The immunology of human and animal cysticercosis: a review

Bulledn of dth World Health Organizaton, 57 (5): 839-856 (1979) The immunology of human and animal cysticercosis: a review * A. FLISSER,1 R. PfREZ-MONTFORT,2 & C. LARRALDE3 In this review of the literature concerning the immunology of animal and human cysticercosis, emphasis is placed on whether previous exposure to the antigen confers protection to the host. Statistical analysis of the published data indicates that immunized animals have a lower risk than non-immunized animals of contracting cysticercosis, there being large variations within and between different host-cysticercus relationships. There is no indication as to which antigen is bestfor immunization but, although live parasites in all stages of development, or extracts, appear to give protection, embryos, eggs, and excretions are most frequently used. Antibodies appear to be the principal mediators of resistance, but the action seems to be only upon very young larvae, while fully grown cysticerci are unharmed. Several immunological methods are valuable in the diagnosis of cysticercosis, the choice depending more on thepurpose of the study than on differences in their ability to discriminate between healthy and sick. The presence of anticysticercus antibodies in the serum of up to 50% ofhuman patients indicates that human vaccination may be possible in high-risk areas; the remaining patients pose an interesting problem open to speculation and research on immunological evasion, immunodepression, and the existence of serotypes. Cysticercosis involves a complex host-parasite relationship in which the participation of the im- mune response may be decisive; the disease consti- tutes a serious threat to human health in under- developed countries. It is also a major problem in the production of meat and thus affects the food industry. The high prevalence of cysticercosis among Mexicans is indicated by a frequency of about 2% of cerebral cysticercosis in hospital patients (1-3), among the ambulatory population (4), as well as in autopsies (5-7). Since immune phenomena may be * From Departamento de Biotecnologia, Instituto de Investiga- ciones Biomedicas, Universidad Nacional Aut6noma de Mexico, Apartado Postal 70228, Mexico 20, D.F., Mexico. 1 Research Fellow. 2Research Fellow, CONACyT. 3Chairman. used in the diagnosis, treatment, and prevention of cysticercosis, and because of the pressing need to control the disease, we decided to prepare the present review. In it we have considered mainly Taenia-host relationships, in contrast to earlier reviews dealing with all cestodes (8, 9), and those articles that evaluate the protection conferred to different hosts by immunization or by infection with the corresponding parasite. Further, for all experi- ments that could be reduced to a single response variable, a single statistical analysis has been per- formed in an attempt to evaluate the overall ef- ficiency of establishment of cysticerci in normal and immune hosts, as well as to investigate the variation between different host-Taenia relationships. The mechanisms of immunity and the use of immuno- 3862 - 839 - A. FLISSER ET AL. logical methods in the diagnosis of cysticercosis are also reviewed, though less extensively. We hope this review will clarify some basic questions on immunity to cysticercosis, give an estimate of the validity of some seemingly estab- lished knowledge, and indicate the main require- ments for further work. The authors cited must realize that some of the finer points of their analyses could not be taken into consideration in our at- tempts at simplification-to investigate whether there is immunity to cysticercosis or not. IMMUNITY TO CYSTICERCOSIS It is generally observed with Taenia that exposure of the host to different forms of the parasite's antigens induces resistance to subsequent challenge, although the degree of resistance may vary. The main findings in the different species studied are presented briefly in the following paragraphs. (1) In a study of the pig's immune response to infection with eggs of Taenia solium (10), it was observed that the animals that received a single dose of eggs had more cysticerci than those that received two doses, indicating that larger doses gave better immunity. (2) Protection of cattle against Cysticercus bovis has been attempted in many ways, using as vaccine: live (11, 12) or irradiated embryos (13), viable (14, 15) or irradiated eggs (16), secretions and excre- tions of cysticerci (17, 18), or whole cysticerci (19). In most cases, exposure to the antigens reduced somewhat the number of cysticerci found after challenge. In some experiments, when the vaccine consisted of live or irradiated embryos implanted intramuscularly, there was considerable reduction in the number of cysticerci that established after challenge, but usually the effects of vaccination have been very variable. When live embryos have been used as vaccine they usually developed into cysti- cerci and survived apparently well at the inoculation site, even in those animals that showed resistance to subsequent challenge with eggs (20). The route and the frequency of administration of vaccine seem to be important: the intramuscular route