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Coccidia parasitic in man 
like in their proportions. In comparison with them the oocysts of the human 
parasite are small, colourless, elongate, and generally more fragile in appear- * 
ance. It would be almost impossible, I think, for anybody who had carefully 
compared the forms from the two different hosts to believe that they belong 
to one and the same species 1 . I have no hesitation, therefore, in concluding 
that the parasite of man is not Isos for a bigemina, but belongs to a separate 
and well-marked species. 
The Isospora of man can therefore be referred without much difficult}" to 
its correct systematic position; and such problems as are raised by its nomen¬ 
clature, and by the earlier records of human coccidiosis, are capable of fairly 
simple solution. But can the same be said about the species of Eimeria recorded 
from man? Are the new and old observations equally susceptible of recon¬ 
ciliation? The answer is, as I will now try to show, in the affirmative, in spite 
of the confusion which appears on the surface. 
As we have already seen, the older workers appear to have discovered a 
species of Eimeria which inhabits the human liver. So far as can be ascertained 
from the fragmentary records, this organism resembles Eimeria stiedae, the 
species found in the liver of the rabbit. It is highly probable, however, that 
it is not this species, notwithstanding the fact that it has, by general consent, 
invariably been referred to it. No evidence for such a view has ever been 
brought forward, either experimental or morphological. There is not a particle 
of proof that the spores of the hepatic coccidia of the rabbit are capable of 
infecting man, or that those of the parasite of man can infect the rabbit. The 
allegations 2 which one constantly meets with in this connexion since the time 
of Leuckart are wholly without foundation, and there can be little doubt that 
they are merely text-book traditions of the type familiar to everybody 3 . 
Leuckart’s original reasons for regarding the two forms as identical are too 
puerile to require further discussion. Moreover, his own work furnishes us 
with the only extant figures of the human parasite; and these—assuming that 
they are correct—supply the chief evidence, from a morphological standpoint, 
to prove that the hepatic coccidia of man and rabbit belong to distinct species 
1 Cf. Wenyon and O’Connor (1917): “The isospora of cats is very common in Alexandria, but 
the oocysts are quite unlike those of the isospora of man.” I may add that Isospora is equally 
common in cats in London, where even quite young kittens are almost always infected. I have 
examined more than 100, and have rarely failed to find oocysts in their faeces—often in enormous 
numbers. 
2 E.g. “Green food-stuffs, contaminated with infected rabbit excrement, are infective to man ” 
(Fantham, 1917). 
3 There is a similar tradition, equally devoid of foundation, concerning the “red diarrhoea, ’ 
or dysentery, of cattle. Many authors assert that this is caused by Eimeria perforans (or E. stiedae) 
—the coccidium of the rabbit’s small intestine (or liver). Even Doflein (1911) in the third edition 
of his text-book states this as a fact. Nevertheless, there is not a single fact to support such a 
view, and abundant evidence to refute it. The parasite of cattle, which lives in the large intestine, 
is morphologically quite unlike E. perforans (having small rounded oocysts, for example) and 
appears incapable of infecting rabbits. There can be no doubt that it is a distinct species (E. 
zurnii Rivolta, 1878) having no connexion whatever with the rabbit coccidia. Cf. Zublin (1908) 
and other recent works on this subject. 
