Showing papers in "Parasitology in 1967"

Abstract: After infection with the nematode Trichuris muris 70–75% of mice of the Schofield strain developed an immunity to the parasite and eliminated the worms between the 16th and 19th days after infection. In these mice the acquired immunity persisted for at least 3 months and prevented the establishment of subsequent infections. In 25–30% of the mice immunity was not produced and infections developed into mature worms. These non-resistant mice remained susceptible to further infection. The development and action of the immune response were suppressed completely by the administration of cortisone acetate.

Abstract: Infections are described of Gasterosteus aculeatus in a pond at Leeds with the plerocercoid larvae of Schistocephalus solidus.Very heavy infections were found in the summer of 1962 and early 1963; many fish harboured over 50 worms and the maximum was 130.For each individual host the relative percentage parasite-weight to host-weight has been determined (parasitization index). In more than 50% of the fish the P.I. exceeded 25; the maximum value was 68·5. The relationship between P.I., numbers of worms and weight of worms is described.Some effects of parasitization on the host are described; they include gross distension of the body, reduction of liver weight, reduction of packed cell volume of erythrocytes and delay in oocyte maturation. In some heavily parasitized individuals spawning is apparently inhibited.Concurrent infections with the microsporidian Glugea anomala are described.Our thanks are due to Mr A. Rennie, B.Sc., of the Yorkshire Ouse and Hull River Authority for drawing our attention to the source of infected sticklebacks, to Mr A. O. Holliday for the preparation of photographs and to Mr G. R. Standley for assistance in the preparation of the slide shown in PI. 1, fig. 3. We also wish to thank Dr J. N. Ball, Department of Zoology, University of Sheffield, for his helpful comments on the gonads of infected fish, and Professor J. M. Dodd, Department of Zoology, University of Leeds, for providing facilities and much helpful discussion. One of us (C. A.) gratefully acknowledges the receipt of a Science Research Council Research Studentship and Fellowship.

Abstract: Both histological and histochemical studies have been made of the food, mode of feeding, gut cell structure and digestive sequences in a selected number of digenetic trematodes.The species investigated range from gut-dwelling trematodes feeding on the superficial epithelial tissues and associated mucoid secretions of the host to those forms living within the respiratory and circulatory system and feeding exclusively on blood.The mode of feeding is suctorial, brought about by the muscular pharynx and normal attachment process of the oral sucker. In one case there is evidence that this purely mechanical process is supplemented with enzymic secretions produced by the trematode which have a histolytic effect upon host tissues.On the basis of gut cell structure the species investigated are divided into those with cells and associated microvilli that vary in size and appearance so that the gastrodermal border is irregular in outline, and those with a gastrodermis comprising regular cuboidal or columnar cells bearing microvilli organized into a striated border. There is evidence to suggest that this difference in gut cell structure reflects differences in digestion. In all cases, gland cells are absent from the gastrodermis which is capable of both secretory and absorptive functions.Digestion is predominantly an extracellular process, but the exact sequence varies according to the nature of the food ingested and different degrees of adaptation are shown by the trematodes to the blood-feeding habit. The tissue-feeding species show less modification.I wish to thank Dr J. B. Jennings for helpful discussion and advice throughout the course of this work. The study was undertaken during the tenure of a Studentship from the Department of Scientific and Industrial Research.

