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The development and pathogenicity of Babesia muratovi and Nippostrongylus brasiliensis were studied in Rowett Hooded rats fed on diets deficient or adequate in copper or in vitamin B12. The course of B. muratovi reached its peak in both adequate and deficient rats on day 6 after infection with inoculum 10⁷/20 g B.W. The parasitaemia was higher and the pathogenicity of Babesia was more severe in rats fed on the copper adequate diets than in copper deficient rats but there was no significant difference between rats fed on vitamin B12 deficient and vitamin B12 supplemented diets. The development of N. brasiliensis in rats was dependent on the dose of larvae inoculated. Higher worm and egg counts were obtained in rats infected with 3,000 larvae than in those infected with 1,500 larvae. The worm and egg counts were higher in simple copper deficient rats and gradually decreased with increasing dietary copper. The egg counts persisted for longer periods in copper deficient rats than in those fed higher levels of copper. The rats inoculated with 3,000 larvae had more severe symptoms. Although in thiomolybdate treated rats the worm and egg counts were lower in deficient than in adequate control rats, the pathogenicity of Nippostronqylus was more severe in deficient rats. The worm and egg counts were highest in vitamin B12 deficient male rats and decreased gradually with increasing dietary vitamin B12, but no such linear trend was seen in females. The development of N. brasiliensis was greater and its pathogenicity was more severe in rats fed diets deficient in copper and vitamin B12 than in those fed on diets adequate in these nutrients.

Loch of Strathbeg is a coastal dune lake, about 40 miles north of Aberdeen. The loch is fed by one burn, Burn of Savoch. The River Don tributaries are about 35 miles west of Aberdeen. The main groups of aquatic invertebrates include Hirudinea, Oligochaete, Crustacea, Coleoptera, Diptera, Hemiptera, Ephemeroptera and Mollusca. The terrestrial forms are mainly Coleoptera, Hymenoptera, Diptera and Hemiptera. The loch is also rich in avifauna especially in autumn and winter. Fish species include trout, sticklebacks, flounders and eels. The Don tributaries are comparatively poor in species of inverte-brates. Most insects belong to two orders, Ephemeroptera and ple-coptera. In addition, there are numerous freshwater shrimps, Gammarus pulex, which form one of the major food items of trout and other fish in the area. The piscifauna of the area include trout, salmon, eels, brook lamprey and sticklebacks. Strathbeg trout do not live for more than 4+ years. Growth of trout in Loch of Strathbeg, Burn of Savoch and Don tri-butaries has been calculated. Loch fish grow more than double their length when they enter the loch in their second year (1+) during this year. In the other two habitats, growth is more gradual. This is dearly demonstrated by their specific growth rates (Fig 4 p.23.). The average length of fish at different ages are equal for both Burn of Savoch and Don fish but almost half the size of their loch counterparts. Growth of trout 'at Strathbeg and Don has been suggested to be influenced mainly by the food, temperature and the availability of spawning facilities. The Don area has good spawning grounds and therefore supports more populations of trout which ultimately results in the decrease in the growth rate due to competition for food and space. This even-tually leads to fast growth of fish due to lower competition for food and space. Trout are surface feeders between May and October, and bottom feeders between November and April. Surface food is predominantly of terrestrial origin and emerging insects, while bottom food is both of permanent e.g. molluscs and shrimps, or temporary bottom origin e.g. insect larvae. Terrestrial food is either washed into the water by waves or blown into the water by wind or is captured when flying over the loch (see Table 6). The main food In the loch includes Diptera larvae, pupae and adults, Hemiptera (mainly corixids), Ephemeroptera (mainly Cloeon simile), Coleoptora (mainly terrestrial beetles) and Trichoptera. Molluscs (mainly P jenkinsi) form about 2.8% of the total food consumed by trout in the loch. sticklebacks form an important dietary item of larger trout. A total of 18 helminth species and one molluscan parasite were recorded in Strathbeg and Don trout, with 14 species, including glochidia larvae, in the former and 12 in the latter. Two species of crepidostomum, C metoecus and C farionis, were recovered from the two habitats studied. Attempts have been made to separate the two species. C. metoecus is a parasite mainly of the pyloric caeca while C farionis is mainly a parasite of the intestine and the gall bladder. C metoecus is more abundant than C. farionis. A clear seasonality has been recorded for C. metoecus. Mature worms disappear in July/August and juveniles appear at the same time. Three additional intermediate hosts of Crepidostomum have been recorded. These are Cloron simile, Siphlonurus