The effect of ambient temperature on the ability of Moniliformis dubius to become established in the intermediate host Periplaneta americana, and the effect of temperature on the rate of development of the parasite, has been investigated. Development is postponed at temperatures below 20 °C and infection does not normally occur at 37 °C, the parasites possibly being unable to avoid the defence mechanisms of the host. Both temperatures are tolerated by the host. A marked temperature dependence of rate of development is observed.

Mice which have recovered from infections with P. atheruri are immune to challenge with the homologous parasite and are also strongly resistant to challenge with P. v. chabaudi and, to a lesser extent, B. rodhaini, B. microti and P. b. yoelii. There is little protection against P. v. vinckei and none against P. b. berghei. In the reciprocal experiments, the greatest degree of protection against P. atheruri occurs after P. v. chahaudi infections but over two-thirds of mice are protected by B. rodhaini or B. microti, half by P. v. vinckei or P. b. berghei but hardly any by P. b. yoelii. The considerable degree of cross-protection which exists between P. atheruri and P. chabaudi can be correlated with some antigenic similarities between these parasites but these are not greater than those of parasites which do not protect against one another.

Mice which have recovered from infections with A. garnhami do not succumb to; challenge with the piroplasms B. rodhaini or B. microti and over half such animals are protected against the malaria parasites P. v. vinckei and P. v. chabaudi. This protection does not extend to the malaria parasites P. b. berghei or P. b. yoelii. Mice which have recovered from infections with the piroplasms are protected against A. garnhami, as are those which have recovered from infections with P. v. vinckei or P. v. chabaudi. About half the mice which have recovered from infections with P. b. berghei or P. b. yoelii are also protected against A. garnhami. This interaction between intra-erythrocytic protozoa may influence the incidence of these parasites in the wild. A. garnhami is unlikely to be of any use in the immunization of humans against malaria but could be of use in the protection of cattle against piroplasmosis.

Mice which have recovered from, infections with Trypanosoma musculi exhibit no immunity to challenge with the malaria parasites, Plasmodium v. vinckei, P. v. chabaudi, P. b. berghei, P. b. yoelii or P. atheruri, the piroplasms Babesia rodhaini or B. microti or the dactylosomid Anihemosoma garnhami. Recovery from infections with any one of these parasites confers no immunity against T. musculi. These results indicate that heterologous immunity between intra-erythrocytic protozoa does not extend to trypanosomes.

(1) Spermophilus columbianus, S. franklinii, S. lateralis tescorum, S. richardsonii, S. tridecemlineatus and s. undulatus plesius (ground squirrels) were collected from Alberta and Yukon Territory (S. u. plesius only), Canada. Trypanosome infections were detected in all but S. l. tescorum. (2) All six species of squirrels were experimentally inoculated with trypanosomes from naturally infected S. columbianus, S. franklinii, S. richardsonii and S. tridecemlineatus. The strains from S. columbianus, S. richardsonii and S. tridecemlineatus produced infections in some or all of the other host species, whereas the strain from S. franklinii did not. (3) Natural transmission of trypanosomes has not been demonstrated, but it seems likely that fleas act as vectors. (4) Intensities and durations of experimental infections were variable and depended upon the trypanosome strain, host species and individual. Infections were non-pathogenic. (5) Previously infected ground squirrels could not be reinfected with the same or a different trypanosome strain and immunity appeared to be lifelong. (6) Size comparisons among and within trypanosome strains from naturally and experimentally infected ground squirrels revealed that the strains from S. franklinii and S. tridecemlineatus were similar to one another, but different from the other three trypanosome strains (those from S. columbianus, S. richardsonii and S. undulatus). (7) It is suggested that there is only one species of trypanosome, Trypanosoma otospermophili, in the genus Spermophilus in North America and that differences in size and types of infection are due to host-induced variations (i.e. they are plastoxenodemes). T. spermophili (present in Eurasian Spermophilus spp.) might be synonymous with T. otospermophili.

During the summers of 1969–71, 554 ground squirrels belonging to six species (115 Spermophilus columbianus, 46 S. franklinii, 13 S. lateralis, 327 S. richardsonii, 20 S. tridecemlineatus and 33 S. undulatus) were sampled for trypanosomes. Natural infections were recorded in all but S. lateralis. In 1969 and 1970 the prevalences of infection were approximately equal within, but varied between each species of ground squirrel. In 1971 the prevalences of infection increased for all host species sampled. Trypanosomiasis was detected in twice as many ground squirrels when the haematocrit centrifuge technique was used for diagnosis instead of stained thin blood films. In S. richardsonii the prevalences of infection varied from month to month but showed the same trends in both 1969 and 1970. Prevalence was lowest in May and highest in June. It is suggested that susceptible juveniles accounted for the high prevalence in June.

