Showing papers in "Parasitology in 1980"

Nematode egg-shells

Abstract: The transmission of parasites often involves a high mortality of free-living stages in the environment outside the host. This may be offset by a high biotic potential. In addition, adaptations of nematode eggs and larvae that ensure their survival or increase their chances of infecting a host will reduce the potential wastage rate. Increasing transmission will have an effect equivalent to increasing the fecundity of the parasite and, energetically, may be the more favourable strategy.

Boophilus microplus: the effect of histamine on the attachment of cattle-tick larvae-studies in vivo and in vitro.

Abstract: Circumstantial evidence suggests that the earlier detachment of Boophilus microplus larvae from highly resistant cattle follows the release of histamine at the attachment site. In vivo and in vitro experiments show that a proportion of the larvae will detach following injection or infusion of histamine. Other mediators such as bradykinin, prostaglandin E2, 5-hydroxytryptamine and dopamine have little or no effect on tick behaviour in vivo. Sensitivity to histamine declines as larval attachments stabilize, and repeated injections have no effect on the weight of larvae after 3 days on the host. Response to histamine is discussed in relation to host resistance, histology of the feeding lesion and larval behaviour.

Antigenic variation in clones of animal-infective Trypanosoma brucei derived and maintained in vitro.

Abstract: Eighteen clones of variable antigen type 052 of Trypanosoma brucei stock S.427 were derived and maintained as animal-infective bloodstream forms in vitro for up to 60 days of cultivation. The antigenic composition of such clones was monitored weekly by immunofluorescent analysis of viable trypanosomes, using antisera raised to isolated variant-specific surface glycoproteins of both 052 and variable antigen type (221) which consistently appeared in the first relapse population of type 052 in vitro. The appearance of new variants was detected in 9 of the 18 clones 18--46 days following initiation of the clone and variable antigen type 221 was found in all 9 clones. On one or more occasions in 8 of such clones, viable trypanosomes were found which did not react with either antiserum but were mouse-infective on the 4 occasions tested and probably represent other variable antigen types. The process of antigen variation in vitro appears to resemble the process in vivo except that new variant types are detected earlier in vivo. This possibly results from different growth rates of the trypanosomes in vivo and in vitro, together with the fact that elimination of the initial variant population by the host's immune response facilitates the detection of newly arising variable antigen types in vivo.

Recovery of Schistosoma mansoni from the skin, lungs and hepatic portal system of naive mice and mice previously exposed to S. mansoni: evidence for two phases of parasite attrition in immune mice.

Abstract: New or improved techniques for recovering Schistosoma mansoni from the skin, lungs and liver have enabled us to trace the attrition of a challenge infection in naive (i.e. previously uninfected) and chronically infected mice. Within each experiment, numbers of schistosomes recovered from the skin of naive mice on day 2 after challenge or from the skin and lungs on days 3, 4 or 5, did not differ significantly from the numbers recovered from the liver on days 14, 21, 28 or 35. Approximately 65% of cercariae which penetrated the skin failed to be recovered from naive mice by any of the assays and it appeared that these schistosomes had already died in the skin in the first 24 h. No further significant loss of the infection was detected in naive mice. In chronically infected mice a further attrition of the challenge infection was demonstrated in two distinct phases. An 'early phase' occurred within the first 3 days of exposure and accounted for the death of 30% of the remaining parasites. A 'late phase' occurred between days 6 and 14 and accounted for an additional 43% of deaths. Thus, the two phases of attrition accounted for a loss of approximately 73% of the infection that would have survived in a naive mice. The late phase of attrition could be demonstrated before the primary infection had matured, in contrast to the early phase of attrition which was seen only after egg laying had commenced. We believe that the early phase of attrition takes place in the skin and the late phase occurs after the schistosomes have left the lungs, either en route for the liver or as soon as they arrive in that organ. The results suggest that there are two distinct mechanisms of immunity against re-infection with S. mansoni in mice.

In vitro cloning of animal-infective bloodstream forms of Trypanosoma brucei.

