Inhibited development of Haemonchus contortus was studied in single experimental infections of worm-free lambs. Chilling of the infective larvae at + 4°C was without effect on the percentage of larvae subsequently becoming inhibited and a period of exposure to autumnal conditions was unnecessary to induce a high rate of inhibition. It was concluded that seasonal inhibition of H. contortus in East Anglia is brought about primarily by an environmental stimulus acting upon the preparasitic stages but that, unlike Obeliscoides cuniculi and Ostertagia ostertagi, this was not cold. It could be provided in a culture kept in the dark at 25 °C for 12 days. While the age of the host did influence the phenomenon, in that larvae were less inclined to inhibition in very young animals, it was concluded that this was not a primary factor in the aetiology.

The ultrastructure of the seminal vesicle, ejaculatory duct, cirrus sac and cirrus is described. The epithelium of the seminal vesicle consists of a single layer of squamous to cuboidal cells. The apical ends of the cells have thin polymorphic lamellae and long narrow pits, both of which enclose normal spermatozoa. The cells have a moderate amount of GER and Golgi complexes which produce a lucid secretory body. The ejaculatory duct epithelium is composed of cuboidal to columnar cells between or through which project the terminal parts of the ducts of the unicellular prostate glands. The apical surfaces of the epithelia are extended into triangular or filiform projections having thin sinuous lamellae. The cytoplasm contains GER cisternae and Golgi complexes which synthesize a dense ovoid secretion. The cirrus sac and cirrus are covered by a thin modified tegument. The cirrus has many spines and the normal ratio of T1 to T2 type of secretory bodies, whereas the cirrus sac has few spines and the T2 type of secretory body predominates over the T1 type. The significance and possible functions of the structures observed in the three tissues are discussed.

Antigens sharing determinants with surface membranes and soluble proteins of adult Schistosoma mansoni have been detected in culture media after incubation of radioactively labelled worms. The relative quantities of these antigens were measured with specific antisera raised in rabbits and with serum from an immune rhesus monkey. It was found that 12–16% of TCA-precipitable radioactivity in the culture medium consisted of membrane antigens and 6–8% consisted of antigens sharing determinants with proteins found in the soluble fraction of adult worms. Over half the membrane antigens were present in particulate form, while other antigens were present in solution. Surface labelling the adult worms with [125I]confirmed that some of the particles in the culture medium were derived from the surface membrane of the adult worm and electron microscope examination of such particles showed that large membrane fragments were present. These results support the hypothesis that anti-bodies against schistosome membrane antigens are induced by particulate membrane antigens released by the parasite.

In one (1c) and six (6c) cysticercoid primary infections of Hymenolepis diminuta in NIH (inbred) and CFLP (outbred) male mice 6±1 weeks old > 85% of the worms established but were rejected (destrobilated or expelled) subsequently. Rejection occurs more quickly in 6c infections than in 1c infections. Considerable worm growth occurs in 1c and 6c primary infections but worms from 6c infections weighed less than worms from 1c infections on all days studied.

Expulsion of H. diminuta does not occur more rapidly in secondary infections than in primary infections; loss of 6c secondary worms occurs at the same rate as 6c primary worms but 1c secondary worms survive longer than 1c primary worms. Although worms are not lost more quickly in secondary than in primary infections, they are affected at an early age by the immune response which stunts their growth. Increasing the intensity of primary and secondary infections increases the severity of stunting of secondary worms.

The results are discussed and it is suggested that immune responses to Hymenolepis spp. in rodents are common but that thresholds of worm numbers exist below which appreciable worm loss does not occur. Stunting due to crowding, which generally is attributed to inter-worm competition, may be in part immunologically mediated. For future immunological studies attempting to induce secondary responses to H. diminuta in mice, worm growth, not survival, is the criterion to evaluate.

Protopolystoma xenopodis (Price, 1943) Bychowsky, 1957 is a parasite of the African clawed toads, species of Xenopus. Adult worms in the host's urinary bladder produce up to 15 eggs/24 h; these are voided with the urine and hatch after 20–24 days (at 26 °C). The swimming oncomiracidia enter the cloaca of the toad and travel via the urinary ducts to the kidneys. Larval development in the kidneys lasts 8–12 weeks (at 22 °C) and the juvenile parasites then migrate to the urinary bladder where maturity is reached after a further 3–4 weeks, i.e. 3–4 months after invasion. Egg production is continuous and parasite longevity has been found to exceed1½ years. Rarely, the adult worms may undertake a return migration from bladder to kidneys.

