Host-Parasite Relations in Amebiasis 29
tropical countries were returning to Europe and North America, medical
authorities were much concerned about the possibility of the introduc-
tion and spread of clinical amebiasis among the local inhabitants: yet
nothing happened. It is possible that convalescent carriers of exotic
strains are present among us, but no relapses are occurring owing to the
absence of predisposing factors. On the other hand, it is known (De-
schiens, 1939, 1941; Chang, 1945; Neal, 1956a) that a virulent strain may
become attenuated after prolonged cultivation, though its virulence can
be restored by animal passages (Neal & Vincent, 1956). Since in a temper-
ate climate the amebae usually live in the lumen of the gut in association
with the conventional flora, it is conceivable that exotic strains of E. his-
tolytica may gradually lose their virulence—as they do in vitro—and this
might account for the scarcity of clinical cases in countries with a colder
climate.
Invasion. There has been much speculation about the mechanism of
tissue invasion by E. histolytica. The histological picture of early lesions
indicates that the parasite elaborates a substance causing histolysis of the
tissues, and various views have been advanced regarding its nature. While
some authors (Rees, 1929; Meleney, 1944; Frye & Shaffer, 1948) thought
that the secretion was a toxin, others found no signs of toxic effects (e.g.
necrosis) in the lesions, nor were they able to reproduce symptoms of in-
toxication in rabbits inoculated with extracts of amebic cultures (West-
phal, 1938). Those who held that penetration of the tissues by the amebae,
and their pathological effect, are due to fermentative action sought to
identify the enzymes responsible. Thus, over 30 years ago Craig (1927)
discovered in E. histolytica a cytolytic substance (“cytolysin”); later Rees
et al. (1953) demonstrated production of a gelatinase, and Deschiens
(1950b) noted its ability to hydrolyse proteins in vitro. Recently Bradin
(1953) discovered the presence of hyaluronidase in this ameba and sug-
gested that it promotes tissue invasion, but DeLamater et al. (1954) failed
to detect this enzyme. Finally, Neal (1956b) demonstrated the presence of
proteolytic enzymes in this ameba.
It is thus seen that the evidence regarding the nature of the enzymes
responsible for the invasion of the tissues by E. histolytica is still frag-
mentary and inconclusive. However most observers are agreed that this
ameba produces some proteolytic enzyme, which Bquefies the intestinal
mucosa at the point of application, thereby enabling it to penetrate into
the tissues, which continue to break down by histolysis as the amebae
multiply and extend through the ulcers. In addition to the fermentative
process, there can be no doubt that the progress of the amebae within the
tissues is aided in a purely mechanical manner by their locomotion (Hoare
& Neal, 1955).