Chub and eels were experimentally infected via intermediate hosts harbouring cystacanths, with Pomphorhynchus laevis alone, or Acanthocephalus anguillae alone, or simultaneously with mixtures of both species in varying proportions, and sampled at 7, 56 or 112 days post-infection. Examination of chub revealed that both species showed low establishment and growth rates, differing markedly from British field data, where chub is apparently one of the most important hosts, and preventing further meaningful experiments. Both species showed higher establishment rates in eels, but A. anguillae both grew faster and matured to a greater extent than P. laevis, reflecting field data. No evidence of any interactions, competitive or otherwise, was found in the mixed infections in eels at low or high infection levels. The results were interpretated with regard to British field data and current theories of the role of interspecific competition in structuring fish helminth communities.

1. A description of the morphology and life history (excluding metacystic development) of Entamoeba muris is given.

2. The descriptions of species of amoebae found in rats (Rattus norvegicus and R. rattus) and mice (Mus musculus) by previous authors are critically discussed, and it is concluded that one species, Entamoeba muris, is present.

3 Experiments on cross-infection of rats and mice with E. muris are described, together with observations on the course of these infections.

4. The transference of an amoeba morphologically similar to E. muris from Cricetus auratus to rats and mice is described.

5. An account is given of unsuccessful attempts to establish an infection of Entamoeba coli in rats and mice.

6. Observations on the incidence and hostrestriction of E. muris are discussed.

7. The affinities of E. coli and E. muris are discussed and it is concluded that, although E. muris resembles E. coli morphologically, it differs from E. coli on physiological grounds.

Considerable diversity has been found in the reproductive behaviour of benedeniine (capsalid) monogenean parasites. Mating has been observed in Benedenia sp. 1 from the gills of Lutjanus carponatatus; externally attached spermatophores are not involved and parasites indulge in mutual cross-insemination with intromission. In contrast, there is evidence of spermatophore involvement in Benedenia sp. 2 from the fins of Lethrinus miniatus; mating was not observed but an individual was found carrying an external ovoid spermatophore attached by a stalk lodged in the vagina. In specimens of Benedeniella macrocolpa and B. posterocolpa, in which the male reproductive system was functional and the female system not fully developed, the everted cirrus was seen to be lodged in the parasite's own uterus, with, in some individuals, the tip of the cirrus inside the ootype. This is the first time that the copulatory organ has been observed inside the female reproductive tract of the same individual, not just in benedeniines but in monogeneans in general, and is also the first demonstration that monogeneans are capable of self-insemination via the uterine route.

A morphological study of the fine structure of the spores of Minchinia chitonis, a haplosporidian parasite of the chiton, Lepidochitona cinereus, is described. The spores contained a single nucleus, mitochondria, haplosporosomes, smooth endoplasmic reticulum, ribosomes and a large spherule (presumed Golgi apparatus). The spore wall was discontinuous at the spherule end, forming an opening covered by a lid which rested on a circumscribed flange. The flange of the spore wall and the lid were continuous in only one area which served as a hinge. The entire spore was encapsulated by epispore cytoplasm bounded by a strengthened membrane and extended to form 2 long projections, one at either end.

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.

Mice which have recovered from infections with the avirulent piroplasm Babesia microti are also resistant to challenge with the virulent malaria parasite Plasmodium vinckei. In mice infected with P. vinckei before the peak of the B. microti infection the numbers of malaria parasites in the blood increase until that peak and are then eliminated at the same time as the piroplasms. In mice infected with P. vinckei at or after the peak there is no apparent multiplication and the malaria parasites begin to disappear from the blood immediately. The malaria parasites in doubly infected mice show signs of degeneration similar to those seen in mice pre-treated with Corynebacterium parvum and it is suggested that a common mechanism exists in homologous and heterologous immunity and in immunity following pre-treatment with C. parvum or BCG.

