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30,530 specimens of freshwater molluscs belonging to eleven genera and eighteen species including gastropods and bivalves, have been examined for infection with larval trematodes. Thirty species of cercariae have been found. Of these two are described as 'Monostome', three 'Gymnocephalous', one 'Pleurolophocercous', eleven 'Furcocercous', one 'Microcercous', one 'Cystocercous', five 'Echinostome', and six Xiphidiocercaria'. These specimens were collected on different dates from 21 different places in West Yorkshire. Eighteen of these cercariae are considered to be new to science, one is described for the first time in Great Britain and the remaining eleven have already been described in this country by previous authors. A short review of each group of cercariae found during this study is presented. The life cycles of eight species have been completed under laboratory conditions. The species involved are: Echinoparyphium recurvatum (Linstow, 1873) Hypoderaeum conoideum (Bloch, 1782) Notocotylus imbricatus (Looss, 1893) Szidat, 1935 Notocotylus attenuatus (Rudolphi, 1809) Plagiorchis farnleyensis n. sp. Plagiorchis kirkstallensis n, sp. Sphaeridiotrema wintersettensis n. sp. Dolichosaccus rastellus (Olsson, 1876) The life cycle of the latter species is the first record from Great Britain.

Chapter 2 Natural concurrent infections of rats with H. diminuta and M. moniliformis may increase the chances of individual insects being infected by both parasites simultaneously. Although previous work has shown H. diminuta and M. moniliformis can co-exist in the small intestine of the rat, it was considered important to determine the length of time the two species would be simultaneously patent. This time period would occur during maximum rate of egg production by both parasite species, determined from numbers of eggs found the rat's faeces. It was concluded that although eggs of both species were found in the faeces for proximity 11 weeks post-infection, the chances of an insect acquiring an infection would be highest at the time of peak egg production. This was approximately 6 weeks in the mid-patent period of M. moniliformis. Chapter 3 The initial process leading to an infection of the intermediate host by H. diminuta and M. moniliformis were examined; i.e. egg ingestion, hatching, passage along the insect gut and penetration of the midgut wall. Adaptations by the larvae of both species of parasite to infect the intermediate host were shown to influence host-specificity. H. diminuta and M. moniliformis hatched in the guts of a wide variety of insect species, but only M. moniliformis acanthors penetrated the gut wall of P. americana. H. diminuta oncospheres only penetrated the gut walls of the locusts, Schistocerca gregaria and their natural hosts, the flour beetles Tribolium confusum and Tenebrio molitor. Transit time for food material passing along the gut was found to be important in the synchronisation of parasite hatching with arrival at the site of gut penetration. P. americana holds food items and parasites in the foregut (crop) until partially digested before allowing them into the midgut. Thus any H. diminuta oncospheres stimulated to hatch by the insect's mouthparts only progressed as far as the crop until initial digestion was completed; the time taken for crop contents to be passed in to the midgut tended to exceed the time larvae remained active and capable of gut penetration. In contrast, M. moniliformis acanthors hatch over a longer time period and therefore enter the midgut in a state capable of gut penetration. The abilities of the two species of parasite to tunnel through the midguts of different insect species was compared in vitro using a qualitative assay technique. H. diminuta oncospheres were unable to penetrate the tissues of P. americana midgut. Chapter 4 Eggs from both species of parasite were fed simultaneously to insects. It was initially proposed that oral infections of cockroaches with M.moniliformis might facilitate penetration of the cockroach gut by H. diminuta oncospheres, if the gut tissues were sufficiently disrupted by the former parasite. However, only locusts could be simultaneously infected with both species orally, infected, and viable H. diminuta oncospheres fed to cockroaches were found to adversely affect the success of a simultaneously offered dose of M. moniliformis eggs (acanthors). A hypothesis was put forward to explain this result; that H. diminuta oncospheres perturbed the midgut tissues in their unsuccessful attempt to burrow through the gut wall, thus initiating a wound-healing response by the hosts immune system. This resulted in gut-penetrant M. moniliformis larvae being killed by a melanotic encapsulation