Abstract: The oncomiracidium of the monogenean skin parasite Entobdella soleae finds its flatfish host, Solea solea, by chemoreception. The free-swimming larva responds to a specific substance secreted by the skin of the common sole, attaches itself to this skin and immediately sheds its ciliated epidermal cells. Larvae respond in the same way to agar jelly which has been in contact with the skin of S. solea.The oncomiracidia attach to S. solea skin in preference to that of other soleid fishes (Buglossidium luteum and Solea variegata), pleuronectid fishes (Limanda limanda, Pleuronectes platessa) and elasmobranch flatfishes (Raia spp.).Larvae respond strongly to isolated epidermis from Solea solea but show no response to the fish's cornea, indicating that the attractive substance is produced by the mucus cells in the fish's epidermis.The larvae attach with equal readiness to skin from the upper and lower surfaces of S. solea. Thus the preponderance of young parasites on the upper surfaces of soles is due not to a preference for the upper skin but to the fact that the lower skin is in contact with the substratum and cannot be reached by the larvae.These results led to speculations on the way in which host specificity evolved in the Monogenea.I am indebted to Mr J. E. Green of the Plymouth Laboratory for setting up a tank containing infected soles and for maintaining the tank and feeding the fishes for many months.I am also grateful to the Directors and Staff of the Plymouth Laboratory and the Fisheries Laboratory, Lowestoft, for hospitality and assistance. I am particularly grateful to Mr J. Riley of the Lowestoft Laboratory for providing various flatfishes.

Abstract: Some factors controlling differentiation of the protoscoleces of Echinococcus into strobilate or cystic stages, in vitro, have been identified. Development in a strobilate direction requires (a) evagination of the scolex, (b) contact of the evaginated scolex with a suitable nutritive substrate. These conditions are satisfied by treating fresh protoscoleces with pepsin and by evaginating them with trypsin + pancreatin + dog bile and subsequently culturing in diphasic media.The most satisfactory diphasic system so far developed consisted of a base of bovine serum (coagulated by heating) with a liquid phase of Parker 199 medium +20% hydatid fluid and a gas phase of 8% O2+5% CO2 in N2. In this diphasic system, protoscoleces have been cultured to the 3-proglottid strobilate stage with the anlagen of the male and female genitalia formed: the first proglottid appeared after 30 days culture in vitro compared with 16 days in vivo; three proglottids were formed after 60 days. Development in vitro stopped just short of mature egg-producing worms.That an intimate contact of a ‘placental’ nature takes place between the parasite and the solid proteinaceous base is shown by the fact that strobilization never took place in cultures lacking bases or in those in which the serum base was separated from the protoscoleces by a cellulose membrane. Cytological and ultra-structure studies of an evaginated protoscolex revealed that fully developed microtriches were present only in the rostellar region and not in the post-sucker region. It is concluded that it is this anterior region which makes initial contact with the nutritive substrate—a contact which appears to provide the ‘strobilization stimulus’. It is not known if the ‘strobilization stimulus’ is stimulatory (i.e. nervous) or nutritive or both. Contact with a non-nutritive base (i.e. agar) does not induce strobilization.In any one sample of protoscoleces, after a high initial evagination, only about 30–40% finally remained evaginated and continued to show sucker activity. The degree of evagination appears to be characteristic for any sample of protoscoleces and may be related to the source of the hydatid cysts. It may prove to be a genetically determined character.Evaginated protoscoleces became inactive under certain conditions and the degree of activity seems to be related to stimulatory and nutritive factors. The activity of evaginated protoscoleces was used as a criterion for assessing the effectiveness of a culture medium or condition. Evaginated protoscoleces which lost their activity developed small posterior bladders as did the majority of unevaginated protoscoleces. In both cases, posterior bladders enlarged and secreted a PAS-positive layer which formed the laminated membrane of a miniature hydatid cyst.The well-known appearance of vesicular forms in cultures appears to be related to unfavourable culture conditions in vitro, such as a low or high pH or a low or high O2 tension.It is pointed out that since the morphogenesis of Echinococcus protoscoleces can now be controlled to a considerable extent in vitro and since the contents of any one cyst have the same genotype, this organism provides exceptionally interesting material for studies on morphogenesis.Acknowledgement is made to the Australian Wool Board, the National Institutes of Health, U.S. Public Health Services Grant No. AI-04707–03 and the World Health Organizati on for generous financial assistance in support of this work.The skilful technical assistance of Mr A. B. Howkins and Mrs Mary Barton is also acknowledged.

