Engorged, parasitic water mites were observed to decrease the growth rate and to increase the stadium length of nymphal backswimmers. Unengorged mites were found to have no effect on the host. The extended stadium lengths of parasitized hosts should reduce the instantaneous rate of natural increase of populations of parasitized hosts because this rate is sensitive to any factor that delays the onset of reproduction.

Ten albino rats injected with hydrocortisone were infected with 36–66 cystacanths of Corynosoma semerme in order to study the course of infection in a warm-blooded animal. Surveys were carried out 2, 5, 8, 14, 18, 21 and 28 days after infection. Recovery decreased from 60% after 2–5 days to 22% after 28 days. The proportion of males dropped markedly before the 8th day of infection. The worms were concentrated in the latter half of the intestine, the males being in a more anterior location on average. The males matured between the 2nd and 5th day of infection. Disintegrated ovaries were observed in some of the 2-day-old females, but mature free ovaries were seen only in 5-day-old females. By the 14th day many females contained fully developed eggs, but it was only after the 18th day that the number of eggs increased considerably. Male worms were not encountered in the 4-week-old infection. The females were longer than the males. In the present study, experimental infections are compared with other acanthocephalan infections in both birds and mammals.

On a coastal farm in New South Wales where beef and dairy cattle production was carried on side-by-side, separate pasture plots were contaminated with eggs of Ostertagia ostertagi by calves from each production system in autumn, winter or spring. Successive groups of parasite-free tracer calves grazed on the plots for 14 days at 4-week intervals and were then killed for worm counts 14 days after removal from pasture. On all plots, the proportion of inhibited early 4th-stage larvae in tracer calves reached a maximum in spring, and was consistently and very significantly higher in calves which grazed plots contaminated with O. ostertagi of beef cattle origin. Factors which may be responsible for this difference between beef and dairy cattle populations of O. ostertagi are discussed.

Populations of Ostertagia ostertagi established from fresh larvae and from larvae stored at 4 °C for up to 12 weeks contained about 10% inhibited early 4th-stage larvae. This value rose to 21% after 16 weeks, due to an increase in numbers of inhibited larvae while adult numbers remained unchanged. Storage at 15 °C had no effect. In the case of larvae stored at 4 °C for 8 weeks, increasing the larval dose to 90000 had no significant effect on the proportion of the dose which was inhibited. The response to low temperature storage was much weaker than in previously reported studies on British populations, which accords with our earlier conclusion that low temperature cannot be the effective stimulus for inhibition in Australia.

Faeces containing eggs of Hyostrongylus rubidus were deposited on grass plots each month throughout 1980. Eggs in faeces deposited on plots from May to October developed into infective larvae but in May development was completed only in artificially moistened faeces, the weather was warm and dry and natural faeces dried out rapidly resulting in the death of eggs and pre-infective larvae. For the remainder of this period the weather was sufficiently warm and wet for development to be completed. More eggs developed into infective larvae on plots with well-grown herbage than on plots with short herbage. In April a few eggs survived and developed into lst-stage larvae but no further development took place. In the laboratory development was completed at temperatures ranging from 10 to 27 °C but no development took place at 4 °C. Out-of-doors infective larvae survived on herbage for up to 10 months while in the laboratory, infective larvae suspended in tap water survived even longer at 10 and 22 °C, but the larvae were rapidly killed by continuous freezing and by desiccation when the relative humidity was less than 95%. The relationship between climatic conditions and the development and survival of the free-living stages is discussed and a comparison made with the free-living stages of Oesophagostomum dentatum.

The metacercarial cyst wall of Bucephaloides gracilescens is bilayered. It comprises an inner granular layer of pepsin-resistant carbohydrate and protein material secreted by the parasite, and an outer fibrous and cellular capsule susceptible to pepsin digestion and elaborated by the host. The capsule of cysts from the orbits and nasal regions of the whiting host are substantially thicker than those from the cranial cavity. Cells in the outer portion of the capsule resemble fibrocytes engaged in the synthesis and secretion of collagen; collagen fibres form a loose meshwork in the intercellular space. The cells incorporated [3H]proline in vitro, but not [3H]tyrosine. The cells of the inner portion of the capsule are closely packed and possess pyknotic nuclei; desmosomes and cytofilaments are numerous and probably impart strength and elasticity, respectively, to the cyst wall. Host cells are necrotic at the interface with the inner cyst wall, possibly due to compression or isolation from oxygen and nutrients. Differences in the levels of incorporation of trace-labelled substrates by encysted metacercariae were related to the site of infection and to the thickness and permeability of the cyst wall.

