REVIEWS OF INFECTIOUS DISEASES • VOL. 8, NO.2. MARCH-APRIL 1986
© 1986 by The University of Chicago. All rights reserved. 0162-0886/86/0802-0004$02.00
THE GLOBAL PROBLEM OF AMEBIASIS: CURRENT STATUS,
RESEARCH NEEDS, AND OPPORTUNITIES FOR PROGRESS
Amebiasis: Introduction, Current Status, and Research Questions
Richard L. Guerrant
From the Division of Geographic Medicine, Department of
Medicine, University of Virginia School of Medicine,
Charlottesville, Virginia
vances in understanding of the cellular biology of
and the host responses to this invasive, tissuedestroying protozoan parasite. Nevertheless, major
gaps remain in the recognition of infection with
potentially virulent E. histolytica, in the understanding of the genetic basis for apparent differences in
virulence among strains, and in the application of
exciting new in vitro findings to adequate animal
models and to the control of the devastating disease
seen in humans with amebiasis.
This paper will provide a historical, taxonomic,
and biologic definition of E. histolytica, summarize
the range of clinical syndromes it causes, and list current research needs and priorities in amebiasis. The
paper by Dr. Julia A. Walsh [3] that follows addresses
the difficult problems involved in diagnosing and
thus recognizing amebic infection and estimates
(from published reports) the magnitude of the global
problem of annual morbidity and mortality due to
amebiasis. While the best available data remain
imprecise and limited because of currently inadequate diagnostic tools, it can reasonably be estimated
that there are nearly 500 million amebic infections
Although several parasitic diseases have attracted
renewed investigative attention in the last five to 10
years, few have become more researchable in this
period than amebiasis. With the substantially improved capacity for in vitro cultivation of axenic Entamoeba histolytica [1] and with the discrimination
among several different virulent and avirulent strains
of E. histolytica on the basis of isoenzyme patterns,
or zymodemes [2], a new era of investigative interest
in amebiasis began. We are now witnessing rapid adThis paper was presented at a workshop on amebiasis sponsored by the Edna McConnell Clark Foundation and organized
by Drs. Joseph Cook, Richard L. Guerrant, and Julia A. Walsh.
We gratefully acknowledge the assistance of the Edna McConnell Clark Foundation and of Dr. Joseph Cook, who stimulated
and supported the preparation of this series. We also appreciate
the help of Ruth Jolly, Susan Davis, and Deborah Payne in the
preparation of the manuscripts in the series and the support supplied to the Division of Geographic Medicine by the Rockefeller
Foundation.
Please address correspondence to Dr. Richard L. Guerrant, Division of Geographic Medicine, Department of Medicine,
University of Virginia School of Medicine, Charlottesville, Virginia 22908.
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This series of five papers on the worldwide problem of amebiasis, its current status, research needs, and opportunities for progress has grown out of a renewalof investigative
interest in amebiasis. The recent development of in vitro culture methods and of means
to distinguish strain differences and the application of modern tools of cellular biology
and biochemistry have raised new questions regarding strain definitions, virulence traits,
host defenses, and the mechanism of invasive disease sometimes caused by Entamoeba
histolytica. The clinical manifestations of amebic infections range from prolonged asymptomatic carriage to extensive, invasive intestinal and extraintestinal disease. The virulence
traits of the parasite, whether stable in each strain or alterable by environmental or genetic
factors, and the host factors involved in the development of disease and in protection
have been investigated. From this series of reviews on the definition of amebiasis, its
manifestations, its global magnitude, the methods for its detection, and the current understanding of its epidemiology, pathogenesis, cellular biology, and host defenses, a list
of keyquestions that can now be addressed in work on amebiasis has been derived. Amebiasis is the third leading parasitic cause of death in the world. The need for work in
this field is great, and the time is ripe for the application of new research tools to a better
understanding of this remarkable tissue-lysingprotozoan parasite and to the control of
the disease it causes.
Amebiasis: Overview and Questions
Definition of Amebiasis
Amebiasis is classically defined as infection with
E. histolytica, with or without overt clinical symptoms. The diagnosis of amebiasis raises two keyquestions about the parasite and host, respectively: (1)
How can we accurately identify the causative agent,
virulent E. histolyticat (2) What host factors determine the range of clinical manifestations of infection with this parasite?
Historical overview. Dysentery, its potentially infectious nature, and its occasional association with
Table 1. Estimated magnitude of amebiasis.
Variable
Global
Developing world
Mortality
75 thousand (range,
74 thousand (range,
39-109)
42.4 million (range,
35-50)
450 million (range,
10010 to ~40010)
Approaches 100010
Morbidity
Prevalence
40-110)
42.6 million (range,
35-50.1)
480 million (1'\J12010
of population)
Incidence
?
in some
populations
"hepatic flux" have been recognized since the time
of Mosaic law and were discussed by Hippocrates.
The parasite was described in fecal specimens in 1869
by Lewis. LOsch first described amebic dysentery in
1875in a patient from St. Petersburg, Russia [7-10].
He further reproduced colitis in dogs given the patient's stool, either orally or rectally. Invasion of tissue by amebas was described in 1887by Robert Koch
[11], and further pathologic descriptions of amebic
infection of bowel and liver werepublished four years
later by Councilman and Lafleur [12]. Cysts of the
amebic parasite were first recognized in 1893 by
Quincke and Roos [13], and in 1903 (as has been
mentioned) the organism was named Entamoeba
histolytica by Schaudinn because of its apparent ability to lyse tissue [14]. In 1912 Rogers described the
efficacy of the root of Cephaelis ipecacuanha (which
was brought to Europe from Brazil by Piso in 1658
and used in France by Helvetius to treat King Louis
XIV) against E. histolytica in vitro and in infected
patients [15]. Walker and Sellards fed E. histolytica
cysts to volunteers in now-classic experiments in 1913
[16], and the organism was first cultured by Boeck
and Drbohlav in 1925 [17]. Recent major advances
include the development of TYI-S-33 medium for
the growth of axenic amebas by Diamond in
1968-1978 [1].