Abstract: Clones of animal-infective bloodstream forms of Trypanosoma brucei (stocks S.427 and LUMP 227) were made by transferring a single organism from bloodstream-form cultures into each well of Microtest II Tissue Culture Plates containing bovine fibroblast-like feeder cells. When the number of trypanosomes increased to 10(2)--10(3)/well on days 4--16, they were transferred into plastic T-25 culture flasks also containing feeder cells and fresh medium. Cultures were thereafter maintained by partially replacing the trypanosome suspension with the same volume of fresh medium (diluting the density to 2--5 x 15(5) trypanosomes/ml) every 24 h. Sub-cultivations could be made by transferring 1--2 ml of the trypanosome suspension to a new culture flask at 4--5 day intervals. A total of 42 clones in the 3 series TC221, TC52 and TC227, carrying variable antigen types (VATs) 221, 052 and ILTat 1.4, respectively, have been established. Average population doubling times for clones of TC221, TC52 and TC227 were 8.7, 14.5 and 15.5 h respectively. Of 35 populations examined, 34 clones retained the original specificity of their VATs for at least 8--32 days from cloning. One cloned population of TC52 consisted of 99.8% VAT 052 and 0.2% VAT 221 at the time when the first VAT test was made on day 18.

Factors affecting the ability of isolated Plasmodium knowlesi merozoites to attach to and invade erythrocytes.

Abstract: Plasmodium knowlesi merozoites were prepared by the polycarbonate sieving method of Dennis, Mitchell, Butcher & Cohen (1975). Merozoite function was assayed by their attachment to and invasion of rhesus erythrocytes at 37 degrees C. The early merozoites from the culture chamber were the most invasive, although maximum numbers of merozoites appeared later. Merozoites were most stable when incubated at room temperature (23 degrees C). At 37 and 0 degrees C invasiveness rapidly declined to zero. Attachment was rapidly lost at 37 degrees C but was retained at 0 degrees C. Attachment was unchanged in the pH range 6.8--7.9 but invasion was reduced at pH 7.9. The presence of L-fucose, D-galactose, D-glucose, D-mannose, N-acetyl-D-galactosamine or N-acetyl-D-glucosamine did not reduce invasion. Attachment and invasion were greatly reduced or abolished by the presence of 2.5 mM EDTA or EGTA, by lactoperoxidase-catalysed iodination of the merozoites, or by treatment of the merozoites with trypsin at a concentration of 1 micrograms/ml or greater for 10 min at 23 degrees C.

The survival of the cercariae of Schistosoma mansoni in relation to water temperature and glycogen utilization

Abstract: Survival curves for cercariae of Schistosoma mansoni maintained at different temperatures were determined. Between 15 and 35 degrees C the curves were of reverse sigmoid form and the probit transformation gave a statistically good fit. The slope of the regression lines fitted to the probit transformation increased exponentially as the temperature rose. Above 35 degrees C heat intolerance became important and increased exponentially as the temperature rose. Below 15 degrees C, cold intolerance caused an initial high level of mortality followed by an extended period with insignificant mortality. The glycogen content of whole cercariae and separated cercarial bodies and tails was measured during ageing. The glycogen content of whole cercariae declined exponentially during ageing. This was a product of exponential decreases in the glycogen content of both the body and the tail of the organism. The cercarial tail was shown to contain slightly over half of the total glycogen content. The rate of glycogen use was higher in the tail than in the body. A computer simulation model was used to demonstrate that the observed exponential use of glycogen could generate survival curves similar to those observed experimentally.

Schistosoma mansoni : a scanning electron microscope study of the developing schistosomulum

Abstract: The surface morphology of schistosomula extracted from the skin, lungs and hepatic portal system (hps) of mice was investigated from Days 0 to 18 post-infection. Skin schistosomula and newly arrived schistosomula from the hps were of similar dimensions but were morphologically distinct. Lung schistosomula were considerably elongated with an estimated 53% increase in surface area compared to skin schistosomula. The pitted tegument of lung schistosomula was formed into ridges and troughs. These were compressed together in contracted individuals recovered from the hps on Day 10. The annular ridges were cross-linked by longitudinal septae which possibly prevent further elongation of the body. A regression of the spines between the mouth and the ventral sucker was observed in Day 2 skin schistosomula. In lung schistosomula only the spines at the anterior and posterior of the body remained. New spines were formed after the schistosomula reached the hps. It is suggested that the spines in the mid-region of the body are selectively disassembled and that their loss facilities migration along the lumina of capillaries with the residual spines acting as anterior and posterior anchors. The mouth opening was enlarged in schistosomula from the hps recovered from Day 10 onwards. Skin schistosomula lost the cercarial apical tegumentary ridges between 24 and 48 h after penetration but a spineless protrusible area remained. After arrival in the hps this area became integrated into the anterior surface as the oral sucker developed around the sub-terminal mouth. The cercarial ciliated papillae were lost on penetration. The migrating schistosomulum had few visible sensory papillae but following arrival in the hps new papillae were observed.