The morphology of the oncomiracidium is described and compared with available accounts of other polystomatid genera. Consideration of data for the genus Polystoma suggests that the arrangement of the epidermal ciliary cells is relatively constant at the inter-specific level. Comparison of Protopolystoma with Polystoma, Diplorchis and Oculotrema reveals that whilst the ciliary cell patterns are clearly related there are important differences in both cell number and disposition at the inter-generic level. Differences are also evident in the distribution of epidermal sensillae in Protopolystoma and Polystoma. Protopolystoma is distinguished from all other polystomatids infecting anuran hosts by the presence of two pairs of haptoral hamuli.

The double isotope labelling method of Arias, Doyle & Schimke (1969) and Dehlinger & Schimke (1971) was used to determine the relative rates of turnover of proteins in various tissue fractions and in the culture medium of adult Schistosoma mansoni. The majority of proteins in the surface membrane turned over at about the same rate. This is consistent with a model for membrane assembly and degradation involving multi-protein units. The proteins in the other fractions examined, except frozen-thawed supernatant fluid, also turned over at about the same rate. In the frozen and thawed supernatant fluid, the higher molecular weight (>40000 daltons) proteins turned over at a much greater rate than the lower molecular weight proteins (<40000 daltons). The antigens in the culture medium, both particulate and soluble, had a higher turnover rate than the worm tissue proteins. It is concluded that the culture medium antigens are released as the result of two distinct processes: (a) surface membrane turnover and (b) a rapid secretory process. It is possible that the high molecular weight proteins found in the frozen-thawed supernatant fluid are involved in the rapid secretory process. The culture media of 6-day and 16-day schistosomula were also examined by the double isotope method. The culture medium of the adult worm and 16-day schistosomula contains more material presumed to be secreted than does that from the 6-day schistosomula. This may explain the poor immunogenicity of young schistosomula.

Mice infected with Trichuris muris during lactation were unable to expel the infection at the normal time, but expulsion occurred when lactation was terminated. Suppression of expulsion was uniform in mice suckling more than five young but variable with smaller litters. Mice exposed to a primary infection while lactating were shown to have serum antibodies capable of passively transferring immunity to recipient mice and showed near normal immunity to a secondary infection given after lactation had ceased.

Acquired immunity to T. muris was also suppressed by lactation, but the worms which became established in lactating resistant mice were fewer and smaller than those in non-lactating, non-resistant controls.

It is suggested that the suppressive effect of lactation in this hostparasite relationship is exerted on the second, lymphoid cell-mediated phase of worm expulsion.

Oncospheral agglutination given by sera immunized with Hymenolepisnana eggs is described as a new way of assessing H. nana infection. All sera of mice which possessed acquired protective immunity against reinfection by H. nana eggs had the potency to induce oncospheral agglutination in vitro. Only oncospheres, which had been hatched, agglutinated, no agglutination occurred in sera from uninfected mice. Oncospheral agglutination was carried out by mixing 0·1 ml of serial two-fold dilutions of serum and 0·1 ml of Hanks' balanced salt solution containing about 600 hatched oncospheres. Titre of agglutinins was indicated as a reciprocal of the final dilution capable of giving agglutination clusters made of three or more oncospheres. Agglutinins developed within 14 days after a primary infection with 500 shell-free eggs. There was no rapid increase of agglutinins within 4 days following a secondary infection. The titre increase coincided with the increase in dosages of eggs. Agglutinins were thought to be immunoglobulins, because the potency of the serum to agglutinate oncospheres was extinguished after absorption of globulins with rabbit anti-mouse globulin serum.

Agglutinins were produced in rabbits by intravenous injections of shell-free eggs. The titres of the rabbit sera were much higher than those of mouse sera.

Natural leishmanial infections among feral mammals in the Republic of Panama were studied in relation to gross skin alterations. Three dermatotropic species of Leishmania, L. braziliensis, L. mexicana and L. hertigi, isolated from 230 sylvatic animals of 5 mammalian orders were involved. In 205 (89%) of the cases there were no gross skin alterations. Cryptic infections of all three leishmanial species among wild animals were confirmed by culture and hamster inoculation. This type of infection was always present in the sloths, Choloepus hoffmanni and Bradypus infusatcus, infected with L. braziliensis and in the porcupine, Coendou rothschildi, infected with L. hertigi. Suggestive skin alterations were encountered in 10 (4%) of infected animals. Only 15 (7%) of infected animals exhibited typical leishmanial lesions. It was possible to correlate skin depigmentation with the course of leishmanial infection in one animal, a young kinkajou, Potos flavus.