Taxonomic classification. E. histolytica belongs
to the subkingdom and phylum of one-celled
animals, Protozoa; the subphylum Sarcodina (with
motility dependent upon pseudopods); the superclass
Rhizopoda (class Lobosea); and the order Amoebida [18]. Separated from the free-living amebas,
Naegleria and Acanthamoeba (Hartmannella), the
family Entamoebidae encompasses the organisms
Endolimax nana, Iodamoebabuetsch/ii, and Dientamoeba fragilis as well as the genus Entamoeba,
which includes the species histolytica, hartmanni (a
noninvasive, antigenically distinct, "small-race"
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worldwide (excluding China), with 8010-10010 of these
infections causing clinical disease predominantly in
Asia, Africa, and Latin America (table 1). If one accepts the conservatively estimated mortality of
75,000 annually (0.2010 of illnesses), amebiasis becomes the third leading parasitic cause of death on
a global scale, behind only malaria and schistosomiasis. In the third paper in this series, Dr. George R.
Healy [4] reviews the immunologic tools used in the
diagnosis of amebiasis and offers an overview of the
epidemiology of this disease in recent years in the
United States.
The roles of adherence, contact-dependent cytolysis, and proteolytic and toxic products of virulent
amebas in the pathogenesis of intestinal and extraintestinal invasive amebiasis are reviewed by Dr. Jonathan I. Ravdin [5]. Named for its ability to lyse tissue, E. histolytica has long been known to produce
several proteolytic enzymes and to lyse host cells in
vitro and in vivo. Dr. Ravdin reviews several recent
advances in our understanding of the mechanism by
which virulent E. histolytica cells adhere to and lyse
target mammalian cells on contact.
Finally, despite the apparently relentless progression of untreated amebic infection in some individuals and the impressive ability of virulent amebas
to shed or ingest antibody and to destroy phagocytic
neutrophils, evidence from animal models and from
epidemiologic studies of human disease suggests that
protective immunity does develop. Indeed, recent
findings - reviewed by Dr. Robert A. Salata and Dr.
Ravdin - show that with the activated macrophage
even virulent E. histolytica may have finally met its
match. They describe host defenses against amebic
infection and discuss the promising concept that cellular immunity may in fact contribute to the effective control of virulent amebic infections and invasive disease.
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endoplasm, and helical rods of ribosomes, which associate to form chromatoid bodies [31].
Scanning and transmission electron microscopy
studies have revealed that this anaerobic protozoan
is elongated and polar, with an irregular surface and
many phagocytic and acid phosphatase-staining
lysosomal vacuoles [32-35]. E. histolytica has helical ribosomes, poorly understood cylindrical bodies, and micro filaments but no apparent mitochondria, centriole, or (as mentioned above) endoplasmic
reticulum or Golgi apparatus [32, 33]. Carbohydrate,
lipid, and protein constituents have been demonstrated in the plasma membrane, and chitin is found
in the cystwall [36, 37]. Virulence has been correlated
not only with the cytolytic properties referred to earlier, but also with the rate of erythrophagocytosis
[38]; the virulence of E. histolytica increases with
passage in animals [39] and with bacterial association [40].
With the emergence of new concepts of virulence,
new epidemiologic settings, and new strain "markers," it increasingly appears that only certain strains
of E. histolytica are capable of tissue invasion and
lysisof cellson contact. Like the Laredo strain, which
has distinct biologic markers and is considered nonpathogenic, isoenzyme patterns (zymodemes) described by Sargeaunt and Williams [2] suggest that
certain strains of E. histolytica are associated only
with asymptomatic carriage, while others are associated with invasive disease.
The concept that striking differences in virulence
exist among amebic strains is further supported by
the widely disparate clinical manifestations of amebiasis in different epidemiologic settings. For example, in contrast to amebic infections in Mexico or
South Africa, amebiasis among sexually active male
homosexuals is rarely associated with extraintestinal invasion. Indeed, the latter group (up to 40070
of whom may be infected) often have no more enteric
symptoms when they are infected with E. histolytica than when they are not infected [41].
The interaction of amebic strains with human
polymorphonuclear neutrophils (PMNs) and with
tissue culture cells in vitro also demonstrates striking strain differences that correlate with virulence
in animal models. In contrast to the virulent,
cytolethal effect of E. histolytica strain HM1, the less
virulent strain 303 is attacked, dismembered, and ingested by human PMNs in vitro via apparently
nonoxidative mechanisms [42,43]. Although amebas
have been shown to be susceptible to alternative path-
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ameba with cysts <10 JAm in diameter), polecki (a
uninucleate ameba that infects pigs), coli, gingivalis
(a commensal ameba without a recognized cystic
stage), and moshkovski (a Laredo-like ameba that
may be free-living in sewage).Although not officially
granted separate species status, the Laredo-like strain
of Entamoeba that grows in culture at 25°C-30°C
and survives in hypotonic media has a distinct isoenzyme pattern and is believed to be nonpathogenic.
Biologic characteristics of E. histolytica. The
trophozoites of E. histolytica are facultative anaerobic, uninucleate organisms that have a 120-A
double-layered limiting membrane. A uroid area with
vesicles external to the cell membrane [19-21] is surrounded by a fuzzy, external, 20- to 30-nm glycocalyx [22]. The trophozoites require a low pH (6.0-6.5)
and complex medium for growth. E. histolytica has
apparent micro filament-like structures and actin
[23-26] but no evident cytoplasmic microtubules or
mitochondria. The negative surface charge may be
somewhat less on virulent trophozoites, which have
also been shown to have concanavalin A receptors
[27-29]. Trophozoites observed in tissue often measure 20-60 JAm and frequently contain ingested red
blood cells. In contrast, trophozoites measuring 7-30
JAm may be seen in nondysenteric stools of asymptomatic individuals. Apparently, under adverse conditions, trophozoites develop into precystic and cystic stages, which are followed by two nuclear divisions
that produce two to four characteristic nuclei with
central punctate karyosomes and delicate peripheral
chromatin. Infected individuals are the reservoir of
the organism and may shed up to 45 million cysts
per day; the shed cysts may survive outside the host
for several weeks in a moist environment.