Migration of the schistosomula of Schistosoma mansoni from the lungs to the hepatic portal system.

Abstract: The pattern of recovery of schistosomula from the lungs of mice, hamsters and rats is described following normal percutaneous infection via the abdominal skin. Peak numbers were found in hamsters and rats on day 5 and in mice on day 6 post-exposure. Schistosomula were recovered from the lungs up to approximately day 20 post-exposure in all 3 species. None were found in pleural washings and only 2 were recovered following mincing and incubation of host diaphragms. The pattern of recovery of schistosomula from the lungs of mice was also described following the injection of a pulse of parasites into the tail vein. Approximately 54% of these injected schistosomula eventually reached maturity in the hepatic portal system. The ability of schistosomula to migrate and mature following their injection into unusual locations was tested. Small numbers reached maturity in the hepatic portal system following injection into the pleural cavity or subcutaneously, more when injected intra-peritoneally and largest numbers following intravenous injection. Small numbers of schistosomula were recovered by mincing and incubation of systemic organs such as the kidney and spleen. Schistosomula injected into the left ventricle of hamsters were able to migrate via the systemic organs to the lungs and 47% eventually matured in the hepatic portal system. The pattern of schistosomulum accumulation in the hepatic portal system of mice, hamsters and rats is described. Following normal percutaneous infection, schistosomula were first detected in this site from day 6 to day 9 depending on the host species. In mice, migration was complete around day 20 but continued in hamsters up to day 40 post-exposure. Following injection of schistosomula into the tail veins of mice, the first arrivals in the hepatic portal system were detected 12 h later and were found to lose their ability to migrate shortly after arrival. However, a proportion of lung schistosomula injected directly into the hepatic portal system were able to traverse hepatic sinusoids and reach the lungs. It was concluded that the route of migration of schistosomula from lungs to liver was entirely intravascular, with potentially several passages round the pulmonary-systemic circulation, before chance entry into arteries leading to the hepatic portal system. The proportion of schistosomula exiting from the pulmonary-systemic circuit was estimated as 0·14/day. A computer simulation produced values of 11 h and 5 h for the duration of migration through pulmonary and systemic capillary beds. The time for 1 complete circuit would thus be 16 h and the proportion of schistosomula exiting per circuit would be 0·095. This approximates to the proportion of cardiac output going to splanchnic organs in the resting rat (0·128). The narrowness of hepatic sinusoids may be one factor contributing to the sequestration of schistosomula in the hepatic portal system.

The influence of Hymenolepis diminuta on the survival and fecundity of the intermediate host, Tribolium confusum

Abstract: An experimental study of the effects of parasitism by H. diminuta on the intermediate host, Tribolium confusum, is described. No density-dependent constraints on parasite establishment within individual hosts are evident, although a reduction in cysticercoid size at high parasite burdens is demonstrated. The relationship between parasite burden, host mortality and host fecundity is investigated. Host mortality is linearly related to parasite burden, whereas the relationship between parasite burden and host fecundity is non-linear. There is no difference in viability between eggs from infected and uninfected females. The generative causes of these effects are not investigated experimentally, although it is postulated that survival is related to the degree of damage to the midgut wall caused by parasite penetration, and fecundity to the biomass of parasites harboured by the host. The significance of these effects is discussed in relation to the overall dynamics of the host-parasite association.

Characterization of the exposed carbohydrates on the surface membrane of adult Schistosoma mansoni by analysis of lectin binding.