proved to be more effective than the subcutaneous one for em- bryos as well as for eggs (20), while repeated injections were more efficient than a single one (14). Some authors favour the concept that cattle acquire natural immunity to cysticercosis, based on the fact that younger cows, especially those not exposed to natural infections are more susceptible to challenge with T. saginata than older ones (21, 22). These observations suggest that cattle acquire immunity to cysticercosis during their life as a result of fortuitous contacts with the parasite (23). (3) Live Cysticercus fasciolaris introduced into the rat's peritoneal cavity (24, 25), as well as extracts (26, 27) or secretions and excretions (27) of the larvae, induce some resistance to oral challenge with eggs of T. taeniaeformis. (4) Partial resistance was induced in rabbits in 1932 by previous contact with eggs or extracts of Taenia pisiformis (28, 29). More recently complete resistance to challenge was induced in rabbits by introducing live embryos by the subcutaneous (30) or intramuscular (31) routes. In this case also, cysticerci that developed from the vaccine were found at the inoculation site in spite of the induced resistance. In other studies in rabbits, variable resis- tance to Cysticercus pisiformis was induced by immunization with developing cysticerci (32) or with their secretions or excretions (33). (5) Sheep can also be partially protected against Cysticercus ovis and Cysticercus tenuicollis by im- munization with live embryos either enclosed within diffusion chambers (25, 34, 35) or in direct contact with the host's tissues (36-38). Complete resistance of sheep has been obtained with live, formalin fixed, or frozen embryos of T. hydatigena, and partial resistance with those of T. ovis, but not with sonically disrupted embryos (39). In sheep kept in pastures contaminated with eggs of T. hydatigena, resistance to infection was demonstrated after im- munization with embryos of T. hydatigena, but only when immunization was done one week before the natural challenge represented by the eggs in the pasture. Because sheep kept in contaminated pas- tures were more resistant to challenge than those kept in sterile zones, it was suggested that a natural immunization process takes place (40-42). (6) Several trials to evaluate cross-resistance have been done with T. pisiformis, T. ovis, and T. hydatigena. These studies clearly demonstrated cross-reactivity between embryos of T. ovis and of T. hydatigena, but not convincingly for T. pisiformis. Cross-resistance was always less than that conferred by the homologous species (41, 43-47). The cross- immunity experiments are particularly difficult to interpret because of the metabolic restrictions im- posed by the hosts on various parasites in addition to antigenic difference. (7) We do not know of any attempt to evaluate the effects of immunization against cysticercosis in man. Nevertheless, the presence of specific anti- bodies in a significant proportion of the people who 840 CYSTICERCOSIS IMMUNOLOGY 841 suffer brain cysticercosis (see page 847) indicates that humoral responses do develop. In view of the protective action of antibodies in experimental ani- mals (see pages 844-846) it may be inferred that they are also important in the host-parasite relation- ship in man. Preliminary studies in our laboratory of the anticysticercus antibodies of parasitized human subjects suggest a heterogeneous response of man to the cysticercus antigens: human subjects seem to differ in the antigen they recognize (48), as well as in the class of immunoglobulin synthesized. The his- tological picture of the inflammatory phenomena around the parasite, particularly around dead para- sites, but quite appreciable around apparently live ones (49, 50), also points to man's immunological awareness of established cysticerci. The small number of cysticerci generally found in humans-frequently less than 10 and usually less than 100 (51)--contrasts with the thousands found in the skeletal muscles of pigs and with the scores or hundreds found in the pig central nervous system (52). Although these differences may indicate differences in the immunological reactivity of man and pig, they are probably related to the lower probability of human subjects ingesting large num- bers of eggs. Even though the participation of cellular immunity in man cannot be definitively discounted, the results of skin reactions (53-55) and blastoid transformation in vitro (56) do not indicate its involvement in the man-cysticercus relationship. (8) In an effort to evaluate the protective effect of immunization against cysticercosis in all host-Taenia relationships, three approaches have been followed: (a) Fig. 1 illustrates the number of experiments reported in the literature in which a certain number of cysticerci became established following the ad- ministration of different numbers of Taenia eggs to normal or immune hosts, respectively (for all host- parasite relationships and all methods of immuniza- tion). Regression analysis reveals significant differ- ences between immune and normal conditions and points to lower efficiency of establishment in the immune animals (normal Y = 0.019 X + 385, r = 0.444, P < 0.01; immune Y = 0.007 X + 150, r = 0.488, P <<0.01). One should also note the very much higher frequency of zero establishment of cysticerci in immune hosts in comparison with nor- mal ones. (b) Summarizing the data from all the reviewed literature, Fig. 2 shows the cumulative frequencies of the efficiencies of establishment of different Taenia species in normal and immune hosts. Very few data are available for T. solium, but, in general, the data A SWA ~~~~~~~~~~~~~~~~OFORATAm 3 4 6 I 3 2 6 51 .00 000 2 3 7- g 9 II 16 48 2 2 2 2 3 95 . 