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Department f Epidemiology, niversityof Michigan,Schoolof Public Health, AnnArbor, Michigan 48109
INTRODUCTION
Protozoal and helminthic parasitic diseases remain a significant cause ofmorbidity and mortality in less developedcountries (130). Althoughmany
these agents are worldwiden distribution, their greatest impact on the health
of people and domestic animals is found in developing countries.
Mostof these infectious agents have been important human athogens sinceantiquity, but few have been controlled as have many f the bacterial and viral
agents. Advances n the development f efficacious chemotherapeuticagents,vector control through insecticides and sanitation, provision of clean water,
and elimination of vector habitat have all contributed significantly to the
progress that has been madewith control of parasitic diseases in many arts of
the world. Nevertheless, technical problems such as drug resistance, drug
toxicity, insecticide resistance, financial constraints, and operational difficul-ties have contributed to a reduction in the rate of progress toward effectivecontrol of parasitic diseases.
Vaccinedevelopmentor parasitic diseases has been slow to progress untilrecently. The spectacular technical advances n protein separation, hybridomatechnology, monoclonal antibody production, and recombinant DNAechnol-
ogy are nowbeing applied to parasites for identification and production ofmolecularly defined vaccine candidates. Much f the slow progress can be
attributed to the difficulties of both in vitro and in vivo cultivation of protozoa
and helminths, difficulties that are nowbeing rapidly overcome.Availability of vaccines for the most mportant parasitic diseases will in the
future be of benefit not only by protecting people against those diseases but
also by reducing the detrimental effects of continued, intensive chemical
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applications in the environment o control vectors. Theprevention of infection
and disease by vaccines and other public health measures s generally more
satisfying than the continued, repetitive, and intensive use of chemotherapeu-tic agents in the infected population.
Although several candidate vaccines have been developed, none has yet
met the stringent tests of evaluation in humans.The vaccine for malaria is
nowundergoing human rials. A few vaccines for protozoal and helminthicparasites of domestic animals have been in use for some ime (58, 78, 99).
This review discusses the present status of vaccine development or malar-
ia, schistosomiasis, filariasis, leishmaniasis, and trypanosomiasis.These five
diseases are the primary targets of the WorldHealth Organization’s Special
Programmeor Research and Training in Tropical Diseases. Also consideredare the potential and need for vaccines for other parasitic diseases.
MALARIA
Malaria today remains the most important parasitic disease in the world,causing approximately 1.5 million deaths per year. Although a number of
chemotherapeutic gents and insecticides have been effective in the control of
transmission, many egions of the world remain foci of high endemicity. The
difficulties in control have been attributed to the rapidly emergingPlasmo-
dium falciparum resistance to chloroquine and the widespread resistance to
insecticides by the anopheline mosquito vectors. Combinedwith financial
constraints and operational and administrative failings, malaria is increasing
in Latin America, Africa, the Indian subcontinent, and Southeast Asia.Although four species of Plasmodium re responsible for humanmalaria, P.
falciparum is the most important, as it is responsible for almost all of the
mortality in malaria, is prevalent worldwide,and is the only species that has
developed drug resistance. P. vivax is also widely prevalent and causes
extensive morbidity. The remaining species, P. malariaeand P. ovale, are oflesser importance, with much ess prevalence and variable morbidity.
Vaccine Development
SPOROZOITEACCINEhe existence of immunity to reinfection has longbeen surmised from analysis of the epidemiological data of endemic egions.
Moreover, the experimental demonstration of vaccination against a murine
malaria species, P. berghei, by the use of either inoculated irradiated sporozo-
ites or irradiated, infected (sporozoite-bearing) anopheline mosquitoeses-
tablished that protective immunity could be readily engendered (90-93).Continuedworkon this irradiated sporozoite as a vaccine led to initial human
volunteer studies with P. falciparumand P. vivax (16, 17, 104, 105). At least6-8 exposures to irradiated mosquitoes over a 3-10 monthspan were required
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for effective protection. The precise quantitation of the exposure n terms of
the numberof irradiated sporozoites entering the host could not be de-
termined. These imited studies clearly demonstrated hat vaccination againstP. falciparumand P. vivax could be achieved with irradiated sporozoites. The
studies also pointed out the difficulty that wouldoccur if this vaccine was to
be utilized: obtaining a supply of viable sporozoites to prepare the live,
radiation-attenuated vaccine.The advent of monoclonal antibody and recombinant DNA echnology
provided he tools for specific identification of the sporozoite protein respon-
sible for the engendered mmunity nd for cloning in bacteria the Plasmodium
gene coding for the protein. These ools provide a virtually unlimited sourceof antibody reagents and the protein for complete analysis.
