In the late 18th century, spontaneous generation was still a widely accepted hypothesis for parasites. For instance, physician Marcus Elieser Bloch stated that parasitic worms were destined to live in very specific locations within the host (Farley, 1992). He hypothesized that these parasites could not possibly arrive there by chance, rather, they had to arise from the host itself. In the early 19th century, Johann Bremser stated that parasitic worms did not occur elsewhere than inside the bodies of other animals. Therefore, he argued that these worms must be produced inside the body. As vectors and intermediate hosts were not yet known, opponents of the spontaneous generation theory could not substantiate their hypotheses.
By the 19th century, scientists realized that animals could develop in various ways previously unimagined. For instance, certain insect life cycles were observed for the first time. The larval and pupal forms were a departure from the well-known egg-to-adult pattern observed in many vertebrate life cycles. Other life cycles were also observed in benthic invertebrates, where larval stages had a contrasting morphology compared with the adult animals, for which they first had to undergo a complex metamorphosis. In 1842, Japetus Steenstrup, a Danish naturalist, made a crucial discovery: alternation of generations, also known as metagenesis or heterogenesis, in parasites (Farley, 1992). This life cycle involves transitioning between two forms, the asexual stage and the sexual stage. Steenstrup observed that the “immature” stage did not directly transform into a subsequent stage, but instead reproduced to generate more than one member to the next stage (Steenstrup, 1845).
Key elements for our story are his observations in flukes (Trematoda). It was known that flukes developed from cercariae, free-living larvae. He concluded that these larvae arose in large numbers from saclike bodies in snails (Steenstrup, 1845). These saclike bodies did not resemble the adult stage. As he was not interested in parasites per se, he was not aware that he introduced two major concepts for parasitology: the intermediate host and the parasitic life cycle. This field of research was investigated further by, amongst others, Friedrich Kuechenmeister, Carl von Siebold, and Pierre-Joseph van Beneden a few years later. They researched the life cycle of Cystica, a taxon of parasitic worms. As the Cystica worms lacked reproductive organs and were found nowhere else than muscles or the digestive system, they were the most obvious examples of spontaneous generation in worms (Farley, 1992). Their feeding experiments with tapeworms showed that Cystica was in fact a collection of larval stages of terrestrial tapeworms and that these worms have a complex life cycle (Farley, 1992).
Historical definitions of parasites
Understanding the word parasite requires context. Coined almost simultaneously by Albert Bernhard Frank (1877) and Heinrich Anton de Bary (1878), the term “symbiosis” is defined as a spectrum of interspecies interactions, ranging from mutualism to commensalism to parasitism. While some researchers have used the term symbiosis to refer only to mutualistic, or mutually beneficial relationships, there is precedent within mycology and other disciplines to deploy the term when referring to a spectrum—ranging from parasitism to commensalism to mutualism (Trappe, 2005; Leung and Poulin, 2008; Horton, 2015)—and this is the meaning invoked here. That being said, there is additional confusion around the term symbiosis, which, depending on the framework, could be considered an exception to biological norms, or a fundamental condition of life for most multicellular organisms.