00 7 4 13 3 2 2 33 3 24 19 II 6 12 13 5 2 94 2' 0 5 9 I Is 4 Number of odministered eggs B - ~~~~~~~~ ~~~~~~~OFDATA lAOIKB~~~~~~ ~ ~ >HX~~~ 1 8 12 2' 16 60 3 Z S0 14 _ .14 _ 5 69 _ 103 4 2650 38 4 07 18 4 5 l6 3991 030 41 31 5 98 33 5 2 3 248 226t8 0 l}o38 44 i20 518 45} 639§ | Number of administered eggs Fig. 1. Data taken from published sources to show the relation between the number of cysticerci established and the number of eggs administered in different host-parasite relationships. The numbers in the grid represent the number of reported experiments in which the variables fell within the ranges shown. A. Normal hosts. B. Immune hosts. indicate significantly greater susceptibility of normal hosts than of immune hosts. (c) Table 1 summarizes all the available values for efficiency of egg establishment classified accord- ing to host-parasite relationship and immune condi- tion.a A two-way analysis of variance with unequal numbers of observations would appear to be valid, where the first variable has six levels, corresponding to the different Taenia species, and the second variable has only two, the normal and immune conditions. The results of this analysis are given in Table 2 and show the significantly lower susceptibili- ty of immune hosts and the significantly different a Original data from which these statistics were calculated are available from the authors on request. A. FLISSER ET AL. T solium C.> z LLJ I-. -LJ z>C r hj'delifmaa r suginule T pisifo,ais 2 6 20 >20 4 8 12 16 20 >20 4 8 PERCENT EFFICIENCY OF ESTABLISHMENT Fig. 2. Cumulative frequency of all reported efficiencies of establishment of different taenias in their respective normal (open circles) and immune (solid circles) hosts. The main effect of immunity consists in shifting the distribution towards lower efficiencies, although the effect is quite variable among the different taenias. Table 1. Basic statistics on the efficiency of egg establishment in normal and immune conditions of different hosts to various taenia speciesa Pig Cattle Sheep Sheep Rabbit Rat T. solium T. saginata T. ovis T. hydatigena T. pisiformis T. taeniaeformis NORMAL HOSTS X 3.33 2.20 6.25 11.13 13.32 17.00 a2 5.33 17.29 122.86 111.12 336.79 210.37 SE 1.32 0.59 2.09 1.73 2.07 1.82 n 3 49 28 37 78 63 IMMUNE HOSTS X 0.12 0.39 2.10 5.63 3.55 5.97 (2 0.03 2.31 7.36 31.60 106.03 89.00 SE 0.08 0.16 0.27 0.44 0.53 0.60 n 4 82 94 161 365 272 The complete data from which these statistics were calculated are available by request to the authors, together with details of the sources of the data. T ovis r hresiveformis 842 CYSTICERCOSIS IMMUNOLOGY Table 2. Analysis of variance (non-orthogonal) Source of Degrees of Sum of Mean F ratio Confidence variation freedom squares square level Mean 1 37200 37200 392.6 1.0 State of host 1 1732 1732 18.2 1.0 Taenia species 5 9412 1882 19.8 1.0 Interaction 5 2341 468 4.9 1.0 Error 1194 113112 94 Total 1206 169948 levels of efficiency of establishment for the various host-Taenia combinations. The most important conclusion from the above analysis is that immunization reduces the risk and/or the severity of cysticercosis in most, if not all, host- Taenia relationships, with great variation within and between the different Taenia species. Another inter- esting finding is the tendency towards a linear relationship between the number of established cysticerci and the number of eggs administered in the normal hosts that indicates an overall average percentage efficiency of establishment of 1 %, a very low figure when one considers the mechanism of acquiring cysticercosis. Such low efficiency minimizes the relevance of massive egg distribution by means of air, water, or handling, where a great dilution factor is expected, and indicates the import- ance of transmission phenomena that ensure the ingestion of many eggs, perhaps achievable only by the ingestion of clusters of eggs, as in ingestion of whole proglottids. CYSTICERCAL ANTIGENS In 1936, Taenia taeniaeformis and its cysticercus were fractionated by several methods and different protein, nucleoprotein, and polysaccharide fractions with immunogenic activity in the rat were obtained (26). However, at that time, attempts were not made to purify, identify, or localize the antigens responsible for protection. Subsequent studies with T. pisiformis in rabbits (32) showed that protection was conferred mainly by antigenic fractions from oncospheres and young cysticerci, containing six protein bands in polyacrylamide electrophoresis, two of which corresponded to glycoproteins; four were shared between the adult worm and the cysticercus, and two were exclusive to the cysticercus (33). In other studies, protection in rabbits against Cysticercus