W. M. Hutchison

The life cycle of the coccidian parasite, Toxoplasma gondii, in the domestic cat.

Abstract Earlier work in which the coccidian nature of the protozoan parasite Toxoplasma gondii was revealed has been confirmed and extended in a series of experiments using only specific pathogen free (SPF) cats. All of these cats, prior to our experiments, were negative for both Coccidia and Toxoplasma. Experiments on 17 of these cats are described. 10 were fed with mouse brains containing tissue cysts of Toxoplasma. 8 of these shed Toxoplasma oocysts in their faeces within 5 to 10 days of infection. 2 cats had shed no oocysts before they were killed on the 3rd and 4th day after infection. 5 cats were fed with Toxoplasma oocysts but gametogonic development resulting in the production of oocysts was observed in only 1 individual; oocysts appeared in the faeces 9 days after the infective meal. 2 cats were examined as controls; 1 received no addition to its diet, the other was fed “negative” brains. Neither showed any evidence of infection. Examination of the gut of infected cats showed that shizogonic and gametogonic stages develop usually in the ileum although the entire length of the small intestine can be affected. The parasites were observed in the epithelium of the tips of the villi, developing within vacuoles, and lying between the nucleus and brush border of the cell. All stages of schizogony and gametogony may be found together, occurring in no particular sequence along the epithelium of the villus; multiple infections of the host cells are common. Schizonts are characterized by granular, basophil cytoplasm and vesicular, karyosomic, Feulgen-negative nuclei which become dense and Feulgen-positive when merozoite formation takes place. The merozoites vary from 4 to 29 per schizont and measure 3·5-4·5μ by 1μ in sections; in smears their dimensions are 4·9 (SD = 1·0)μ by 1·5μ. They form clusters about a residual body and may appear to radiate fan-wise from it in sections. The microgametocyte is spherical, about 10μ in diameter and gives rise to 12–32 microgametes which form on its surface, are slender and crescentic and about 3μ in length, excluding the flagella. The macrogametocyte has, throughout development, a vesicular, karyosomic and Feulgen-negative nucleus. At the commencement of development, the cytoplasm contains many basophil and few PAS-positive granules; at its completion this condition is reversed. No granules were seen to be associated with the formation of the oocyst wall. The oocyst itself measures 12·7 (SD = 0·7)μ by 10·4 (SD = 0·8)μ. Sporulation, which does not occur within the gut of the cat, gives sporocysts measuring 7·8 (SD = 0·9)μ by 6 (SD = 0·5)μ. These coccidian stages of Toxoplasma are compared and contrasted with those of Isospora spp. of the cat. Despite its significant resemblances to Isospora, the retention of the existing nomenclature of Toxoplasma gondii is supported.

Behavioural abnormalities in Toxoplasma-infected mice.

Observational methods, using video recordings and computer-assisted data analysis, were used to investigete the behaviour of Toxoplasma-infected mice. Infection had a selective effect, increasing the amount of general movement but decreasing the amounts of rearing and digging. In addition infection affected the pattern of bouts of behaviour, increasing the number of shorter bouts, and this was found to underlie a variety of specific behavioural changes. The results indicate that Toxoplasma infection probably affects the animals response to its environment and the stimulation arising from it, and may even affect endogenous regulatory processes in the brain.

The effect of congenital and adult-acquired toxoplasma infections on activity and responsiveness to novel stimulation in mice

Activity and responsiveness to novel stimulation were assessed in three groups of mice infected with Toxoplasma. One group was infected when adult; two groups were infected congenitally, one born to dams infected during gestation, the other to dams chronically infected prior to mating. Each mouse was tested in a box, the floor of which was marked off into 16 equal squares, and its activity was measured over ten minutes by counting the number of times the mouse entered each square. Infected mice were more active. In addition, infected mice showed a smaller relative preference for the more novel central area of the box, especially towards the end of the observation period. These differences were independent of emotionality (as measured by defaecation counts), general health (as measured by subjective health ratings and body weight) and the number of Toxoplasma tissue cysts in specified brain regions. We suggest that differences arise from pathological changes caused by proliferating toxoplasms in the brains of the infected mice; an immunopathological reaction due to the presence of tissue cysts in the brain may also be involved. Other possible factors contributing to observed deficits in behaviour are also discussed. We suggest that such deficits may render Toxoplasma-infected mice more susceptible to predation by the domestic cat, the definitive host of Toxoplasma.

