The morphological similarity between the members of the genera Theileria
and Gonderia in domestic animals makes their differentiation extremely difficult.
In making a differential diagnosis veterinarians and zoologists often are compelled
to take the pathogenicity and the epizootology into consideration. In doubtful
cases it may even be necessary to resort to in vivo cross-immunity tests before a
final identification can be made. From this it becomes evident that a great deal
of work will have to be done before the "intermediate stages" of the Theileria
spp. referred to by Du Toit (1930) can be identified. From the observations on
the artificial transmission of East Coast fever (Theiler, 1912; Theiler and Du Toit,
1929) and of Tzaneen disease (Gonderia mutans infection) (De Kock, Van Heerden,
Du Toit and Neitz, 1937) it becomes apparent that the infectious agents cannot be
maintained by serial passages as has been possible in the case of Gonderia annulata
(Sergent, Donatien, Parrot and Lestoquard, 1945). The opinion is expressed that
the undetermined species referred to by Du Toit (1930) may behave in a similar
way. For systematic studies, therefore, it will be necessary to determine the
vectors of these parasites. From this it is obvious that many years will elapse
before the classification of this group of micro-organisms can be placed on a
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