An ultrastructural examination of Fellodistomum fellis has revealed that the caecal lining consists of 2 distinct components: (a) cup-shaped digestive cells and (b) a pleomorphic layer of cytoplasm which supports and separates individual digestive cells. The digestive cells sequester host blood components within apical pockets formed by lamellated extensions of the cell surface, and undergo asynchronous, cyclical transformations in morphology associated with extracellular digestion and with the extrusion to the gut lumen of pigmented digestive residues. Histochemical tests and elemental analysis of the pigment suggest that it is a ferripor-phyrin, haematin. In the anterior portion of the caecum the supporting cytoplasmic layer is in continuity with the oesophageal tegument and snares the same ultrastructure. It is concluded that the digestive cells are supported by an extension of the foregut tegument.

Colour changes that occur during tanning of intact and punctured Globodera rostochiensis females and of isolated cuticle are described. Experiments with catechol and enzyme inhibitors show that the tanning enzyme, a phenoloxidase, is located in the cuticle, soluble in aqueous media and relatively abundant. Substrate for the enzyme is limited in the cuticle. Pseudocoelomic fluid of yellow females promoted tanning and contains either phenolic substrate(s) for the enzyme or part of a biochemical system responsible for release of the substrate. The substrate or biochemical system is thermolabile. Apparently, the tanning enzyme and its substrate(s) are partitioned in living females but the partitioning breaks down after death and the cuticle tans to form the cyst.

F1, Quackenbush (Q) × Wild (W) Mus musculus were assayed and selected for their level of immunity to secondary infections with 100 Nematospiroides dubius using faecal parasite egg counts. A significant positive linear correlation was found between the number of parasite eggs voided in the faeces and the number of N. dubius recovered after secondary infections in mice. Selection for immunity was not influenced by the conformation of the mice. High (H) and low (L) responder colonies together with a randomly (Rd) bred colony of mice were established over 5 generations. Adaptive immunity was more efficacious in protecting female than male mice and appeared to be controlled by a few dominant genes; the heritability (h2) of the trait was estimated at 0·56 for the parental stock and 0·55 for the F5 Rd colony and the realized heritability was 0·49. Higher anti-N. dubius antibody titres were detected in H than L mice. H mice harboured fewer and less fecund parasites than Rd colony mice; L mice had more worms with greater fecundities than Rd mice. The levels of primary infection in H and L mice related to the levels developed after secondary infection from F4. This suggested at least partial linkage of genes controlling innate with genes controlling adaptive immunity against N. dubius.

The intestinal flagellates, Giardia muris and Spironucleus muris, cause similar infections in CBA mice as determined by trophozoite and cyst counts. Both parasites occur all along the small intestine with G. muris, being mainly present in the anterior part and S. muris towards the posterior. The early stages of infection are similar in all strains of mice examined and peak levels of both trophozoites and cysts occur 1–2 weeks after infection. All strains of mice overcome the infection but the rate of recovery varies considerably between strains, being most rapid in BALB/c and slowest in A and C57BL.B10. Outbred mice are more variable in their recovery than inbred mice. After recovery, mice are partially resistant to reinfection with the homologous but not the heterologous parasite. Resistance to reinfection with S. muris is greatest in those strains that eliminate the primary infection most rapidly. Giardia muris and S. muris cause similar changes in the mucosa of the small intestine of BALB/c mice with increased intra-epithelial lymphocyte counts from 3 weeks onwards corresponding with the start of the elimination of the parasites from the gut. A reduction in villus height and increase in crypt depth is also characteristic of these infections.

In mice infected with the intestinal flagellates Giardia muris or Spironucleus muris, together with the blood parasites Babesia microti or Plasmodium yoelii, there is a temporary decrease of flagellate cyst output coincident with the peak of the blood parasite infections, followed by a rapid return to normal levels. This decrease in cyst output is correlated with decreased numbers of trophozoites in the small intestine. The effect on S. muris is more marked than that on G. muris. Neither blood parasites has any effect on the total duration of the flagellate infection and the flagellates do not affect the blood parasites. In mice infected with G. muris or S. muris and P. berghei there is also a decrease in cyst output but this is less apparent than in infections with B. microti or P. yoelii because of the fatal nature of the P. berghei infection. It is suggested that the decrease in cyst output is probably due to changes in the contents of the small intestine or to non-specific immunological factors rather than to specific immunological changes.