These methodsand the advent of an easy, reproducible in vitro methodof
cultivating P. falciparum (44, 124) permitted study of the P. falciparum
immunogens. t became vident that the sporozoite protein responsible forengendering immunitywas situated on the surface of the sporozoite and wasthe only protein at this site (32-34, 87). It has been designated as the
protein for circumsporozoite, ndicating its location when dentified by anti-
bodies. The gene coding for this P. falciparum polypeptide has been clonedand sequenced 21). The polypeptide is about Mr44,000 and, interestingly,
contains a large block of 41 tandem epeats of the tetrapeptide, asparagine-
alanine-asparagine-proline, or NANP. his knowledge allowed the produc-
tion of synthetic peptides containing various repeats of this tetrapeptide forsubsequent vaccination and serological studies (88, 140). A polypeptidecomprisedof three repeats of this tetrapeptide, (NANP)3, as found to serve
as antigenic epitope measuring the CSantibody in malaria sera (140). Theepitope (NANP)3 ccurred in all isolates of P. falciparum from around theworld (88, 89). Furthermore, DNAxtracted from 23 P. falciparum isolates
was found to hybridize specifically with a DNA robe corresponding to the
NANPepeat region (133). These data indicated that this putative vaccinewouldbe effective in all regions withP. falciparurn transmission. Great effort
is nowbeing made to develop this molecularly defined sporozoite-derived
vaccine. Whenhe synthetic peptide (NANP)3s coupled to a carrier protein,immunizationproduces antibodies in experimental animals. These react with
live sporozoites, producing a typical immuneeaction, the circumsporozoiteprecipitation, and neutralizes .sporozoite infectivity in vitro (3, 139).
This sporozoite-derived vaccine will probably be the first to be used for
human rials. The vaccine is currently undergoingphase I and soon phase IIclinical trials in human olunteers. However, urther experimental workhasrevealed another problem. The synthetic peptide and recombinant product
when used as an immunogen ith adjuvants will stimulate immune esponsesonly in strains of mice bearing the H-2b major histocompatibility complex
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haplotype (23, 138). All other strains of mice tested failed to produce
significant titers of antibodies (23,138). Thebasis for this was dentified tocontrolled by the immuneesponse genes, specifically that the responsiveness
is restricted to the I-Ab allele (23). But previously nonrespondermice,
immunized with (NANP)3 oupled to a carrier protein and mixed with
adjuvant, will readily produce antibodies (23). This indicates the need forhelper T-lymphocyteepitopes to provide antibody-stimulating help to the
B cells specific for the (NANP)3epitope (36). If this (NANP)3
munodominantpitope is to becomea vaccine for human se, suitable carrier
proteins are needed such that the vaccinated individual will be able to boostthe specific immuneesponse to (NANP)3uring natural exposure to sporozo~
ites from P. falciparum-infected mosquitoes. Otherwise the specific anti-body titers wouldbe too low to be protective. That repeated stimulation is
necessary to effect a favorable anti-sporozite antibody titer is shown nseroepidemiologic studies. The anti-sporozoite (anti-NANP3)iters are very
low or nonexistent in up to 60%of sera from 5-8-year-old children, where-
as over 90% f adult sera have significant anti-sporozoite antibodies (86,87).
It has been suggested that the unresponsiveness in children in endemicregions maybe partly related to the immuneesponse genes, similar to the
murine antibody responses (24). Such data clearly indicate the need forresearch to identify a more immunogenic porozoite-derived vaccine. This
need was further demonstratedby a recent experimental study that found that
the whole, radiation-attenuated sporozoite vaccine in mice was muchmoreefficacious in engendering protection than the synthetic peptide-carrier or
recombinant DNA-producedubunit vaccines (31). This study further demon-
strated that the live vaccine and subunit vaccines producedequivalent anti-
sporozoite antibody titers but that the former induced an effective cellularimmune esponse, which has been demonstrated to be of importance in
protection in experimental murine malaria (117, 134).
Theprospects for a sporozoite-derived vaccine that will be available for usein endemic regions are good nevertheless. Such a vaccine will be able toprevent the establishment of a P. falciparummalarial infection by eliminating
the initial exoerythrocytic phase of infection, and thus producesterile immun-ity. But this vaccine is clearly species- and stage-specific (85, 125). The
vaccine will have no effect on individuals with patent infections or infectionswith other species of parasites. Several laboratories have endeavored to
develop vaccines for the schizogonic (erythrocytic) phase and the gametocytephase.
