After hamsters have been infected with Dipetalonema viteae for about 4 months, microfilariae disappear from the circulation but the adult worms remain alive and active. Such hamsters are said to harbour a ‘latent’ infection. Additional adult worms implanted into such hamsters produce no microfilariae, but adult worms recovered from hamsters with latent infections resume microfilariae production when surgically transplanted into new hosts. In vitro, worms from hamsters with latent infections renewed microfilariae production in the presence of serum from uninfected but not infected hamsters. Serum from infected animals had a marked suppressive effect on the microfilariae production of adult worms implanted into passively immunized hamsters. This suggests that microfilarial release is regulated by a serum factor. Microfilarial production in in vitro and in vivo experiments was more strongly inhibited by serum taken from hamsters with latent infections and hamsters infected for only 30 days (pre-patent) than by serum taken 90 days after infection (whilst microfilariae were in the circulation of the serum donors). A cytotoxic effect of sera from infected hamsters on the microfilariae could not be demonstrated.

In animals immunized by extracts of adult worms or microfilariae, microfilaraemia was considerably reduced. In contrast, animals immunized repeatedly with the extract from small numbers of microfilariae during the pre-patent phase of an infection had an enhanced microfilaraemia. Adult worm numbers were not affected by immunization except in animals given extract from a single female worm which harboured fewer worms than controls.

There were no differences in mean intestinal worm burdens 8 days after subcutaneous injection of 4000 infective larvae of Strongyloides ratti into rats in dioestrus, pro-oestrus, oestrus and metoestrus. Thus, changes in the hormonal environment of the migrating larvae dependent on the oestrous cycle did not alter the worms' destination or affect their potential for development. In particular, the results are prima facie evidence that prolactin is not, on its own, responsible for the re-orientation of larvae in the tissues of nursing mothers. Other sources of variability in experimental S. ratti infections are analysed and the ‘exact dose’ technique offered as a corrective for some procedural errors.

Mature male Dipetalonema viteae released a substance(s) which caused eahanced microfilaraemia in infected hamsters. In hamsters implanted with female D. viteae, the microfilaraemia of a subsequent infection was suppressed. The microfilaraemia of female worms implanted in hamsters was depressed within 5 days when the animals were given a further infection with infective larvae.

The freeze fracture technique has been used to quantify changes in the integral components of the double outer membrane of Schistosoma mansoni during the 6-week period of development within the mouse. The intramembraneous particle (IMP) density on the P1 face begins to rise Within 6 h of host penetration, reaches a maximum at day 4 and then falls rapidly after day 9, so that it is at a low level between 3 and 6 weeks. The E1 face IMP density follows the same course as that of the P1 face except that maximum particle density is recorded on day 1 and the counts begin to fall on day 5. The IMP density on the P1 face remains at a consistently low level throughout development. The E2 face IMP density rises gradually to a peak at day 4, when the parasites have migrated to the lungs, and remains thereafter at a similar level, so that by 6 weeks the E2 face has a higher IMP density than the other three fracture faces. The E2 face IMP show a marked increase in size on day 4. Morphological studies indicate that a different type of inclusion body makes a transient appearance in the tegument of the lung worms, and immunocytochemical techniques show the lung worms to be nonimmunogenic. It is suggested, therefore, that the E2 face IMP may represent complexes of parasite antigens and acquired host antigens. The tegumental membranes of cultured specimens have also been examined by freeze fracturing and the IMP densities compared with those obtained from in vivo parasites; the cultured schistosomula have a lower E2 face particle density than the in vivo specimens.

Changes in the ultrastructure of the daughter sporocyst and contained metacercariae of Microphallus pygmaeus during culture in artificial seawater and modified Medium 199 were examined and compared with variations in oxygen consumption. The very early commencement and rapid progression of autolysis in the sporocyst body wall as compared with little or no change in most metacercariae in both media, together with the elevated oxygen consumption in the nutrient medium, suggests that most of the oxygen is consumed by the contained metacercariae. The almost simultaneous onset of body wall degeneration in both media suggests that the nutrient medium is not suitable for the maintenance of healthy daughter sporocysts.

The size and form (length, width, and length: width ratio) of eggs of Diphyllobothrium dendriticum, D. ditremum and D. laturn vary considerably among individual worms within each species. The size of eggs varies with host species and a decrease in egg size with increasing intensity of infestation is indicated. The egg size of D. laturn increases during the first 10–12 days of egg production. For single worm infections in golden hamsters the mean egg length and width of D. ditremum are significantly smaller than the corresponding means of D. dendriticum and D. latum, while D. latum has significantly wider eggs than D. dendriticum. As taxonomic characteristics, egg size and form may contribute to species delimitation at the population level. For identification at the individual level the best possible accuracy is about 80%. This accuracy is considerably reduced when variation in host species and intensities of infestations are introduced. Scanning electron microscope studies did not reveal any differences among eggs of the three species.

