Sheep which had been either previously infected with O. circumcincta or maintained worm-free, were surgically prepared with separated fundic pouches and abomasal cannulae and subsequently infected with 20000 O. circumcincta larvae three times weekly.

A reduction in food intake and increases in total acid output from the pouches and plasma pepsinogen levels were evident in both groups of sheep 4 days after repeated infections commenced; effects which increased in severity after 12 or more days. Except for a transient period of slight failure, previously infected sheep retained the capacity to acidify their abomasal contents whereas previously worm-free sheep lost this capacity. These changes were reversed between 2 and 7 days after treatment with thiabendazole (88 mg.kg−1).

Secretory capacity of the fundic pouches was tested with histamine (40 μg.kg−1), the histamine antagonist (burimamide 8mg.kg−1) and atropine (100 μg kg−1). Ostertagiasis reduced or abolished the stimulatory effects of histamine. An increase in secretion volume and acid output was obtained after food was freshly provided, even though as little as 25 gm was consumed. Atropine and burimamide both caused a profound decrease in pouch secretion and acid output.

These data are consistent with the hypothesis previously stated that in ostertagiasis the hypersecretion from fundic pouches is due to increased levels of circulating gastrin.

The primary trophozoites of Malpighamoeba locustae excyst in the crop and midgut of the Desert Locust (Schistocerca gregaria) and then penetrate the epithelial cells of the midgut and caeca where they grow and multiply slowly. After about 10–12 days they leave the gut epithelium passively when the cells in which they have grown degenerate and are extruded. From the lumen of the gut they make their way into the lumen of the Malpighian tubules where they feed upon the brush border as extracellular parasites. No primary or secondary trophozoites were found in the haemocoele of the host.

During the first few days in the Malpighian tubules the secondary trophozoites undergo a rapid series of divisions which enables them to double their numbers every 24 h so that by day 20–22 the lumen of the tubules is packed with cysts and trophozoites. The trophozoites phagocy-tose small pieces of brush border and numerous food vacuoles are present in their cytoplasm. The cytoplasm of the primary trophozoite contains the normal complement of cell organelles, a notable feature of which are the large, numerous mitochondria with their tubular cristae. When the trophozoites are ready to encyst they round up and begin to lay down a series of membranes which totally cover the trophozoite. Numerous storage granules are found in mature trophozoites which are about to encyst and these may be utilized in cyst wall production.

The parasite reaction phenotype of 80 lambs was determined with natural infection on pasture. These lambs were then assigned to outcome groups and randomized into treatment groups. Developing some simple but unusual analytical techniques it was possible to demonstrate that mixed natural infections of trichostrongyles with the chief biomass component Haemonchus contortus cayugensis protected against superinfections with 35000 infective larvae but protected against superinfections with 1000 infective larvae per day only within the limits characteristic of the enclosing barn ecosystem. The limits of the barn ecosystem were less constraining on more diathetic sheep than the natural pasture infection. Internal evidence from the experiment suggests that the interaction of sheep × pasture × infective trichostrongylid larvae constrains the parasite biomass in the host in more diathetic animals at significantly lower levels. Anaphylactoid ‘self-cure’ did not occur in this experiment but something like premunition certainly did.

The behaviour of four species of Eimeria was studied in lambs which were given either monospecific or multispecific infections. In the presence of other species the patency of oocyst production of E. ovina and E. weybridgensis was extended and the total number of oocysts produced by all species except E. ninakohlyakimovae was increased. Clinical symptoms were observed only in lambs which received E. ninakohlyakimovae.