After ingestion the quadrinucleate cysts reach the
intestinal tract, where they develop into a metacystic stage and undergo an additional nuclear division;
thus, eight new uninucleate trophozoites emerge to
complete the life cycle[30]. The cysticstage is responsible for fecal-oral transmission via food, water, or
direct person-to-person contact.
The organism has no cytochromes, no classic
Embden-Meyerhof metabolic pathway, no rough endoplasmic reticulum, no cytoplasmic tubulin, and
no Golgi apparatus; it does have a malate dehydrogenase system, alcohol dehydrogenase, and a limited
capacity to consume up to 5% O 2 (provided its ironsulfur proteins are intact and reducing agents, such
as cysteine and ascorbic acid, are available). Amebas
also contain glycogen, digestive vacuoles, ectoplasm,
Guerrant
Amebiasis: Overview and Questions
Host Range and Determinants of Susceptibility
Although E. histolytica is a pathogen that primarily affects humans and although the human host
doubtless represents the major reservoir and source
of spread, a few other animal species have been
reported to be naturally or experimentally infected
with this parasite. Asymptomatic natural infections
have been reported in macaque monkeys and pigs
and symptomatic experimental infections have been
described in dogs and rats [47]. Among nonhuman
primates, only macaque monkeys have been shown
to spontaneously harbor E. histolytica in nature, and
the identity of the amebic strains affecting macaques
and humans has been demonstrated by cross-infection [48].
Both infection with colonic or hepatic involvement
and asymptomatic cyst excretion (syndromes described by LOsch) have been recognized in dogs [49,
50]. Spontaneous infections in dogs have been described in parts of North Africa, India, Indochina,
Indonesia, China, and the United States [47]. Because of the coprophagic habits of dogs, spontaneous canine amebiasis is likely of human origin. Despite the potential for transmission from dogs to
humans, this route is not widely recognized.
Both natural and experimental infections with
E. histolytica- with mild superficial ulceration in the
cecum - have been described in rodents, particularly
rats; it has been postulated that such infections play
a role in transmission of amebiasis to humans [51].
In rare instances in Africa, cattle have also been
reported to have symptomatic infections presumably transmitted by human feces. Experimental infections have been reported in cats, dogs, guinea pigs,
and rabbits; however, rodents - particularly young
rats, guinea pigs, and hamsters - have been the most
useful models [52].
In humans, the influence of a range of conditions,
such as age, nutritional status, sex, geographic origin, and immune status, on host susceptibility is well
recognized and raises important questions about host
factors and defenses against amebiasis. In contrast
to many infections (such as giardiasis, where rates
are highest among children), infection with E. histolytica increases steadily in frequency throughout
life in endemic areas; this pattern would seem to indicate that there is little effective immunity [53].
Likewise, repeated invasive infections are well recognized. However, mortality rates are highest among
young children with invasive amebiasis, and visitors
to endemic areas appear to be more susceptible to
invasive disease than are native residents.
Major geographic differences, however, remain
unexplained and probably reflect geographic differences in the parasite itself. For example, invasivedisease develops in an estimated one of every five infections in Mexico but in only one of every 100-1,000
infections in temperate areas, such as the United
States.
Reactions in invasive amebiasis in humans and
animals include the rapid appearance of circulating
antiamebic antibodies, which are detectable by counterimmunoelectrophoresis, indirect hemagglutination, or enzyme-linked immunosorbent assay [54,
55]. Cellular immune responses are apparent from
skin test reactions and from in vitro macrophage
migration inhibition and lymphocyte mitogenesis after amebic infection or antigen exposure; such responses are also reflected by the protection of
animals against challenge with different amebic antigens [56, 57]. Sepulveda [58] and Martinez-Palomo
et al. [59] have noted that, despite the moderate inflammatory response that characterizes early amebic
lesions, necrosis ensues with remarkably little scarring as amebic lesions heal.
Other host determinants of susceptibility remain
poorly understood. For example, although the overall
rates of infection in males and females are roughly
equal, a striking predominance of males among
adults with hepatic invasive infection is well recog-
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way complement activation, to hydrogen peroxide,
and to cytotoxic lymphocytes, killing of amebic strain
303 by PMNs occurs readily in heat-inactivated serum and in the presence of chronic granulomatous
disease neutrophils, which lack the necessary enzymes for normal oxidative pathways. In contrast,
virulent E. histolytica strain HMI is capable of killing a considerable excess of human PMNs, even at
ratios of up to 3,000:1. This cytolethal effect of
amebas appeared (on the basis of cinemicrographic
evidence) to involve direct contact with PMNs (i.e.,
PMNs that did not come into contact with amebas
remained alive and active); in addition, the lethal effect was independent of serum but dependent on intact microfilament function. Although extra-amebic
lysis of PMNs was documented, PMN fragments
were phagocytosed by the virulent amebic trophozoites [43]. These findings have long-recognized
practical application in the differential diagnosis of
amebic vs. bacillary dysentery [44-46].
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nized. Hepatic invasion is associated with alcoholism, and worsened invasive infections are associated with malnutrition, iron overload, and
corticosteroid treatment both in experimental
animals and in patients. (In addition to the series
of papers in this issue [3-6], other reviews of the immunology and pathogenesis of amebiasis are available [60-62].)
The clinical syndromes of amebiasis that result from
the varied host and parasite conditions range from
asymptomatic infection to a relentless, invasive, disseminated, fatal disease.
Asymptomatic infection (luminal colonization).
