Subliminal parasites and zombie snails

    Tuesday, 06 August 2019
    Credit: Wikimedia
    Snail with the parasite Leucochloridium paradoxum inside its left eye stalk
    Credit: Wikimedia

    Camouflage is usually tiny organisms’ first line of defence. Snails are no different.

    By blending in the colour of soil and leaves, these laggards avoid detection by predators. In fact, some snails practise active camouflage: a certain species chews lichen and applies it to the surface of its shell to conceal itself.

    Imagine yourself as a hungry bird desperately looking for a meal. In the green and yellow, you sight two dangling strands. You descend to take a closer look. Against the contrasting backdrop of leafy green, you spot two wiggling figures. Voila! It’s your favourite food: a pair of succulent caterpillars.

    But they are more colourful than usual! It is as if, they are ‘asking for it’. You swoop down-in and pluck them off. It was easy, as they made no effort to escape. Moreover, your eyes are attuned to detect motion, such as their writhing. They seem to have a deviant taste. What an unexpected catch, you wonder!

    Trematodes are flatworms that infect molluscs and vertebrates. Certain species as the ‘Banded Broodsacs’ (Leucochloridium sp.), ubiquitous in Europe, and identifiable with colourful bands and spots, live in the guts of birds. Their eggs and larvae are thus found in bird droppings. When snails consume it, the little parasitic worm enters their body. It journeys to the digestive system, where it develops into the next stage of its life cycle i.e. “sporocyst”.

    It undergoes a drastic transformation called ‘metamorphosis’ to transition to this new form. All such larvae gather together to form an encased agglomeration. This is called a “sporocyst”. It grows into long tubes to form swollen “broodsacs”. These tunnel into the eyestalks of snails, exhibiting a preference for the left one. Once in, it builds fat, throbbing broodsacs there, in which it pumps hundreds of embryos.

    Unlike the previous broodsacs, which were elongated to penetrate, these are intentionally bulky. The snail’s eyestalks become bright colour-banded and start pulsating, resembling decorative lights. The tubular sac is responsible for the swollen appearance. The snail now cannot, as usual, withdraw its eyestalks at will.

    The snail is but an intermediate host. The juvenile parasite needs a bird’s body to mature and complete its life cycle in. To this end, it pursues a predator by inviting it to maul up its host.

    The squirming stalks thus become prominent beacons that mimic caterpillars, the birds’ staple food. Now that the parasite has overridden the first defense measure of the snail, it proceeds to disable the next: Hiding in the dark. Snails instinctively seek the dark. This is obviously an evolutionary habit to avoid being hunted. The parasite craftily manipulates the snail’s tiny brain, erasing its aversion to light.

    Research suggests that the eye-dwelling parasite impairs the snail’s sight (light perception ability). The snail is now likelier to end up in an open, exposed area, where it is visible and susceptible to a looming predator. What’s more amazing is the fact that the eye-tentacles only pulse when in light, and the pulsation increases with increasing intensity of light.

    Thirdly, with its mental faculties compromised, the snail’s final and most obvious defence: retraction into its shell in response to a threat, is effectively disabled. The snail becomes a zombie, inert, unresponsive and single-minded. It totally loses it conscience and sense of its good and harm.

    With all its defenses down and featuring stark red, yellow and green patterns, the snail is soon devoured. The ingested parasite settles in the digestive system of the bird. It completes the remnant stages of its life cycle inside the bird. It ultimately lays eggs in its host’s rectum, which make it to another unassuming snail, via its droppings. The cycle keeps going so forth.

    Often, the snail’s infected eyestalk(s) are plucked out, and it survives. The snail, if left with one eyestalk, will have to undergo the predicament once again. This is because the parasites enter in large numbers and mostly, some always persist in the snail’s digestive gland.

    It was recently discovered that the schistosome parasite, the pathogen of schistosomiasis, an ailment that infects 200 million worldwide, further slows down snails. It makes them prone to being mauled up by crayfish and prawns.

    It is not the only instance of mind-control and suicidal zombification in the Animal kingdom. Certain rats have been occasionally observed to walk right into cats. Do you smell a rat?

    The microbe Toxoplasma gondii infects and “hacks” rat brains, removing their innate fear of cats! It needs to enter a cat’s body, in order to complete its life cycle. Infected rats “forget” their general instinct to run away from larger creatures. They become confrontational and casual. In fact, the rats seem to be “attracted” by the cat’s smell and chase after them.

    Interestingly, animals aren’t the only things manipulated by the one-celled trickster. Upon consumption of undercooked meat, or contact with cat faeces, humans can contract T. gondii as well. Upon entering animal bodies, as humans, the parasite “fools” the immune cells and assumes their reins.

    Taking control of the very sentries of the body, it rides them to spread around. It utilises these cellular defenders, tasked with elimination of threats, to act as its ‘Trojan Horse’ emissaries, spreading the infection throughout. It is also reported to affect human brains and cognitive function.

    Pitamber Kaushik