A new haemogregarine, Schellackia landauae sp.nov., is described in the Brazilian lizard Polychrus marmoratus (Linn) from Pará State, north Brazil. Sporozoites are found principally in the red blood cells (84%) in the peripheral blood but also occur in lymphocytes and monocytes. Experimental transmission to three uninfected P. marmoratus was achieved after feeding them with laboratory-bred Culex pipiens fatigans which had engorged on an infected lizard 14 days previously. The cycle of development in the small intestine of P. marmoratus takes approximately 30 days: schizogony, gametogony and fertilization of the macrogametocytes is in the epithelial cells of the gut, with zygotes penetrating the lamina propria, where the mature oocysts develop. Living oocysts average 14·3 × 13·3 μm, and are approximately 10·0 μm as seen in histological sections. During the period of intestinal development, the parasite also undergoes asexual multiplication within cells of the spleen and liver by a process which appears to be endodyogeny. The exact time required before the first invasion of the peripheral blood by the sporozoites remains to be ascertained, but is some time within 30–45 days after the lizard ingests the infected mosquitoes. Morphology of the sporozoite in the vertebrate host is variable and depends on the host cell occupied; there may be 1 or 2 refractile bodies. Reduction or fusion of these to a single refractile body in those sporozoites within the gut cells of the infected mosquito suggests that the bodies may represent some form of energy source that is used up during this latent phase in the insect vector. Accumulation of sporozoites takes place in the reticulo-endothelial cells of the viscera, in particular the pigmentladen cells of the liver and lung. Attempts to infect other species of lizards, Tropidurus torquatus (Iguanidae) and Ameiva ameiva (Teiidae) failed.

The effect of chemical abbreviation of the primary infection dose (PID) of 160 infective larvae of Metastrongylus apri on the immune status of the guinea-pig host was studied. The criteria used for assessing the status of immunity consisted of clinical manifestations following administration of a challenge infection dose (CID) of 800 infective larvae of M. apri, the rate of worm recovery 15 days post-CID and the rate of mortality following administration of CID.

Among the guinea-pigs of the main experimental group, where 15-day-old PID was abbreviated by two parenteral doses of levamisole, a strong immunity to CID given 35 days post-PID was built-up. Against this, all the guinea-pigs of a control group, which did not receive PID, died between 16 and 22 days post-CID.

The increase in serum gamma-globulin level of the guinea-pigs, where the PID was abbreviated chemically, suggested that the rise of this globulin fraction in the serum could be in some way related to the resistant state of guinea-pigs.

Plasmodium falciparum parasites infecting Aotus trivirgatus erythrocytes were cultured in media (Harvard and TC199) augmented with human, foetal calf, or other sera. Conditions were established which supported growth of parasites and allowed cyclical multiplication when fresh erythrocytes (from Aotus or Homo) were added in sub-culture (mean multiplication rate: x 3). Immunoglobulin G pools, prepared from plasma collected in endemic malarious areas in Africa and from unexposed Britons, were tested for effects on the in vitro growth (measured by incorporation of tritiated leucine) and multiplication of parasites. Whilst non-immune IgG was without effect, IgG from both East and West Africa inhibited the multiplication of East African (Uganda-Palo Alto strain) parasites.

The fine structure of egg shells of four different genera belonging to the order Tylenchida has been examined. The species examined were Meloidogyne javanica, Rotylenchulus reniformis, Tylenchulus semipenetrans and Pratylenchus minyus. They are all similar in their basic structure, being composed of vitelline membrane, chitin and lipid layers, but there is considerable variability in the thickness of these layers.

We have retained the conventional nomenclature because of its convenience, but it is clear that these layers have a variety of chemical components. However, they do appear to contain the compounds from which they take their name. Thus chitin occurs in the chitin layer, and lipid in the lipid layer. The latter is removed by the technique used in isolating the shell from the egg. Chemical analysis of the hydrolysis products of these shells has revealed a high (35 %) proline content which appears to be a characteristic of those nematode egg shells which have been examined so far. These analyses and treatment with enzymes indicate that the chitin layer is a chitin–protein complex.