The vast majority of human infections with E. histolytica (80% to >99%) are completely asymptomatic. These infections are usually detected by fecal
screening examinations for cyst excretion or by serologic surveys. Cysts may be excreted by 2 % to >40%
of the population, with the exact figure depending
on the area and the level of sanitation and hygiene
[63, 64]. Whether these cysts represent biologically
different strains that may reside over an extended
period as harmless commensals in some settings remains to be proved. Differences may exist between
the isoenzyme patterns of amebas that infect asymptomatically and those of amebas that produce invasive, symptomatic disease [65]. Whether one amebic
form might be transformed into the other under particular environmental, host-related, or transmissible
genetic influences remains unknown, but this question should be answerable with current genetic and
culture methods. Such issues are of great importance
as one attempts to understand and control disease
caused by E. histolytica.
Rates of excretion of E. histolytica cysts in the sexually active male homosexual population often exceed 15%-20% [66, 67]; however, the rates of cyst
excretion by asymptomatic and symptomatic individuals in this population are similar [41]. Many sexually active homosexual men who are infected have
negative serologic test results (with the rate of such
results depending on the setting and the particular
test), while others have positive serologic results in
the absence of overt clinical symptoms. Whether the
latter finding reflects nonspecific exposure to related
antigens or a host response to subclinical E. histolytica infection is unclear.
Symptomaticinfection limitedto thegastrointestinaltract. Whether in the course of prolonged infection (perhaps when host defenses are impaired)
or after ingestion of cysts of virulent E. histolytica,
symptomatic rectocolitis may develop. In the outbreak setting the incubation period is usually one
to four weeks [68] but may be as long as one year
[69]. The onset may be insidious, with lower abdominal pain, or overt, with fulminant fever, bloody diarrhea, dysentery (with or without tenesmus), or
typhloappendicitis. The risk of a more fulminating,
serious disease is associated with youth, pregnancy,
malnutrition, underlying systemic disease, and corticosteroid therapy [70-73]. In the milder form of
disease, the onset may be gradual and diarrhea mild,
with only slight constitutional symptoms. The tenesmus of amebic dysentery is usually less severe than
that of bacillary dysentery (shigellosis). In certain
regions, such as Mexico and Venezuela, 2%-15% of
all cases of acute diarrhea in children requiring hospitalization have been associated with E. histolytica
infection [74-76]. Although systemic leukocytosis is
common, fecal PMNs may be pyknotic or nonexistent in amebic dysentery (in contrast to shigellosis)
[44-46], probably because of the lethal effect ofvirulent E. histolytica on leukocytes [42, 43].
Complications of intestinal amebiasis develop in
10,10-4% of cases [77]. The commonest complication
is bowel perforation and peritonitis. A few individuals may develop an anular colonic inflammatory
mass, or ameboma, that is sometimes tender and
may be indistinguishable radiographically from colonic carcinoma. Rarely, infection will extend from
severe colitis to the perineal or (with genital-rectal
contact) the penile skin. Another uncommon complication of amebic recto colitis is a chronic irritable
bowel (or even ulcerative colitis-like) syndrome,
which has been termed postdysenteric ulcerative colitis [77].
Extraintestinal amebiasis. Extraintestinal amebiasis may develop within days of amebic dysentery,
may follow dysentery by months or even years, or
(in up to 500/0 of cases) may be associated with no
clinical history of intestinal amebiasis [78].
The liver is the commonest organ of extension of
intestinal amebiasis. Presenting symptoms may involve weight loss, weakness, and low-grade fever or
an acute, hectic, febrile illness. Pain may include
vague, right-upper-quadrant discomfort; point
tenderness between ribs on palpation; or pleuritic
discomfort with referral to the right shoulder. Ane-
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Disease Syndromes Resulting from
E. histolytica Infection
Guerrant
Amebiasis: Overview and Questions
Current Needs, Priorities, and Opportunities in
Amebiasis Research
Entamoeba histolytica is capable of a relentless,
progressive invasion of the intestines and of extraintestinal tissues (such as liver, brain, lungs, and skin)
in humans; it is a major parasitic cause of morbidity
and death. Why, however, does a parasite that infects well over 40010 of the population in some areas
cause fulminant disease in only a low proportion of
those infected (1OJo-100J0)? Differences in the virulence of the parasite and in the susceptibility of the
host are important determinants of the range of
manifestations from asymptomatic to fatal infection.
With the development of in vitro cultivation methods
and rapid advances in cellular biology, genetics, enzymology, and immunology, tools are now at hand
to convert our questions, which are outlined below,
into opportunities for progress. Despite the magnitude of the problem and the availability of appropriate technology for the study of this disease, relatively few laboratories throughout the world are
productively working on amebiasis, with a total
funding of less than 1.5 million dollars (principally
from a few governments, universities, and foundations). The ideas that follow reflect the collectiveconsiderations of the authors of this series of papers
[3-6] and of several collaborators in the field.
(1) What constitutes virulent E. histolytical Todefine amebiasis, we must know the answer to this
question. Are subgroups of avirulent and progressively more virulent strains of E. histolytica found
in asymptomatic, intestinal, and extraintestinal infections (as is suggested by different zymodemes
[79])? Or is a single strain of E. histolytica transformed under particular environmental, host-related,
or transmissible genetic influences from an aviru-
lent into an invasive form (as is suggested by bacterial reassociation and animal passage experiments
[40])?Are strains that are commonly carried asymptomatically in many groups (e.g., populations living
in temperate climates and some populations of sexually active male homosexuals) different from strains
that cause tissue invasion? Several questions arise
regarding the extent to which amebic disease is due
primarily to the parasite or its products and that to
which the disease is due to the host's reaction to
amebic infection. Further pathologic, animal, and
field studies should help distinguish the relative roles
of parasite and host in disease. Additional studies
of differences in strains from asymptomatic and
symptomatic individuals and from different geographic regions (as well as of the transformation of
strains in vitro and in vivo from virulent to avirulent and back to virulent) will be of great importance
to our understanding of the pathogenesis and
epidemiology of amebiasis. In addition to the correlation of different markers with virulence in vitro
and in vivo, the application of genetic technology
to amebic nuclear and extranuclear genetic codes is
particularly timely and may shed new light on this
complex area.