When adult Nippostrongylus brasiliensis were maintained in vitro they became damaged. Using the criteria of ultrastructural morphology, acetylcholinesterase isoenzyme pattern and the behaviour of the worms after transfer to a normal rat, this damage appeared to be similar to that produced by the in vivo action of antibodies.

Antibodies were shown to be responsible for the anterior migration of adult worms which occurs during primary infections in mature rats and in the prolonged infections seen in lactating and immature rats.

Antibody damaged worms and worms unaffected by antibodies were equally able to stimulate the immune response required for worm expulsion. Apparently antibody damage is not required for the initiation of the second immune component necessary for expulsion of this parasite.

Fixation of carbon dioxide has been demonstrated for extracts from Crithidia fasciculata, Trypanosoma mega and Trypanosoma brucei brucei bloodstream and culture forms. The enzymes involved in this fixation were found to be ADP-stimulated phosphoenolpyruvate carboxykinase (E.C. 4. 1. 1. 32), ‘malic’ enzyme (E.C. 1. 1. 138–40) and pyruvate carboxylase (E. 0. 6. 4. 1. 1). The subcellular localization of these enzymes has been investigated in all three organisms. Products of short and long term fixation experiments were separated and identified.

The importance of carboxylation reactions is discussed in relation to the maintenance of oxidized and reduced coenzyme levels.

A culture chamber fitted with a polycarbonate sieve has been used to isolate Plasmodium knowlesi merozoites as they are released from schizonts. A 3 μm pore-size sieve allows passage of normal erythrocytes and red cells containing rings and trophozoites and can be used to concentrate schizonts from a mixed cell population. A 2 μm pore-size sieve retains normal and parasitized cells and provides uncontaminated merozoites in high yield (5 × 1010 merozoites per ml schizonts). Merozoite viability diminishes rapidly during 30 min after isolation. These preparations should prove valuable for studies of the biochemical, physiological and antigenic properties of this transient extracellular phase of the malaria parasite.

The epidemiology of Haemonchus contortus was studied in north east England in lambs reared under field conditions with infected ewes. The parasite population was monitored by pasture larval counts, faecal egg counts and lamb postmortems. From 25 June to 11 November, eight lambs were slaughtered each month, together with eight worm-free tracer lambs. Postmortem worm counts showed that on 25 June, small numbers of adult H. contortus were present but no inhibited larvae. However, subsequently the number of worms recovered rose rapidly to 1000–5000 and the percentage inhibition increased to 57% in July, 75% in August and virtually 100% in September, October and November. On each sampling occasion no significant differences in worm count or percentage inhibition were recorded between experimental and tracer lambs. These results suggest that neither autumn climatic effects nor host immunity were responsible for inhibition in this strain of H. contortus.

The endogenous and exogenous stages of a species of murine Eimeria were examined and compared with descriptions of other species isolated from Mus musculus. Developing stages of the parasite were found in the epithelial cells of the caecum and colon. Three asexual generations were observed, the third becoming apparent on the fourth day concurrently with gametogeny. The total length of the prepatent period was 5 days. Oocysts of the coccidian, identified as Eimeria falciformis, were administered to a group of germfree mice. Despite a dose level of 1 ü 106, the coccidium was not established. Histological examination indicated the failure of the sporozoite to penetrate its host epithelium. The possible causes of this phenomenon were discussed.

Hydrocortisone treated male and female mice, given a primary infection with Aspiculuris tetraptera, did not reject the worms during the third week of infection. Mice given hydrocortisone during the first week of infection had elevated worm burdens on day 10, suggesting that some worm loss was encountered during the anterior migration in control mice. Furthermore, this temporary period of treatment was sufficient totally to suppress rejection and to allow the parasite to persist until day 28. Methotrexate also significantly delayed rejection, but larval growth was retarded in treated mice. These results, it is suggested, add strength to the hypothesis that the loss of A. tetraptera in a primary infection in mice, is an immunological phenomenon.