MEROZOITEACCINEn addition to diversifying the vaccine strategies, a
merozoite vaccine might be of importance when sporozoite vaccine is not
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totally effective. Anysporozoites that escaped the defenses induced by the
sporozoite vaccine wouldreadily lead to a patent infection. Additionally,someantigens fromP. falciparumschizogonic stages share epitopes with the
immunodominantporozoite protein (20, 47). It is possible that a vaccine
prepared from these cross-reactive proteins from the asexual stage couldengender protection against both infective sporozoites and morozoites.
Muchecent research has been devoted to cloning P. falciparumgenes that
encode for the various surface membrane roteins and identifying those
proteins that are immunodominant y utilizing humansera from endemicregions (14, 67, 114, 118). Several vaccine-candidate antigens have been
identified (83, 102, 106, 109). Two olypeptides of approximately Mr 75,000
and 100,000 have been separated fromP. falciparumblood-stage proteins and
used successfully to vaccinate squirrel monkeys gainst a homologous hal-
lenge (30). A second candidate is a Mr155,000 glycoprotein (98, 129) that
thought to be released by the merozoitc when he erythrocyte is invaded (7).This protein is a ring-infected erythrocyte surface antigen (RESA).RESAas
been used successfully to protect nonhuman rimates against challenge P.
falciparum infections (4, 19).
Successful vaccination in these experimentalstudies has required the use ofcomplete or incomplete Freund’s adjuvant or other adjuvants. These are too
toxic for human se and so great effort is being made o utilize the variouswater-soluble derivatives of muramyldipeptide (110, 111). Although thesearch for molecularly defined candidate vaccine antigens will no doubt be
successful, such proteins will have to be used in combinationwith adjuvants
to stimulate the immuneesponse. Analternative strategy is to clone the gene
that codes for the vaccine protein in a carrier vaccine such as vaccinia virus
(115) and perhaps in the newly developed live oral typhoid vaccine (29).
GAMETOCYTEACCINEince each stage--sporozoite (exoerythrocytic),
schizogonic (asexual, erythrocytic), and gametocytic--will stimulate immun-ity that is stage-specific, a potential gametocytic vaccine wouldhave im-portance in preventing transmission but wouldbe without effect on the other
stages in humans.The initial demonstrationof this type of vaccine was made
in avian, rodent, and simian malarias (12, 41, 76). Through the use
monoclonal antibodies, passive immunity against P. falciparum could beconferred on experimental animals (43). Such antibodies wouldalso readily
prevent mosquitoes from becoming nfected if antibodies were mixed with
infected blood in cultures used to membraneeed the vectors (63, 103). The
antigens to which such monoclonalantibodies are directed appear to be Mr45,000 and 48,000 (126). Current effort is directed at cloning the genes
encoding or these antigens for large-scale production and subsequentanalysisand vaccine testing (122).
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Approximately200-250 millions of the world’s population are afflicted by
schistosomiasis caused primarily by Schistosomamansoni, S. japonicum,and
S. haematobium. n more restricted foci, S. intercalatum in Africa and S.mekongi n Southeast Asia are the remaining human pecies. This helminthic
trematode parasite lives as an adult in the mesenteric venules and vesicalplexus and discharges eggs containing larvae in the excreta. All species
require a freshwater intermediate snail host in which the larvae (miracidia)
from eggs will transform and multiply into thousands of infective larvae or
cercariae, whichemerge rom the snail and penetrate human kin during water
contact. Theparasites enter the skin and migrate via the circulation to develop
in the liver; ultimately they migrate to the venousvasculature of the gut andbladder. The adult worms re generally not the direct cause of morbidity. The
eggs laid by the femalesare trapped in the liver, intestines, and urinary tract;these are responsible for disease due to the cellular immuneesponse (granu-
loma) directed at each egg. The cumulative effect of the granulomas is
fibrosis, which leads to the clinical problems commonly ncountered.Morbidity is correlated with the tissue egg burden and thus with the adult
worm urden (131, 132). If only a few worms re present, the likelihood
the infected individual’s suffering clinical disease is relatively slight. In
control strategies, therefore, the reduction of transmission and of the adult
worm urden will ultimately lead to a reduction in morbidity even without asignificant impact on prevalence. The ideal vaccine would provide the vac-
cinee with solid (sterile) immunity,but in schistosomiasis we must consider
the practical alternative of a vaccine that wouldprevent the establishment of
large wormburdens. All current experimental approaches to vaccination
result in significant protection but never sterile immunity 22).