reaction. Unfortunately, light and electron microscopy has revealed little evidence of such a wounding in the gut which might have initiated such an immune response. Chapter 5 H. diminuta oncospheres were injected directly into the cockroach haemocoele, as it has been previously shown that a small number of parasites survive. By repeatedly passaging the few surviving cysticercoids from each infection through the rat/cockroach system it was hoped to raise a cockroach-infective strain of H. diminuta. However, their infectivity to cockroaches did not increase in successive generations; several explanations for the possible failure of this selection programme have been put forward. Intrahaemocoelic injections of pre-hatched H. diminuta oncospheres or M. moniliformis acanthellae into the host made it possible to by-pass the gut and thus investigate concurrent haemocoelic infections of cockroaches with both species of parasite. When H. diminuta was injected into M. moniliformis-infected cockroaches, prevalence and intensity of the former were significantly elevated compared to naive controls, indicating that a putative immunosuppressive action from M. moniliformis facilitated H. diminuta development. In some instances, H. diminuta was found to have burrowed through the envelope surrounding M. moniliformis and continued normal development within, unmolested by the host's haemocytes. This was considered as further evidence for the protective nature of the acanthocephalan envelope. Chapter 6 In Chapter 6, assays for aspects of haemocyte behaviour was performed on insects (in particular, P. americana) experimentally infected with either M. moniliformis or H. diminuta. It was found that the phenomenon seen in Chapter 4, whereby H. diminuta adversely affected the success of M. moniliformis when fed simultaneously to cockroaches, appears to be a direct consequence of the stimulatory effect of H. diminuta on the immune system. Conversely, the developing larvae of M. moniliformis were shown to depress haemocytic activity; possibly explaining why elevated numbers of injected H. diminuta survive in M. moniliformis-infected cockroaches. To investigate the affects of immune stimulation on the survival of parasites, locusts were injected with Zymosan, a derivative of yeast cell walls containing 1,3-glucans. H. diminuta oncospheres, injected into Zymosan-stimulated locusts appeared to be partially encapsulated, resulting in a temporary arrest in their development when compared to controls. An assay was devised to observe the encapsulation of materials in vitro by the haemocytes of P. americana. This method was used to show differences in the haemocytic encapsulation reaction to the different larval stages of M. moniliformis and H. diminuta. Both the gut-penetrant stages of each parasite (i.e. oncospheres and acanthors) were encapsulated, whereas the haemocoelic stages (i.e. cysticercoids and acanthellae to cystacanths) remained free of haemocytes. Chapter 7 Finally, in Chapter 7, a model for the alternative pathways leading to success or failure of parasitism by H. diminuta and M. moniliformis in the insect host has been discussed. The results presented here contribute to the fuller understanding of how immune stimulation and immunosuppression affect the survival of helminth parasites, particularly in the cockroach host. These two phenomena have also been shown to be effected by the parasites themselves.

Attempts were made to vaccinate mice against infection with Hymenolepis diminuta, a tapeworm rejected from the se hosts by an immunologically- mediated mechanism. Various putative antigenic preparations from the strobilated worm were tested. Marginal protection, as compared with that induced by an oral infection from cysticercoids, was obtained when mice were orally vaccinated with multiple doses of whole worm antigens. Marginal protection was also obtained when tegument antigens were given by this route. Results from two other experiments with tegument antigens, however, did not confirm this finding. No protection was obtained following vaccination with antigens ,from sonic disruption of somatic cells, exoantigens, saline extracts or egg antigens. On the basis of evidence from other experimental models, it is proposed that this failure to evoke strong protective immunity by vaccination with killed worm antigens was possibly due to one or more of the following factor s: (i) the tapeworm protective antigens were not pre sent initially in most of the preparations injected or that they were pre sent in too Iowa quantity to stimulate immunity (ii) the worm protective antigens were highly labile and they were destroyed during preparation, probably by enzymes released by the disintegrating worms themselves, or inactivated by chemical reactions