24 The Rice Institute Pamphlet
small race is sufficiently characterized both morphologically and biologi-
cally to be regarded as a distinct species or subspecies, under the name
E. hartmanni or E. histolytica hartmanni, respectively (Burrows, 1957;
Hoare, 1949, 1952,1957). '
The exclusion of E. hartmanni leaves only the large race (E. histolytica
proper) for consideration as a pathogen. In the past various hypotheses
have been proposed to explain variations in the course of amebic infec-
tions. Thus, some authors maintain that, although in many cases the large
race of E. histolytica lives in the lumen of the gut as a commensal (“mi-
nuta” form), feeding there on bacteria and Saprozoically, it is potentially
always pathogenic but its virulence (or invasiveness) remains dormant
until activated by extraneous factors (such as harmful bacteria, functional
disturbances, host’s diet, which will be considered below) which affect
the host’s health, thereby lowering his resistance and impairing the struc-
tural integrity of the intestinal wall.
However, in discussing the possible factors affecting the pathogenicity
of E. histolytica the characteristics of different strains have usually not
been taken into consideration, though it is a well-known fact that the
course of amebiasis varies considerably in different parts of the world.
Thus, although the incidence of infection in countries with a temperate
climate is comparable to that in hot countries, manifestations of disease
(including dysentery) are common in the tropics but rare in temperate
regions, where the infections are as a rule symptomless. Until quite re-
cently it was generally assumed that E. histolytica is represented by one
race of pathogenic amebae and that variations in the course of amebiasis
observed under different climatic conditions were determined by various
injurious factors mentioned above, which in the tropics tend to lower the
host’s resistance to invasion by the amebae. It is true that some observers
suggested that the types of amebiasis prevalent in regions with different
climates might be due to the existence of virulent strains in the tropics
and of avirulent ones in temperate regions. However, it was thought that
the invasive power was not an innate peculiarity of the amebae but de-
pended on the conditions previously experienced by them in the host.
Thus it was suggested that the virulence of a strain might be enhanced
as the result of adaptation to life in the tissues (Reichenow, 1931), a view
supported by experiments in which the virulence of E. histolytica was
apparently increased by repeated passages through animals (Faust &
Swartzwelder, 1935; Meleney & Frye, 1937). And, conversely, it was be-
lieved that prolonged sojourn in the lumen of the gut might reduce the
invasiveness of this ameba (Westphal, 1950).
Dualism. This unicistic concept was opposed by Brumpt (1925, 1928,
1949) who propounded a dualistic hypothesis of the etiology of amebiasis.