Radiation-Attenuated Vaccine
In 1961, the first report appeared on the induction of immunity n mice byvaccination with 6°Cobalt-irradiated S. mansonicercariae (1.28). Since that
time, a plethora of studies have confirmedand extended hese findings (6, 22,
52, 84, 101,123). There is general agreement hat high dose radiation, 20-50
Krad, keeps organisms from surviving to adulthood but allows the live,attenuated larvae to stimulate immunity n mice, rats, guinea pigs, rhesus
monkeys, and baboons (51, 53, 97, 119, 120). While these studies have
evaluated various efficacy parameters such as dose of irradiated cercariae
(500 cercariae), numberof doses (3-5 doses), duration of immunity overyear in mice), and minimum ffective time of immunization (1-2 weeks),
little is known f the vaccine’s safety. Many f these studies indicate that
none of the vaccine organisms grew to adulthood, but inoculating large
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numbersof animals with the vaccine disclosed that one mouse n 50 harbored1-3 stunted adult worms G. I. Higashi, unpublished observations). The
histopathologic response to the irradiated vaccine has been fully described
(9). The organismsdo induce a strong immunenflammatory esponse (9)
resolves without significant residual lesions within six weeks G. I. Higashi,
.unpublished observations). Improvedsafety and efficacy through the com-bined use of 6°Co-radiation and radiosensitizer chemicals is currently being
evaluated (H. J. Gomberg nd G. I. Higashi, unpublished observations).
Since it appears that this radiation-attenuated vaccine could serve as an
experimental modeland that any human accine for schistosomiasis should bemolecularly defined (35), current research on improving he live vaccine has
diminished.The vaccine clearly induces in murinehosts an array of antibodies directed
at various antigens of the schistosome arval surface (95, 113, 114). A recent
study has demonstrated that such serum antibodies will passively transferimmunityo naive murinehosts (74). Here also, the protection transferred
still partial. The additional role that cell-mediated immunityplays in thisvaccine modelhas not been discounted, as macrophages ctivated by specifi-
cally sensitized T lymphocytes in response to schistosome antigens are
helminthotoxic (61, 62).
Veterinary application of the radiation-attenuated vaccine has been success-ful. S. bovis and S. japonicumn cattle producea disease similar to that inhumans.6°Co-radiatedS. bovis cercarial vaccine protects cattle in the Sudan
from acquiring large worm urdens and improves the health of those vaccin-
¯ ated (8, 56). Initial effort in protecting cattle in Chinawith an X-irradiatedjaponicum ercarial vaccine also appears effective (55). Large scale applica-
tion has not been carried out.
Molecularly Defined Vaccines
Significant progress has been made n the identification of antigens derivedprimarily from the surface membranetegument) of the early larval stage, the
schistosomulum 112). That this stage is important in induction of immunityis evident from all of the live radiation-attenuated vaccine studies and recent
studies in whichkilled schistosomula, either in combinationwith an adjuvant
like Bacille-Calmette-Guerin (BCG) 59) or alone (54), engenderedprotection. Indeed, most of the positive reports of protective monoclonal
antibodies and/or active immunizationhave mostly come rom analysis of the
surface antigens of schistosomula (Table 1). Thus, an impressive number
defined antigens that confer some egree of protection have been identified. Itis not knownwhether any of these of similar molecular weights from differentlaboratories are identical. The Mr >200,000, 38,000, and 28,000 antigens
maybe identical (27, 66). Indeed, monoclonal ntibodies recognize the same
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ness) and lymphatic obstruction (elephantiasis) from Wuchereriabancroft~,
Brugia malayi, and B. timori. Over 200 million individuals are at risk of
infection by these filarial worms.Although ery little progress has been made
toward vaccination against onchocerciasis due to the absence of a reliableanimal model nd difficulty in laboratory maintenance f O. volvulus, signifi-
cant developments have been made for the lymphatic dwelling parasites,
particularly B. malayi.Irradiated infective stage larvae of B. malayi and the related zoonotic B.
pahangi have been shown o confer somedegree of resistance to challenge
infections in monkeys135), jirds (137), and cats (94). In jirds, protection
91%can be readily induced (137) by vaccination with 75 6°Co-radiated (15
Krad) infective stage larvae. Reducingas muchas possible the establishment
of adult worms n the lymphatics is of crucial importance in this vaccinemodel, as these are the primary cause of pathology, since the microfilariaelaid by the females circulate with minimalassociated morbidity. Of particular
importance is that the vaccination does not incite lymphatic obstruction,
although studies to date have not fully characterized this feature.
