A brief history of parasitology - Veterinary Practice
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A brief history of parasitology

From the Egyptians through the Middle Ages to Redi and van Leeuwenhoek, how did parasitology become an established field of study?

Stories of veterinary medicine: 1 of 4

With their many morphologies and the variety of symptoms produced in their chosen animal hosts, parasites confused and puzzled early humans. They had been recognised from very early times but were believed to be the products of spontaneous generation, and no one knew their often extraordinary life cycles. Now they are classified as ectoparasites, living in or on the skin (mites and ticks), and endoparasites, found in the alimentary tract, body tissues or blood (nematodes, trematodes, cestodes and protozoa).

Early identification

The first known record is in the Egyptian Ebers papyrus (c.1550 BC) which described the larger ectoparasites and what is now recognised as Dracunculus (guinea worm) nematodes in humans, but without understanding their life cycles. It has been suggested that “the plague of fiery serpents” that afflicted the Israelites in their wanderings was in fact guinea-worm infestation (Deuteronomy 8.15). 

Aristotle (384-322 BC), our first veterinary scientist, recognised what we would now call helminths in animals and described the cysts of Cysticercus cellulosae in the tongues of pigs as being like hailstones. As these cysts were alluded to in an Aristophanes (c.446-386 BC) comedy they were obviously well known in those days. It is possible that the Hebrew prohibition on pig meat consumption was related to knowledge of the cysts.

As these cysts were alluded to in an Aristophanes (c.446-386 BC) comedy they were obviously well known in those days. It is possible that the Hebrew prohibition on pig meat consumption was related to knowledge of the cysts

In following years there were many observations of both human and animal parasites, usually helminths and the finding of hydatids. Aretaeus (81-138) wrote of “small and numerous bladders full of fluid”. Galen (130-210) recognised three types of human helminths and the intestinal regions they frequented. Paulus Aegineta (625-690) then described these three types of worm in the human intestines, “the round, the broad and those called Ascarides”, now termed Ascaris, tapeworms and ascarides, the small irritant worms found in the rectum.

Gradually there was a recognition of helminth parasites. Alexander of Tralles (c.526-605), a leading Byzantine physician, wrote De Lumbricis (“on worms”), now seen as the first parasitological treatise. Progress, however, was slow as it was realised that the presence of parasites resulted in disease but there was no knowledge of where they had come from or how to control them. Abbess Hildegard of Bingen (1098-1179), a writer on human and animal health, appears to have recognised that scabies was caused by a mite at the same time as this was described by the Islamic Moroccan physician, Ibn Zuhr (1094-1162). 

Furthering parasitology understanding in the Middle Ages

Albertus Magnus, in De Animalibus in 1478, recorded parasitic worms in horses, dogs, falcons and fish. But the most interesting observation made in the Middle Ages was by Sir Anthony Fitzherbert in A newe Treate or Treatise most profytable for all Husbandmen (1532). He gave the first recognisable description of the liver fluke. The condition of “rotte” in sheep and its association with wet land, the presence of small snails and “worms” in the liver was well known, but not the life cycle relationship. Fitzherbert described how to “know rotten shape” and that “if thou cut the lyver, therein wyll be lytell quickens lyke flokes”. The word fluke comes from the Old English floc meaning a flat fish, especially a flounder. The French word used was fasciole possibly derived from a vernacular word for the leaves of a plant that the sheep had swallowed. Linnaeus described Fasciola hepatica as a kind of leech, the young of which lived in water, in his first classification. Its life cycle was not resolved until 1882.

The generally held belief was that parasites were the result of spontaneous generation in or on the body of animals and humans, part of a wider and generally accepted belief in spontaneous generation throughout nature

Basic knowledge, however, remained limited, although many extraordinary and imaginary parasites were described and drawn. The Chinese believed that a man should possess at least three worms to maintain good health, and in Europe for a time it was believed to be beneficial for children to have worms! The generally held belief was that parasites were the result of spontaneous generation in or on the body of animals and humans, part of a wider and generally accepted belief in spontaneous generation throughout nature. Aristotle had respected this opinion but wondered why certain insects, if they were created by spontaneous generation, had male and female forms that produced offspring? 

