Parasite Rex

Almost every animal on the planet will one day play host to a parasite. And yet, despite their ubiquity, parasites remained a mystery for scientists for thousands of years.

Back in the days of the ancient Greeks, parasites were thought to have generated spontaneously within and by the bodies they are inhabiting. It seemed like the most plausible explanation at the time since parasites had only ever been observed within the bodies of their hosts, never actually as they entered the body.

The ancient Greeks’ erroneous beliefs about parasites were due primarily to a lack of information: the life cycles of parasites are highly complicated and unlike anything humans knew, which made them more elusive.

This all changed in the 1830s, when Johann Steenstrup began studying the mystery of flukes: scientists knew that flukes laid eggs, but nobody had seen a baby fluke in its host.

In his experiments, Steenstrup showed that the fully grown, leaf-shaped flukes found in the livers of sheep (or another host) were actually the final stage of a single animal’s complex life cycle.

Streenstrup observed that the eggs laid by adult flukes within their hosts actually escaped the hosts’ bodies and later hatched in water. Once hatched, they appeared to be covered by fine hairs and swam around in the water until they penetrated a snail.

Once inside the snail, these parasites transformed yet again into something Streenstrup thought resembled a shapeless bag, swollen with embryos of yet more flukes.

These embryos, called the King’s yellow worms, then transform yet again into missile-tailed cercariae, i.e., swimming parasite larvae. The cercariae then look for another host – another snail or a vertebrate host like a sheep – inside of which they finally develop into mature flukes.

Armed with the knowledge of these strange transformations, scientists could finally discard the notion that parasites generated spontaneously.