Abstract: Observations on the movement of fluids in the vicinity of the mouthparts of actively feeding female Dermacentor andersoni were made with the aid of trans-illuminated live-hamster pouch host-tissue. Following tick attachment and the invasion of ‘attaching cement’ into the outer skin layers, there appeared a progressive release of host blood as the tick settled into a feeding pattern. Short periods of sucking, each followed by an immediate ejaculation of saliva, became lengthened as feeding progressed, as did the resting intervals between sucking. Other movements brought about by the tick included a ‘toying’ of the tissue fluid, a rapid vibration of the same, and a distinct regurgitation of host-blood material. A sudden haemorrhage usually appeared at the feeding site about 2½ h after attachment of partially fed ticks.Speculations are made concerning the part played by the release of tick saliva and tick water loss, its probable relationship to host paralysis, and its apparent role in tick nourishment.

Abstract: The anatomy and function of the pharyngeal ridges, which occur in the larvae of saprophagous Cyclorrhapha but are generally absent in biontophagous forms, were investigatedThe ridges extend throughout the length of the ventral aspect of the pharynx and lie in an evagination of its floor They enclose channels which narrow and disappear posteriorly but are open anteriorly Each ridge consists of a longitudinal plate projecting dorsally from the floor of the pharynx, the bifurcated upper edge of the plate bears, one on each side, a row of lateral lamellae, each about 0·3 μm thick They are set vertically at regular intervals of just less than 1 μm The lateral lamellae of adjacent ridges are closely apposed and completely roof over the channels between the ridges Anteriorly the upper arms of the ridges are fused togetherThe lamellae form a sieve which retains particles of 0·6 μm and above in size as the relaxation of the pharyngeal dilator muscles allows the roof to descend so forcing liquid downward between the lamellae In Calliphora vicina (Syn C erythrocephala) the sieve concentrates a suspension of 2700 particles of 4–7 μm per mm3 between five and six times The natural food consists largely of bacteriaFungus spores and pollen grains of selected sizes were used to demonstrate the movements of particulate material in the pharynx and to define the upper size limit of the particles ingestedSwallowing is effected by simultaneous relaxation of the anterior dilator and transverse muscles of the pharynx, with contraction of the posterior dilator muscles; the transverse muscles are otherwise maintained in a state of contraction closing the posterior exit from the pharynxStarved third-instar larvae of C vicina can ingest 50% of their body weight in 6 minThe larvae of the Cyclorrhapha which possess pharyngeal ridges are essentially particle feeders The ridges form a sieve by means of which food is concentrated and the amount of non-nutritious liquid ingested is reduced The lack of pharyngeal ridges in larvae feeding on living material is correlated with the constant and high nutritive value of both the solid and the liquid fractions of the foodI should like to thank Dr P Tate for his help and encouragement during the course of this work which was carried out during the tenure of a Postgraduate Studentship awarded by the Agricultural Research Council Thanks are also due to the staff of the Plant Pathology subdepartment of the Botany School, Cambridge, for their help in obtaining fungus spores of suitable sizes

Abstract: The cyst wall of Parorchis acanthus Nicoll, consists of an outer wall of three layers and an inner wall of two layers. The outer wall is hemispherical, complete, attached to the substratum and extends as a flange around the margin. The composition of its three layers differ dorsally and ventrally. Dorsally the layers are: (1) acid mucopolysaccharide and neutral mucopolysaccharide, (2) protein and lipoprotein, (3) acid mucopolysaccharide, neutral mucopolysaccharide and glyco-protein. Ventrally the layers are: (1) acid mucopolysaccharide, protein and lipoprotein, (2) acid mucopolysaccharide. The middle layer in both is secreted by the ventral granular cystogenous glands and the outer and inner layers, dorsally and ventrally, by the dorsal and ventral agranular cystogenous glands respectively. The inner cyst wall is oval in surface view. It consists of two layers, both of neutral mucopolysaccharide, the outer giving a more pronounced reaction. Both are secreted by the dorsal granular cystogenous glands. The ventral wall is strengthened by a secretion from the plug-forming gland cells and consists of neutral mucopolysaccharide. All the layers of the cyst wall show the same histochemical reactions as the glands which secrete them. Studies on the fine structure of the cercaria have revealed mammalian-type synapses in the neuropile of the central nervous system, rind cells around the ganglia, an elaborate arrangement of muscles in the wall of the oral sucker, the flame cell structure and the structure of the granules contributing to the formation of the cyst wall.