pisiformis was achieved only with the complete culture medium in which cysticerci had been grown, or with the fraction passed through a filter that excluded material of relative molecular mass greater than 300 000, while culture media fractionated through G200 or A50 Sephadex were ineffective (57). These findings could be explained if the antigen responsible for immunity is of low relative molecular mass (RMM) and is loosely associated with a macromolecule (RMM < 300 000) that dissociates during chromatography but not during filtration. An antigen (RMM = 140 000) that induced complete protection in rats was purified recently from a homogenate or from the medium in which Cysticercus fasciolaris had been cultured (27). Rabbits produce antibodies against protein and polypeptide-polysaccharide fractions of C. pisifor- mis but their role in immunity has not been estab- lished (58). An allergen of C. fasciolaris, responsible for passive cutaneous anaphylaxis, has been purified: it has a negative net charge, a relative molecular mass of about 50 000, is present in cysticerci and in smaller quantities in adult worms, but is not found in culture media (59). Two of five antigens of C. tenuicollis have been demonstrated by precipitation with hyperimmune sera of rabbits, but they reacted poorly or not at all with the sera of infected sheep (60). By immunizing sheep and rabbits with extracts of C. cellulosae and Freund's complete adjuvant we have found evidence of many antigens: some are shared by the scolex, the wall, and the fluid, while others seem to be present in one or other structure of the cysticercus (Fig. 3) (48). It should be noted that host antigens contaminate most preparations of cysticercus and seem to correspond to immuno- globulins and albumin (61). Using this kind of antigenic extract of C. cellulosae, we have found that persons with naturally acquired cysticercosis have antibodies to several of the antigens, but that not all the patients have antibodies against the same one 843 A. FLISSER ET AL. Fig. 3. Results of immunoelectrophoresis (3a) and double immunodiffusion (3b) of the antigenic fractions of scolex (E), wall (P), and fluid (L) of C. cellulosae against the respective rabbit antisera (SaE, SaP, and SaL). Some antigens are shared between fractions, while others seem exclusive: scolex and wall have essentially the same antigens while the fluid contains at least one exclusive antigen, since the reaction between L and SaL has one band not present in the reaction of L with SaE or SaP in double immunodiffu- sion. Reproduced from reference 48. (Fig. 4) (48). Even though these antigens certainly stimulate the production of antibody under experi- mental and natural conditions in man and other species, their importance in the induction of resist- ance has still to be determined. No systematic serological classification of cysti- cerci is available. A survey of the documented cross- reactions indicates that there is extensive antigenic overlap between cestodes, and even other parasites, although no precise statement is possible owing to variability in the methods employed. Biagi and his coworkers (62, 63) report important differences among various preparations of antigens of C. cellu- losae in their reactions with sera of human patients and of pigs, as well as differences between the reactions of C. cellulosae and C. racemosus antigens with cerebrospinal fluid of human patients. Al- though these observations suggest the existence of serotypes among cysticerci, they are inadequate as a basis for classifying the parasites that affect humans. Likewise, we have proposed the existence of sero- types as an explanation of the large fraction of patients with confirmed cysticercosis whose sera did not react with extracts of C. cellulosae (see page 847) but other possibilities have not been excluded.. The finding of a cysticercus from T. crassiceps in a human eye (64) proves that man can harbour cysticerci of Taenia species other than T. solium: thus, apart from the problem of different serotypes within species, the serology of human cysticercosis is further complicated by its multiple etiology. Thus, in summary, eggs, oncospheres, taenias, and cysticerci contain many antigens, apparently mainly proteins or carbohydrates, some of which cross-react extensively with those of other cestodes and perhaps of other parasites. Several of these antigens may induce protection against cysticercosis; if only one is responsible it has not yet been isolated and may vary between species. MECHANISMS OF IMMUNITY TO CYSTICERCOSIS Numerous studies of different cysticercoses have shown that antibodies are capable of conferring immunity, either naturally by the transplacental or oral routes (65-68), or artificially in the sera of immune animals. Successful passive transfers have been performed with immune sera against C. fas- ciolaris (69-74), C. pisiformis (30, 75), and C. tenuicollis (76). The studies with C. bovis and C. tenuicollis were notable in that colostrum and serum failed to protect calves (13, 77), and lambs (42, 78). Recently, however, successful transfer of immunity against C. bovis has been demonstrated in cattle by means of colostrum (79) and serum (80). Research aimed at evaluating the class