A consideration of the tick-activity data for South Wales reveals that there are usually two peaks of female activity in this region, one occurring in spring, the other in autumn. On an experimental area, divided into eight plots, on the Margam Moors, Glamorgan, significantly varying population levels are present on the different plots, although the peak of activity on all of them is attained around the same period. The seasonal activity of the nymphal and larval forms in South Wales is essentially comparable to that recorded by Milne (1945a) in the north of England. There is reason to believe that in mid-Wales a single summer peak of tick activity exists.

The engorgement period of female ticks on cattle varies from 7 to 13 days. The tick shows a distinct preference for the hind region of cattle for feeding purposes. The differences in the mean populations of ticks on different breeds of cattle are not statistically significant, at least, on the six breeds tested in 1944 and 1945. In the case of cattle, tick density and fecundity of the female are not correlated with the physiological condition of the host.

In South Wales, the populations of ticks on sheep are significantly lower than those on cattle, even where both hosts are pastured on the same ground for similar periods. The seasonal activity of female ticks on horses follows the same general trend as that noted on cattle.

The development of Archigetes limnodrili in species of Limnodrilus is described. There is no free-living larva and eggs are ingested by the tubificids. Growth and development is completed within the body cavity of the annelid, and egg liberation is accomplished by release of the parasite and decay of its body.

Breeding of A. limnodrili takes place throughout the year. In the localities investigated there was no evidence that a fish host was required in the life-cycle. Progenesis was the only type of development encountered in Britain.

A. limnodrili exhibits an unusual degree of host specificity, being found only in species of Limnodrilus. It is suggested that this is due to differences in the composition of the coelom or intestine of Limnodrilus compared to other genera.

The degree of infection in all localities is very low, and shows no regular seasonal variation. There is no similarity in the seasonal changes in different localities.

The relationship between the host and parasite is a stable one, and there is little mutual damage. Factors contributing to this stability are discussed.

The development of A. limnodrili is compared with that of other species of Archigetes, and the life-history discussed with particular reference to the phenomenon of progenesis.

I wish to thank Professor R. J. Pumphrey in whose Department this work was carried out, and Dr J. C. Chubb for his constant advice and criticism. I also wish to thank Dr K. H. Mann and the University of Reading for provision of specimens and permitting me the use of their facilities. The work was carried out during the tenure of a Nature Conservancy Research Studentship.

A murine model using Heligmosomoides polygyrus and Trypanosoma congolense has been developed for studying the effects of concurrent chronic gastrointestinal nematode and trypanosome infections. Female outbred mice were infected either with 500 infective larvae (L3) of H. polygyrus or with 104 bloodstream forms of T congolense or both. In concurrent infections, animals were dosed with both parasites simultaneously or the trypanosomes were injected 5 or 10 days after the mice were infected with the nematode. The course of infection was monitored by routine parasitological and immunological techniques for 30 days after the H. polygyrus infection. Concurrently infected mice were severely compromised, except when T. congolense was superimposed on a 10-day-old (adult) H. polygyrus infection. In H. polygyrus-infected mice, simultaneous or subsequent infection with trypanosomes did not markedly influence worm establishment or fecundity, but the female worms were slightly stunted. Surviving mice displayed a markedly reduced antibody response to H. polygyrus antigens and a slightly reduced antibody response to T. congolense antigens.

The activity of lysophospholipase of human erythrocytes increased by about 3 orders of magnitude upon infection with Plasmodium falciparum. The apparent Km for hydrolysis of lysophosphatidylcholine by this enzyme was 50 ± 7μM and the apparent Vmax 6·8±0·6 nmol/h × 106 cells. The activity was Ca2+ independent and had a broad pH maximum at pH 8. The enzyme was insensitive to such anti-malarials as mefloquine and arteether and was only weakly inhibited by chloroquine, with a 50% inhibition concentration (IC50) of 70 mM. The anti-malarials quinine and quinacrine were more efficient inhibitors, with IC50s of 2·6 mM and 0·7 mM, respectively. The sulphydryl agents p–hydroxymercuribenzoate (pHMB) and thimerosal were considerably more potent, inhibiting the plasmodial lysophospholipase with IC50s of 18 μM and 10 μM, respectively. When present at 10 μM prior to invasion, both pHMB and thimerosal arrested the growth and reinvasion capacity of P. falciparum in culture. In a synchronized P. falciparum culture the continuous presence of 5 μM thimerosal dramatically decreased total parasitaemia and, within 4 days, totally abolished the capacity of the surviving parasites to reinvade. Thus the plasmodial lysophospholipase may represent a potential new target for anti-malarial chemotherapy.