lacustris and Bphemerella ignita. Two species of diplostomuli have been observed in the lens and humour of trout. Infection of trout at Strathbeg with both meta-cercariae takes place in the loch. Infection of trout with humour specimens is significantly higher than that of the lens specimens in both Strathbeg and Don fish. Infection with both species of parasites increases with the age of fish. Infection increases with the age of fish from 1+ to 4+. Younger fish (0+) are not infected by these tapeworms. It is concluded that intermediate hosts and their consumption by trout are the major factors affecting the intensity of infection of these fish. It is added that when considering the parasite fauna of a stocked body of water, the history of the fish and the habitat before stocking should be taken into consideration, Factors that affect parasite transmission to trout include availability of hosts, temperature of the environment, feeding behaviour of trout and movement of trout to and from infected areas. From the result of this work, infection of Strathbeg trout with both Diphyllobothrium and Eustrongylides is to a large extent responsible for the total disappearance of fish older than 4+ in the loch.

The reasons for the declines in red grouse numbers that occur periodically on the heather moors in the British Isles, have been uncertain for a long time. During some declines many grouse are found dead in poor condition, bearing thousands of tiny threadworms Trichostrongylus tenuis in their intestinal caeca. The purpose of my study has been to find whether threadworms alter the health of well-fed grouse. Although 90% of wild grouse that I examined carried threadworms, old birds had 30 times as many as young birds, and most of the worm population was carried by a few heavily infected grouse. This highly skewed distribution of parasites within hosts is common in parasite infections. Grouse become infected with threadworms when they eat worm larvae on heather. During the present study, adult worms were well tolerated by grouse and produced eggs which were passed in the birds' faeces for over a year. Nevertheless, the immune system of the birds seemed to be reacting to the presence of either larvae or worms, in that the numbers of circulating eosinophils and heterophils rose as did the concentration of alpha, beta and gamma globulin. The pathogenic effects of worms varied amongst individual grouse, and some grouse carrying 3000 - 4000 worms died while others survived with many times more. Infected grouse developed a mild anaemia and the levels of serum albumin in their blood fell. Body weight did not change but muscle masses shrank. These physiological changes were associated with a reduction in dominance of captive grouse at least in the short term, although two wild grouse dosed with trichostrongyles maintained their territories and breeding status. The idea of a link between trichostrongyles and either the dominance of grouse or their ability to hold territories should be retested. In conclusion, trichostrongyles reduce the condition of grouse and other physiological variables usually associated with good health, although they do not kill many well-fed grouse. In the wild, these effects might contribute to declines in population, especially if associated with poor nutrition.

The ecology of Polydora, epp was reviewed and the infestation of molluscs by the boring, spionid polychaete, Polydora ciliata, was studied in detail. X-radiography showed that, P. ciailt a infested a wide variety of molluscan substrates, but it . occurred in greatest abundance on large, rough-surfaced, epibenthic shells, which were collected from the-sublittoral zone and the level of extreme low water spring tides, of shores sheltered from wave action. P. ciliata was contagiously dispersed on mussel--shells. Monthly radiographs of mussels indicated that maximum settlement of P. ciliata larvae in the river Fowey, 1 occurred in May. The effect of heavy infestations of P. ciliata on limpets P. vul ata , mussels (Eytilus edulis), and oysters (Crassostrea is) was investigated. At certain times of the year, heavy infestations of P. ciliata reduced the dry weight condition and fecundity of limpets and mussels, but did not seem'to adversely affect C. i ae. Compression tests showed that P. ciliate weakened mussel shells. Predation experiments, with Cancer a s, indicated that this might increase the mussels' vulnerability to predation. Heavy P. ciliata infestation also reduced the mussels' resistance to loads which tended to force the mussele' valves apart. This may decrease the mussels' phyeico-chemical tolerance, and increase their vulnerability to starfish predation. Limpet adhesion, measured on the shore with a simple dynamometer, was impaired by P. ciliata infestation. This may reduce the limpets' resistance to wave action, and increase their vulnerability to predation. Reasons for the adverse effects of P. ciliata infestation on limpets and mussels are suggested, and the ecological and commercial implications of the infestation are discussed.