(2) How does intestinal colonization occur? Are
there specific adherence receptors that can be
blocked biochemically or altered immunologically?
If so, are these receptors responsible for species
differences in susceptibility to colonization or to invasion? Several recent advances in lectin-carbohydrate receptor biochemistry are now being applied
to the study of E. histolytica adherence characteristics. An appropriate model of human intestinal infection is needed to clarify the effects of ameba-host
cell ligand interactions, mucus, and amebic strain
and host differences on susceptibility to colonization. This area holds promise for the control not only
of disease but also of infection and the spread of
the organism itself.
(3) How do tissue invasion and diarrhea occur?
What steps are involved? Can these steps be altered,
either biochemically or immunologically? In vitro
models have opened up ameba-host cell interactions
to study via several powerful new tools in cellular
biology. The nature of tissue destruction and of specific amebic products and functions can now be dissected, and the appropriate factors can be altered
either pharmacologically or immunologically for
correlation with effects in new models in vivo and
(ultimately) in humans. Other properties of the para-
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mia, leukocytosis, and elevated alkaline phosphatase
concentrations are often noted, but jaundice, striking transaminase elevation, and eosinophilia are unusual. Diagnosis is commonly made by liver scan,
which classically may reveal a large single defect in
the right lobe. Some clinicians suggest that a pyogenic liver abscess can be distinguished from an
amebic abscess by the greater isotope uptake on a
gallium scan in pyogenic infection. Direct or
metastatic extension to pleural or pericardial spaces
or to more distant sites (e.g., brain, lung, or kidney)
may occur, especially if there is rupture into a hepatic vein.
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nus toxoid vaccines. Although it might not prevent
infection, such an amebic vaccine might employ virulence or adherence factors as antigens. If this type
of vaccine were to be efficacious, every susceptible
individual would need to be vaccinated; unvaccinated
individuals would remain susceptible to infection and
disease. A second type of vaccine protects not only
the individual vaccinated but also - as a result of
transmission of the vaccine strain - many other people; live polio vaccine is an example. Could an avirulent but infectious amebic strain provide protection
against infection or disease caused by virulent
E. histolytical Could such a vaccine strain provide
some degree of herd immunity if enough individuals were protected? The third type of vaccine prevents transmission without preventing disease. If
means were available to prevent encystation or excystation, transmission of E. histolytica could be
greatly reduced.
With regard to the spread of amebiasis, epidemiologic studies are dependent upon improved methods
of case and disease detection. The major vehicles for
transmission of E. histolytica in field situations have
not been wellidentified. While improvements in overall sanitation and socioeconomic conditions are appropriate long-range goals, such changes are difficult, costly, and not immediately foreseeable for
many people. In the meantime, specific, selected interventions, such as hand washing, fly control, improved water quality and quantity (e.g., via household vs. village standpipes), and improved sanitary
facilities (e.g., in-house latrines) must be examined,
with at least one to three years of follow-up. Evaluation of the impact of these measures on not only
amebic but other enteric and even respiratory infections will greatly enhance our understanding of the
transmission of the etiologic agents and will expedite interim, short-range control measures.
(6) Finally, with regard to therapy for asymptomatic and symptomatic infections, in vitro tools should
be applied to establish the frequency and nature of
drug resistance as well as the mechanisms of antiamebic drug action. Improved therapy for recurrent
intestinal amebiasis is particularly needed.
Conclusion
Major constraints currently impede progress in amebiasis. Although much can now be done with available tools, funding for laboratory and field work is
severely limited. Other constraints are the geographic
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site that may be related to its pathogenicity need
to be delineated; possibilities include the abilities to
produce enterotoxin, to associate with bacteria,
and to phagocytose target cells. The role of possible
enterotoxic products - suggested by the results of
rabbit ileal loop studies and by a serotonin-like secretory effect - must now be confirmed and extended
[80, 81].
(4) What are the host determinants of susceptibility to disease? Does effective immunity develop?
If so, what type? Are there ways to augment or impair host defenses against tissue invasion by E. histolytica? Why do males predominate among adult
patients with hepatic abscesses due to E. histolytica infection while other types of infection in adults
and hepatic infection in children are fairly equally
distributed between the sexes? New in vitro and animal models can be used for the dissection of important variables in the pathogenesis of tissue invasion
by E. histolytica that have been noted clinically, such
as increased susceptibility during pregnancy, malnutrition, or steroid therapy. The roles of specific
amebic antigens in the pathogenesis of amebiasis and
in protection against the disease must now be defined; in addition, the role of the separate components of cellular immunity in protection against amebiasis must be elucidated.
(5) How is E. histolytica spread? What are the appropriate short- and long-term measures for its control? Among the greatest needs for adequate
epidemiologic studies are greatly improved diagnostic methods that are sensitive, specific, and simple
enough for widespread field application [82]. The
microscopic diagnosis of E. histolytica infection is
difficult at best and requires a highly skilled observer
for the avoidance of false-negative and false-positive
interpretations. A serologic test for IgM antibody
would be a particularly useful method for detecting
evidence of acute infection and for monitoring therapeutic results. With regard to the transmission of
E. histolytica, further development of defined media should clarify requirements for encystation and
excystation and should provide much-needed diagnostic tools for different parasitic stages. Longitudinal studies are essential to evaluations of cyst excretion patterns, acquisition of immunity, protection
by breast milk, and epidemiologic patterns of
disease.
Vaccine development might proceed in three ways.
