ants to climb up vegetation and to clamp their mouth-parts shut, locking themselves in an exposed position. This makes it more likely that the ant will be swallowed by a larger animal, such as a herbivorous mammal grazing on the plants. The behaviour is obvious and the reasons behind it are plausible, but until now we have not know how the parasite manages to manipulate the ants into this suicidal behaviour.
Very fine detailed scanning of ants, and their heads in particular has released more details of the life cycle of these flukes. The first host of the fluke is a snail; the eggs hatch in this host and the miracidia larvae move into the snail's digestive gland. There they reproduced asexually and their offspring (cercariae) move into the lung. From there they escape in the snails slime. This slime is eaten by ants and in the ant some cercariae break through the ant's digestive system and migrate into the brain. The other larvae become dormant in the ant's abdomen. It is the ones in the brain that are interesting; one lodges in the suboesophageal ganglion. The new scanning work has shown that the fluke does not destroy the brain or take over a large part of it, but just causes a slight deformation. This part of the brain controls movement, both general activity and movement of the jaw muscles. Interfering with this reduces activity and closes the jaws. Thus, an ant with fluke cercaria in the suboesophageal ganglion will stop walking and will clamp its jaws closed. If this happens during foraging the ant is stuck on the vegetation, unable to avoid being eaten by larger animals. This observation also seems to explain one further oddity, namely that if the temperature rises above 20 C the ant will resume normal activity and return to the nest, only to repeat the suicidal behaviour the next day. This probably arises because temperature changes cause the fluke to move inside th ant's brain, temporarily removing the compression on the suboesophageal ganglion. Similar processes occur in other parasites, such as wasps that will paralyse the ant by injecting toxin into the same part of the brain.
Source: Daniel Martin-Vega et al. 2018. 3D virtual histology at the host-parasite interface: visualisation of the master manipulator, Dicrocoelium dendriticum in the brain of its ant host. Scientific Reports 8; 8587 https://www.nature.com/articles/s41598-018-26977-2
Photo: from Martin-Vega et al. (2018) under Creative Commons 4.0