This research study is divided into 4 major sections. Section A deals with the world-wide distribution of the haematoprotozoan parasites of marine fishes. These are tentatively divided into 3 major groups: the Haemogregarines, the Trypanosomes and the Trypanoplasms and one group of ill defined and controversal organisms including Haemohormidium sp. and Dactylosoma sp. The results of extensive surveys of the coastal waters of France, Scotland and Wales with added examples from the Mediterranean Sea substantiate the zoogeographical distribution of these parasites. Two species of haematoprotozoan parasites Haemogregarina simondi and Trypanosoma soleae are re-described, earlier accounts dating from the beginning of the century being considered incomplete. A new species of haemogregarine is described from the farmed turbot (Scophthalmus maximus) as Haemogregarina sachai n. sp. and an unidentified species of Haemohormidium found occasionally in turbot and Dover sole (Solea soles) is also described. In addition Haemogregarina sp. were encountered in certain wild marine fish from the Atlantic coast of France e. g. Zeus faber, Sebastes sp., Trisopterus luscus, Pagellus bogaraveo and Raja sp. and from the coastal waters of Malta e. g. Peristedion cataphractum and Oblade melanura. However, they were not found in sufficient numbers to allow a definite description. The value of surveys of wild fish populations is discussed in the light of zoogeographical distribution, the apparent periodicity of these parasites and a seasonal variation of parasitism. Section B attempts to summarize the knowledge of the mode of transmission of marine haematoprotozoan parasites by piscicolid leeches as intermediate hosts and vectors. The developmental stages of a trypanosome, probably Trypanosoma murmanensis from the Atlantic cod Gadus morhua are described in the marine leech Calliobdella nodulifera. Stages of a haemogregaririe were observed in the same leech. The development of the turbot haemogregarine Haemogregarina sachai n. sp. in artificially infected leeches is also described up to the 20th day post infection. Transmission of this haemogregarine to apparently uninfected turbot via this leech was not successful. Various stages of development of Haemogregarina simondi are described in its apparently natural vector, the marine leech Hemibdella soleae, and transmission with infected leeches to apparently healthy hatchery reared soles was achieved. Thus it was shown for the first time that marine leeches can serve also as vectors for haemogregarines. Stages of this haemogregarine are also described in the blood-sucking ectoparasitic copepod Lernaeocera sp. parasitizing the haemogregarine infected soles. These results are discussed in relation to the feeding behaviour and migration patterns of the fish hosts, the periodicity of the parasites and possible other vectors or other ways of acquiring an infection with these haematoprotozoan parasites. In Section C the pathogenicity of the haematoprotozoan parasites of marine fishes is summarized from previous accounts among wild fish populations and compared with the situation in aquaculture. The pathogenicity of the two haemogregarines, Haemogregarina simondi and Haemogregarina sachai n. sp., accidentally introduced into several fish farming establishments connected with the effluent cooling waters of a nuclear power station is described with special reference to the possible source of the infection. The results of therapy trials and control programs are discussed in the light of the periodic reappearance of the parasites, the possibility of carrier fish existing, the immune status of the host and the possible role of an intermediate host or vector in maintaining the infection. Possible means of controlling the pathogenicity of marine haemogregarines and perhaps other haematoprotozoan parasites when they occur in farmed marine fishes are also discussed. In Section D are described for the first time, the ultrastructural characteristics of various stages of the haematoprotozoan parasites of marine fishes in fish hosts and vectors. The electronmicroscopical studies are limited to Haemogregarina sachai n. sp. and Haemogregarina simondi, for which the ultrastructure of schizonts from the spleen and blood, intracellular merozoites and free gametocytes were contrasted. In addition stages of H. simondi were demonstrated in Hemibdella soleae and Lernaeocera sp. The fine structure of the various organelles encountered was compared with that of related organisms from other cold-and warm-blooded vertebrates. In conclusion attention is drawn to the need for more investigations in this field of host-parasite relationship of marine haematoprotozoan parasites and their vectors and their pathogenic-action as seen in a confined and artificial environment such as the marine aquaculture.

