Profitable Wonders James Le Fanu
The parasite, ‘One who eats at the table of another’, is a lowly creature as, with all its bodily needs catered for, it requires none of the specialised organs of those that must make their own way in the world. And the lowliest of all, or so it would seem, are the flukes and nematodes that live contentedly, well provisioned for, within the lubricious intestines of their generous hosts.
They thus have no need for a brain or nervous system, nor – living perpetually in the dark – sight or hearing or the senses of touch or taste. Absorbing nutrients directly through their outer walls, they can do without a digestive tract. Firmly attached to the lining of the gut, they require neither limbs nor muscles.
And yet, in one of those deeply perplexing paradoxes of the natural world, the life cycle of these primitive decerebrate, insensate, inert, helminthic worms could scarcely be more complicated – or bizarre. Their progeny, before returning as adults from where they originated, must undertake a hair-raising journey through the bodies of intermediaries, compared with which Ulysses’s adventures might seem almost uneventful. Their chances of survival are minuscule but there is strength in numbers. And they have a remarkable ability to bend to their will the minds of animals vastly more sophisticated than themselves.’
Three instances must suffice, starting with the (relatively) simple life cycle of Sphaerularia bombi. This one penetrates the skin of a bumble bee queen while she hibernates through the winter months in her mossy den. Once safely ensconced within her body, Sphaerularia’s eggladen uterus grows prodigiously in size, everts – with its ovaries – through its vagina to become one hundred times larger than the worm itself which, reduced by now to a mere appendage, withers and dies. Soon after, the uterus bursts, liberating millions of eggs that, maturing into larvae, move into the bumble bee’s intestines. She by now has emerged from her hibernating state but, rather than seeking out a burrow to hatch and tend her own offspring, is condemned by the parasitic larvae in her gut to become – as the Dutch naturalist Anton Minderhoud described it – ‘ de
eeuwige zoeker’ (the eternal seeker). She flies from dawn to dusk close to the ground, stopping every so often to dig a shallow hole into which she voids her unwelcome guests. She eventually succumbs from exhaustion, by which time the Sphaerularia larvae she has deposited in the soil have matured into adults and, several months later, are ready to inflict the same fate on another unwary hibernating bumble bee queen.
The life cycle of Leucochloridium, an intestinal parasite of the hedge warbler and other small birds, is more remarkable still. First, a hungry snail must eat its eggs concealed in the droppings of an infected bird. These then metamorphose into a brightly coloured, cylindrical ‘broodsac’ resembling a caterpillar containing thousands of larvae. The broodsac then migrates from the snail’s gut into its protuberant eye stalk, adversely affecting its perception of light.
So, rather than evading potential predators by sheltering in some concealed spot, the snail gravitates to exposed sites such as the upper surfaces of leaves. There it is spotted by an inquisitive warbler that, mistaking the pulsating multicoloured broodsac in its eyestalk for a nutritious meal, snaps it off and eats it. The larvae contained within, having completed their journey back into the gut of their primary host, then mature into adults and the cycle repeats itself. Meanwhile, the snail regrows its missing eye stalk, which is then occupied by another of those caterpillar-like broodsacs, allowing, as it were, for a repeat performance.
The life cycle of the liver fluke Dicrocoelium dendriticum, which inhabits the bile ducts of sheep and cows, is the most astonishing of all. Here, too, a snail must first eat its eggs, concealed in the droppings of an infected herbivore. Maturing into larvae, they migrate, this time up into its lungs, to be covered by a thick layer of mucus and coughed up as a ‘slime ball’.
The next stage of the Dicrocelium’s odyssey requires that a passing ant should chance upon the slime ball and carry it back to its colony to share with its fellows. The next evening, as the air cools, they disperse and, clambering up a blade of grass, attach themselves to the tip with their powerful jaws. There they will remain throughout the night, returning to the colony the following morning.
This nocturnal ritual continues until a passing herbivore eats the blade of grass to which one of those parasitised ants is fixed. And so, by this circuitous route, the larvae of Dicrocoelium return to their natural host.
These primitive parasites, it must be presumed, take over the minds of their intermediary hosts by altering the electrochemistry of their brains – but how they do so, no one can tell.