Abstract: The surface architecture of adult male Schistosoma mansoni was explored using a range of lectins with differing carbohydrate specificites. Highest specific binding was achieved with concanavalin A and the agglutinin of molecular weight 60000 from Ricinus communis; the binding of wheat germ agglutinin was mostly non-specific. Small amounts of peanut agglutinin and soybean agglutinin binding were observed and the binding of these lectins was increased by pre-treating the parasite with neuraminidase. The fucose binding protein of Lotus tetragonolobus failed to bind. These results indicate that mannose and/or glucose, galactose, N-acetylglucosamine, N-acetyl-galactosamine and sialic acid are exposed on the surface of the adult male schistosome.

The development of Trypanosoma cruzi in macrophages in vitro. Interaction with lysosomes and host cell fate.

Abstract: The interaction between mouse peritoneal macrophages and ‘Y’ strain Trypanosoma cruzi bloodstream forms was studied at optical and electron microscopical levels. The method of marking lysosomes with Thorotrast, either before or after infection of cell monolayers with parasites, revealed that secondary lysosomes fused with phagosomes shortly after trypanosome interiorization. In spite of this, 24 h later most parasites were no longer in a vacuole but lay free within the host cell cytoplasm, multiplying actively. At this time, and up to shortly before 96 h when parasites escaped to the external milieu, most parasitized cells were not lethally injured, as revealed by the Trypan blue dye-exclusion test. Only when parasites were released into the external medium was this situation reversed and infected macrophages took up the dye.

Uptake of purine bases and nucleosides in African trypanosomes

Abstract: Uptake of radioactivity labelled purine bases and nucleosides by suspensions of Trypanosoma brucei and Trypanosoma congolense in bicine buffer was determined at 37 degrees C. With T. brucei, the rate of uptake of adenosine was much greater than that of the other compounds tested, the uptake of which decreased in the order adenine, inosine, guanosine and hypoxanthine. With T. brucei, adenosine uptake increased with concentration in a manner suggesting two mechanisms, one with high and the other with low affinity for adenosine. The uptake of adenine increased with concentration only up to about 1.5 microM while the uptake of guanosine increased little with concentration and that of inosine and hypoxanthine not at all. In both species adenosine strongly inhibited the uptake of both of the other nucleosides and of both purine bases. In T. brucei, guanosine and inosine caused small increases in adenosine uptake which was, however, inhibited by them in T. congolense. In T. brucei, each of the purine bases adenine and hypoxanthine inhibited its own uptake maximally but that of each other less effectively. Hypoxanthine was more effective than adenine in inhibiting the uptake of the nucleosides guanosine and inosine, but neither base effected marked inhibition of adenosine uptake. The uptake of adenosine by T. brucei was inhibited by dipyridamole and its analogue, compound RA--233, strongly at 100 microM and slightly at 10 microM. The other dipyridamole analogues, VK--744 and VK--774, were ineffective.

Changes in the membrane microviscosity of mouse red blood cells infected with Plasmodium berghei detected using n-(9-anthroyloxy) fatty acid fluorescent probes.

Abstract: A set of n-(9-anthroyloxy) fatty acids (n = 2, 6, 9, 12, 16) have been used as fluorescent probes to examine the lipid environment at different depths in the outer membrane of normal mouse erythrocytes and red blood cells from Plasmodium berghei-infected blood. Fluorescent polarization experiments with normal mouse erythrocytes have demonstrated a typical gradient in microviscosity from the surface to the centre of the bilayer as a consequence of the motional properties of the C-atoms of the phospholipid acyl chains. The fluorescent probes rotate faster in the membrane of purified pluriparasitized cells (greater than 90% purity) than with the remaining fraction of red blood cells from infected blood (20--40% immature, infected red cells, and uninfected red cells), or normal mouse erythrocytes. This increase in fluidity with heavily infected cells occurs predominantly at the centre of the lipid bilayer, rather than at the membrane surface. A comparison of the polarization values of intact and lysed infected cells indicates that the fluorescent fatty acids preferentially label the plasma membrane rather than the internal membranes of infected cells. The results suggest that P. berghei infection causes a change in the composition and/or organization of the outer membrane of pluriparasitized cells which produces a decrease in membrane microviscosity.

Antigenic variation in trypanosomes: a computer analysis of variant order.