A number of antigens derived from B. malayi recognized by antibodiesfrom humans nd various stages of lymphatic filariasis have been identified
(64, 72, 73), although none has been evaluated with respect to its ability
confer active immunization n experimental hosts. Based on the success of
passive transfer of immunity y vaccinated jird sera (46), it would ppear that
antibodies are responsible for the engendered esistance, but the added role ofcell mediated immunity has not been thoroughly assessed. Cross-reactive
antigens between arval and microfilarial stages mayalso be protective, as B.
malayi microfilariae were shown o induce immunity n jirds to a challenge
infection (65). Clearly, muchmore needs to be done before any candidate
molecularly defined vaccine can be proposed.
LEISHMANIASISA diverse array of species infects humans around the world, causing
long-lasting immunitymediated by the cellular immune ystem (48). Histor-
ically, it has been a commonillage practice to allow the purposeful infection
of a part of the body other than the head, neck, and arms to promote the
establishment of the lesion, recovery, and then immunity, eaving a scar thatwouldnot be cosmetically disfiguring. In recent years, the USSR nd Israel
have utilized cultures of L. tropica or L. major o provide a source of infective
organisms (promastigotes) for controlled infections as prophylaxis (39).
Although tens of thousands have been vaccinated effectively in the AsiaticUSSRnd about 5000 in Israel with significantly reduced rates of subsequent
infections, untoward ide effects of secondary nfections, skin allergies, and
even immunosuppressionave been frequently noted (39). Further attempts
employing attenuated strains of Leishmania for L. major in the USSR,L.
braziliensis in South America, and L. donovani(visceral leishmaniasis)Kenya for vaccination have been uniformly unproductive. But recently,avirulent strains of L. major(75) and L. braziliensis (37) have been produced
by exposure to the mutagen, N-methy-M’-nitro-N-nitrosoguanidine n v~tro,with the latter species at reduced temperature. These attenuated strains in-
duced protective immunity n naive mice, although the resistance was notabsolute in all animals. Other L. majoravirulent strains have been selected
that readily induce protective immunityn mice (82). Lethally irradiated (150Krad) L. major cultured promastigotes will confer strong immunity n mice
after intravenous inoculation (49, 50, 71). The immunitycross-reacts with
several other Leishmaniapecies and appears to be due to the induction of thecellular immune ystem. A genus-specific vaccine could be possible if these
studies are confirmedand extended. Nevertheless, these studies suggest that a
molecularly defined vaccine is within reach in the immediate uture. Although
a number of promising candidate antigens have been identified, none have
been isolated and used actively to vaccinate susceptible mice.
TRYPANOSOMIASIS
Trypanosomiasis n Africa is caused by Trypanosoma rucei rhodesiense and
T. b. gambiense,commonlyalled African sleeping sickness, and is transmit-
ted to humans y the bite of the tsetse fly. This disease has been he scourgeofAfrica and has decimated populations. Today it is muchmore limited in
scope, although tens of thousands become nfected yearly with a very high
mortality rate, as there are no safe and totally effective drugs. T. b.rhodesienseand related trypanosomeskill hundreds of thousands of domestic
animals each year, providing a further burden to agricultural productivity.
These protozoan parasites have adapted to their hosts by a strategy ofvarying their outer surface antigenic proteins in successive wavesof parasite
populations (127). Each waveof parasitemia displays an antigen to the host
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different from antigens of previous parasite populations, such that the host
immuneystem is continually evaded. This process is called antigenic varia-
tion and is genetically controlled. Over a hundreddifferent variant surfaceantigens have been identified for each strain (127). Vaccinationwouldbe verydifficult unless the vaccinecontainedall of the variant antigens or at least the
predominant ones. Although a number of early studies have attempted toimmunizeagainst African trypanosomes, no vaccination strategy has been
devised that wouldcircumventantigenic variation.
In Latin America, rypanosomiasis s caused by T. cruzi, which s transmit-
ted to humansby the activities of hematophagous eduviid bugs. No fewerthan 15 million people are afflicted in Latin America.Drug herapy is almost
nonexistent. Thedisease is responsible for chronic cardiac and gastrointesti-nal problems hat are uniformly fatal.