Antibody responses to soluble and cellular antigens have been demonstrated in juvenile and adult plaice, Pleuronectes platessa L. A relatively strong precipitating antibody was produced to calf serum, but not to bovine serum albumin. The onset, and magnitude of antibody production was temperature dependant and physico-chemical characterisation of plaice precipitins indicated they were of the IgM class. Plaice also produced haemagglutinins, following injection of rabbit erythrocytes, with a suggestion of a heightened response upon secondary stimulation with the antigen. An evaluation was made of the humoral immune.response of the fish to three tissue parasites; Rhipidocotyle johnstonei and Cryptocotyle lingua, the metacercariae of which develop in the musculature and. connective tissue of plaice; and Trypanosoma platessae, a haemoflagellate. Elevated levels of β-globulin were detected in the sera of T. platessae infected plaice, and it is suggested that this may be associated with antibody secretion to the parasite. Further studies were made on the biology of T. platessae, including efforts to maintain the parasite, by passage, in the laboratory. Natural and experimental infections with the metacercariae of C. lingua and R. johnstonei induced a temperature dependant, precipitin response in plaice. Application of the indirect fluorescent antibody technique suggested that the functional antigens of both parasites included somatic elements, however the antigens of R. johnstonei were also clearly associated with secretory tissues. A 'natural' antibody, present in the sera of the majority of test plaice, was found to precipitate in agar gel with an antigenic extract of the nematode, Proleptus obtusus. This was considered of particular interest, as P. obtusus is a parasite specific to the dogfish and is unknown in plaice. The precipitin, first suspected of being non-specific C-reactive protein, was later characterised as 19S IgM. The nature of serological and immunological changes in fish subjected to immunisation and. parasitic infection was reviewed and discussed in relation to the findings of the present project.

Third stage larvae of Brugia pahangi irradiated with 10, 25 and 45 krads. of Co.SO were inhibited in their development beyond the juvenile adult stage, the fourth larval stage and third larval stage respectively. The higher two doses altered the migration pattern of most of the parasites, which were confined to the subcortical sinus of the infected lymph nodes. Hale parasites were more susceptible to irradiation than were females. Repeated infections with irradiated ».pahangi did not really change the architecture of the lymphatics of the e cats. Cats were repeatedly vaccinated with irradiated n.pahangi to determine whether attenuated parasites protected against challenge infections. Cats immunized with parasites irradiated with 10 krads. resisted 60.3-98.5% of the homologous challenge infections; and cats immunized with parasites irradiated with 25 krads. resisted 61 - 93% of challenge infections. The resistance in the immunized animals was mounted against all the stages of the life cycle. Two cats given heterologous challenges with Brugia patei resisted 78.6% and 65.3% of the challenge inoculations. One cat which was infected with normal parasites, and challenged after it had become amicrofilaraemic, also resisted challenge. Jirds vaccinated with parasites irradiated with 45 krads. resisted challenges, whilst vaccination with non-irradiated worms and parasites irradiated with 25 krads. did not protect these animals. Antibody responses to various homologous antigens were higher In cats given repeated infections than in cats given single infections. Antibodies against microfilariae were detected only when the animals had suppressed their microfilariae. No antibodies against adult stages could be detected in animals infected with irradiated larvae. The only significant change in the white blood cell population was eosinophilia. The highest eosinophilia occurred at the time of the onset of microfilaraemia. There were no significant changes in the serum components of cats infected with B.pahangi.