Abstract: Meiogymnophallvs minutus (Cobbold, 1859) comb, nov., from Cardium edule L. and Haematopus ostralegus occidentalis Neumann is shown to be synonymous with Lecithodendrium somateriae (non Levinsen, 1881) Jameson, 1902 in partim, Cercaria margaritae Lebour, 1907, Oymnophalloides oedemiae (Jameson & Nicoll, 1913) James, 1964, Cercaria cambrensis Cole, 1938, and Gymnophallus nereicola Rebecq & Prevot, 1962. The intensity of infestation of cockles, with the metacercariae, increases with the size and particularly with the age of this host and varies with the seasonal migrations of the final host. The connexion between the metacercaria and the adult is demonstrated experimentally. Domestic ducklings used as experimental hosts were susceptible to infection when fed with minced cockles but not when fed on baby chick crumbs. We are very grateful to Professor E. W. Knight-Jones for the provision of working facilities and for help with the manuscript and to the Science Research Council for a grant to one of us (E.A.B.) which made the work possible.

Abstract: The structural relationship between cells of the parenchyma and those of the alimentary and excretory systems, and tegument are described.Junctional complexes are formed in areas of close cell apposition. They occur laterally on invaginations of parenchymal processes which are embedded in the epithelial cells of the alimentary and excretory systems, and on flat plaques between parenchymal cells and the tegument.The ultrastructure of the complexes has been resolved and a three-dimensional interpretation is put forward. It is suggested that the complex is a type of desmosome.The findings reported here add support to the suggestion made in a previous paper that the parenchyma may function as a transport system in this fluke.We wish to acknowledge grants from the Wellcome Trust and the S.R.C. for the purchase of an Akashi TRS 50 and an A.E.I. EM 6B electron-microscope respectively. This research and that reported in a previous paper were carried out while one of us (S.S.E.G.) was in receipt of a Training Studentship Grant from the Wellcome Trust.We also wish to thank Mr W. Ferguson and Mr A. Lyness for photographic and technical assistance.

Abstract: Five new species of Eimeria are described, viz. E. ablephari from Ablepharus boutonii (Scincidae), E. egerniae from Egernia whitii (Scincidae), E. gehyrae from Gehyra variegata (Gekkonidae), E. lampropholidus from Lampropholis guichenoti and E. leiolopismatis from Leiolopisma challengeri. The characters of these species and the other coccidia from lizards are tabulated and a host check-list is given. Gall bladder parasitism is discussed as is the significance of complete sporogony occurring within the host's body. Some ideas concerning the possible events occurring in the evolution of the Haemosporina are presented.I wish to thank Miss J. Arnold of the Zoology Department, University of Western Australia, for her identification of many of the lizards and Miss S. Curtis for her drawings. Finally I wish to thank Dr H. M. D. Hoyte, Department of Parasitology, University of Queensland, for his interest and helpful criticism of the manuscript.

Abstract: The germ-cell cycle and method of reproduction in the daughter sporocysts of the bucephalid Cercaria bucephalopsis haimeana (Lacaze-Duthiers, 1854) and the gymnophallid Cercaria dichotoma Lebour, 1911 (non Muller) from the edible cockle, Cardium edule L., have been investigated by the examination of stained sections. A review is given of the principal interpretative theories which attempt to explain the nature of the reproduction and the homology of the germinal sacs of the Digenea. It is suggested that the life-cycle includes a cyclic alternation of homologous generations. Of these, the miracidium/mother-sporocyst and the daughter germinal-sac generations usually reproduce by apomictic parthenogenesis but polyembryony may also occur secondarily in some species.We are very grateful to Dr Gwendolen Rees and Professor E. W. Knight-Jones for considerable help with the manuscript.