The antigenic differences and similarities between culture forms of Trypanosoma (Schizotrypanum) hedricki, Trypanosoma (Schizotrypanum) myoti, Trypanosoma (Schizotrypanum) vespertilionis and Trypanosoma (Schizotrypanum) dionisii, all isolated from bats, were compared using the double-diffusion technique. Antiserum to each trypanosome species was produced in rabbits by inoculating them with sonicated culture forms. Two isolates of T. hedricki were antigenically identical to each other, as were 2 isolates of T. myoti. There were several common antigens between the species. However, at least 1 antigen of each species was distinct from those of the other species.

Since the in vitro production of trypomastigote stages of Trypanosoma cruzi in cell culture is frequently limited by (1) host cell overgrowth and (2) by the unequal redistribution of parasites after cell division resulting in asynchronous release of trypomastigotes, a culture system was devised in which host cell mitosis was inhibited by irradiation prior to parasite infection. L-6 rat myoblast cells when exposed to 3000 rad. of gamma radiation lost their ability to divide but remained susceptible to infection with, and capable of supporting the intracellular growth of, T. cruzi. Using this approach it proved possible to have virtually 100% of cells infected and achieve much better synchronization of trypomastigote release than with conventional culture systems. Additionally, the total number of parasites provided approached 1 × 109 trypomastigotes/ 150 cm2 flask, a significant increase over other culture systems. Preliminary studies with Plasmodium fallax and Eimeria tenella indicate that irradiated host cells may be utilized to advantage for the cultivation of other intracellular protozoa.

[14C[ethidium bromide has been used to determine drug levels in tissues and body fluids of rabbits and calves following intramuscular injection. Uninfected and Trypanosoma brucei- or Trypanosoma congolense-infected animals were studied. Blood and tissue fluid levels reached a maximum within 1 h and then fell rapidly; after 96 h 80–90% of the radioactivity injected had been excreted, approximately one third in urine and two thirds in faeces. By 1 h after injection of 1 mg [14C]ethidium/kg into a T. congolense-infected calf, 70–80% of the radioactivity in blood was found to be bound to trypanosomes. Doses of 1 or 10 mg/kg were found not to be curative for T. congolense or T. brucei infections in rabbits; drug treatment resulted in a period of sub-patent parasitaemia which was always followed by a relapse. Examination of the prophylactic action of ethidium in rabbits showed that the drug extended the pre-patent period following trypanosome inoculation but provided no absolute protection. A period of ‘apparent’ prophylaxis observed after drug treatment of infected rabbits has been correlated with the presence of anti-trypanosome IgG in the serum.

Histological studies using horse-radish peroxidase (HRP) as a marker, injected iontophoretically through a recording electrode, have indicated the origins of potentials encountered upon advancement of the electrode into and beneath the dorsal surface of adult male Schistosoma mansoni. The first potential encountered, having a value of – 51 ± 0·6 mV originates across the outer tegumental membrane. The next potential has a value of — 28 ± 0·6 mV and originates in the muscle masses underlying the tegument. Finally, a potential having the value — 10 ± 0·5 mV originates within the basal lamina and the interstitial fibres and extracellular space surrounding the muscle. Altering ion concentrations in the bathing medium (i.e. high K+, low Na+, zero Ca2+, low Cl−, high Li+) depolarizes all three potentials. External applications of ouabain and the anti-schistosomal, praziquantel, also cause depolarization of the potentials. It appears that the muscle potential and the tegumental potential are primarily K+-dependent. The depolarizing effects of ouabain, LiCl and low Na+ suggest that active transport is important in the maintenance of the muscle potential, just as is the case for the tegumental potential. There appears to be a close correlation between changes in the tegumental, muscle and extracellular space potentials. The correlation between tegument and muscle potential changes might be explained by junctional complexes between tegumental cell bodies and muscle cell bodies.