Abstract: African trypanosomes can undergo antigenic variation and evade the host immune response. Whether the antigenic variants arise in an ordered sequence or randomly has been in dispute but has not been statistically tested. The coefficient of concordance (W), a statistic designed to detect similarities between sequences of objects, was applied to the literature data. The tendency towards a reproducible order of variants was strong, although in several of the studies the number of experimental animals was so low that no conclusions could be drawn. A computer model was used to determine whether this degree of order could arise with random generation of variants followed by selection. The model simulated a trypanosome clone with 90 possible variants, widely differing variant-specific growth rates, random variant origin and variant eradication by an anamnestic host immune response. Parameters varied were maximum parasitaemia, growth rate differential between 'fast' and 'slow' variants, and parasitologist ability to detect minor variants. Random generation and selection by growth rate alone could not produce the degree of variant orderliness reported in the literature. However, experiments with larger numbers of host animals and direct investigation of variant growth rates and competitive interactions are necessary before the random generation-selection hypothesis can be proven or disproven.

Prospects for the biological control of plant-parasitic nematodes

Abstract: Biological control is understood here in the classical sense, which is precisely defined by De Bach (1964) as ‘the action of parasites, predators or pathogens in maintaining another organism's population density at a lower average than would occur in their absence’. This account consists of a survey of the principal causes of disease in nematodes, with a summary of the efforts made to use certain pathogens in practise and a discussion of nematode pathology with reference to destruction of plant pathogenic nematodes.

The involvement of cytokinins in a host-parasite relationship between the tomato (Lycopersicon esculentum) and a nematode (Meloidogyne javanica).

Abstract: The influence of Meloidogyne javanica on cytokinins in the host Lycopersicon esculentum has been studied at different stages of the nematode's life-cycle. Marked differences were detected in cytokinin content of root homogenates between infected and control plants, particularly at the 3rd (32 day), 4th (39 day) and 5th (55 day) harvests. Most of the cytokinin detected appeared to be associated with root homogenates in which the nematode was in the rapid post-moult growth stage. The influence of these nematodes on cytokinins in the host's xylem exudate was not nearly so pronounced. The freshly hatched 2nd-stage infective larvae of M. javanica were themselves capable of exuding cytokinin-like substances.

The contribution of Ascaris limbricoides to malnutrition in children.

Abstract: There is a need for more applied and experimental research to determine more precisely the relationships between nutrition, particularly childhood malnutrition, and intestinal parasitic infections, particularly Ascaris infection. Nevertheless, it is clear that in certain communities Ascaris infection is associated with poor growth in malnourished children and that deworming improves growth. Periodic deworming of children using a mass treatment approach may be needed to control soil-transmitted helminths in areas where parasites and protein energy malnutrition are highly prevalent. The main aims of treatment should be to reduce parasite loads below the level of clinical significance for the individual child and to reduce future environmental contamination with infective faeces for the sake of the community.

Metabolic changes associated with the migration of the schistosomulum of Schistosoma mansoni in the mammal host.

Abstract: A variety of measurements was made on small samples of schistosomula recovered from the skin, lungs and hepatic portal system of percutaneously infected mice, on the basis of which development could be divided into a migration and a growth phase. The density of schistosomula, estimated using a Ficoll gradient, was found not to vary significantly between Day 0 and Day 24 post-infection, having a mean value of 1.077. The reduced weight was measured by means of a Cartesian diver balance and used in conjunction with density to estimate the change in wet weight of schistosomula. Wet weight was found to decline slightly during the migration phase and to increase exponentially following arrival of schistosomula in the hepatic portal system. No change in nitrogen content was detected during migration but there was a rapid increase after arrival in the hepatic portal system. The rate of oxygen consumption, measured by Cartesian diver respirometer, declined significantly during migration and then increased exponentially after arrival of worms in the hepatic portal system. No change was detected in the lactic dehydrogenase activity of migrating worms but again, an exponential increase occurred after entry into the hepatic portal system. It was concluded that the migrating schistosomulum is in a semi-quiescent metabolic state, although it possesses the ability to take up nutrients and undergoes morphological changes. Growth is triggered by an unknown mechanism after arrival in the hepatic portal system. Examination of the various sets of data suggests that it is initiated between 10 and 11 days post-infection in the most advanced schistosomula.