One type of vaccine prevents disease in the individual; examples are found in diphtheria and teta-
Guerrant
Amebiasis: Overview and Questions
References
1. Diamond LS, Harlow DR, Cunnick CC. A new medium for
the axenic cultivation of Entamoebahistolytica and other
Entamoeba. Trans R Soc Trop Med Hyg 1978;72:431-2
2. Sargeaunt PG, Williams JE. Electrophoretic isoenzyme patterns of the pathogenic and nonpathogenic intestinal amoebae of man. Trans R Soc Trop Med Hyg 1979;73:225-7
3. Walsh JA. Problems in recognition and diagnosis of amebiasis: estimation of the global magnitude of morbidity and
mortality. Rev Infect Dis 1986;8:228-38
4. Healy GR. Immunologic tools in the diagnosis of amebiasis: epidemiology in the United States. Rev Infect Dis
1986;8:239-46
5. Ravdin JI. Pathogenesis of disease caused by Entamoeba
histolytica: studies of adherences, secreted toxins, and
contact-dependent cytolysis. Rev Infect Dis 1986;8:247-60
6. Salata RA, Ravdin JI. Review of the human immune mechanism directed against Entamoeba histolytica. Rev Infect
Dis 1986;8:261-72
7. Kean BH, Mott KE, Russell AJ, eds. Tropical medicine and
parasitology: classic investigations. Ithaca, NY: Cornell
University Press, 1978:71-168
8. LOsch F. Massenhafte Erwickelung von Amoeben im Dickdarm. Virchows Archiv Pathol Anat Physiol Klin Med
1875;65:196-211
9. Stilwell GG. Amebiasis: its early history. Gastroenterology
1955;28:606-22
10. Bloomfield AL. A bibliography of internal medicine: amebic
dysentery. J Chronic Dis 1957;5:235-52
11. Koch R. Berricht tiber die Thatigkeit der zur Erforschung der
Cholera im Jahre 1883nach Egypten und Indien entstandten Kommission. Arbeiten aus dem Kaiserlichen Gesundheitsamte 1887;3(Anlage VI):63-78
12. Councilman WT, Lafleur HA. Report in pathology. I. Amoebic dysentery. Johns Hopkins Hospital Reports 1891;2:
395-548
13. Quincke H, Roos E. I. Ueber Amoben-Enteritis. Berliner
Klinische Wochenschrift 1893;30:1089-94
14. Schaudinn F. Untersuchungen tiber die Fortpflanzung einiger
Rhizopoden (Vorlaufige Mittheilung). Arbeiten aus dem
Kaiserlichen Gesundheitsamte 1903;19:547-76
15. Rogers L. The rapid cure of amoebic dysentery and hepatitis
by hypodermic injections of soluble salts of emetine. Br
Med J 1912;1:1424-5
16. Walker EL, Sellards AW. Experimental entamoebic dysentery. Philippine Journal of Science. B. Tropical Medicine
1913;8:253-331
17. Boeck WC, Drbohlav J. The cultivation of Endamoeba
histolytica. Am J Hyg 1925;5:371-407
18. Levine NO, Corliss JO, Cox FEG, Deroux G, Grain J, Honigberg BM, Leedale GF, Loeblich AR III, Lorn J, Lynn 0,
Merinfeld EG, Page FC, Poljansky G, Sprague V, Vavra
J, Wallace FG. A newly revised classification of the protozoa. J Protozool 1980;27:37-58
19. Fletcher KA, Maegraith BG, Jarumilinta R. Electron microscope studies of trophozoites of Entamoeba histolytica.
Ann Trop Med Parasitol 1962;56:496-9
20. El-Hashimi W, Pittman F. Ultrastructure of Entamoeba
histolyticatrophozoites obtained from the colon and from
in vitro cultures. Am J Trop Med Hyg 1970;19:215-26
21. Pittman FE, EI-Hashimi WK, Pittman Jc. Studies in human
amebiasis. II. Light and electron-microscopic observations
of colonic mucosa and exudate in acute amebic colitis. Gastroenterology 1973;65:588-603
22. Lushbaugh WB, Miller JH. Fine structural topochemistry
of Entamoeba histolytica Schaudinn, 1903. J Paras itoI
1974;60:421-33
23. Rosenbaum RM, Wittner M. Ultrastructure of bacterized and
axenic trophozoites of Entamoebahistolytica with particular reference to helical bodies. J Cell Bioi 1970;45:367-82
24. Martinez-Palomo A, Gonzalez-Robles A, Chavez de Ramirez B. Ultrastructural studies of various Entamoeba strains.
In: Sepulveda B, Diamond LS, eds. Memorias de la Conferencia Internacional sobre Amibiasis. Mexico City: Instituto Mexicano del Seguro Social, 1976:226-37
25. Michel R, Schupp E. Cytoplasmic fibrils and their relation
to amoeboid movement of Entamoeba histolytica. In:
Sepulveda B, Diamond LS, eds. Memorias de la Conferencia Internacional sobre Amibiasis. Mexico City: Instituto
Mexicano del Seguro Social, 1976:300-10
26. Kettis AA, Lidman K, Fagraeus A. Actin in Entamoeba
Downloaded from http://cid.oxfordjournals.org/ at Penn State University (Paterno Lib) on September 18, 2016
isolation of investigators and the limited data available for many parts of the world. Adequate information remains difficult for many workers to obtain. The lack of adequate models of human
intestinal infection and disease remains a substantial problem, as does the lack of availability of
amebic strains and materials for in vitro study. Finally, there is a great need for field-study areas in
which controlled and standardized longitudinal
investigations - with follow-up - can be conducted.
