Methods for screening compounds against Eimeria tenella in cell culture and in the chicken are described. An analysis for incidence of anticoccidial activity and host cell toxicity has been made of 11550 compounds screened in vitro, and some correlation of activity in vitro and in vivo has been attempted. The results show that screening compounds in tissue (cell) culture is not a satisfactory or reliable alternative to screening in chickens, although in vitro studies undoubtedly have a part to play in the evaluation of an active drug.

Rhesus monkey erythrocytes when incubated in vitro under similar conditions to those used for the cultivation of Plasmodium knowlesi-infected erythrocytes in vitro, exhibit an increase both in their osmotic fragility and in the activity of their acetylthiocholinesterase. No effect was observed on the catabolism of glucose through the glycolytic pathway or through the primary dehydrogenases of the pentose phosphate pathway. The ATP content of normal monkey erythrocytes was also unchanged during incubation in vitro. These observations indicate that incubation of erythrocytes in vitro primarily causes membrane changes. Infection of normal erythrocytes by P. knowlesi was reduced markedly by preincubation in vitro at 37 °C for 24 and 48 h. These results suggest that the maintenance of integrity of the surface of the eythrocyte in vitro is a necessary prerequisite for an efficient culture system for the malaria parasite.

Fasciola hepatica miracidia labelled with radioselenium were used to study aspects of their host-finding capacity by determining radioactivity subsequently taken up by exposed ‘target’ snails (Lymnaea truncatula). Possible interfering effects exercised by a number of non-host snails and bivalves were examined in linear test channels. The infection rate (radioactivity) among ‘target’ snails was markedly lowered when non-host Lymnaea species (L. pereger, L. palustris, L. stagnalis) were interposed as ‘decoys’. The prosobranch Bithynia tentaculata and the bivalve Sphaeriwm corneum exhibited a slight decoy effect whereas pulmonate species like Anisus vortex, Gyraulus albus, Planorbis planorbis, Physa fontinalis did not interfere with miracidial host-finding.

Other experiments showed that miracidia are more strongly attracted towards L. truncatula than L. pereger. Miracidia are not able to penetrate intact egg clusters of L. truncatula.

Microvilli are developed on the surface of Schistosoma mansoni schistosomula during penetration of the host skin; they form rapidly but are lost approximately 90 min after penetration. Identical microvilli are also formed on schistosomula which have penetrated a mouse skin preparation in vitro, and on schistosomula prepared by mechanical separation of the tail from the body of the cercaria. The microvilli, which are limited by the trilaminate tegumental membrane of the cercaria, eventually degenerate and are cast off from the surface of the tegument. There is little change in the surface area of the schistosomulum at this time, and the formation and loss of microvilli coincides with the replacement of the cercarial tegumental membrane by the new heptalaminate membrane. It is suggested that during the cercaria/schistosomulum transformation, some intramembraneous components of the original cercarial membrane may migrate into the new heptalaminate membrane and thus be retained, while other peripheral components such as the glycocalyx are almost certainly lost.

Infections of Raillietina cesticillus in the small intestine of the fowl caused a mononuclear leucocyte infiltration, which occurred in the tissues around the worm scolices and was especially prevalent in challenge infections. There was no evidence of vascular congestion or of a neutro-philic response, and the cellular reaction seemed to consist of a local proliferation of lymphoid tissue. Mast cell and pyroninophylic cell numbers increased in the intestinal mucosa of birds with longstanding primary infections and with secondary infections, but very little eosin-lophil infiltration was detected and the globule leucocyte response was minimal. The infections had no effect on the numbers of thelio-lymphocytes and goblet cells. Challenge infections did not result in an increased level of serum antibodies, but more cells containing globulin seemed to be present in the intestinal mucosa of these birds. The interrelationships of the various cell responses to the cestode and the origin of the globule leucocyte are discussed in the light of previous work on helminth infections.

Proteocephalus ambloplitis was recovered from largemouth bass inhabiting different thermal regimes, ranging from normal to greater than 10 °C above normal, on the Energy Research and Development Administration's (ERDA) Savannah River Plant near Aiken, South Carolina. Possible alterations in the mechanisms initiating plerocercoid migration as proposed by Fischer & Freeman (1969) were noted. According to these authors, a rise in temperature from 4 to 7 °C and above stimulated migration of parenteric plerocercoids into the gut. The lowest water temperature recorded for the reservoir for the entire sampling period was 8·0 °C. Theoretically, this should have terminated the life-cycle of this tapeworm since migration temperature was never attained. At Savannah River, the appearance of adult tapeworms coincided with a decrease in water temperature, while the appearance of adult tapeworms in Canada was correlated with an increase in water temperature. It is postulated that the decline in water temperature in southern latitudes and the increase in water temperature in northern latitudes initiates the same response, that is, migration of plerocercoids from parenteric to enteric sites where maturation to the adult form ensues.

The equilibrium, of vulvar phenotypes in Haemonchus contortus cayugensis was changed by infecting lambs with either a selected strain of smooth or of linguiform worms. Subsequently the lambs grazed the same pasture and observations were made on changes in worm populations in the lambs, and on pasture it appeared that the seasonal change in the relative frequencies of A and B linguiform worms was related to density-dependent factors in the host. In the spring, when lambs first become infected, the worm population is small and the majority of linguiform worms are A-type. As the season progresses the density of the parasite population increases. Once the population becomes crowded selection begins to favour the B-type females. The effects of crowding on linguiform A females can be seen as a positive correlation between the percentage of linguiform A and their weight. Population density also affected the survival of A-type worms since there was a negative correlation of percentage A-type on total worm numbers. There was a similar negative correlation between percentage A-type and total worm numbers in both the continuously infected and tracer lambs.

Population density did not affect the proportion of smooth worms. However, smooth worms increased in the tracers from, approximately 10% at the beginning of the experiment to 55% at its conclusion 18 weeks later. The latter percentage is almost within range of the local subspecies, which has 60% or more smooth worms. It appears therefore that the proportion of smooth to linguiform worms is a stable equilibrium maintained by natural selection.

Vulvar phenotypes and the rate of development of eggs over a range of temperatures were used to compare Haemonchus contortus populations in New York State and Ohio. These parameters indicated that the westernmost boundary of the subspecies Haemonchus contortus cayugensis is the Chautauqua valley in New York. The Haemonchus ecotype in Ohio had a vulvar phenotype formula similar to that described for the subspecies Haemonchus contortus contortus. The relationship between slope and intercept of the regression of rate of egg hatch on temperature was different for morphs within ecotypes as well as between the New York and Ohio ecotypes. Linguiform-A appeared to be the cold-adapted morph in both ecotypes. Smooth was the warm-adapted morph in New York with linguiform-B filling that niche in Ohio.