The effect of temperature on times to hatching of eggs of the nematode Ostertagia circumcincta.

Abstract: A delayed gamma distribution satisfactorily described the distribution of times to hatching of Ostertagia circumcincta eggs incubated in 0.1% saline at temperatures between 6 and 20 degrees C. Below 6 degree C hatching of eggs was extremely variable. The relationship between rates of development and temperature within the range 10 to 20 degrees C was more closely described by a non-linear function than a linear one. The non-linear function was incorporated into a temperature summation equation which satisfactorily predicted the hatching of eggs incubated under conditions of alternating temperatures.

Egg-shell permeability and hatching of Ascaris suum.

Abstract: When eggs of Ascaris suum were transferred from glass-distilled water into 0.1--0.4 M NaCl solutions, the water content of the unhatched juveniles fell with increasing concentration of solute. The effect was reversible. The egg-shell was thus permeable to water. The osmotic pressure of the egg fluid was osmotically equivalent to between 0.1 and 0.2 M trehalose. In hatching experiments in Fairbairn's medium containing 0.1 or 0.2 M trehalose, only 5 and 3% respectively of the eggs hatched; 83% of the eggs hatched in the absence of trehalose. The evidence suggests that loss of solutes from the egg fluid permits an increase in the water content of the unhatched juveniles and that this may be responsible for ending their quiescence.

The development of Theileria=Cytauxzoon taurotragi (Martin and Brocklesby, 1960) from eland in its tick vector Rhipicephalus appendiculatus.

Abstract: The sexual cycle of Theileria taurotragi was identified in the gut lumen of replete Rhipicephalus appendiculatus nymphs maintained at 28°C which had fed on eland with rising parasitaemias Macro- and microgametes developed from ring-form piroplasms within 24 h after repletion The microgamonts were elongate with 2 or more lateral projections The nuclei of the microgamonts divided into 4 and the microgamonts differentiated into 4 thread-like microgametes each with a central nucleus Round macrogametes developed at the same time and stages indicative of the fusion of the macro- and microgametes were observed after 48 h The resultant zygotes were detected in the gut wall cells by day 4, many being rounded and vacuolated with a peripheral nucleus By day 7 the zygote cytoplasm became dense and they lay in clusters still within the cells Binucleate zygotes were observed at this stage The zygotes increased in size and by day 12 began to transform into kinetes by invagination By the time the nymphs moulted into adults (about day 14), the kinete straightened to a broad anterior end and a tapering posterior with a mean length of 22·1 μm Kinetes were detected in the haemolymph by day 16 By day 20 the kinetes had penetrated the salivary gland acinar cells where they underwent schizogony until the infected acinar cells were filled with multinucleated sporoblasts The nuclei of the infected acinar cells became greatly enlarged soon after penetration of the kinete Division of the sporoblast nuclei was stimulated by feeding of the adult ticks From the 2nd day of attachment of the ticks to rabbits the sporoblasts underwent a process of schizogony to produce cytomeres Each nucleus of the cytomere divided to produce several small nuclei which differentiated into uninucleated sporozoites

Sub-cellular fractionation of Trypanosoma brucei. Isolation and characterization of plasma membranes.

Abstract: A procedure is described for the isolation of sub-cellular fractions from bloodstream forms of Trypanosoma brucei. The method leaves intact most of the nuclei, mitochondria and microbodies. All the fractions have been chemically characterized and tested for 10 enzymatic markers. About 5% of total cell protein was isolated as a microsomal fraction containing mostly plasma membranes and endoplasmic reticulum vesicles. Plasma membranes were purified by high-speed centrifugation on magnesium-containing Dextran, and on linear sucrose-density gradients. The yield of membranes was approximately 0·3% of the total cell protein. The purified material had a sucrose density of 1·14 g/cm3 and consisted of smooth vesicles. Specific activity of the membrane markers Na+, K+, ouabain-sensitive ATPase and adenylate cyclase were 26-and 20-fold higher, respectively, than in total cells. Neither DNA nor RNA was detected. The sum of the cholesterol and phospholipid content was 0·99 mg/mg protein. The cholesterol/phospholipid molar ratio was 1 : 2.

