26 The Rice Institute Pamphlet
volunteers with a carrier strain, found that it produced in all of them
symptomless infections persisting in some cases up to 14 months.
Nво-dualism. Since the invasiveness of a strain cannot be altered ex-
perimentally, its characteristics appear to be fixed, indicating the exis-
tence of two types of E. histolytica-. (1) the non-virulent one, correspond-
ing to Brumpts E. dispar, and (2) the virulent one, corresponding to his
E. dysenteriae. In the light of these observations our concept of the host-
parasite relations and etiology of amebiasis takes on a “new look”: though
corresponding to Brumpt’s original views, it leaves the question regarding
the validity of the specific independence of the two races open. This neo-
dualistic hypothesis thus postulates that the human entamebae with 4-nu-
cleate cysts are represented by the following categories: the small non-
pathogenic E. hartmanni and the large E. histolytica, which in its turn
comprises two types or races that are morphologically identical but differ
in virulence, one being a non-pathogenic commensal, the other a patho-
gen capable of invading the host’s tissues with the production of the well-
known clinical manifestations of amebiasis. The invasive large race is re-
stricted to warm and hot countries, where clinical amebiasis is endemic,
whereas E. hartmanni and the avirulent large race of E. histolytica are
apparently cosmopolitan and are the only indigenous parasites in coun-
tries with a temperate climate, though clinical cases of exotic origin are
occasionally introduced there. In this connection it is significant that in a
number of European countries (e.g. Britain, Germany, Holland, France)
there is either direct proof or strong circumstantial evidence that local
cases of amebic dysentery are due to importation of virulent strains of
E. histolytica or have been in contact with extraneous sources of infec-
tion (cf. Hoare, 1950; Boyd, 1957).
It has been estimated (cf. Hoare, 1952) that the average prevalence of
infection with E. histolytica (sensu lato) throughout the world is about
20% of the population, representing a global incidence of more than 400
million cases. Of this number roughly 80% have no signs or symptoms of
disease, about one-third of them harboring the inoffensive small race
(E. hartmanni) and the rest being infected with the large races. Among
the symptomless infections with the large amebae, a high proportion are
cases of non-pathogenic amebiasis harboring the avirulent race, whereas
those parasitized with the virulent race are carriers in whom—under
favorable conditions—clinical manifestations are liable to develop at any
time. If this interpretation of the host-parasite relations in amebiasis is
correct, the only parasite concerned with the etiology of the clinical dis-
ease is the virulent race of E. histolytica, but, at the present stage of our
knowledge, in cases without clinical manifestations it can be differenti-
ated from the avirulent race only by animal experimentation.