Standard intracellular microelectrode techniques were used to determine the electrical properties of the tegument and sub-tegumental regions in male Schistosoma mansoni. Three distinct compartments of electrical potential were observed. The resting potentials recorded in these compartments of –45·9±2·5 mV (Eteg), –22·0±1·1 mV (E2) and – 4·7±0·3 mV (E3) corroborate those previously reported by Fetterer, Pax & Bennett (1980) and Bricker, Pax & Bennett (1981). Input resistance was measured in each compartment and was found to be 4·5 MΩ (tegument), 9·2 MΩ (E2) and 3·5 MΩ (E3). Time-constants for the tegument, E2 and E3 were 0·24±0·01 msec, 0·25±0·01 msec and 0·13±0·01 msec, respectively. Multiple electrode experiments revealed that the tegument and E2 compartment are electrical syncytia with similar current-spreading capabilities. Low resistance pathways also appear to connect the tegument and E2 region, since electrotonic signals initiated in either of those compartments experience only a 15–25% reduction upon passing into the other. Injecting large (> 200 nA) depolarizing current pulses into the tegument or E2 compartment often resulted in the initiation of active membrane responses. These spikes were highly variable, ranging from 4 to 75 mV in magnitude (occasionally overshooting zero potential by as much as 25 mV) and from 10–40 msec in duration. The responses were not actively propagated along the parasite, and their decay over distance was approximately equal to that predicted on the basis of length constant values obtained from electrotonic signals. The addition of a non-diffusible solute to the recording medium resulted in a significant reduction in the current-spreading capacity of both the tegument and E2 compartment. Coupling ratios between the tegument and E2 compartment were decreased, and the input resistance for both compartments increased, while resting potentials remained constant. Active responses could not be evoked in schistosomes exposed to the hyperosmotic medium.

Twenty-one day old Fasciola hepatica were recovered from the livers of infected mice and cultured for 5–7 days in a serum-free medium containing either [C]leucine, [C]isoleucine or [S]methionine, or in a medium containing [C]leucine and serum from a sheep vaccinated with excretory– secretory (ES) antigens of juvenile F. hepatica. All three labelled amino acids were incorporated into fluke proteins. Labelled proteins also appeared in the culture medium. Three major polypeptides detected in the culture media had apparent molecular weights of 26000, 24000 and 23000. All were immunoprecipitated from [C]leucine-labelled culture medium using antisera against fluke somatic antigens raised in rabbits or from sheep vaccinated with ES antigens of juvenile F. hepatica. A polypeptide of molecular weight 27 000 was also prominent in the culture medium when [C]isoleucine was used. This polypeptide was present as a minor component when [C]leucine and [S]methionine were included in the culture media; it did not appear to be immunoprecipitated by the above antisera from [C]leucine-labelled culture medium. In the presence of serum from vaccinated sheep, the ES antigens formed immune complexes which contained the polypeptides mentioned above, together with several higher molecular weight polypeptides. Additionally, a number of minor bands of varying molecular weight were present. After micro-Ouchterlony gel immunodiffusion, 2 precipitin lines formed between the labelled ES antigens and antisera. Electrophoresis of these indicated that the 23000, 24000 and 26000 Dalton labelled polypeptides were present in each. The higher molecular weight and the 27000 Dalton labelled polypeptides were also present in one of the lines.

Viruses are frequently referred to as the supreme parasites and yet it is now more than 20 years since any paper on this topic was published in Parasitology. This deficiency probably reflects the great emphasis placed during the last two decades on those aspects of virology christened by Sir Christopher Andrewes ‘dream virology’ (Andrewes, 1973), namely the molecular and genetic properties of viruses, in contrast to the more clinical and biological aspects of ‘steam’ virology. Many modern virologists select a virus as a convenient model system with which to investigate such things as genetic organization or control of transcription, with little regard for the interaction with the host cell that supports its replication. Paradoxically, it is this very emphasis on the detailed molecular mechanisms of virus replication that has now put us in a better position to understand the relationships between these highly specialized parasites and their host systems than for any other type of parasite. Recent advances in cell biology coupled with an understanding of the molecular basis of viral replicative mechanisms mean that new insight is possible into the interactions of a virus with its host. For example, we are just beginning to appreciate why a virus should infect one individual and not another, or why it should multiply only in certain tissues at certain stages of development of a multicellular organism. Indeed, study of such tropisms may frequently tell us as much about the host cell as about the virus.