of immuno- globulin responsible for protection indicates that IgG and IgA are the antibodies concerned with resistance to C. pisiformis and C. fasciolaris (66, 67, 72, 81-85); the role of IgM is not clear (24, 72, 85). IgE, which is active in passive cutaneous anaphy- 844 CYSTICERCOSIS IMMUNOLOGY (Fig. 4. See legend overleaf.) laxis, has not been shown to contribute to the development of resistance (59, 86). Little information is available concerning the kinetics of antibody production. In cattle, antibodies against C. bovis reach detectable concentrations in serum approximately one month after the experi- mental infection, the titre being very variable and the rate of development depending of the system studied and the calf's age (13, 15, 87-89). The rabbit also produces a humoral immune response against C. pisiformis about two weeks after the first infec- tion, the titres in the passive haemagglutination test being very high (82, 90, 91). In the rat, antibodies against C. fasciolaris appear also in the second week 845 A. FLISSER ET AL. Fig. 4. Results of immunoelectrophoresis of the antigenic fractions of scolex (4a), wall (4b), and fluid (4c) from C. cellulosae against various sera from patients with cysticercosis. The precipitation patterns point to heterogeneity in man's immune response to cysticercus. Reproduced from reference 48. and rise to a peak around the sixth or seventh week (27). In the rabbit (86), reaginic antibodies (IgE) for C. pisiformis and C. fasciolaris appear a little earlier than in the rat (59), the highest titre being reached one month after infection with a subsequent decrease. Antibodies somehow inhibit the early develop- ment of the oncospheres in vitro, causing them to aggregate and leading to the formation of halos, like those of immune precipitates, around their surface and preventing the development of the next larval stage (30, 91). In addition to their direct effects, it is possible that anticysticercus antibodies exert their antiparasitic action via other mechanisms, for exam- ple protective anticysticercus IgG2, has been found to increase the number of circulating eosinophils (92), cells that are presumed to have a protective action in cattle infested with C. bovis (13, 87). Also, antibodies have been found to modify the permea- bility of the embryo and interfere with its metabo- lism (93). Complement is probably involved in the toxicity of the antibodies, as indicated by studies in rats in which animals became sick if they were subjected to anticomplementary treatment, adminis- tered decomplemented immune serum, and later challenged with eggs of T. taeniaeformis, while normal controls were protected by the same immune serum (72). In most studies, antibodies do not have overt effects in vitro or in vivo upon adult larvae (24, 30, 33, 71, 72, 75), although occasionally some crude effects have been noted (94). As a consequence of the fact that they act upon young embryos and have little or no effect upon larvae, antibodies are effec- tive for only a very brief period under natural conditions in vivo, and this may limit their protective action in the intestine (37, 43, 59, 66, 85, 86, 95) and in the circulation. Such a brief period of activity and restricted suceptibility of the parasite could explain the development of cysterci from the vaccine in immunized animals capable of resisting a chal- lenge with eggs. The proposition that the main site of action of the antibodies is in the intestine is defended by some because T. pisiformis embryos administered via the mesenteric vein established in similar num- bers in normal and in immune animals, while the immune hosts were resistant to a challenge with eggs by the oral route (95). This point is debatable, however, since other data provide evidence of pro- tection against parasites introduced by the parental route (13, 72, 75, 76), and because the transient susceptibility of embryos to the action of antibodies 846 CYSTICERCOSIS IMMUNOLOGY could account for the apparent resistance of the injected embryos: injected embryos might have already reached the resistant stage when they were administered, while the eggs still had to pass through the susceptible stage before becoming established in the target organ. In any event, immunologically protective mechanisms located in different organic compartments are not mutually exclusive, and it is clear that a very competent mechanism exists in the intestine, at least against T. pisiformis. The fact that immunity protects against the chal- lenge but not against the cysticerci from the vaccine, which establish freely, has caused some confusion. The reason for this is now very clear: fully devel- oped cysticerci are resistant to immune attack while early larvae are susceptible (24, 70, 75), and the fact that the peak of the immune response occurs slightly after vaccination permits the development of para- sites from the vaccine but not of those from the challenge dose. The question of how established cysticerci elude the action of the immune response is a different matter, all kinds of hypotheses-some fantastic, others reasonable-have been advanced: (a) antigenic variation occurs more rapidly than the immune response (96); (b) the existence of blocking antibodies