Gland cells surrounding the oesophagus in the redia of Fasciola hepatica are described. These cells produce electron-dense, membrane-bound vesicles which have an unusual mechanism for secretion of their contents.

A method for detecting antibody activity in vitro against the invasive stages of E. tenella and E. maxima is described. Treatment of cultured chick kidney monolayers with serum (or a globulin fraction thereof) from immunized birds caused a reduction in the numbers of intracellular stages 24 or 48 h after inoculation of sporozoites. This reduction was seen when serum was incorporated in the culture medium at the time of sporozoite inoculation and also, but to a lesser extent, when the monolayers had been pretreated with the antibody-containing medium which was washed off prior to infection. The results correlated reasonably well with those obtained when the sera were tested in embryo (E. tenella) or in vivo (E. maxima). Similar treatment of cultured CAM monolayers in which infection with E. tenella was well established (101 h), did not greatly affect the development of the parasite. This suggests that the anti-coccidial activity is either effective against the sporozoite stages only, or is unable to affect parasites within cells.

Detailed descriptions are given of the larvae of the three species of British metastriatan ticks, with particular reference to the arrangement and dimensions of setae.

A modification of the ‘floating raft’ method of in vitro penetration study is described. This allows replication of results and additional conveniences in operation. Filter paper membranes have shown characteristics similar to those of skin and have been used for quantitative studies. A method of digesting penetrating larvae from skin has been used to reduce the variability inherent in whole skin. Neither chloroform soluble skin products nor gravity were found to have a significant effect on the number of larvae entering membranes. Exsheathment of larvae was not essential prior to penetration and no specific stimuli for exsheathment appear to be necessary. Scanning electron micrographs have shown that entry may be by either hair follicles or desquamations of the stratum corneum. No evidence of enzymic activity during the invasion process has been found so far and the results suggest that a mechanical rather than chemical system obtains.

The distribution of larval ticks in relation to vegetation cover was studied on two coastal farms in the Port Alfred district of the Cape Province. The following five species were found: Boophilus decoloratus (Koch), Amblyomma hebraeum Koch, Ixodes pilosus Koch, Haemaphysalis silacea Robinson and Rhipicephalus evertsi Neumann. B. decoloratus predominated in short protected vegetation, I. pilosus and H. silacea in short covered vegetation and A. hebraeum in medium-to-tall protected vegetation. R. evertsi was collected in too small numbers to allow any correlation to be established. Both I. pilosus and H. silacea demonstrated activity peaks during the winter months. Microclimatic measurements indicated that larval ticks were not usually collected in microhabitats which experienced midday saturation deficits in excess of approximately 10 mm Hg. Behavioural studies on larval ticks climbing glass rods demonstrated the possible association of larvae with a definite vegetation height. The optimal vegetation heights were correlated with field data. The water balance of some tick species was studied and it was found that at 26 °C a relative humidity of 70% or more (i.e. above 7·53 mm Hg saturation deficit) was required by these larvae. Larvae lost water to the atmosphere at humidities lower than this value and took up water vapour from the atmosphere at values higher than 70% R.H. They were shown to be able to imbibe water through the mouthparts, and this possibly has survival value.

Toxocara vajrasthirae is described from the hog-badger (Arctonyx collaris) of Thailand. Of the species in the genus Toxocara, it most closely resembles T. canis. It differs in the possession of proportionately shorter and narrower cervical alae, relatively longer spicules, a coarse, reticulate pattern on the eggshell and an undivided part of the uterus which is longer than the divided part.

The behaviour of the third-stage larvae of Nippostrongylus brasiliensis has been studied on rat, mouse and human skin. Locomotion in thin films of moisture is by two-dimensional, undulatory propulsion on skin, and by three-dimensional, undulatory propulsion on hairs. In drops of water or in thick films of moisture locomotion is less efficient than in thin films. It is suggested that the thin film of moisture around larvae on the skin may become coated with a thin monolayer of lipid which could reduce evaporation from the surface of the nematode and from the film of moisture, thus extending the period before desiccation begins. Studies with the electron microscope have shown that the larvae move horizontally into the stratum corneum and later penetrate the epidermis and dermis by separating the constituent cells of these tissues. In the dermis there is dissolution of collagen around the larvae. It is suggested that this separation of cells and dissolution of collagen need not be brought about by enzyme action, as similar effects can be brought about by changes in pH or ionic composition of the bathing medium.