in the stomach or intestine when the se preparations were given orally or intraduodenally. (iii) . -the physicochemical characteristic s of the se protective antigens had been altered during preparation (iv) the presence of a wide array of worm antigens, e. g. in a homogenate, dissipated the host immune response and masked the presence of the protective antigens possibly as a result of antigenic competition (v) the route of antigen administration was the crucial factor that militated against the induction of functional immunity rather than the antigens themselves (vi) the duration of antigenic stimulation was not long enough and a longer period. analogous to that required for the development of immunity from an enteric H. diminuta infection, was needed (vii) the regimes of vaccination described were conducive to the induction of tolerance. To elucidate the validity of these assumptions, experiments were carried out with live worm antigens. The rationale behind each approach is described separately in the text. Strobilate a-day-old worms. apparently capable of surviving for appreciable periods of time when implanted subcutaneously or intraperitoneally, did not immunize mice against challenge. Implantation of a strobilate 8-day- day old worm surgically into the duodenum conferred only weak protection. The in vivo process of excystation, which is by passed when immunization is performed by implantation of worms directly into the duodenum, had no influence on the ability of the worm to stimulate immunity. The se findings suggest that the failure to induce strong immunity by parenteral implantation of a live a-day-old was possibly due to -the fact that the 8-day- old worm is, in itself, inefficient in inducing a pronounced protective response against challenge, even when presented enter ally • The fact that a weak protective response was induced by the intraduodenal administration of the 8-day-old worm and not by parenteral implantation of the se worm s suggests that the enteric route is more efficient in the induction of functional immunity against the tapeworm than either the subcutaneous or intraperitoneal routes. In other experiments it was established that the young worm is more efficient in the stimulation of protective immunity against challenge than older worms. It is of interest that the older worms, when implanted surgically into the duodenum, pre sent the host with significantly larger amounts of strobili antigens per unit time than do the younger ones. This observation casts doubt on the significance of the strobila as the major source of H. diminuta protective antigens . The logical explanation for the failure to immunize mice by vaccination with the killed worm antigens is that this is possibly because the antigens used were derived mainly from strobilar tis sue obtained from worms even older than the 8-day-old parasites whose poor immunizing potential was demonstrated. Live excysted worms, which provide only scolex and neck antigens J were capable of inducing a protective response when administered intraperitoneally. Irradiated worms, incapable of growing strobilae, were as immunogenic as worms of the same age which were not irradiated. Immunization of mice with an irradiated vaccine is advantageous in the sense that the immunizing infection can be denoted as self-limiting resulting in the prevention of propagation of the parasite to the intermediate host, at a time when specific protective immunity is raised in the definitive host. Two independent investigations were undertaken to locate the origin of H. diminuta protective antigens. The technique s of chemical abbreviation of immunizing infections and irradiation were used for this purpose. The re suits provided evidence that the induction of functional immunity against H. diminuta in mice is independent of the presence of a strobila: is determined by the duration of an antig e nic stimulus deriving from the scolex and/or neck regions. The degree of this immunity is also determined by the number of worms in the immunizing infection. The conclusions drawn from the present investigation as to the origin of H. diminuta protective antigens and the immunogenic potential of irradiated worms are at variance with those reached by other investigators. In the second part of this thesis the mechanism of the immunologically based rejection of Raillietina cesticillus from chickens was investigated. Chickens whose ability to produce antibodies was abrogated by bur sectomy and irradiation developed protective immunity against the tapeworm as did the controls with specific anti-worm antibodies in their sera. It is suggested that antibody is not the crucial component of the mechanism affecting the growth and development of R. cesticillus in the immune chicken.