that compete with cytotoxic antibodies (97, 98); (c) the parasite is surrounded by a coat of self or host components that is impervious to anti- bodies or cells (99-101); (d) the parasite induces tolerance (89, 102). There is no evidence of great antigenic differences in cysticerci, and the similar precipitation patterns that we observe in most batches of antigen preparations from different in- fected pigs indicate the absence of significant anti- genic differences in cysticerci or in the larval stage of the parasite, for it is most unlikely that all cysticerci collected would have had the same residence time in their respective hosts. The idea of blocking anti- bodies is just another way of saying that most antibodies do not affect the outer coat of the fully developed parasite. Tolerance is out of the question, since at least 50% of parasitized persons and other infected animals do have circulating antibodies. Research on the role of cellular immunity in resistance has been limited to some attempts to transfer immunity with cells. The results are far from convincing, either because the experimental designs were not satisfactory or because of the great varia- tion observed (24, 57, 75, 91, 103). In cysticercosis induced in rats by C. fasciolaris, a delayed-type hypersensitivity has been clearly demonstrated but its participation in protection was not studied (27). IMMUNODIAGNOSIS OF CYSTICERCOSIS The production of antibodies by infected animals implies the possibility of diagnosing the disease by examination of serum or some other component. Nevertheless, the absence of antibodies does not rule out cysticercosis since the humoral immune response could be brief or too small to detect. Practically all the conventional serological tests have been used in the search for anticysticercus anti- bodies, and in almost all cases there has been a significant proportion of sick individuals without demonstrable antibodies (Table 3). In man, for example, immunoelectrophoresis has shown that about 40% of the patients with confirmed brain cysticercosis do not have precipitating antibodies at the time of testing (48). There may be different explanations for the negative cases, the most notable being: (a) the immunodepressive effects of the anti- inflammatory treatment frequently administered to these patients; (b) the different degrees of infec- tion; (c) the low sensitivity of the method used; (d) the existence of serotypes among the cysticerci; or (e) avoidance of immune recognition. Even though it would be of value to investigate the importance of each of these factors, the existence of serotypes is the first possibility that should be investigated be- cause it may help to distinguish different types of host-parasite relationship, and different factors affecting this relationship, and because it is of great importance in the evaluation of epidemiological data and in the consideration of the types of vaccine. On the other hand, selection of an optimal im- munological test for cysticercosis depends on a combination of variables: technical ease, cost, quantity of collateral information that it provides, sensitivity, and the main objective of the study- hospital or community test or research purposes. Thus, for example, the agglutination test (104-135), which seems to be the most sensitive of the conven- tional tests, is very variable with different batches of sensitized particles, and does not readily distinguish cross-reactions. In addition, because of its greater sensitivity, this test gives a high proportion of false- positive results. If the titre required for positivity is increased the advantage of this test over others is nearly nullified. Sensitivity should be regarded more as the discriminatory power of a test than its ability to detect small concentrations of reactants. These two factors are of course related but probably not in such a simple way as is indicated by present practice in immunological diagnosis. Complement fixation tests (53, 104, 113, 116, 117, 123, 136-142) have 847 A. FLISSER ET AL. Table 3. Immunodiagnosis of various cysticercosis Percentage of positive cases Cysticercosis Test Confirmed Probable Neurological References Normal cysticercosis cysticercosis patients Human indirect haemagglutination 4-25 10-92 8-53 10-50 104-113 double immunodiffusion 0-5 15-57 0-59 5-21 48, 110, 112, 113 immunoelectrophoresis 0-13 54-87 47-83 25-24 4, 48 counter electrophoresis 0 0 25 113, 145 precipitation 0-10 7-100 7 62, 104, 138 complement fixation 0-8 40-80 38-45 0-46 53, 104, 113, 136-142 indirect immunofluorescence 0-4 80-89 109, 112, 120, 152, 153 skin reaction 5 66 87 5 53-55 blastoid transformation 56 Porcine indirect haemagglutination 3-7 66-100 104-114 double immunodiffusion 0 0 114 precipitation 0-6 3-100 62, 104, 143, 144 complement fixation 87 68 104 Bovine indirect haemagglutination 8 46-96 115-117, 121-124 latex agglutination 0-5 45-98 123, 125-130, 135 double immunodiffusion 0 0-41 117 complement fixation 16 62 116, 117 skin reaction 0 57-100 115, 130, 147-149 Ovine and indirect haemagglutination 5-18 70-100 118-119 caprine latex agglutination 54-100 131 bentonite flocculation 132, 133 the same disadvantages as agglutination tests. Pre- cipitation tests (4, 7, 48, 