The sensory organelles and some components of the nervous system of the adult filarial worm Dipetalonema viteae have been studied by means of electron microscopy. The sensory organelles contain modified cilia innervated by nerve axons. The cilia lie within cuticle-lined channels. The amphids each contain nine such cilia, but the phasmids and the papillae contain only one cilium. The amphidial and phasmidial cilia are exposed to the external environment and it is suggested that I they function as chemoreceptors. The cilia within the cephalic and caudal papillae are modified so as to produce a greater area of sensitivity. The ciliary channels in the cephalic papillae do not open to the exterior and these papillae therefore probably function as mechanoreceptors. The caudal papillae of the male worm are structurally similar to the cephalic papillae, except that the ciliary channels are open to the external environment. It is suggested that these papillae function both as mechanoreceptors and chemoreceptors. The terminal pair of caudal papillae differ from all the other papillae. The ciliary channels of all the sensory organelles are surrounded by supporting cells, but multivesicular cells are only associated K with the amphids, phasmids and cephalic papillae. The nerve-ring encircles the muscular region of the oesophagus; it is composed entirely of nerve axons. The nerve axons contain two kinds of vesicle. The nervering is surrounded by an ensheathing cell. The major nerve bundles are carried by the hypodermal cords, but they are not embedded in the hypodermal cytoplasm. The nerve axons at the surface of the cord form synapses with the innervation processes of the somatic muscle cells. Electron cytochemical techniques have been used to investigate the distribution of cholinesterases in the structures described. Acetylcholinesterase has been identified in the amphids, phasmids, cephalic papillae and nerve-ring; it is suggested that acetylcholine functions as a chemical transmitter at these sites. Pseudocholinesterase has been identified in the amphids, phasmids and cephalic papillae, but not the nerve-ring; it is present in considerably smaller quantities than acetylcholinesterase.

A technique is described for obtaining consistent reinvasion and multiplication of the erythrocytic stages of Plasmodium falciparum from humans in vitro. Subsequent to this, P. falciparum was subcultured through almost three cycles in vitro, during which multiplication of the parasite was obtained in the first but maintenance or only slight increase in numbers was obtained in the second subculture. The effect of sera from adult Gambians on the reinvasion of P. falciparum was measured in vitro both morphologically and by the incorporation of [14C]isoleucine into parasite protein and by the incorporation of [3H]adenosine into parasite DNA and RNA. Two sera reduced the amount of reinvasion.

The ultrastructure of the body wall of the redia of Sphaeridiotrema globulus is described. The tegument, which possesses numerous microvilli, is shown to be a syncytial, cytoplasmic layer connected to underlying subtegumentary ‘cells’ by cytoplasmic processes. Between the subtegumentary ‘cells’ are located the normal parenchyma cells of the body wall, which are believed to be important in carbohydrate storage. Mitochondria, Golgi complexes, endoplasmic reticulum and beta glycogen granules are located in the tegument and subtegumentary ‘cells’. In addition, large whorled bodies and dense secretory granules appear to be formed within the subtegumentary ‘cells’ prior to their passage into the outer tegument. It is suggested that these whorled structures are contributing to the growth of the tegument. Small membrane-bound ‘vesicles’ are also seen in the tegument and some of these may be pinocytotic in nature. Following incubation, horseradish peroxidase tracer was localized within ‘vesicles’ in both the tegument, subtegumentary ‘cells’ and parenchyma cells, which indicates that the redial body wall may be important in the uptake of nutrients. Phosphatase enzymes are abundant within the tegument, particularly in association with the plasma membranes and microvilli. These enzymes, which appear to have their origin in the endoplasmic reticulum of subtegumentary cells, are believed to be associated with the uptake of nutrients through the redial tegument.

A comparison was made of the immunogenicity of normal worms, derived from a primary infection, and immune-adapted worms, derived from a trickle infection, after surgical transfer into previously uninfected rats. Exposure to 300 normal worms for 2 days was sufficient to induce resistance to challenge infection 3 weeks later in four of eight rats. The immune-adapted worms were, however, significantly less immunogenic, only one rat in ten developing resistance to challenge after 21 days exposure to 300 worms. These results offer further evidence that the protective antigens produced by normal worms are in some way altered in adapted worms, so that adapted worms have a reduced immunogenicity.