Abstract: Three new species of Isospora from Australian lizards are described, viz. Isospora amphiboluri from Amphibolurus barbatus (Agamidae), I. egerniae from Egernia whitii (Scincidae), and I. ablephari from Ablepharus boutonii (Scincidae) and Heteronota bineoi (Gekkonidae). A further Isospora sp. is mentioned from Phyllurus cornutus (Gekkonidae). A table of comparative data of lacertilian Isospora is given, and also a host check-list.I would like to thank Miss J. Arnold of the Department of Zoology, University of Western Australia, for her identification of many of the lizards, and Miss S. Curtis for her drawings. Finally I wish to thank Dr H. M. D. Hoyte, Department of Parasitology, University of Queensland, for his interest, and helpful criticism of the manuscript.

Abstract: The acquired immunity of twenty-eight splenectomized and twenty-three intact cattle following primary inoculation with either of two strains (the Y and the R strain) of Babesia bigemina was investigated. Some of these animals were carriers, while others had ceased to be infected. Observations were made on the response of splenectomized animals challenged up to four times with the original infecting (homologous) strain and finally with the heterologous strain.In the splenectomized animals, a total of sixty-three homologous challenges produced parasitaemias in only twenty-seven instances, and these were generally delayed and of lower intensity than in control animals. Fifteen negative transmission tests performed shortly after thirty-two challenges showed that it was difficult to re-establish the homologous strain in some animals that had achieved self-cure. Substantial homologous immunity had developed within 16 weeks of the primary infection in animals which were no longer carrying B. bigemina.All twenty-one splenectomized animals subjected to heterologous challenge developed well-marked parasitaemias. Nine of these had been highly immune to homologous challenge 2 weeks earlier. Eight of the twenty-one were carriers at the time of challenge and these showed much lower parasitaemias than the self-cured animals which almost invariably were acutely affected.Challenges of twenty-three intact steers confirmed that the two strains differed immunologically. Six of twelve challenged homologously failed to show detectable parasitaemia, whereas all eleven challenged heterologously developed parasitaemias.

Abstract: The egg shell of Fasciola was investigated by electron microscopy and found to consist of a fine reticulum of fibrils. No evidence for concentric lamellae was discovered. The amino acids of the shell were identified using chromatography and it was suggested that the original protein was of a simple fibrous type. An unidentified spot was thought to represent a hydrolysis product of the cross-linkage. The effect of sodium hypochlorite on the shell, was also studied. The ‘vitelline membrane’ of the fully developed egg was found to consist of a layer of peri-vitelline material beneath which were two closely opposed unit membranes. The origin of these layers was discussed. The permeability of the shell and vitelline membrane was studied using simple physical techniques and radiotracers. The shell was found to be freely permeable to small molecules. The vitelline membrane complex was thought to represent the main barrier to permeability but an uptake of phosphate ions amounting to 8% of total phosphate, was recorded. This may have been more apparent than real as most of the activity was probably associated with egg shells rather than contents. I would like to thank Professor O. W. Richards in whose department this work was carried out, Dr F. Call for supervision of the work and Professor B. G. Peters for his many helpful criticisms. Lastly, I am indebted to Miss M. Morris without whose expert assistance the electron microscope studies would not have been possible.

Abstract: Rats thymectomized at birth were infected with Nippostrongylus brasiliensis at 6 weeks of age. In some, but not all, rats egg production was greatly prolonged after the primary infection. Thymectomized rats were less resistant than controls to second infections. In rats infected at birth the development of resistance was significantly delayed. Neonatally infected rats, when they were 6 weeks old, were less resistant to reinfection than adult controls. Rats infected three times in the neonatal period were more resistant to challenge at 6 weeks of age than rats infected once at birth, but less resistant than adult controls. The degree of immunity induced by a single neonatal infection was variable; some rats showed little or no resistance to reinfection 6 weeks later. Antibody formation, measured by the occurrence of reagins, was altered in the same way by both neonatal thymectomy and neonatal infection. In the thymectomized (immunologically deficient) and newborn (immunologically immature) rats, reagin formation following initial infection was delayed; there was a reduced anamnestic response after challenge; mean titres were lower and more variable than in control animals; and in some rats circulating reagins were never detected. In thymectomized rats the capacity to demonstrate anaphylaxis was as variable as in control rats, but in the few thymectomized rats in which egg production was prolonged, anaphylaxis was slight at a time when most intact rats had developed the capacity to show anaphylaxis. Rats infected at birth and adult rats infected at the same time were equally susceptible to systemic anaphylaxis 6 weeks later, although by this time there was a marked difference in the degree of resistance to the parasite developed by the two groups. In adult rats splenectomy before, during or after initial infection had no effect either on the rate of development, or the degree of immunity induced by primary or secondary infections, or on antibody formation. We would like to thank Miss W. Griffiths and A. J. Edwards for their willing technical assistance. This work was done whilst one of us (B.M.O.) was the holder of a Wellcome Fellowship of the Animal Health Trust.