Studies were conducted on the host-parasite relationship in Strongyloides ratti infections in rats. During initial infection, rats acquired a strong immunity against this parasite and 1. production and the expulsion of adult Infected rats were strongly resistant to reinfection and this immune response lasted for a period of three months, independent of the residual worm population. The immune response during initial infection caused a reduction in the size of adult worms after day 18 PI. Adult worms, which occupied the anterior half of the small intestine in the first 18 days, shifted to the posterior half before they were expelled. The Immune response affected the adult worm in the intestine during a primary infection, the migratory larvae in the lung during a secondary infection and the skin stage following multiple infections. In heavily infected rats, larvae disseminated and were detected in the brain tissue. A full infection with S. ratti induced complete resistance to reinfection, but adult worms were a major souroe of protective antigens, while migratory larvae did not stimulate an immune response against reinfection. "Normal" worms obtained on days 8-16 PI were able to continue their larval production when transferred to new, clean, rats; "damaged" worms obtained on days 18-25 PI were unable to do so. Worm damage during initial infection was irreversible. Short non-fertile worms found in rats early after challenge were able to resume their egg production and survive in new hosts. The possibility of adaptation was discussed. 22 day-old worms were examined under the electron microscope and were compared with 10 day-old worms. Damaged worms were smaller in size and showed vacuolation and tissue disorganisation, particularly the gut cells. Peculiar whorls were observed inside the gut lumen of damaged worms, and were suggested to be immune complexes. Precipitates were persistently observed surrounding the mouths of damaged worms. It was proposed that worm damage is instigated by immune complexes plugging the mouth and causing a state of starvation through inhibition of food intake. Worm expulsion was demonstrated to be an imrtunological event. Antibody mediated worm damaqe, in S. ratti, as an essential step in worm expulsion, was argued immume MINC alone, given on days 6 or 12 PI, were capable of inducing worm expulsion. Serum alone repeatedly induced auto-infection. A variety of factors were suggested to explain the action o f transferred antibodies. Co-operation between immune MLNC and serum in inducing worm expulsion was only evident when MLNC were given on day 12 PI. The tissue eosinophils Increased in the mucosa of Infected intestines immediately before worm expulsion, during initial and challenge infections. The percentage of degranulating mesenteric mast cells increased during the period of expulsion, and the introduction of histamine into the gut of infected rats caused a significant acceleration of worm damage and expulsion. Aspirin, a potent inhibitor of prostaglandins, delayed the action of expulsion for 5 days suggesting a possible role of prostaglandins in this process. Corticosteroid treatment caused the long persistanoe of S. ratti worms in the rats and indicated that worm damage was due to an immune response, and not to the worm's senescence. Levels of tissue eosinophils and degranulated mesenteric mast cells were also suppressed. No auto-infection was demonstrable when cortico steroid treatment commenced with the infection , but it was evident when the treatment started immediately prior to worm expulsion. Corticosteroids suppressed all manifestations of acquired immunity to reinfection. An attempt was made to render rats unresponsive to S. ratti infection by treating with corticosteroids at strategic periods during an initial infection.

Aspects of the host parasite relationships which exist between larvae of the filarial nematode Brugia patei and two mosquito intermediate hosts are explored with the aid of an autoradiographic technique and selected histochemical staining methods. In Aedes togoi normal development of the worm occurs following a blood meal taken from a domestic cat infected with microfilariae of Brugia patei. Conversely Anopheles labranchiae atroparvus is refractory to infection with Brugia patei and it has been established previously that in the latter mosquito the parasite exhibits a number of abnormalities in its pattern of growth, and it rarely develops successfully to the second larval stage. The present study therefore concentrates largely on the principal histological events which take place in both the parasite and its two mosquito hosts during this early critical period of development. The results describe the incorporation of [H3]-labelled nucleosides, amino acids and carbohydrates in normal and abnormal worms and include autoradiographic and histochemical features of the host reaction elicited by Brugia patei when parasitic in Anopheles labranchiae atroparvus. Autoradiographic observations are also recorded for both host mosquitoes. Particular emphasis is given to the alimentary canal, fat body, heart, pericardial cells and female reproductive tract during the period when the mosquito is parasitised by the nematode. Differences between parasitised individuals and uninfected controls are noted. These findings are discussed in relation to our general understanding of host parasite relationships in insects and entomophilic nematodes.