The ultrastructure of the apical surface of the gastrodermal cells of Fasciola hepatica has been investigated by transmission (TEM) and scanning electron microscopy (SEM). The surface is composed of triangular or rhomboidal lamellae which may be subdivided at their margins. The lamellae may have a smooth or wrinkled surface and apical microvilli of varied length and number. Spherical bodies may be attached to the sides or tips of the lamellae.

The lamellae of individual cells are grouped to form either a series of overlapping, concentrically arranged leaflets or a series of sinuous lamellae running parallel with the cell's long axis. Lamellae may also divide, anastomose, or cross from one cell to an adjacent cell.

The lamellae have an internal skeleton consisting of laterally orientated ‘fibres’ which are more or less evenly spaced and parallel with the cell's apical plasma membrane. There is some evidence that the ‘fibres’ are composed of a number of very small tubules in a dense matrix. The ‘fibres’ are connected to the plasma membrane and to each other by dense amorphous material.

Groups of the snail Lymnaea truncatula, maintained at a range of spatial densities, were exposed to different densities of miracidia of Fasciola hepatica. The resulting degree of parasitization was measured 3–4 weeks after infection. The relationship between parasite densities and parasitization appeared to be curvilinear. The lower than expected parasitization at high parasite densities could be explained by the multiple infection of some snails by two or more miracidia.

The level of parasitization was not related exponentially to the temperature at which infection was carried out. This was thought to be due to the inverse relationship between miracidial longevity and swimming speed, with respect to temperature. A depth of free water overlying a mud surface was an absolute requirement for miracidia to successfully infect snails.

In the fully formed daughter sporocyst of Microphallus similis (Jäg.) the tegument is divided into a wide, outer cellular epithelium and a narrow, inner electron-dense anucleated microvillous syncytium. The narrow subtegument consists of circular and longitudinal muscle, nervous, synthesizing and germinal cells. It is suggested that during development of the tegument, an undifferentiated nucleated syncytium becomes divided by the formation of membranes between the nuclei and between the narrow, inner electron-dense microvillous and the outer cellular regions. The outer nucleated region of the tegument is homologous to the nucleated investing syncytium which surrounds redial and cercarial embryos and which is shed later in development. The retention of the nucleated region in some daughter sporocysts is, therefore, an example of paedogenesis.

Synergy between clopidol and methyl benzoquate against Eimeria maxima was shown to be supra-additive. Collateral sensitivity to these drugs could not be demonstrated in resistant lines of this parasite. Resistance to methyl benzoquate and clopidol was not transferred when lines of E. maxima, resistant to the respective drugs, were propagated together. The failure to demonstrate this phenomenon was judged not to be due to synergy between the drugs. Attempts to induce simultaneous resistance to both drugs failed, although resistance to methyl benzoquate was readily acquired by a line of E. maxima resistant to clopidol. Induced resistance to clopidol in a methyl benzoquate-resistant line required numerous passages.

Theileria mutans (Aitong) isolated from cattle exposed in the Narok District of Kenya and blood-passaged through cattle 8 times, appeared to have lost its original pathogenicity for cattle. It was demonstrated that the parasite was trans-stadially transmissible by the tick Amblyomma variegatum but not by Rhipicephalus appendiculatus. Four tick–bovine passages were made using A. variegatum, and infective parasites were also harvested from A. variegatum nymphs which had been fed for 5 days on rabbits. Blood containing piroplasms, or lymphoid cells infected with schizonts taken from cattle, at the 2nd tick–bovine passage were shown to be infective on inoculation.

The course of the T. mutans infections in the cattle was studied. In tick-induced infections macroschizonts occurred transiently, persisting longest in circulating lymphoid cells. Microschizonts were rarely detected. The macroschizonts were morphologically distinct from those of other Theileria species described from East Africa.

In tick-induced infections, the piroplasm parasitaemia increased rapidly and there was evidence of restored pathogenicity of the parasite since high piroplasm parasitaemias were associated with a marked anaemia.

Larvae of Boophilus microplus were labelled with [32P] and used to study the rejection of larvae from British breed cattle with different levels of resistance. On animals of high resistance the loss of larvae due to grooming ranged from 9–54% during the first 24 h of infestation, and more time was spent grooming by these animals. Most of the grooming activity could be attributed to the presence of tick larvae and the presence of older tick stages did not increase the percentage of larvae lost. Grooming was directed to attached larvae and these could be removed. Animals of low resistance did not lose a significant number of larvae as a result of grooming, but all previously infested hosts lost a proportion of the larvae (18–39%) which could not be accounted for by grooming. This loss was greater than the total loss of larvae (11%) on animals not previously exposed to B. microplus.