Experiments on the permeability of eggs of M. javanica at different temperatures indicate that changes in permeability are not due to the melting of a single lipid with a distinct melting point as had been thought in the past. We have found that Arrhenius activation energies calculated from the two slopes of an Arrhenius plot were 17·8 kcal/mol and 43·0 kcal/mol respectively, the transition from one to the other taking place at 62°C. We think that these changes are due to changes in the properties of lipoprotein membranes in the lipid layer. These membranes appear to be of paramount importance in controlling the permeability of the nematode egg shell.

In snails maintained at 20 °C rediae of Fasciola hepatica emerge from sporocysts from 11 days after infection onwards. The number of mother rediae rises steadily thereafter until at least 40 days after infection. Daughter rediae are seldom observed in mother rediae dissected from snails maintained at 20 °C. Their production can, however, be stimulated by subjecting the snail host to starvation, to low, and to high temperature shocks. The parasite is susceptible to stress from immediately after infection for about 16 days, when maintained at 20 °C. In general, the more extreme the shock, the greater is daughter redial production. Increasing the length of the period of stress from 12 h up to 9 days does not increase the production of daughter rediae, nor does repeated on/off cold shocks or continuous maintenance at 10 °C. Daughter rediae develop more rapidly than cercariae and leave the mother rediae several days earlier. There is no evidence that presence of daughter rediae coincides with the suppression of cercarial production. The findings are discussed with reference to possible mechanisms by which parasite development might be controlled.

A number of common freshwater molluscs were exposed to Fasciola hepatica miracidia labelled in vivo with radioselenium. Radioactivity was rapidly incorporated in all lymnaeid species tested (L. auricularia, L. columella, L. glabra, L. natalensis, L. palustris, L. pereger, L. stagnalis, L. tomentosa and L. truncatula) whereas no radioactivity could be demonstrated in non-lymnaeid pulmonate species (Aplexa hypnorum, Physa acuta, Physafontinalis, Anisus vortex, Gyraulus albus and Planorbis planorbis) or in the prosobranchs Valvata cristata and Valvata piscinalis. However, the prosobranchs Bithynia leachii, Bithynia tentaculata and Potamopyrgus jenkinsi showed a slow but significant uptake of radioactivity which was presumably explained by uptake of labelled miracidia through a ‘filter-feeding] mechanism. Sphaerium corneum, a true filter-feeder, showed a similar course of uptake of radioactivity.

Oogonia of Meloidogyne javanica are radially arranged around a central rachis to which they are attached by cytoplasmic bridges. As the oocytes mature the rachis disappears and the oocytes pass through the oviduct in tandem. The oviduct-spermatotheca valve is constructed of two rows of tightly packed cells of which there are four per row. The nuclei of these cells are large and contain balloon-shaped cytoplasmic invaginations. The spermatotheca is characterized by microtubules which extend to its lumen and by invaginations of plasmalemma. Cells of the distal uterine region contain large intracytoplasmic spaces bordered by endoplasmic reticulum whereas proximal uterine cells have dense cytoplasm and large areas of compact endoplasmic reticulum. Egg-shell formation begins in the spermatotheca with the modification of the oolemma to form the vitelline layer. The chitinous layer begins in the distal portion of the uterus and appears to originate from the egg. Proline-containing protein is incorporated into the chitinous and lipid layers as the egg passes through the mid-region of the uterus and formation of the lipid layer in this region completes egg development.

A prominent filamentous and granular cell coat is associated with the plasma membraneof haptomonads of Crithidia fasciculata attached to the cuticular lining of the hindgut of Anopheles gambiae. This surface material is completely absent from morphologically identical haptomonads from the mosquito foregut and from rosettes in culture. On the basis of these observations, it is suggested that cell coat formation is dependent on suitable environmental conditions. A cytochemical staining technique indicates the presence of carbohydrates in (a) the cell coat, (b) the outer leaflet of the plasma membrane of haptomonads from all locations and (c) small cytoplasmic vesicles and multivesicular bodies in all haptomonads. It is suggested that these cytoplasmic organelles represent stages in the intracellular pathway for material ingested by pinocytosis rather than elements involved in surface coat formation.