The maintenance of adult schistosomes in vitro is characterised by transient production of eggs, limited survival and a visible deterioration in worms during prolonged maintenance. Empirical, or trial and error attempts at improving media for the in vitro maintenance of adult S. mansoni have proved largely unsuccessful. The present study employed a more logical, biochemical approach to in vitro maintenance by quantifying the deleterious changes occurring in. vitro, in an attempt to identify the causes of the rapid cessation of egg production and the concomitant worm deterioration. C2) In Medium 199 supplemented with 10% newborn calf serum, adult mansoni produced a total of 400 eggs per pair during the first 10 days in vitro. Egg deposition was rarely observed beyond this period even though worms survived for up to 60 days. (3) There was a decrease in schistosome biomass during maintenance in vitro. Over 10 days the dry weight of males decreased by 30% while the dry weight of females fell to 50% of its level at isolation from the host. C4) During 5 days vitro the protein content of male schistosomes fell by 30%. There was a corresponding decrease in the protein content of females of 60%. The loss of schistosome gut contents in vitro would account for some but not all of this decline. (5) At least 50% of the glycogen of male S. mansoni was depleted during the first 6 h in vitro. Female glycogen content was less affected by vitro maintenance. The glycogen content of both male and female worms decreased during prolonged in vitro maintenance. The level of glycogen depletion during the initial 24 h in vitro was apparently unaffected by the glucose concentration in the incubation medium. (6) Studies of the uptake and incorporation of glucose, adenine, and the amino acids leucine and alanine from the incubation medium were used to examine the metabolic differences between worms incubated as pairs and worms incubated following separation. (7) Male S. mansoni were capable of fulfilling their biosynthetic amino acid and adenine requirements in copula through trans-integumental uptake whereas paired females were not. (8) Allowing for the obscuring of the tegumental surfaces in copula and for the loss of gut contents, the amino acid uptake and incorporation rates of male and female S. mansoni in vitro reflected the changing somatic requirements of the worms. (9) Amino acid uptake rate and the rate of incorporation of amino acids into schistosome protein were not related to reproductive activity in vitro. CIO) The rates of uptake and incorporation of adenine also reflected somatic rather than germinal requirements, and were not related to reproductive activity in vitro. (11) Adult paired S. mansoni removed the equivalent of twice their combined body dry weight of glucose from the medium during each day of the initial 3 days in vitro. (12) The rapid depletion of male glycogen in vitro appeared to be caused by a decrease in the rate of incorporation of glucose into glycogen by the male worms during the first hour jin vitro. A concomitant decrease was not observed in female schistosomes. (13) Increases in both the rate of uptake of glucose and the rate of its incorporation into glycogen were effected by increasing the glucose concentration in the medium and by the incubation of separated male and female worms. (14) It is proposed that the cessation of egg production and the observed decrease in worm biomass in vitro are due to the failure of adult schistosomes to feed on red blood cells in vitro and that the rates of uptake and incorporation of metabolic precursors in vitro merely reflect the decreasing worm biomass and have little significance to reproductive function in vitro.