Post-larval mortality of the endoparasitic isopod castrator Portunion conformis (Epicaridea: Entoniscidae) in the shore crab, Hemigrapsus oregonensis , with a description of the host response

Abstract: The shore crabs, Hemigrapsus oregonensis and H. nudus, sometimes kill the female endoparasitic entoniscid isopod, Portunion conformis, a parasitic castrator. Studies of host populations from Baja California, Mexico to Vancouver Island, Canada, show that the incidence of parasitized hosts with dead parasites and the percentage of the parasite population found dead vary markedly with locality but only occasionally with season. Both higher incidences of hosts with dead P. conformis and higher proportions of the total parasite population found dead are associated with (1) high prevalence of parasitism, (2) female hosts and (3) large hosts. Within a host, the proportion of the parasites that are dead is not related to the degree of multiple infection. Typically, either all or none of the parasites in a multiple infection are dead. Supernumerary juvenile parasites do not suffer differential mortality. The developmental stage of the female parasite does not seem to influence the likelihood of death. The presence of dead parasites may not confer an acquired immunity to re-infection. These features suggest that parasite death is typically the result of activation of a successful host defensive process rather than indicative of a defect on the part of the parasites. Parasitized female hosts can regain their reproductive capabilities following death of the parasite. Post-parasitic broods are smaller than normal. Reproductive recovery is presumed to provide the selective pressure favouring evolution of a lethal host response. The host-produced sheath surrounding female parasites is a haemocytic response. Sheaths enclosing dead parasites are thicker and more electron dense than those containing healthy parasites. The sheath of a healthy P. conformis may actually protect the parasite from a more intense host response.

Variable expression of virulence in the rodent malaria parasite Plasmodium yoelii yoelii.

Abstract: The expression of the virulence character in the virulent line (YM) of Plasmodium yoelii yoelii was investigated. The level of virulence was measured by counting the parasitaemia of the mature red blood cells. Several sub-clones were isolated from the virulent line YM and each was tested for its level of virulence. Out of 10 sub-clones 1 showed a marked decrease in virulence. However, transmission of this sub-clone through mosquitoes fully restored its virulence. A clone isolated from the progeny of a cross between mild and virulent parents had an intermediate level of virulence. A sub-clone isolated from this intermediate virulent line exhibited greatly reduced virulence. Mosquito transmission of this sub-clone also restored its virulence to a level comparable with the virulent line YM.

Total intestinal absorption of glucose and L-methionine in broilers infected with Eimeria acervulina, E. mivati, E. maxima or E. brunetti.

Abstract: The in vitro absorption of glucose and L-methionine in the intestine of broiler chickens was measured 7, 14 and 21 days post-inoculation (p.i.) with sporulated oocysts of Eimeria acervulina, E. mivati, E. maxima or E. brunetti. The small intestine of each bird was divided into 8 regions of equal length and absorption was measured on 3 tissue disks of equal size from each region. The absorption rate of each substrate with each coccidial species was measured based on (1) an equal area from each region, (2) an equal weight from each region, (3) the total absorption in each region, and (4) the total potential absorption in the intestine. Comparisons of absorption rate of equal areas in each intestinal region demonstrated that infected birds at 7 days p.i. absorbed significantly less substrate per unit area in the regions of maximum infection than uninfected controls. Malabsorption was less apparent when the weight of the region was used as the unit of measurement. Compensatory absorption was seen in some uninfected regions with E. acervulina. The total potential intestinal absorption at 7 days p.i. was reduced with E. mivati, E. maxima and E. brunetti but not with E. acervulina. At 14 days p.i., total L-methionine and glucose absorption in some regions of the intestine was significantly increased with E. acervulina but not with E. mivati, E. maxima or E. brunetti. No absorption differences were seen at 21 days p.i. with any species.

Studies on Trypanosoma (nannomonas) congolense III. Antigenic variation in three cyclically transmitted stocks.