Abstract: A survey was made of the helminth fauna of 657 herring gulls (474 ‘adults’ and 183 chicks) in northern Caernarvonshire and Anglesey during the period July 1962 to November 1964. A total of 31 species of helminths were identified (10 species of Trematoda, 11 of Cestoda and 10 of Nematoda), including 1 new species, four new host records and three new British records. Two species of Mallophaga were identified from the birds. In three cases (Gymnophallus deliciosus, Paricterotaenia porosa and Capillaria contorta) a significant difference was found in the burden carried by male and female birds. ‘Adult’ birds were generally found to be more heavily infected with helminths than the chicks, with the exception of the nematode Cyathostoma lari, which was found more frequently in the chicks. A new intermediate host for Capillaria contorta was noted, namely Eiseniella tetraedra.Three species of Trematoda, two of Cestoda, and two of Nematoda were found to be cyclic (seasonal) in their time of appearance. Tetrabothrius spp. appeared most frequently in winter, whilst Anomotaenia larina was found only during the summer months. C. lari showed a primary peak of appearance in summer, with a secondary peak in winter. Capillaria contorta appeared in high numbers throughout the year.The distribution of helminths within the birds was also examined. Gymnophallus deliciosus was generally found in the gall bladder, whilst Parorchis acanthus was found only in the rectum. Intestinal trematodes were normally found in the mid-third of the small intestine (e.g. Spelotrema excellens, Himasthla elongata). In the case of C. lingua, however, peak numbers were found in the duodenum, whilst Brachylaemus fuscatus was found most frequently in the posterior third of the small intestine. Cestodes appeared most frequently in the anterior or posterior thirds of the small intestine (e.g. Tetrabothrius spp., Anomotaenia micracantha). Hymenolepis cirrosa showed two peaks of infection along the length of the small intestine, a small one in the anterior third, a much larger one in the posterior third. Attempts were made to correlate the helminth burden found with the food eaten by the birds and the availability of intermediate hosts.I should like to thank Mr J. Hobart and Dr I. V. Herbert for all the help that they have given me in the preparation of this manuscript, and also the bodies that gave me permission to shoot birds on their land, particularly the Nature Conservancy. Finally, I would like to thank the S.R.C. for the grant that made this work possible.

Abstract: The wall of the trunk, that of the praesoma, and the lemnisci of Acanthocephalus ranae have been studied by electron microscopy. Striations visible in sections of the body wall under the light microscope do not correspond with the ‘striped layer’ revealed by the electron microscope.A new region, the ‘canal layer’, has been described. This contains canals running into the body wall from cuticular pores.Structurally the wall of the trunk and that of the praesoma are similar. The lemnisci resemble the ‘inner layer’ of the praesoma wall. However, it is suggested that the wall of the trunk differs physiologically from that of the praesoma, and from the lemnisci. The possible roles of the wall of the praesoma and the lemnisci in fat excretion or uptake have been discussed.The body wall of A. ranae has been compared with that of the other acantho-cephalans studied with the electron microscope.Grateful acknowledgement is made to D.S.I.R. (now S.R.C.) for a research grant to the Department of Zoology for the purchase of a Huxley ultramicrotome, a vacuum coating unit, and an AEI EM 6 electron microscope.I am grateful to Dr D. A. Erasmus for reading and criticizing the manuscript, and to Mr T. Davies for valuable technical assistance.