Lethal irradiation (850 rads) of mice made resistant to Trichuris muris markedly depressed their ability to expel a challenge infection. Expulsion was restored within 7–10 days when MLNC from uninfected mice were transferred on the day of infection, but no significant restoration was evident after transfer of immune serum. Transfer of BM alone had no restorative effect within 10 days and no synergism was seen when both BM and MLNC were transferred. MLNC from uninfected donors did not restore challenge expulsion when transfer was delayed until day 7 and the mice were killed 3 days later, although MLNC from resistant donors were effective within this time. When irradiated mice were given BM and the challenge infection allowed to continue for 15 days expulsion was restored, as it was when challenge was delayed for 7 days after BM transfer in thymectomized mice. The results confirm that expulsion of T. muris involves both antibody-mediated and lymphoid cell-mediated phases and offer no evidence for the involvement of other cell types.

Larvae of Taenia pisiformis were cultured in vitro in medium containing 2·5, 5 or 20% (v/v) of normal rabbit serum (NRS). Greatest development occurred in 20% NRS, and the potency of antigens collected in medium from each culture tested by intradermal (i/d) skin tests in infected rabbits paralleled the in vitro growth rate of larvae. ‘Culture’ antigens from 5% NRS stimulated good immunity in rabbits to a challenge infection with T. pisiformis eggs, although they were poorly reactive in skin tests.

T. pisiformis larvae were also cultured in 10% (v/v) of nitrates of serum reduced to one-half of its volume by passage through 300 000 MW cut oif (XM300F) or 100000 MW cut off (XM100F) ultrafiltration membranes. Larvae cultured using XM300F had growth rates comparable with those cultured in 20% NRS, and the antigens released into the culture medium had equal potency in i/d tests and in stimulating protective immunity in rabbits. Larvae did not develop in XM100F orproduce skin-reactive or protective antigens.

Crude ‘culture’ antigen from cultures in 20% NRS was separated into 4 fractions by nitration on Sephadex G200. All of these fractions gave i/d skin reactions in infected rabbits. Fraction 3 (F3) was the most active, but was shown by acrylamide gel electrophoresis and immunoelectrophoresis to be highly contaminated with rabbit serum proteins. F3 was separated into fractions on DEAE-Sephadex A50, and the third fraction from this was as active as the original culture medium in i/d skin tests, but had only 5% of the original protein concentration. Electrophoresis demonstrated few serum contaminants, and 2 indistinct protein bands that were not present in a similar fraction of NRS.

Neither Sephadex G200 F3 nor DEAE-Sephadex F3 stimulated protective immunity in rabbits, suggesting that antigens stimulating immunity against the establishment of T. pisiformis in rabbits and those provoking cell-mediated immune reactions may be different.

In young (6- to 8-week-old) NTH strain inbred mice expulsion of a primary infection of Trichinella spiralis began on day 8 and was virtually complete by day 11·5. In older mice expulsion occurred 1 or 2 days earlier. Experience of a primary infection elicited strong immunity to challenge, whether the challenge was given immediately after worm expulsion (day 14) or delayed (day 42). Challenge infections were expelled rapidly, the majority of worms being lost during the first day. Immunity to challenge was elicited by low-level primary infections and was effective against large challenge infections. These results are discussed in relation to conventionally accepted parameters of immunity to T. spiralis in mice which, it is considered, are applicable only to mice with a genetically-determined low-level of responsiveness to the parasite.

Changes in the amount and distribution of acid and alkaline phosphatase, non-specific esterase, glycogen, lipid and acid mucopolysaccharide in the caecal wall and liver of turkey poults infected with Histomonas meleagridis have been studied histochemically. A microdensitometer was used to measure changes in activity and distribution of acid phosphatase and non-specific esterase in the caecal mucosa. During the course of the infection there is a marked reduction in activity and distribution of acid phosphatase and non-specific esterase but little change in the amounts and distribution of alkaline phosphatase, glycogen, lipid and acid mucopolysaccharide in the wall of the main part of the caecum. Similar, but smaller, changes occurred in the wall of the neck region of the caecum. In the liver most changes occurred in the immediate vicinity of the parasites. Initially, there was a reduction in the amount of glycogen in the parasitic lesions but later in the infection there was a marked loss of glycogen in all regions of the liver. Changes in the caecum are apparently brought about by the parasite prior to and after invasion of the caecal tissues; changes in the liver occur after it has been invaded.