62, 104, 110, 112-114, 117, 123, 124, 134, 138, 143-146) have the power to distinguish easily among different antigenic systems, are simple to perform, and make it possible to study the immunoglobulin classes participating in the reac- tion and other related events (complement fixation for example) but they do not detect low antiboldy levels. The skin reaction (53-55, 115, 124, 130, 147- 150) has been studied extensively in cattle, where it is successful in 85-100% of the cases, but its application in man is limited because of the firmly established belief that in performing it there is a risk of starting an allergic reaction, either generalized or localized to the site containing the cysticercus. Other tests, like blast transformation in vitro (56), have not been successful and indirect immunofluorescence (109, 112, 120, 134, 139, 151-156) is quite subjec- tive and variable. Nothing has been published yet concerning the use of a radioimmunoassay in the search for specific antibodies or antigens in patients with cysticercosis. Our preliminary data suggest that it is more sensitive than other methods in detecting serum antibodies, especially when employing the cysticercus fluid as a source of radioactive antigen. In our opinion the objective of the study is all important in deciding which is the best immunologi- cal test: for epidemiological surveys and cattle evaluation, where large numbers of sera are hand- led, the technical ease, reproducibility, and ability to detect cross-reactions of the precipitation test com- pensate for its lack of sensitivity; while for hospital medicine all the tests seem adequate until the radioimmunoassay is fully developed. Some remarks on the antigen preparations used in the serological diagnosis of cysticercosis are pertinent. No systematic survey is available of the cross-reactions of different antigens with important human parasites but several instances of cross- reactivity have been documented. Thus, serological diagnosis may give rise to some ambiguity in heavily parasitized communities. Other important points concerning antigens for immunodiagnosis are: (a) since most antigenic preparations are crude extracts of the parasite they are quite variable in concentra- tion, are not necessarily representative of the origi- nal antigens, and vary in their degree of proteolysis, and consequently, in their ability to react with antibody; (b) since the antigenic preparations are frequently contaminated with materials from the host, particularly in the case of cysticerci (61), this may lead to some false positive diagnoses in persons allergic to the host's proteins; and (c) since the antibody response of human subjects to the recog- nized antigens is extremely heterogeneous there appears to be a need for complex antigen mixtures instead of purified preparations. Perhaps, purifica- tion of the main antigenic components in conditions that inhibit proteolysis followed by remixing at equivalent concentrations would be the ideal proce- 848 CYSTICERCOSIS IMMUNOLOGY dure for the standardization of antigen preparations to be used in the serological diagnosis of cysti- cercosis. GENERAL CONCLUSIONS In spite of the great differences among the various host-parasite relationships of the cestoda and the different experimental designs that have been used for their study, the following general conclusions and considerations would appear to be appropriate: (1) It is possible to induce a state of resistance to cysticercosis by means of. immunization but, as with other vaccines (157), the degree of resistance varies in the fraction of vaccinated animals that develop it, and varies in the number of parasites that become established after challenge. (2) That the induced resistance is at least partly due to the presence of antibodies has been clearly established by means of successful natural and experimental passive transfers of resistance with immune serum, colostrum, and immunoglobulins. (3) That only certain stages of the parasite-em- bryos and early larvae-are susceptible to the action of antibodies is indicated by the frequent finding of vaccine-derived live cysticerci in animals resistant to a challenge with eggs, and by in vitro experiments showing a progressive loss of susceptibility to the action of antibodies by larvae during their develop- ment. One study indicates that antibodies require complement to exert their toxic action on the parasite. (4) That man can establish an immune response to a natural infection with Taenia eggs is demon- strated by the presence of anticysticercus antibodies in a considerable fraction of the patients with confirmed cysticercosis, even though the response is heterogeneous with respect to both the antigens recognized and its magnitude. As the antibodies are mediators of immunity in experimental animals, it is reasonable to propose that in man too the host- parasite relationship is affected by immunological factors. Thus, it is reasonable to be optimistic about the chances of preventing human cysticercosis by immunization in persons with a high risk of contract- ing the disease. (5) The antigens characteristic of each develop- mental stage of the parasite have still not been identified, and their relation to immunity has still