Infections with the intestinal flagellates Giardia muris and Spironucleus muris are accompanied by a depression in the ability of mice to mount an immune response to a thymus-dependent antigen (sheep red blood cells) but not to a thymus-independent antigen (TNP-lipopolysaccharide). The number of splenic IgM plaque-forming cells and haemagglutination titres, of both IgM and IgG, to sheep red blood cells decreased between days 10 and 21, which correlated with the time of maximal trophozoite levels in the small intestine. The number of background IgM plaque-forming cells to sheep red blood cells or DNP was not significantly different from controls in either infection. No evidence for systemic macrophage activation was associated with these infections. In fact, adherent peritoneal exudate cells (PEC) from infected mice were slightly less cytostatic against target tumour cells than adherent PEC from normal mice, at a time when the parasites were being eliminated from the small intestine.

The fore-gut of the redia of Parorchis acanthus is ingestive in function. It consists of a buccal cavity, suctorial pharynx and an oesophagus divided into anterior and posterior regions. The syncytial lining is continuous with that covering the body surface, except for the posterior region of the oesophagus which is lined by a separate syncytium. This latter has different cytoplasmic inclusions and the unusual feature of a single ventral nucleus in the outer cytoplasmic epidermis. Intrinsic and extrinsic muscles are responsible, respectively, for the fairly rapid passage of food through the fore-gut and for support. Secretion bodies in the epidermis are probably discharged into the lumen and pass with food into the intestine where they may be involved in extracellular digestion. Young rediae feed mainly by mouth while older, immotile rediae absorb nutrients mainly through the body surface.

The intestine of the redia of Parorchis acanthus is lined by a gastrodermis and surrounded by inner longitudinal and outer circular muscles. The gastrodermis consists of alternating rings of dense and lucent cells of similar dimensions, with similar nuclei and with microvilli on the lumenal surface. The dense cells contain ribosomes, dense secretion bodies, vesicles and a few lipid bodies. Ribosomes, dense bodies and lipids are absent from the lucent cells but vesicles are present which, as in the dense cells, pass along the microvilli to be discharged into the lumen. Both types of cells are secretory, absorptive and transportive. The lucent cells, in addition, store glycogen in areas of the cytoplasm devoid of organelles. Tips of processes from the parenchyma cells are engulfed in pockets in the basal plasma membrane, of both, facilitating transport of nutrients from the gastrodermis directly into the parenchyma

Swiss CD-1 mice infected with cysts of Giardia muris are protected against a challenge infection. A temporal study of this phenomenon was undertaken using two different approaches. First, the time needed for mice to acquire resistance was assessed by terminating the first infection using metronidazole on days 3, 6, 12, 24 or 48. These animals received a challenge infection 7 days later. Second, the duration of the protection was determined by giving mice a challenge infection on days 30, 60, 90, 120 or 150 after the primary infection. In all cases, a significant reduction in both cyst output and trophozoite numbers in the small intestine was observed after challenge. The reduction in cyst output following challenge was similar to the reduction in trophozoite numbers. The acquired resistance of some inbred strains of mice to G. muris was similar to that of CD-1 Swiss mice. These results show that mice can acquire a significant resistance against G. muris even after a short period of contact with the parasite (3 days) and that the resistance may last up to 21 weeks.

Available data in the literature pertaining to the life-history characteristics of all known species of mammalian schistosomes have been gathered, and correlations between such variables as length of pre-patent period, adult worm size, rate of progeny production and progeny size have been explored. Accommodation of the schistosome life-cycle to the constraints imposed by certain host characteristics such as life-expectancy and size is discussed. Of the 23 known species of mammalian schistosomes, 20 species apparently rely to a major extent on relatively large-bodied and long-lived mammals such as primates, ungulates and proboscideans for their transmission. Only 1 species, Schistosomatium douthitti, is exclusively dependent on rodents for its transmission. S. douthitti attains maturity within its definitive host faster than any other mammalian schistosome, and is the only species known to be capable of producing viable eggs by facultative parthenogenesis. For all species of mammalian schistosomes, adult worm size, as estimated by female length, is positively correlated with the number of uterine eggs contained within the female (r = 0·682). For the 7 species for which data exist, rate of egg production/worm pair/day is positively correlated with uterine egg counts (r = 0·873) and inversely correlated with egg length (r = −0·787) and miracidium length (r = −0·953). Length of the pre-patent period is positively correlated with egg length (r = 0·503). With respect to the molluscan host, the number of cercariae produced by snails is positively correlated with the shell size of the snail (r = 0·657). For the 5 species for which data exist, the rate of egg production is inversely correlated with shell size of the intermediate host (r = −0·955) and the common logarithm of the number of cercariae produced (r = −0·893). Comparisons between species suggest that exceptionally low rates of cercariae production in the intermediate host may be compensated for by rapid rates of egg production in the definitive host, implying a degree of integration in the schistosome life-cycle not previously appreciated. Most species of mammalian schistosomes have long-lived definitive hosts, and snail hosts capable of producing many cercariae; compensatory relationships are therefore less obvious in such species. Additional quantitative data on all aspects of schistosome life-histories, particularly rate and duration of egg production, are needed to confirm or refute the relationships discussed above.

Age-dependent survival and infectivity characteristics are described for the cercariae of Echinoparyphium recurvatum. At 18 °C the maximum life-span of the cercariae was 48 h and 50% survival occurred at 30·5 h. Infectivity of cercariae to the second intermediate host, Lymnaea peregra was maximal approximately 2 h after emission from the first intermediate host and it subsequently declined to zero at 19 h. It is suggested that the period of sub-maximal infectivity at the beginning of the cercarial life-span may represent a phase during which dispersal is an important function of the larvae. The relationship between infective stage density and establishment success was linear up to densities equivalent to 5000 cercariae/1. At higher cercarial densities the proportion of parasites establishing in second intermediate hosts declined progressively with increasing cercarial density. The mean number of parasites establishing/host increased linearly with increasing host size.

Flubendazole or thiabendazole were administered orally to mice harbouring larvae of Angiostrongylus cantonensis, Trichinella spiralis, Diphyllobothrium erinacei or Hymenolepis nana 5–10, 28–33 and 21–26 and 1–6 days post-infection respectively. All the mice infected with A. cantonensis were completely cured after treatment with flubendazole at 5 mg/kg/day for 6 days. No noticeable damage was found in the cerebral hemispheres of the mice treated with flubendazole and examined under a dissecting microscope. On the other hand, larvae were found in or on the cerebral hemispheres showing obvious haemorrhage in the control mice. The mice treated with flubendazole gained weight while the control mice lost weight. No larvicidal effect of thiabendazole on A. cantonensis was found at a dose of 10 mg/kg/day for 6 days. The mean reduction of larval T. spiralis in mice treated with flubendazole at 5, 50 and 100 mg/kg/day for 6 days was 64, 100 and 99% respectively. In comparison, thiabendazole showed no efficacy against T. spiralis larvae at 50 or 100 mg/kg/day. Mice harbouring plerocercoids of D. erinacei or H. nana larvae were not cured with either flubendazole or thiabendazole at 50 or 100 mg/kg/day for 6 days.