In sum, the needs are great and the time is ripe
for major advances in our understanding and control of amebiasis. Although the epidemiologic data
are limited, it is reasonable to estimate that amebiasis is the third leading parasitic cause of death and
a major cause of morbidity on a global scale. Largely
because a method for in vitro cultivation of the organism has become widely available during the last
five years, new cellular biologic, immunologic, and
genetic tools can now be applied with considerable
promise in investigations of the nature of virulent
E. histolytica. Improved understanding not only will
open new avenues for the control of amebiasis but
probably will also reveal novel basic concepts relevant to cytolytic mechanisms and host defenses. Still,
the amount of funding and the number of workers
in this field remain grossly inadequate for the tasks
at hand. Perhaps this brief review, along with the
others in this series [3-6], will further enhance the
level of interest in this remarkable tissue-lysing protozoan parasite and in the fascinating and important questions it poses.
225
226
Guerrant
histolytica trophozoites revealedby human actin antibodies.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
referenceto the cytology of bacillary dysentery and its bearing on early and accurate diagnosis. Lancet 1921;2:998-1002
Callendar GR. The differential pathology of dysentery. Am
1 Trop Med 1934;14:207-20
Bercovitz Z. Studies in the cellular exudates of bowel discharges. III. The diagnostic significance of cellular exudate studies in chronic bowel disorders. Ann Intern Med
1941;14:1323-40
Hoare CA. Reservoir hosts and natural foci of human protozoal infection. Acta Trop 1962;19:281-317
Dobell C. Researches on the intestinal protozoa of monkeys
and man. IV. An experimental study of the histolyticalike species of Entamoeba living naturally in macaques.
Parasitology 1931;23:1-72
Ganapathy MS, Alwar VS. Amebic dysentery in dogs (case
report). Indian Veterinary Journal 1957;34:427
Brumpt E. Precis de parasitologie. 6th ed. Vol 1. Paris: Masson et Cie, 1949
Neal RA. The duration and epidemiological significance of
Entamoeba histolytica infections in rats. Trans R Soc Trop
Med Hyg 1951;45:363-70
Mattern CF, Diamond LS, Keister DB. Amebal viruses and
the virulence of Entamoeba histolytica. Arch Invest Med
(Mex) 1978;9(Suppl 1):165-6
Oyerinde lPO, Ogunbi 0, Alonge AA. Age and sex distribution of infections with Entamoeba histolytica and
Giardia intestinalis in the Lagos population. Int 1
Epidemiol 1977;6:231-4
Sepulveda B. Immunology of amebiasis. In: Larralde C,
Willms K, Ortiz-Ortiz L, Sela M, eds. Molecules, cells, and
parasites in immunology. New York: Academic Press,
1980:163-77
Bos Hl, Van Den Eijk AA. Enzyme-linked immunosorbent
assay (ELISA) in the serodiagnosis of amebiasis. In:
Sepulveda B, Diamond LS, eds. Memorias de la Conferencia Internacional sobre Amibiasis. Mexico City: Instituto
Mexicano del Seguro Social, 1976:721-7
Ortiz-Ortiz L, Zamacona G, Sepulveda B, Capin NR. Cellmediated immunity in patients with amebic abscess of the
liver. Clin Immunol Immunopathol 1975;4:127-34
Landa L, Capin R, Guerrero M. Estudios sobre immunidad
celular en la amibiasis invasora. In: Sepulveda B, Diamond
LS, eds. Memorias de la Conferencia Internacional sobre
Amibiasis. Mexico City: Instituto Mexicano del Seguro Social, 1976:654-60
Sepulveda B. Progress in amebiasis. Scand 1 Gastroenterol
[Suppl] 1982;77:153-64
Martinez-Palomo A, Tanimoto-Weki M, Tena-Bentancourt
E. Evolucion de las lesiones producidas en hamsters por
inoculation de entamoeba histolytica. Arch Invest Med
(Mex) 1980;11(Suppl 1):169-72
Trissl D. Immunology of Entamoeba histolytica in human
and animal hosts. Rev Infect Dis 1982;4:1154-84
Ravdin 11, Guerrant RL. A review of the parasite cellular
mechanisms involved in the pathogenesis of amebiasis. Rev
Infect Dis 1982;4:1185-207
Sepulveda B. Amebiasis: host-pathogen biology. Rev Infect
Dis 1982;4:1247-53
Elsdon-Dew R. The epidemiology of amoebiasis. Adv
Parasitol 1968;6:1-62
Downloaded from http://cid.oxfordjournals.org/ at Penn State University (Paterno Lib) on September 18, 2016
J Parasitol 1977;63:581-3
27. Martinez-Palomo A, Gonzalez-RoblesA, de la Torre M. Selective agglutination of pathogenic strains of Entamoeba
histolytica induced Con A. Nature [New Bioi] 1973;
245:186-7
28. Bos HJ, van de Griend RJ. Virulence and toxicity of axenic
Entamoeba histolytica. Nature 1977;265:341-3
29. Trissl D, Martinez-Palomo A, Arguello C, de la Torre M, de
la Hoz R. Surface properties related to concanavalin A-induced agglutination: a comparative study of several Entamoeba strains. J Exp Med 1977;145:652-65
30. Barker DC, Swales LS. Characteristics of ribosomes during
differentiation from trophozoite to cyst in axenic Entamoeba sp. Cell Differ 1972;1:297-306
31. Report of a WHO Scientific Group. Intestinal protozoan and
helminthic infections. WHO Tech Rep Ser 1981;666
32. Martinez-Palomo A, Gonzalez-Robles A, de la Torre M, de
la Hoz R. Fijacion e inclusion in situ de e. histolytica:
aplicaciones en estudios de morfologia y citoquimica
ultramicroscopica. Arch Invest Med (Mex) 1974;5(Suppl
2):283-92
33. Kettis AA, Lidman K, Fagraeus A. Actin in Entamoeba
histolytica trophozoites revealedby human actin antibodies.
