HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



Protozoology is one of the youngest of the sciences but 

 nevertheless has had an interesting and important his- 

 tory. Protozoa were first discovered by Anton von Leeu- 

 wenhoek (1632-1723) in 1675. This famous Dutch 

 microscopist not only saw free-living protozoa, but, as 

 Dobell (1920) has pointed out, was the first to observe 

 intestinal protozoa, having provided an account of a 

 human intestinal flagellate, Giardia lamhlia, that is easily 

 recognizable. In 1681 Leeuwenhoek announced in a letter 

 to the Royal Society of London the discovery in his own 

 stools of "very prettily moving animalcules, some rather 

 larger, others somewhat smaller than a blood corpuscle, 

 and all of one and the same structure. Their bodies were 

 somewhat longer than broad, and their belly, which was 

 flattened, provided with several feet with which they 

 made such a movement through the clear medium and the 

 globules that we might fancy we saw a pissabed running 

 up against a wall. But although they made a rapid move- 

 ment with their feet, yet they made but slow progress" 

 (from Dobell, 1920). It is also clear from Leeuwenhoek's 

 words that he discovered the fact that the vegetative, 

 motile giardias, thus described, appear only in loose stools, 

 and that the number of specimens in the stools varies 

 from time to time and is no indication of the extent of 

 the infection. 



For many years after Leeuwenhoek's discoveries large 

 numbers of excellent biologists were engaged in describ- 

 ing and classifying protozoa and soon hundreds of species 

 were known. Knowledge of the life-cycles, physiology 

 and behavior of the protozoa also accumulated. Most of 



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