That some breeds of sheep and individuals within these breeds thrive better than others in parasite-infested localities has probably been recognised by stockmen for centuries, but it is only within the past 50 years, and largely as a result of field surveys that the existence of genetically-determined differences in host resistance to parasitic infections has become widely recognised. As yet remarkably little is known about the genotypes responsible for resistance or susceptibility and even less of the mechanisms involved. Within the past decade several reports have suggested that resistance to some parasites is associated with the animal's haemoglobin type, sheep with HbA being more resistant as judged by faecal egg counts and venous haematocrits than those with HbB. Since the frequency of these haemoglobin types varies in different breeds it has been tacitly assumed that inter-breed variations in resistance are related to differences in the relative haemoglobin type gene frequencies, but none of these studies has indicated whether such resistance is expressed primarily as a resistance to parasite establishment or resistance to the parasites' specific pathogenic effects. In view of its potential practical significance it was considered worthwhile to examine the concept of breed and haemoglobin type resistances to parasitic infections and their possible relationship to the immunological status of the host under carefully controlled experimental conditions. The work represented in this thesis is essentially a study of the role of genetic factors in resistance to two important gastrointestinal helminth parasites of sheep, i.e. Haemonchus contortus and Ostertagia circumcincta. The first section deals with the inter-relationship between haemoglobin type and breed and the response of sheep to primary infections with H. contortus. For this purpose worm-free Scottish Blackface and Finn Dorset sheep with different haemoglobin types were each infected with 350 H. contortus larvae/kg bodyweight and their responses monitored and compared by a combination of clinical, radioisotopic and parasitological techniques. The results obtained showed that sheep with HbA developed less severe clinical and pathophysiological disturbances, passed fewer eggs and harboured fewer worms at necropsy than animals with HbB and that Scottish Blackface sheep exhibited similar advantages over Finn Dorsets with the same haemoglobin type. Since variations in the severity of the disease as judged by pathophysiological effects correlated closely with worm numbers it was concluded (a) that genetic resistance operates at the level of parasite establishment which in turn is controlled by the immune response elicited, (b) that although HbA is a useful genetic marker for resistance, the degree of protection with which it is associated is very much influenced by other, and as yet undefined "breed" characteristics, and (c) on the basis of a second experiment demonstrating that sheep of each haemoglobin type were equally susceptible to the establishment and pathogenic effects of H. contortus when heavily infected (1400 larvae/kg) , it would appear that the magnitude of the larval intake is an additional factor involved. The second and third sections are devoted to an examination of the influence of breed and haemoglobin type on acquired resistance to H. contortus, the former dealing with the acquisition of resistance from primary infections terminated by anthelmintic treatment, the latter with the well-known "self-cure" phenomenon. The results of the third section demonstrated that individuals and breeds with high resistance to primary infections, i.e. animals with HbA and belonging to the Scottish Blackface breed are also more resistant to reinfection than for example HbB and Finn Dorset sheep. The final section examines the genetic control of antibody production to non-parasitic antigens. No difference was observed between HbA and HbB type sheep, or between Scottish Blackfaces and Finn Dorsets in respect of immune elimination of horse gamma globulin, but on tlie basis of a significantly better response to human serum albumin, and suggestive evidence of a better response to rabbit red cells in HbA than in HbB Scottish Blackface sheep, it seemed reasonable to conclude that the advantages exhibited by the former in relation to parasitic infections were associated with a superior immunological competence.

Certain aspects of the relationship between small mammals and their epifaunistic communities were investigated in a habitat supporting three rodent species. Preliminary studies indicated that, in an area of rough grass and bramble shrub, a community of rodents existed harbouring a wide range of flea ectoparasite species, as well as the apparently rare beetle Leptinus testaceus Muller. Techniques were developed, and are described, for the accurate assessment of the level of epifaunistic infestation of rodents. The infestation of various host species, sex and age categories were compared during the different phases of the host's annual population cycle. The rates of re-infestation of hosts and the levels of infestation with fleas with respect to home range size and migratory/ sedentary host categories were also investigated. The underlying cause of certain observed similarities in the level of infestation of some host categories was studied by the use of mark and recapture of fleas. The possibilities offered by this method of study of ectoparasites and their hosts are discussed. In order to contrast the situation observed in the flea infestation of small mammals a study of the association between hibernating hedgehogs and their flea ectopara-sites was undertaken. A method was developed for the assessment of the alimentary relations of a flea population with its host and this approach, together with the use of other techniques, indicated certain behavioural modifications which allowed the fleas to successfully parasitise their hibernating hosts.