1 Parasitol 1977;63:581-3
34. Weinbach EC, Diamond LS, Clagget CE, Kon H. Iron-sulfur
proteins of Entamoeba histolytica. 1 Parasitol 1976;
62:127-8
35. Martinez-Palomo A. The biology of Entamoeba histolytica.
(Tropical medicine research studies.) Chichester, England:
Research Studies Press, John Wiley and Sons, 1982
36. Aley SB, Scott WA, Cohn ZA. Plasma membrane of Entamoeba histolytica. 1 Exp Med 1980;152:391-404
37. Arroyo-Begovich A, Carabez-Trejo A, de la Torre M. Tincion de quistes de entamoeba invadens, entamoeba histolytica y entamoeba coli con aglutinina de german de trigo
marcada con particulas de oro coloidal. Arch Invest Med
(Mex) 1980;11(Suppl 1):25-30
38. Trissl D, Martmez-Palomo A, de la Torre M, de la Hoz R,
Perez de Suarez E. Surface properties of Entamoeba: increased rates of human erythrocyte phagocytosis in pathogenic strains. 1 Exp Med 1978;148:1137-45
39. Lushbaugh WB, Kairalla AB, Loadholt CB, Pittman FE. Effect of hamster liver passage on the virulence of axenically cultivated Entamoeba histolytica. Am 1 Trop Med
Hyg 1978;27:248-54
40. Wittner M, Rosenbaum RM. Role of bacteria in modifying
virulence of Entamoeba histolytica:studies of amebae from
axenic cultures. Am 1 Trop Med Hyg 1970;19:755-61
41. Keystone lS, Keystone DL, Proctor EM. Intestinal parasitic
infections in homosexual men: prevalence, symptoms and
factors in transmission. Can Med Assoc 1 1980;123:512-4
42. 1arumilinta R, Kradolfer F. The toxic effect of Entamoeba
histolytica on leukocytes. Ann Trop Med Parasitol
1964;58:375-81
43. Guerrant RL, Brush 1, Ravdin 11, Sullivan lA, Mandell GL.
Interaction between Entamoeba histolytica and human
polymorphonuclear neutrophils. 1 Infect Dis 1981;
143:83-93
44. Anderson J. A study of dysentery in the field. With special
Amebiasis: Overview and Questions
74. Gutierrez-Trujillo G. Acute infectious gastroenteritis. Etiology and its correlation with clinical manifestations and
fecal mucus. Arch Invest Med (Mex) 1979;10:135-45
75. Donta ST, Wallace RB, Whipp SC, Olarte J. Enterotoxigenic
Escherichia coli and diarrheal disease in Mexican children.
J Infect Dis 1977;135:482-5
76. Romer H, Perez de Suarez E, de Lares A, de Davila D, Gomez MA, de Galindo M, de Fermin C, Yamin Y, de Torres
B, Esparza J, Alvarez N, Torres P. La amebiasis intestinal
en el nifio, I. Estudio etiologico de las lesiones de colon
rectosigmoideo. Arch Invest Med (Mex) 1978;9(Suppl
1):375-80
77. Adams EB, Macleod IN. Invasiveamebiasis. I. Amebic dysentery and its complications. Medicine 1977;56:315-23
78. Adams EB, Macleod IN. Invasiveamebiasis. II. Amebic liver
abscess and its complications. Medicine 1977;56:325-34
79. Farri TA, Sargeaunt PG, Warhurst DC, Williams JE, Bhojnani R. Electrophoretic studies of the hexokinase of Entamoebahistolyticagroups I to IV. Trans R Soc Trop Med
Hyg 1980;74:672-3
80. Lushbaugh WB, KairallaAB, Cantey JR, Hofbauer AF, Pittman FE. Isolation of a cytotoxin-enterotoxin from Entamoeba histolytica. J Infect Dis 1979;139:9-17
81. McGowan K, Kane A, Asarkof N, Wicks J, Guerina V, Kellum J, Baron S, Gintzler AR, Donowitz M. Entamoeba
histolytica causes intestinal secretion: role of serotonin.
Science 1983;221:762-4
82. WHO Scientific Working Group. Parasite-related diarrhoeas.
Bull WHO 1980;58:819-30
Downloaded from http://cid.oxfordjournals.org/ at Penn State University (Paterno Lib) on September 18, 2016
64. Meerovitch E, Healy GR, Ambroise-Thomas P. Amoebiasis
survey in Calcutta (India), Bangkok (Thailand), Medellin
(Colombia), and San Jose (Costa Rica). Can J Public
Health 1978;69:286-8
65. Sargeaunt PG, Williams JE, Bhojnani R, Campos JE, Gomez A. The epidemiology of Entamoeba histolytica in rural
and an urban area of Mexico. A pilot survey II. Trans R
Soc Trop Med Hyg 1982;76:208-10
66. Schmerin MJ, Gelston A, Jones Te. Amebiasis: an increasing problem among homosexuals in New YorkCity. JAMA
1977;238:1386-7
67. Medical Staff Conference, University of California, San Francisco. Venereal aspects of gastroenterology. West J Med
1979;130:236-46
68. Bundesen HN, Connolly n, Rawlings ID, Gorman AE,
McCoy GW, Hardy AV. Epidemic amebic dysentery: the
Chicago outbreak of 1933. National Institutes of Health
Bulletin, no. 166, 1936
69. Craig CF. The occurrence of endamebic dysentery in the
troops serving in the EI Paso district from July, 1916, to
December, 1916. Military Surgeon 1917;40:286-302, 423-34
70. Lewis EA, Antia AU. Amoebic colitis: review of 295 cases.
Trans R Soc Trop Med Hyg 1969;63:633-8
71. Abioye AA. Fatal amoebic colitis in pregnancy and puerperium: a new clinico-pathological entity. J Trop Med Hyg
1973;76:97-100
72. Wagner VP, Smale LE, Lischke JH. Amebic abscess of the
liver and spleen in pregnancy and the puerperium. Obstet
Gynecol 1975;45:562-5
73. Kanani SR, Knight R. Relapsing amoebic colitis of 12 years'
standing exacerbated by corticosteroids. Br Med J
1969;2:613-4
227