A comparative study has been made of the factors influencing the assessment of anticoccidial potency in vitro against Eimeria tenella using established anticoccidials and arprinocid and some of its analogues. Drugs whose potency depended upon medium composition were amprolium, lasalocid and halofuginone. There was a difference in strain sensitivity with robenidine. Host cell type had an important effect on potency of monensin, decoquinate, arprinocid and its analogues. Arprinocid was active in chick liver cell systems but totally inactive in chick kidney cell systems, although its N-oxide was active in both cell types. Arprinocid-containing medium, conditioned by supporting the growth of chick embryo liver cell cultures, had an anticoccidial effect on E. tenella, growing in chick kidney cells. It is deduced that the anticoccidial activity of arprinocid in the chick is due to a metabolite.

A 196 bp fragment of cloned kinetoplast (k) DNA of Leishmania infantum has been sequenced and shown to be diagnostic for this species. The DNA from 104–105 promastigotes was routinely and specifically detected in crude dot blots of whole cells lysed in situ, and there was no cross-hybridization with 106 cells of L. major, L. tropica, L. killicki, L. aethiopica, L. braziliensis and L. mexicana. A low degree of hybridization was found with L. donovani and L. chagasi. This probe will be of particular use in Mediterranean countries, such as Italy, in which dermotropic zymodemes of L. infantum can be difficult to isolate and grow in numbers sufficient for isoenzyme typing.

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.

The genus Dujardinia was proposed by Gedoelst (1916) for the reception of a form considered to be identical with Ascaris helicina Molin, 1860. In the same paper another species, Ascaris nigra, was described by Gedoelst, who mentions that it has close affinities with A. helicina, but does not actually refer it to Dujardinia. The present writer (1920) assigned Ascaris halicoris Owen, 1833, to this genus. More recent work renders it somewhat doubtful whether this species really belongs to Dujardinia, for reasons which will be pointed out later, and though for the present it is retained, it must be regarded as a somewhat aberrant form.

Chicks infected with the Weybridge strains of Eimeria maxima and E. acervulina were not protected by the normal levels of amprolium, but the sporulation of the oocysts was inhibited. With higher drug concentrations, fewer oocysts were produced. E. brunetti was not so markedly affected, although oocyst sporulation was reduced by the higher dosage levels. The effects were not enhanced by the inclusion of ethopabate.

With dinitolmide the phenomenon was not so marked, although oocyats of E. maxima and E. acervulina were reduced in numbers by the normal drug concentration and sporulation was reduced when this level was increased. Much higher drug levels were required to obtain these effects with E. brunetti. Monensin at 120 ppm affected neither oocyst numbers nor their sporulation in any of the species tested. The significance of the effects of anticoccidial drugs on gametogony and sporogony is discussed.

Eighty-seven of 175 laboratory raised F1 generation brook charr (Salvelinus fontinalis) were resistant to experimental Cryptobia salmositica infection. Susceptibility to the pathogen was not related to age nor to route of infection. Plasma of resistant charr lysed the parasite under in vitro conditions while those of susceptible fish did not. Sperm from 4 susceptible and 4 resistant male charr, from 8 F1 families, were used to fertilize eggs from 2 hatchery raised susceptible charr. Susceptibility of progenies (F2 families) was determined 4 weeks after inoculation of the parasite. All progenies (seven F2 families) of 4 susceptible males were susceptible to experimental infection and their plasma did not lyse the parasite. It was presumed that these susceptible progenies were homozygous recessive (rr), and that the resistant allele was dominant. All progenies (two F2 families) of 1 resistant male (presumed RR) and 2 susceptible females were all resistant to infection and their plasma lysed the parasite. The ratio of resistant to susceptible charr in 6 other F2 families from the other 3 resistant males (presumed Rr) and two susceptible females was about 1:1. The results suggest a single Mendelian locus which determines innate resistance to C. salmositica infection in brook charr and that it is possible to breed Cryptobia-resistant fish.