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Questioning spontaneous generation

Increasingly, the concept was questioned. William Harvey (1578-1657), the discoverer of the function of the heart and circulation of the blood, also wrote On Animal Generation (1651). In this treatise, he rejects the idea of spontaneous generation, with his dictum omne vivum ex ovo (all life comes from the egg). Jan Swammerdam (1637-1680), a Dutch biologist and microscopist, demonstrated with insects that the egg, larva, pupa and adult were all forms of the same animal, and therefore that spontaneous generation did not exist.

Francesco Redi (1626-1697), an Italian physician known as the “father” of parasitology, was one of the most important researchers of the 17th century. He described and drew lice and ticks from animals and humans, and described the donkey louse (because Aristotle had claimed they were never affected). In 1668 he wrote “everything which we know in past or present times… came solely from the true seeds of the plants and animals themselves”. Redi destroyed the concept of spontaneous generation with a very simple but highly effective experiment: he took two pieces of fresh meat, put a cover over one and watched; within a few days flies had landed on the uncovered one and soon it was crawling with maggots, whereas the covered one remained maggot-free. 

A decade later Antonie van Leeuwenhoek (1632-1723), the Dutch microscopist now known as the “father” of microbiology, confirmed Redi’s work. Leeuwenhoek was the first to observe and draw a single-celled protozoan pathogenic parasite. Now termed Giardia lamblia, it colonises the small intestine in dogs, cats and humans, causing diarrhoea. It also exists in a non-replicative cyst stage enabling survival and contaminating food.

Edward Tyson (1651-1708), an English physician, performed a detailed examination of Ascaris lumbricoides which showed that there were two sexes; his conclusion that sexual reproduction took place in such simple animals was an observation that again refuted the idea of spontaneous generation in helminths.

A growing interest in parasitology

Suddenly Europe seemed to wake up to the study of parasites. In 1687 Cosimo Bonomo, an Italian microscopist, described and drew the “itch mite”, the cause of scabies. Nicolas Andry studied the known human helminths in 1695, and in 1782 Johann Goeze described Ascaris suum from pigs. In 1766 Peter Simon Pallas, a German zoologist working in Russia, reported on human hydatid cysts and the cat tapeworm Taenia crassiceps,and almost worked out the lifecycle. C A Rudolphi, a Swedish-German botanist, published three books in 1819 that laid out a system of parasite orders and species which was the standard reference for years. Rudolph Virchow, a German physician/pathologist, demonstrated in 1863 how to prevent the trichinosis public health hazard by efficient pig meat inspection before sale. In the 1830s Theodor Schwann, a German physician and biologist, proved that microorganisms do not spontaneously generate, and in 1862 Louis Pasteur ensured that the idea of spontaneous generation was a total fallacy.

Britain appears to have neglected parasite research until in 1854 T C Cobbold, a physician, dissected a giraffe and found a fasciola in the bile ducts that was double the size of Fasciola hepatica in sheep. He named it F. gigantica and began to research the European parasite literature, as well as undertaking his own studies. In 1864 he published a notable book, Entozoa, an Introduction to the Study of Helminthology, and was elected FRS in the same year. In 1872 he was made professor of botany and helminthology at the Royal Veterinary College, London. At a meeting of the Linnean Society in 1878 he presented, on behalf of Patrick Manson, the remarkable discovery of an embryo filaria in the body of a mosquito – out of this arose the mosquito–malaria concept. Then, in 1880, Griffith Evans, a veterinary officer in the Indian Army, identified and described the first pathogenic trypanosome (named Trypanosoma evansi) as the cause of “surra”, a serious tropical disease of horses and camels. His army superiors at first rejected his work, but it was published in 1881. 

An established field of study

At last parasitology had come of age, and a multitude of parasites were being recognised and classified. Increasing research was elucidating life cycles, and effective means of control, treatment and prevention were enabled

At last parasitology had come of age, and a multitude of parasites were being recognised and classified. Increasing research was elucidating life cycles, and effective means of control, treatment and prevention were enabled. There was recognition and control of the serious public health threats from cestodes/hydatids and trichinellae. There was much more to be investigated and controlled – cattle trypanosomiasis, lungworms, liver fluke, coccidia, Babesia, haemonchiasis in sheep, livestock roundworms and many others, but the pioneers had shown the way. Veterinary medicine had a new discipline, and in the process incidentally helped to banish the belief in spontaneous generation!

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