A numberof attempts at vaccination have been tried using whole organismsor various extracts of these protozoans; findings have been inconclusive
(107). Even though vaccinated animals can be shown to have diminished
parasitemias from challenge infections, very few studies have assessedwhetherall parasites are eliminated from the host. The chronic phases of the
disease are the central issue in Americanrypanosomiasisor Chagas’Disease.
A further complication is that there appear to be cross-reactive antigens
between he T. cruzi parasite and mammalianeart and neuronal antigens, thetwo primary sites of pathology (121, 136). Thus any attempt at vaccination
mayresult in a form of autoimmunization,with resultant autodestruction ofthese tissues, a decidedly unfavorable outcome. Antigens must be sought that
do not cross-react with human issues and must be isolated free of any
extraneous components. t is plausible that protective immuneesponses canbe induced in humans y the use of the correct antigens, as specific com-
plement-dependentytic antibodies have been dentified in patients’ sera (69).
The antigen(s) inducing these antibodies have not been identified. Greater
attention is needed o characterize and isolate surface antigens. A vaccine isclearly needed,since other formsof control are of little practical value for thisdisease of the economically poor.
OTHER PARASITIC DISEASES
The literature is replete with reports of studies attempting the active im-
munization of experimental animals against almost every parasite of humanand veterinary importance (15, 57, 80, 81). Vaccinations have consisted
whole organisms, disrupted, homogenized,and fractionated, or of drug- orradiation-attenuated larval stages. In most situations, the use of radiation-
attenuated organisms has led to significant protection, such as withTrichinella spiralis in rats (38), Dictyocaulusviviparus in cattle (96), and
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Ancylostoma aninum n dogs (77), to nameonly a few. The D. viviparus live
vaccine is still in use, particularly in Englandand Europe. The A. caninum
was withdrawn rom the market for commercial easons unrelated to its safetyand high efficacy (77). Very little work has been done to identify the
important unctional, protective antigens in these parasites for potential isola-tion and use in vaccination. The greatest advance has come rom study of T.
spiralis larvae, wherein he specific protective antigens have been identified
and isolated (25, 26). Contemporarytudies of other helminthic parasites arestarting to elucidate the sites of immune ttack as well as the immunogenic
antigens (11, 79, 100).
Amonghe other protozoan parasites of humans, most studies on im-
munization have been directed at Entamoeba istolytica amebiasis (108) andToxoplasmagondii infections (68). Encouraging results are noted but much
more research is needed to define a useful vaccine model and then tomolecularly characterize the immunogensesponsible for the resistance.
CONCLUDING REMARKS
The introduction of monoclonal antibody and recombinant DNAmethodolo-gies has been primarily responsible for the burst of data on vaccine develop-
ment in parasitic diseases. The extreme difficulty of in vitro and in vivocultivation that retarded the identification and recovery of candidate vaccine
antigens from the important humanand veterinary parasites has to a largeextent been circumvented with these technological advances.
The rapidity with which the malaria sporozoite vaccine has been developedsuggests that humanrials in endemicareas will be conducted n the next few
years. If such a vaccine were to, if not eliminate falciparum malaria, but
drastically reduce the morbidity and mortality currently seen, it wouldhave
accomplished its goal. Despite the significant progress toward a malaria
vaccine, concurrent research is needed to develop better adjuvants and novelstrategies to improve he immunogenic ualities of the molecularly defined
(synthetic or recombinantprotein) vaccines. Analternative strategy that hasnot been fully investigated is to search for anti-idiotype antibodies that upon
inoculation into a host would nduce the production of antibodies specific for
the parasite epitope responsible for protection.Progress too has been made n the search for a defined antigen vaccine for
schistosomiasis. Several antigens are clearly protective, but the lack of solid
(sterile) immunity till provides a significant obstacle. Most tudies with all
vaccine candidates find no more than 60-70%protection. Is this enough toprevent large wormburdens and thus prevent the development of chronicmorbidity?Although t is generally accepted that morbidity is correlated withwormburdens, nothing is known n humansabout the threshold level. Inno-
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vative epidemiological investigations are still needed to ascertain the worm
burdens that trigger the onset of clinical morbidity. It is unfortunate that the
live, attenuated vaccine is not developed enough for application.
The technological advances described above will afford greater attention to
the other important parasitic diseases of humans. The developing world’s
inhabitants suffer extensively from a multitude of parasitic infections (poly-
parasitism), which in aggregate contribute to significant morbidity. A need
exists for a more diversified armamentarium of parasitic vaccines.
ACKNOWLEDGMENT
The author thanks Anne Glas for expert secretarial assistance in the prepara-
tion of this manuscript.
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