The Niallophaga, a suborder of the Pthiraptera, are a group of obligate ectoparasites living on birds and mammals which present interesting problems of evolution and phylogeny.+ The present distribution of the avian Niallophaga suggests that these insects became parasitic on the birds early in the evolution of the latter class and that they evolved with their hosts. In a group of related host species, each species may have allopatric species of a number of sympatric genera of Niallophaga common to the host group (VI: tables 1+, 5, 6), and in addition, sympatric species of one or more of these genera. In many cases, therefore, a single host species may have a considerable number of genera and species of Mallophaga (1 :279). The problem is to find an explanation of the presence of often closely related genera and species in what is the equivalent of a restricted and isolated geographical area. A study of the morphology of the Mallophaga and their present distribution both on a single host individual and throughout the class Aves makes it possible to deduce some of the factors which may have influenced speciation in this group of ectoparasites and to compare these factors with those influencing groups of free -living animals. Apart from the intrinsic interest of evolutionary problems in a group of ectoparasites, it is necessary when attempting to formulate a natural classification of the Mallophaga to have some understanding of the possible steps in the evolution of the group.

Following a general introduction, the extensive literature on fascioliasis is reviewed. The distribution, importance, pathogenesis and pathology of the disease are considered, with special emphasis on Fasciola gigantica infections. Field studies are reported on the epidemiology and control of F. ßiganti.ca in East Africa and a survey described of infections with Fasciola spp. in African wild mammals. This survey was undertaken in order to assess the importance of wild mammals in the epidemiology of this disease, in view of the increasing development of game parks and ranching schemes. The techniques used in the experimental studies are outlined with emphasis on those modified during this work. The techniques described include the cultural methods by which the infective agents were obtained, with special reference to the large scale production of metacercariae of F. gigantica and a detailed discussion of the problems which had to be overcome before this was possible. Other techniques described include those used in haematological examinations, faecal egg counts, liver function tests, serum enzyme assays and other biochemical studies. The difficulties which had to be overcome to obtain and maintain experimental cattle and sheep free of intercurrent diseases in Kenya are discussed. Observations are described which were made in the course of several experiments using a total of 35 sheep and 46 cattle in Edinburgh and in Kenya. In Edinburgh the experiments comprised studies on the pathogenesis of acute and chronic fascioliasis (Fasciola hepatica) in sheep. In Kenya chronic fascioliasis (F. gigantica) was studied in sheep with special emphasis on the economic importance of this condition. It was found that the infected animals gained weight more slowly than the uninfected controls and that this arose mainly from differences in the amount of body fat. Several experiments were carried out in Kenya on cattle infected with single doses of 500, 1000 or 2000 metacercariae of F. gigantica or with two doses of 500 metacercariae ten weeks apart. The pathogenesis of the disease and the parasitological findings in these animals are described in detail. Other studies described include observations on long term chronic fascioliasis in cattle. The gross pathology of P. giçantica infections in cattle from 2 -51 weeks after infection is also described. The main finding from these studies is that F. gigantica appeared to be less pathogenic for cattle than reported by earlier workers. There was no evidence of an "acquired self- cure" under the conditions of these experiments, which would support the suggestion that resistance to F. sigantica infections in cattle may be merely related to the fibrosity of the liver parenchyma. Further support was found for this suggestion from the study of the leucocyte levels and other observations. Evidence was found that F. gigantica has a relatively short life in cattle. The dissertation ends with a discussion of the results from the experimental infections.

A preliminary quantitative study on the population growth rate of Theileria parva in the bovine host failed to confirm the constant growth rate independent of size of infective dose reported by earlier workers. Experiments in methodology were performed therefore to standardize the techniques for obtaining (a) accurate estimates of infection rates from biopsy smears, (b) representative samples from a superficial lymph node source. A further experiment was carried out to establish that such samples were representative of the total parasitic biomass. These techniques were applied in a wider ranging quantitative study using four infective doses at ten fold intervals for infecting animals. The resultant growth rates were again dependent on the size of the infective dose. A definitive experiment, using five infective doses at ten fold intervals confirmed the divergence of growth rates. It was also shown that the severity of the clinical reaction, and the survival time was dependent on the size of the infective dose. The implications of these results in relation to immunization of cattle against T. parva are discussed. The standardized methods were applied also in a chemoprophylactic study to observe the growth rates of T. parva as affected by different regimens of tetracycline. Other possible applications of these standardized methods are discussed.