This study has examined the interaction between the viviparous rnonogenean GyrodactYlus and sticklebacks. Six species of Gyrodactylus were collected from Gasterosteus aculeatus and Pungitius pungitius in Britain. Each was narrowly host specific, and was restricted to either the gills or the body surface of the fish. A difference in attachment mechanism was noted between gill parasites (e.g. G. rarus), in which the hamuli were more important, and skin parasites (e.g. G. gasterostei), which relied upon the marginal hooks .. The observed growth rate of G. gasterostei populations upon Gasterosteus aculeatus was considerably slower than the calculated potential rate. In the later stages of infestation the population declined, possibly as a result of an increase in the probability of the ~. parasites becoming detached. This may have been related to an observed increase in the number of goblet mucus cells in the skin of infected fish. Although detached parasites were able to reinfect other fish, infection more frequently resulted from transmission during host-host contact. The abundance of Gyrodactylus spp. in the river Ver, Herts., England, was found to be limited by the annual life cycle of the hosts, which restricted transmission between adults and fry to a short period in midsummer. A comparison was made with the biology of the related Gyrdicotylus gallieni, from the amphibian Xenonus laevis. This parasite, which has a suctorial attachment mechanism, inhabits the mouth of its host, entering this habitat via the nostrils. The slow population growth, and preponderance of older flukes in the population suggests that this parasite may be adapted for persistence in individual hosts. Q. gallieni has a wider host specificity than GyrodactYlus spp., and has been recorded from five species and sub-species of Xenopus. An oviparous monogenean, closely related to the viviparous genera, has been described from the catfish Farlowella amazonum. The origin and evolution of the gyrodactylids from a form similar to this parasite has been discussed

Certain factors controlling the parasitic phase of some nonspecific fish ectoparasites were investigated. Studies were made of the infection patterns exhibited by the branchiuran Argulus foliaceus (L. ), the fish leeches Piscicola geometra (L. ) and Hemiclepsis marginata (Mull. ) and glochidia larvae of Anodonta cygnea (L. ) on freshwater fish. Aspects of the parasites life history, survival, development, seasonal cycles in incidence, dispersion patterns, immigration and site selection were investigated by field and laboratory studies. In adult A. foli. aceus and P. geometra, the initial attacking a phase onto the fish host does not appear to be influenced by densitydependent factors. In larval A. foliaceus, infection is also unaffected by parasite density, but in adults, it is controlled at high parasite numbers when the carrying capacity of the fish is reached. The infection patterns of A. foliaceus appear to reflect the heterogeneity of both host and parasite populations, particularly with respect to variations in host susceptibility and to parasite searching activity. In adult P. georn tra, the parasitic phase is influenced by density-dependent factors with increasing intraspecific competition at high densities. The population processes controlling these parasites are compared to other types of host/parasite systems.

Three spined sticklebacks infected with Schistocephalus solidus plerocercoids were used for in-vivo experiments to determine the relative distribution of 14C-labelled sugars (D-glucose and 3-0-M D-glucose) in fish and worm tissues. A maximum of 23.6% labelled material from D-glucose was found in plerocercoids after 4.5 hours and a maximum of 2.8% labelled material from 3-0-M glucose after 24 hours. Comparison of distribution levels between the two sugars and between infected and uninfected fish tissues did not show significant differences except in a few cases. In-vitro studies with plerocercoids demonstrate D-glucose uptake to be sodium dependent, saturable with an apparent Kt of O.66mM, inversely related to dry weight and absorbed against a concentration gradient. Uptake is also affected by temperature with high rates at 200 and 400C and lower rates at 100 and 30°C. Uptake is inhibited (non-competitively) by phlorizin and to a lesser extent by ouabain and DNP. The nature of inhibition caused by the latter two compounds was not determined. THe effect of 3-O-M glucose on D-glucose uptake was inconclusive. Similarly the effect of D-galactose was inconclusive and also inconsistent,causing both stimulation and inhibition of D-glucose uptake. A gradient of glucose absorption is present along the length of the plerocercoid; the lowest rate being found at the anterior section and the highest at the posterior end. Plerocercoids incubated under aerobic conditions in 14C-D-glucose produced insignificant amounts of labelled CO2. Under aerobic conditions lactate is produced by plerocercoids at the rate of O.13mg/ml/hour but no detectable amounts of pyruvate were found under similar conditions. Glycogen extracts from plerocercoids incubated in-vivo contain increasing levels of 14C-labelled material with increased incubation periods. Host metabolism is significantly affected by the presence of plerocercoids. Lower levels of glycogen are found in the liver tissue of infected fish compared to uninfected fish. Also the rate of D-glucose uptake by fish stomach and intestinal tissues is considerably reduced in infected fish.