Abstract: Cyclical transmission of different variable antigen types of Trypanosoma congolense STIB 228 resulted in the development of metacyclic trypanosome populations which were similar in their variable antigen composition as judged by immunofluorescence and neutralization assays The variable antigen types present in the ingested bloodstream populations were not found in the metacyclic populations The bloodstream populations which were obtained from cyclically infected, irradiated (900 rad) mice contained variable antigen types which were not present in the corresponding metacyclic populations When derivatives of 2 other stocks of T congolense, isolated in different area of Tanzania, underwent cyclical development in the tsetse fly, the metacyclic populations of each stock also had a characteristic variable antigen composition The metacyclic populations of the 3 stocks were, however, completely dissimilar in variable antigen composition Simultaneous infection of tsetse flies with a mixture of different stocks resulted in the concurrent production of metacyclic trypanosomes which contained the characteristic variable antigen types of each stock The effect of cyclical transmission on the process of antigenic variation in T congolense infections is therefore similar to that in T brucei infections

Quantitative modelling and prediction of development times of the free-living stages of Ostertagia ostertagi under controlled and field conditions.

Abstract: The development of the free-living stages of Ostertagia ostertagi incubated at constant temperatures from 5 to 27 degrees C and in dung pats in the field was observed. The death rates were adequately described by a single exponential functional and the distribution of times for development by a delayed gamma density function. The times of appearance of selected development stages such as maximum numbers of embryonated eggs and half the maximum number of infective larvae, were highly dependent on temperature. The relationships between these times and temperature were adequately described by the Arrhenius equation. The parameters of this equation and mean hourly temperature recorded in dung pats in the field were used to predict development times in the field. A comparison of predicted and observed times showed that an initial delay accounted for a large proportion of the time required for development in dung pats, particularly the embryonation of eggs. This delay was attributed to lack of aeration associated with high moisture content of the dung pat.

Light and gravity responses of the oncomiracidium of Entobdella soleae and their role in host location

Abstract: The behaviour of oncomiracidia of the monogenean skin parasite Entobdella soleae, after hatching without chemical stimulation, has been studied in vertical, 4 cm high chambers illuminated from above Most freshly hatched larvae are photo-positive and swim to the top of the vessel, where they remain actively swimming Occasionally larvae become photo-negative and make a brief excursion to the bottom of the vessel As the larvae grow older, these photo-negative excursions become more frequent and larvae spend progressively longer in the photo-negative phase of behaviour, but, even after many hours, photo-positive vertical excursions are still occurring If this pattern of alternating photo-positive and photo-negative vertical movements occurs in the natural environment, these vertical movements, coupled with horizontal transport of larvae by water currents, would provide a search pattern for the host, the diurnally-inactive, bottom-living flatfish Solea solea When eggs are stimulated to hatch in the chambers with urea, large numbers of larvae emerge within a few min, and it was observed that during the period of about 30 min after stimulation, many larvae are photo-negative When eggs on the sea bottom are stimulated to hatch by mucus or urine from a nearby host, such behaviour would greatly enhance chances of contact with the fish When eggs are stimulated to hatch by means of urea in the absence of light, there is evidence that the oncomiracidia are geo-negative It has been shown that larvae are able to extricate themselves in total darkness from sediment containing buried eggs and that, in an experimental situation illuminated only by an infra-red beam, larvae readily attach themselves to the lower skin of soles within 2 min of hatching There is evidence that, in aquaria, some invasion of the host takes place from below

The growth of Trypanosoma cruzi in human diploid cells for the production of trypomastigotes.

Abstract: The use of human diploid cell lines of finite life for the in vitro production of Trypanosoma cruzi is described. Both MRC5 and WI38 cells release trypomastigotes with less than 5% amastigotes. This could form the basis for biochemical and immunological studies, which were previously limited by the problems of separating parasites from blood. By selecting the in vitro passage number of the parasite it is possible to select for either the broad or the slender forms of trypomastigotes, allowing comparative studies of these forms within a single strain of the parasite. It is also possible to isolate amastigotes by disrupting the cells before trypomastigotes appear, and separating them from cell debris with Metrizamide. It is shown that by incorporating [3H]uridine in the cell-culture medium, labelled trypomastigotes are obtained. The release of this label (putative RNA) provides a relatively simple isotopic assay for parasite death. Examples of this assay for testing drug toxicity and in immunological lysis are presented.