The structure and invasive behaviour of extracellular erythrocytic merozoites prepared by a cell sieving method have been studied with the electron microscope.

Free merozoites contain organelles similar to those described in late schizonts of Plasmodium knowlesi. Their surface is lined by a coat of short filaments. On mixing with fresh red cells, merozoites at first adhere, then cause the red cell surface to invaginate rapidly, often with the formation of narrow membranous channels in the red cell interior. As the merozoite enters the invagination it forms an attachment by its cell coat to the rim of the pit, and finally leaves this coat behind as it is enclosed in a red cell vacuole. Dense, rounded intracellular bodies then move to the merozoite periphery, and apparently rupture to cause further localized invagination of the red cell vacuole. The merozoite finally loses its rhoptries, the pellicle is reduced to a single membrane and the parasite becomes a trophozoite. Invasion is complete by 1 min after adhesion, and the trophozoite is formed by 10 min.

A direct life-cycle in Reighardia sternae, a cephalobaenid pentastomid of gulls was investigated: the work was prompted by a report of eggs and larvae recovered from the stomach and intestine of a naturally infected gull.

Infective pentastomid eggs were obtained by surgically transplanting maturing female Reighardia, taken from freshly shot wild gulls, into captive recipients. Faecal material from birds thus artificially infected was collected daily and examined for eggs. Eggs were force fed to 33 hand-reared (from eggs or nestlings) juvenile gulls which were selected at random and sacrificed at intervals thereafter and examined for pentastomids.

One hour after infection, primary larvae appear in the body cavity where they moult immediately. They grow steadily and by 27–35 days are sexually differentiated, and by 66 days have copulated. Fertilized females take a further 116 days to produce eggs by which time they are 7·6 cm long.

The complex migrations undertaken by developing larvae in the gull, and the problems of the mechanism of direct transmission, are discussed.

The distribution of acetylcholinesterase in mother and daughter sporocysts of Schistosoma mansoni was studied histochemically. In young mother sporocysts derived from miracidia cultured in vitro the miracidial neural mass and flame cells were shown to persist. The nerve trunks and commissures, as well as papillae, are apparently lost in the transformation process. In young daughter sporocysts freshly dissected from mother sporocysts there was little enzyme activity except for a sparse distribution in the tegument. After cultivation, intense enzyme activity was associated with developing cercarial embryos. A similar distribution of activity was observed in older daughter sporocysts obtained from the digestive gland of snails. No evidence of flame cells, neural mass, or commissures was detected in daughter sporocysts by the methods employed.

Effect of gamma radiation on oocysts of Eimeria necatrix was investigated. It was observed that oocysts exposed to 200 kR or above did not sporulate. Irradiation at 10–150 kR caused a progressive decrease in sporulation. Irradiation affected normal development of unsporulated oocysts as the zygote protoplasm divided into unequal masses or was shattered into granules.

Increase in the intensity of irradiation of sporulated oocysts resulted in the progressive decrease in severity of the resultant infections in chicks and their effects – mortality, type of lesions developed, total oocyst production and immunity produced – were comparable with infections induced by decreasing the number of unirradiated oocysts. Infection produced by 1000 unirradiated oocysts was comparable with that resulting from 50000 oocysts irradiated at 25 kR. Infection obtained with 20000 unexposed oocysts approximated to that produced by 50000 ooeysts irradiated at 2·5 kR. It was concluded that irradiation abolished infectivity of the oocysts/sporozoites rather than bringing about attenuation of the parasite.

When protoscoleces of Echinococcus multilocularis were cultured in vitro, under axenic conditions in either monophasic or diphasic media, segmentation was suppressed in most organisms, some 70–80% of which developed into unsegmented, monozoic forms with a complete set of sexually mature male and female genitalia. The most striking feature of monozoic worms was the large lateral swelling produced by the cirrus sac the effect being to produce organisms with an unusual asymmetric shape. Worms which did not become monozoic either (a) underwent some somatic growth, developed two sets of genitalia and became ‘pseudosogmented’, i.e. with the inter-proglottid membranes absent or poorly defined, or (b) became vesicular or abnormal. The mechanisms which could be involved in the suppression of somatic growth and the induction of the monozoic condition, are examined in terms of cell lineage. The possible significance of these results in understanding the evolution of the cestodes is discussed.