The acanthocephalan, Acanthocephalus clavula Aujardin, 1845 is an inteatinal. _parasite of freshwater fir-he The iropod, Acellus neridianus Racovitza , is, an, intermediate host. The details of growth and development of A. clavula in A. ceridianue and Cottus robio (L. ) were described from laboratory experimental infection3... The complete life cycle required at least 132 days at 8-13°C; development, in intermediate host took 84 days, and 48, days were required for copulation and maturation in the, definitive, host. Temperature had direct effects on the. growth and development of, _the,. larval-parasite, by increasing the rate of development at, high, temperature (5-19°C). The growth and development of larval parasites in the intermediate host. was affected by crowding. Uniform growth and development were disturbed, if more than 3 ppranites were established in the same isopod.. ;> wz The occurrence of A. clavula in gwyniad, roach, bullhead, eel was affected by a combination of one or more factors of the following: seasonal: iluetuationa, host sex, host length, hont age, feeding habits. The seasonal cycle of A. clavula wan clearly shown in gwyniad, androach4n Bala Lake. The parasites started to establish in late dummer and reached a peak in, Nay and June_in gwyniad and roach respectively, and declined thereafter. until the beginning of Coptember. The incidence of infection in, gwyniad was the same in the aale and female fish for. the, whole year. When the gwyniad data were summed into four 3-monthly periods, the incidence in, male gwyniad had aktendency to be lower than in femalen only during April-June. In roach, the incidence of infection was the same in males and females during April-June, and July-September, and the incidence in male fish was lower than females during October-December, and January-March. The incidence of infection in gwyniad increased with length and age of the fish over the whole year, but when the affect of seasonal fluctuation was minimized, the incidence of infection increased with length of fish. The intensity of infection in male gwyniad was lower :. hsr females from January to October, but much higher in November and December. In roach, the intensity of infection was always lower than 1. A. clavula was able to survive in all regions of the fish intestine, but it was seen that A. clavula established in a particular region of the intestine which was dependent on each host species. In gwyniad, the parasites were established 0-403 from anterior of the intestine; but in eel, the parasite was established 40-100% from anterior of the intestine. The experimental studies showed that superimposed infections occurred in both intermediate and definitive hosts. Overloaded superimposed infections may lead to the discharge of some worms. Host starvation did not cause any apparent discharge of worms from the fish. The shelled acanthors of A. clavula did not hatch in vitro using either wet and dry or applied pressure methods.The in survival times. of, ohelled acanthors and adults of A. clavula, were determined, but the results proved difficult to interpret.

The major part of this work was concerned with the mechanism of protective immunity which acts against the parenteral phase of a Trichinella spiralis infection in mice. Passively transferred immune serum always gave strong protection, to recipients, against an intravenous challenge of newborn larvae, regardless of whether the donor animals were sensitized by single, multiple, full or parenteral infections of T. spiralis. Small doses of immune serum gave significant protection, against challenge, to mice. Sensitized spleen cells and mesenteric lymph node cells, taken from infected donors, varied in their ability to protect recipient mice against challenge. Immune serum was only effective when transferred before a parenteral challenge, suggesting that it affected the newborn larvae and not the muscle larvae. The protective factors in immune serum appeared seventeen days after a single parenteral infection was given; they became progressively more protective after this time and there was evidence of their presence in serum taken five months after infection. The serum factors were protective in the absence of effector T-cells but whether they acted alone, against newborn larvae, or required the cooperation of non-specific cells was not fully established. Genetical studies on the ability of CBA and NIH mice to expel intestinal T. spiralis were made. CBA mice responded poorly to infection whereas NIH mice responded well. Cross breeding between the strains and back crosses with the first generation progeny of these showed that the responder characteristic of NIH mice was dominant and its inheritance was polygeneic. The ability to expel adult worms was not linked to the genes coding for the colour or sex of the mice.