Abstract: E. praeox has been isolated in Britain for the first time and is similar in regard to its oocyst size, prepatent time, life-cycle and the development of host immunity to the reports of Tyzzer et al. (1932). Oocysts had mean dimensions of 20·4 × 17·45 μm and the first oocysts were discharged 83½ h after infection. Oocyst production and life-cycle studies suggest that at least three generations of schizogony precede gametogony and that at least one further generation of schizogony is needed to explain the oocyst reproduction resulting from a small dose of oocysts. Young chickens 1½–3 weeks of age were not such suitable hosts as older chickens (6 weeks) and the reproduction of the parasite was seriously reduced when large numbers (e.g. 10 5 , 10 6 ) were used to infect chickens. A high degree of resistance to reinfection occurred after a single exposure to infection. Sporozoite infections of the caeca or cloaca resulted in the infection of the usual small intestine site; there was no development at the site of inoculation and attempts to infect the embryo allantois did not succeed. I wish to thank Dr M. Elaine Rose for the surgery required for the caecal infections, for her interest throughout and for help with the manuscript; Dr R. F. Gordon for help with the manuscript and permission to publish and Mr B. J. Millard for skilled technical assistance.

Abstract: A group of thirty-nine Merino-Border Leicester cross-bred lambs were given six sensitizing doses each of 3000 infective Haemonchus contortus larvae. Previous studies showed that this treatment produced a state of immunological exhaustion (Dineen & Wagland, 1966a). In the present study the regeneration of the immunological response was followed by challenging the sensitized animals and previously uninfected control animals with 3000 infective larvae at 2, 4, 8 and 16 weeks after termination of the sensitizing infections with an anthelmintic. Faecal egg counts were carried out on infected animals twice weekly throughout the course of the experiment, and differential worm counts were performed on groups of animals slaughtered 48 days after challenge.The results show that maximum regeneration of the immune response occurred when periods of 4 and 8 weeks were permitted to elapse between removal of the sensitizing infections and challenge. At 16 weeks the response of the sensitized animals to challenge infection was not significantly different from that of the control animals.In contrast to results recorded in the previous communication, a high mortality (49%) occurred among the sheep during sensitization. In the earlier experiment the sensitizing infections were given at fortnightly intervals whereas they were given at weekly intervals in the present experiment.Interpretation of the study is based upon the threshold behaviour of immunological responsiveness in parasite immunity, the occurrence of an immunological latent period and the relationship of this period to the developmental rate of the parasite, the susceptibility of the 4th larval stage to immunological attack, and the immunological privilege of the adult worm. These phenomena have been described in previous communications in the present series.We are greatly indebted to Mr Emil Teleki and Misses Helen Giller and Lindy Stothart for their excellent technical assistance.

Abstract: The present material of buffalo–dog origin could not be transmitted to the laboratory animals, such as white rats, white mice, rabbits, golden hamsters, guinea-pigs and chicks. Secondary development took place only in mice. The strobilar phase was infective to a cat but did not show sexual differentiation even at 30 days, and worms were further observed to be at a degenerative stage. The buffalo was highly suited to the perpetuation of the parasite, forming larger cysts and massive production of scoleces; nearly 90% of the cysts were fertile. The strobila had mostly two segments and not more than three. The number and distribution of testes were more posterior to the genital pore and were also observed posterior to the vitellaria, occasionally. The position of the genital pore in the gravid as well as mature segments was both anterior and posterior but with an average of 52.8%. Uterine sacculations were observed to range from 28 to 54. Echinococcus material of buffalo–dog origin does not appear to fall into a definite ‘subspecies’ pattern. It is suggested that it be merely recognized as a further ‘mutant’, the existence of which can be accounted for on theoretical grounds. The authors are thankful to the Director, Indian Veterinary Research Institute, and to Dr H. D. Srivastava, Head of Division of Parasitology, I.V.R.I., for providing the necessary laboratory facilities.