Because of their small size, it was not possible to recognize any protozoa until the invention of the microscope and its use by Antonie van Leeuwenhoek toward the end of the 17th century. The study of parasitic protozoa only really began two centuries later, following the discovery of bacteria and the promulgation of the germ theory by Pasteur and hiscolleagues at the end of the 19th century.
Amoebae and Amoebiasis
Humans harbor nine species of intestinal amoebae, of which only one, Entamoeba histolytica, is a pathogen. The life cycle is simple. The amoebae live and multiply in the gut and form cysts that are passed out in the feces and infect new individuals when they are consumed in contaminated water or food. Most infections are asymptomatic, butsome strains of E. histolytica can invade the gut wall, causing severe ulceration and amoebic dysentery characterized by bloody stools. If the parasites gain access to damaged blood vessels, they may be carried to extraintestinal sites anywhere in the body, the most important of which is the liver, where the amoebae cause hepatic amoebiasis. Supposed evidence that both the intestinal and liverforms of the disease were recognized from the earliest times is circumstantial because there are so many causes of both the bloody dysentery characteristic of amoebiasis and the symptoms of hepatic amoebiasis that many of these records are open to other interpretations (24). With these reservations in mind, the earliest record is possibly that from the Sanskrit document Brigu-samhita, written about1000 BC, which refers to bloody, mucose diarrhea (260). Assyrian and Babylonian texts from the Library of King Ashurbanipal refer to blood in the feces, suggesting the presence of amoebiasis in the Tigris- Euphrates basin before the sixth century BC (24, 148), and it is possible that the hepatic and perianal abscesses described in both Epidemics and Aphorisms in the Corpus Hippocratorum refer toamoebiasis (131). Since epidemics of dysentery by itself are likely to result from bacterial infections and dysentery associated with disease of the liver is likely to be amoebic, laterrecords are easier to interpret. In the second century AD, Galen and Celsus both described liver abscesses that were probably amoebic, and the works of Aretaeus, Archigenes, Aurelanus, and Avicenna toward the end ofthe first millennium give good accounts of both dysentery and hepatic involvement (238). As amoebiasis became widespread in the developed world, there were numerous records of “bloody flux” in Europe, Asia, Persia, and Greece in the Middle Ages (137). The disease appears to have been introduced into the New World by Europeans sometime in the 16th century (51), and with the later development ofEuropean colonies and increased world trade, there are numerous clear descriptions of both the intestinal and hepatic forms of amoebiasis. In the 19th century, several books mainly concerned with diseases in India, including
Researches into the Causes, Nature and Treatment of the More Prevalent Diseases of India and of Warm Climates Generally
by James Annersley, clearly refer to both intestinaland hepatic amoebiasis (6), and it is now generally agreed that this book contains the first accurate descriptions of both forms of the disease. The connection between amoebic dysentery and liver abscesses was described by William Budd, the English physician who discovered the method of transmission of typhoid (30). The amoeba itself, E. histolytica, was discovered by Friedrich Lo¨sch (also knownas Fedor Lesh) in 1873 in Russia (163), and Lo¨sch also established the relationship between the parasite and the disease in dogs experimentally infected with amoebae from humans. Stephanos Kartulis, a Greek physician, also found amoebae in intestinal ulcers in patients suffering from dysentery in Egypt in 1885 and 1896 and noted that he never found amoebae from nondysenteric cases (132)....