Filaroides hirthi lungworm infection was diagnosed by the recovery of 1st-stage larvae from the faeces of dogs with heavy, artificially induced infections using zinc sulphate flotation. Diagnosis of low-grade natural infections was infrequently achieved. Zinc sulphate flotation was demonstrated to be about 100 times as efficient as the Baermann technique in concentrating F. hirthi larvae from dog faeces. Larvae recovered from faeces proved to be infective when fed to a pup and it was concluded that F. hirthi infection can be transmitted directly and immediately by fresh faecal contamination. Mongrel dogs of diverse ancestry were readily infected by feeding 1st-stage larvae from lung tissue. This, F. hirthi infection was shown not to be limited to the Beagle breed by biological restrictions. The observations that 1st-stage larvae pass through the alimentary tract on their way out of the body and that larvae are found in the mesenteric lymph modes long after a single exposure to infection support the hypothesis that there is an autogenous re-infection of the host by a proportion of these larvae.

There is increasing support for the presence of viruses and virus-like particles inside protozoan cells. This study describes viral-like RNA molecules that have been detected in two species of Eimeria that infect the chicken. The RNA molecule identified in E. maxima has been characterized: subcellular fractionation studies have shown that the RNA is present in the cytoplasm, probably as an abundant ribonucleoprotein that is insensitive to RNAse A treatment. Electron microscopy has demonstrated that this RNA molecule is double stranded. In addition, all E. maxima strains examined so far contain this RNA molecule.

The effective use of biological tags in stock assessment relies on the reliable identification of the parasites concerned. This may be compromised if cryptic species are not recognized. Here we review what is known about cryptic species in aquatic hosts and its potential importance in this respect. Although strictly cryptic species may be considered as species which can be distinguished only by molecular data, we accept the far looser but more practical definition of species that cannot be readily distinguished morphologically. Cryptic species appear to have been identified most frequently as occurring in separate host species; this is heartening in that this has no significant impact on tagging studies. But cryptic species have occasionally been identified in single hosts sympatrically and are relatively common in geographically distinct populations of the same host species. Ignorance of both kinds of occurrences has the capacity to undermine the reliability of tagging analysis. We review in detail what is known of intra- and interspecific genetic variation over geographical ranges in the trematodes, based on recent molecular studies. Although the existence of cryptic species and evidence of intraspecific variability may appear daunting, we suspect that these complexities will add, and indeed have already added, to the sophistication of the information that can be derived from tagging studies.

A pleomorphic neoplasm (PN) is described from sections of Onchocerca volvulus worms in nodules excised from Cameroonian patients. PN is confined to older, non-fecund, female worms, and those classed as moribund/dead. It is mainly composed of small, roundish, basophilic cells of diverse sizes, often forming a ‘rosette’ pattern around amorphous eosinophilic centres. The cells have a high nuclear/cytoplasmic ratio and up to 2–3 mitoses/high-power field; some become grossly enlarged, highly polymorphic and contain large, irregular blocks of chromatin. The eukaryotic PN cells first appear posteriorly in the pseudocoelom, probably from ovarian cells; they spread anteriorly, invading or compressing the uteri. Ivermectin treatment increased the prevalence PN from 3.7% of 1422 female worms in 637 patients before treatment to 17.5% of 1134 worms in 511 patients after 3 years treatment. Ivermectin at 400–800 μg/kg annually, or at 150 μg/kg or 400–800 μg/kg 3-monthly, over 3 years, did not increase the PN prevalence significantly, as compared with standard doses of 150 μg/kg annually. In other small series of African patients, PN prevalence increased in worms 2, 4, 6 and 10 months after ivermectin treatment; but there was no increase after treatment with amocarzine, albendazole or diethylcarbamazine and suramin. PN may partly account for the increased macrofilaricidal action of ivermectin on female O. volvulus in patients treated for 3 years at 3-monthly intervals.