not been established. Similarly, much remains to be done in the study of serotypes within and between the species of Taenia that affect man and animals. (6) All common immunological methods are clearly useful in the diagnosis of cysticercosis, the choice depending more on cost, ease of execution, and the purpose of the study than on sensitivity. (7) The overall low efficiency of establishment (about 1%) of Taenia eggs is perhaps of interest when considering the main mechanism by which man contracts cysticercosis. Massive egg-distribution mechanisms (air, water, handling) are probably of little importance, whereas more intimate transmis- sion phenomena that ensure the ingestion of many eggs (accidental coprophagy by children of contami- nated faeces, coprophagy due to massive contamina- tion of food) are more likely, as Gemmell's experi- mental work with T. hydatigena suggests (158, 159). The objectives for future immunological research in cysticercosis are clear: (1) From the purely immunological point of view, the nature and mechanism of immune damage to the parasite, definition of the antigens and antibodies responsible, the part played by complement, confir- mation that only embryos and early larvae are susceptible to the action of antibodies, and the presence of blocking antibodies or other mechan- isms for eluding the immune response by the estab- lished cysticerci, appear to be the most important aspects of this host-parasite relationship requiring further study. However, the insensitivity of larvae to antibodies, and the mechanism and limits of this insensitivity, appear to be most important for an understanding of the host-parasite relationship, especially in relation to the treatment of persons afflicted with the disease. (2) Of medical interest are: the nature of the immunoglobulins synthesized by man, the evalua- tion of the risk of man being parasitized by other species of cysticerci, the development of a vaccine for man, the elucidation of the main mechanism of transmission, and the development of a radio- immunoassay to aid in diagnosis. (3) With respect to cysticercosis in animals for human consumption, there is an urgent need for careful geographical studies followed by treatment of the infected persons and the killing of infected animals in the areas involved. Also, the development of a vaccine on an industrial scale and the standardi- zation of a method of diagnosing cysticercosis before slaughter would probably reduce the prevalence of infection in both man and animals, and reduce the economic impact of the disease on the food industry. 849 850 A. FLISSER ET AL. ACKNOWLEDGEMENTS We wish to acknowledge the fine work done by Mrs Consuelo Cappello, Miss Esperanza Ruiz, Mr Jose Aviles, and Mr Salvador Barbosa in collecting the literature and to thank them for their help in preparing the material used in this article. RtSUMt REVUE DE LA LITItRATURE RELATIVE A L'IMMUNOLOGIE DE LA CYSTICERCOSE HUMAINE ET ANIMALE Cette revue tres complete de la litterature consacree a l'immunologie de la cysticercose, a ete realisee en vue de la recherche d'une solution immunologique a la pr6vention de la cysticercose humaine. De nombreuses etudes ont ete effectuees sur la souris, le rat, le lapin, le mouton et les bovins afin d'etudier l'effet de diverses techniques de vaccination contre les cysticerques de ces differents ani- maux. La plupart des etudes ont fait apparaitre que les animaux vaccines presentaient un moindre risque de con- tracter la cysticercose et developpaient d'ailleurs une affection plus benigne que les temoins; toutefois les resultats ont ete tris heterogenes. Pour evaluer la valeur globale de la vaccination en tant que mesure de protection contre la cysticercose, nous avons ramene toutes les donnees disponibles a une seule variable, a savoir l'aptitude des cysticerques a s'etablir chez l'animal (nombre de cysti- cerques trouves a l'autopsie/nombre d'ceufs administres) et nous en avons etudie les variations chez les animaux immunises et non immunises ainsi que dans differents cas de relations hote/cysticerque. L'analyse de variance a clairement indique que la vaccination protegeait a des degres divers tous les hotes etudies contre leurs cysticer- ques respectifs. On a utilise differents antigenes pour la vaccination: des cysticerques entiers vivants ou tues, des extraits de cysticerques bruts ou purifies, des embryons, des ceufs, tous antigenes qui se sont reveles efficaces. Ce sont les embryons et les aeufs qui ont e utilises le plus frequemment. La protection contre la cysticercose s'effec- tue par le canal d'anticorps comme l'on fait ressortir des etudes de transfert passif utilisant du serum, du colostrum ainsi que des IgG et des IgA purifiees. Toutefois, les anticorps ne sont efficaces in vivo ou in vitro que contre les jeunes larves; les cysticerques parvenus a maturite sont considerablement plus resistants. Techniquement parlant, la plupart des methodes classiques de titrage des anticorps permettent le diagnostic immunologique de la cysticercose, le choix de la methode la plus appropriee dependant davantage de l'objet du titrage (medecine communautaire ou hospitaliere, recherche) que de sa sensibilite. 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