The biological parameters which generate seasonal fluctuations in the size of populations of Caryophyllaeus laticeps are examined in detail. A mathematical model is formulated to describe the dynamics of the adult parasite within the fish host Abramis brama, and the predictions of the model are compared with observed population data.

The seasonality of parasite population size is shown to be caused by the combined effects of a temperature dependent mortality rate and fluctuations in host feeding activity, which controls the immigration rate of larval parasites. The observed overdispersed distribution of adult parasite numbers within the host population is shown to be generated by variability in feeding habits between individual members of the fish population.

The theoretical population model is used as a framework for the investigation of, among other factors, the comparative influences of the immigration and death rates on the dynamics of the adult parasite, the future behaviour of the system under altered environmental conditions and the importance of chance effects in the dynamics of individual parasite populations within a single host.

The stability of the dynamics of the complete life-cycle of C. laticeps is discussed in relation to the inherent biological complexity of host/helminth parasite interactions.

The genetic basis of virulence in a line (YM) of Plasmodium yoelii yoelii was investigated in a cross with a mild line (A/C). The blood forms of the virulent line developed extensively in mature erythrocytes of mice, causing death of the host within 7 days; infections with the mild line were normally restricted to reticulocytes, infected animals recovering after three weeks. Lines YM and A/C differed additionally in enzyme and drug-sensitivity markers. Studies on infections established from each line alone from sporozoite mixtures of the two lines and from the cross between the lines showed that the appearance of virulence had been caused by a genetic change in the parasite, and not by other factors such as a concurrent infection with another organism. An analysis of the characters of 56 clones derived from the cross showed that the virulence character had undergone recombination with the other markers, and appeared to be inherited in Mendelian fashion. Three clones exhibited atypical virulence, although it was not clear whether this had been produced by genetic recombination.

Immunity to the adult stage of Trichinella spiralis, assessed by an acceleration of worm expulsion, was transferred to recipient mice with mesenteric lymph node cells (MLNC) or serum taken from infected donors. Immunity was transferred most effectively by MLNC taken from donors infected for 8 days, i.e. donors actively responding to infection. Transfer of both MLNC and serum brought about a marked acceleration of worm expulsion in all cases, even where MLNC or serum given separately failed to transfer a significant degree of immunity.

The distribution of the acanthocephalan Pomphorhynchus laevis along the alimentary tract of several species of freshwater fish was studied in both natural and experimental infections. The parasite exhibits a clear preference for a particular region of the alimentary canal, which differs from host species to host species, but it is capable of surviving in all regions of the intestine. In some host species, but not in all, its growth rate may be reduced in regions outside of the preferred site. The parasite remains in the same site throughout the period of infection, emigrations occurring only preparatory to complete loss of the parasite from the host. The presence of cestodes in the region of the preferred site has no effect upon the distribution of the parasite. At high levels of infection the range of the parasite's distribution is extended. The feeding of larval parasites to fish by stomach tube also leads to an extension of range, whereas when fish are allowed to feed on cystacanths removed from their intermediate host the parasite establishes in a more anterior position than usual and in a site that it does not occupy to any extent in natural infections. It is concluded that the distribution of P. laevis in the alimentary canal offish is determined primarily by the process of liberation, activation and establishment of the parasite, and that it normally attaches in the first available space and remains there. Unsuitable physico-chemical or feeding conditions affect only its growth, not the site that it occupies. It is suggested that P. laevis may be atypical in this respect.