An animal Infected with schistosome worms is stimulated by antigen associated with the living, adult parasite to mount an Immunological reaction against the surface of invading schlstosomula. The adult worms, however, remain unaffected by the Immune response of the host and the. young schistosomula develop resistance to the host response during the 5 days in which they migrate from the skin to the lungs. The aim of the experiments described In this thesis was to compare the biochemical composition and antigenicity of the surface of the parasite at different stages of development from the cercarla to the adult, in order to understand more about how resistance to the host is acquired and maintained. The initial step was to establish a method for producing sufficient quantities of schlstosomula which were uniformly developed and uncontaminated by host material. Comparative studies were made of several types of artificially transformed schlstosomula and the prototype organisms, namely schistososaila resulting from cercarial penetration of isolated skin (SS). An organism (MS) produced by mechanically separating cercarial tails from bodies with subsequent incubation of the bodies in a defined medium was shown to undergo surface changes similar to those observed during cercarlae-SS transformation. Surface microvilli were produced in the 1st hour, the glycocalyx was lost within 2 hours, there was a change from a trilaminate to a heptalamlnata surface membrane by 2-3 hours and the inclusions sesn in the tegument were similar to those previously described in the SS tegument. Only one morphological difference was noted between MS and SS: the pre-acetabular glands of MS retain their contents for up to 48-72 hours while SS secrete the granular contents within 3 hours of skin penetration. MS were therefore used as the reference organisms In all further studies. Comparisons were made with cercarlae, the in vivo stages of in vitro schlstosomula (CS) and adult worms. Immunological techniques and reagents, i.e. immunofluorescence,125 I-Protein A binding, and radio-labelled myeloma proteins and antiglobulin were adapted to the study of living schistosomes. The use of triple-layer antibody techniques and the use of radioimmunoassays are considered to be significant Improvements over previous studies. A combined study of antibody class and subclass development and the binding capacity of the surfaces of the various organisms revealed differences both in the host response of different mice strains and in the quantity and distribution of antibody bound by the various schistosome stages. With identical cercarlal infections, Parkes mice produced da tec table levels of schistosome surface specific antibody by 10 days while antibody was not detected until 2-3 weeks after infection in CHA mice. IgM levels were shown to appear late and remained high throughout the course of the infection. IgG1 IgG2a, IgG2b, IgG3 and IgD levels were only slightly Increased while IgA levels were moderately Increased over that observed with normal mouse sera. Furthermore, the various schistosome stages differed in the binding of antlbody: cercariae, MS and SS bound significantly more antibody than did the later stages, namely, CS, LS and the adult schlatosomea. Only one morphological difference was noted between MS and SS: the pre-acetabular glands of MS retain their contents for up to 48-72 hours while SS secrete the granular contents within 3 hours of skin penetration. MS were therefore used as the reference organisms in all further studies. Comparisons were made with cercarlae, the In vivo stages of schistosomula collected from the skin (SS) and from the lungs (LS), the in vitro schlstosomula (CS) and adult worms. Immunological techniques and reagents, i.e. immunofluorescence, 125 I-Proteln A binding, and radio-labelled myeloma proteins and antiglobulin were adapted to the study of living schistosomes. The use of radioimmunoassays are considered to be significant Improvements over previous studies. A combined study of antibody class and subclass development, and the binding capacity of the surfaces of the various organisms revealed differences both In the host response of different mice strains and In the quantity and distribution of antibody bound by the various schistosome stages. With identical cercarlal infections, Parkes mice produced de tec table levels of schistosome surface specific antibody by 10 days while antibody was not detected until 2-3 weeks after Infection In CBA mice. IgM levels were shown to appear late and remained high throughout the course of Ota Infection. IgCj, IgG2a, IgC2b, IgG3 and IgD levels were only slightly Increased while IgA levels were moderately Increased over that observed with normal mouse sera. Furthermore, the various schistosome stages differed In the binding of antibody:cercarlae, MS and SS bound significantly store antibody than did the later stages, namely, CS, LS and the adult schistosomes. A phenomena observed during the inmunofluorescence studies was the sloughing of the fluorescent surface coat of MS as the organism moved across the microscopic field. Sloughing could be prevented at A°C; electron microscopy revealed no damage to the tegument, but microvilli were observed In the sloughing surface coat. Attempts to restaln "sloughed organisms" with Immune serum were unsuccessful. Sloughing may represent a mechanism for eliminating host antibodies but further studies are necessary to prove this. The use of radioimmunoassays revealed the adsorption of host Immunoglobulins onto the surface of all the schlstosomula stages. The use of lodlnated myeloma proteins and F(ab)2 fragments indicated that this adsorption was non-specific and did not require the presence of an Fc receptor. Schistosome synthesis of a mouse a2- macroglobulin-like determinant was confirmed using the artificially prepared MS. However,adsorption was also observed In studies using 125 I-Mo a2-macroglobulin. Correlations between parasite antigens and the presence of host antigens on the schistosome surfaces have been observed. The expression of human blood group-llke antigens and mouse erythrocytes antigens was studied by mixed agglutination, Immunofluorescence and immunoradloassay. With all schistosome stages the binding of specific antibody is relative to the amount of host material detected on the surface. A variety of biochemical techniques were used to compare the tegumental surfaces of cercarlae, schlstosomula and adults. These techniques Included lactoperoxldasa lodlnatlon and galactose oxldase- tritlated borohydrlde labelling of surface proteins and glycoproteins, followed by detergent solubilization and separation of components by polyacrylamide gel electrophoresis. Similarities and differences in the total proteins and surface components of the various schistosome stages were noted. The number of proteins obtained by 1° SDS-PAGE was between 45-55, which was considered minimal. The majority of these proteins were common to all stages, two of these proteins migrated at rates similar to actln and myosin. The actln-llke component was labelled by the lactoperoxldase. technique, Indicating that it is exposed to the external surface. The proteins unique to a particular developmental stage included a 160,000-180,000 molecular weight component of cercarlae, three components (58,000-60,000 daltons, 23,000 daltons and 11,000 daltons in adult schistosomes and a protein of about 29,000 daltons in adult females. Carearlas had 8 surface components, which were iodinated by the lactoperoxldase technique, 3 of these components were also labelled by the galactose oxidase- 3H borohydrlde procedure. MS and SS shared 8 lactoperoxldase labelled surface components, 3 of these were similar to cercarlal surface components. Two surface components were also labelled by the galactose oxidase- 3H borohydrlda procedure. LS had 7 components which wars iodinated by the lactoperoxldase procedure; 3 components are also labelled by tritium. CS possess 9 lactoperoxldase labelled components, three of which were tritiated. Adult schistosomes had 8 iodinated surface components, 3 of which were labelled by tritium. It Is concluded that the schistosomes maintain a complex Interaction of acquisition, elimination and mimicry of host antigenic material and thus evade the action of the host Immune response. It is suggested that the development of these evasive mechanisms occurs during the period between skin penetration and migration through the lungs.

Trypanosomiasis caused by Trypanosoma cruzi and transmitted by triatomine bugs is widespread in South and Central America, human Infections, chronic or acute, being known as Chagas' disease. Since blood forms of the trypanosome in man and animals are commonly too few to be found in blood films, conclusive evidence of infection is still by xenodiagnosis. This requires vector bugs to be engorged on a suspected infected vertebrate and the bug faeces examined a month or so later by which time the trypanosomes should have greatly multiplied in the insect gut. It is known that there is variation between triatomine species and between individuals within species in ability to develop T. cruzi infections. The present research investigated the genetic basis of this variable susceptibility in one species, Rhodnius prolixus, by selection for susceptible and refractory bug populations and revealed evidence of polygenic control of susceptibility to infection with T. cruzi; this contrasts with the major gene mechanisms reported for other pathogens in other insect vectors. Male bugs showed significantly higher levels of infection than females and may be more efficient for xenodiagnosis. The radioresistance of triatomine bugs may be related to their chromosome morphology. Male R. prolixus given a sub-sterilising irradiation dose were less sterile than their progeny, this delay resulting from the diffuse structure of the centromeres of triatomine chromosome. These results are discussed in relation 10 possible control of triatomines by genetic manipulation. Investigation of spermatogenesis by autoradiographic techniques male R. prolixus revealed that starvation induced diapause inhibits spermiogenesis. A blood-borne factor produced when the diapausing bug is fed may directly affect the rate of mulosis. The karyotype is described for several triatomine species. A method was developed for distinguishing between individual mitotic chromosomes within the complement of Triatomi infostan., and K. prolixus. the chromosome markers being valuable for further genetic studies of triatomine bugs.