Chronic Toxoplasma infections and familiarity-novelty discrimination in the mouse

Male mice were exposed to a black arm and the stem of a Y-maze; entrance to a white arm was blocked by a transparent door. Toxoplasma-infected mice were significantly less active and tended to produce fewer faecal boluses than uninfected controls. In a subsequent free-choice trial, in which both arms were black, the uninfected mice spent significantly more time in the novel (previously blocked) arm; the infected mice showed no preference for either choice arm. Possible explanations are discussed. In particular, it seems that Toxoplasma-infected mice may be less responsive to novel stimuli. This suggestion has important implications for our understanding of one of the major ways in which Toxoplasma passes from host to host. If Toxoplasma infections impair responsiveness to novel stimuli, then infected mice are more likely to be taken by predators.

The effect of congenital Toxoplasma infection on mouse activity and relative preference for exposed areas over a series of trials.

Previous studies showed that mice with congenital Toxoplasma infections tend to be much more active and tend to show a smaller relative preference for more exposed and novel areas than do uninfected controls. However, in those studies, mice were exposed to behavioural testing procedures once only. Results described here show that these differences (and differences defaecation) are more than just transitory phenomena observed on a first exposure to a novel area. The differences between infected and uninfected mice were clear during each of five separate trials; moreover, they tended to increase from the first to the fifth trial. The possible implications of these findings for the continuation of the life-cycle of the parasite in the environment are considered.

The effect of congenital and adult-acquired Toxoplasma infections on the motor performance of mice.

Motor performance was assessed in three groups of mice infected with Toxoplasma. One group was infected when adult. Two groups were infected congenitally: the first was born to dams infected during gestation and the second to dams which were chronically infected prior to mating. All mice were placed individually on a rotating cylinder and the number of falls from it noted over a two-minute period. Infected mice fell significantly more often than uninfected controls. The difference was independent of emotionality (as measured by defaecation) and general body health (as measured by body weight and a subjectively assessed health rating). There was no significant difference in motor performance between the two congenitally infected groups. However, the offspring of mice infected during pregnancy fell significantly more often than mice infected when adult. There were no significant correlations between motor performance and the actual number of Toxoplasma tissue cysts in the brains (or in separate defined sectors of the brains) of infected mice. We suggest that differences between infected and uninfected mice result from pathological changes caused by proliferating toxoplasms in the brains of infected mice. An immunopathological reaction due to the presence of the tissue cysts may also be involved. Other possible factors contributing to observed deficits in motor performance of infected mice are discussed. We suggest that such interference with the motor performance of Toxoplasma infected mice may render them more susceptible to predation by the domestic cat, the definitive host of Toxoplasma.

Chronic Toxoplasma infections and motor performance in the mouse

Motor performance in Toxoplasma-infected mice and uninfected controls was assessed by placing them, individually, on a rotating cylinder. The infected mice fell significantly more often than the co...

Congenital toxoplasmic retinochoroiditis in a mouse model.

A study of the eyes of adult mice infected in utero with Toxoplasma gondii is reported. The histopathological features of the ocular inflammatory response in the infected mice ranged from minimal damage to complete destruction of the retinal tissue. Notable features such as retinal vasculitis and an almost uniform and highly selective destruction of the photoreceptor layer of the retina suggest a similarity between experimental autoimmune retinitis and the disease process in the retinas of our Toxoplasma-infected mice. We suggest that our mouse model could provide a simple and inexpensive tool for the investigation of immuno-pathological processes in the retina resulting from congenital Toxoplasma infection. The model has the advantage of low post-natal mortality coupled with high ocular morbidity. Furthermore, its aetiology is probably analogous to that of human ocular toxoplasmosis, in that the foetus becomes infected in utero via a mother whose primary infection is acquired during gestation.

Ultrastructural studies on the sporulation of oocysts of Toxoplasma gondii. II. Formation of the sporocyst and structure of the sporocyst wall.

The ultrastructural changes observed during sporocyst formation and the structure of the sporocyst wall was examined in oocysts which had been allowed to sporulate for between 12 and 48 hours at 27 degrees C. As the spherical sporoblast developed into the sporocyst the cytoplasmic mass became ellipsoidal in shape although no change was noted in the organelle compliment, which cosisted of two nuclei plus a number of polysaccharide granules, lipid globules, mitochondria, Golgi bodies, and some rough endoplasmic reticulum. The sporocyst wall consisted of a thin outer layer (15-20 nm) which was formed from two limiting membranes of the sporoblast and an inner layer (40-50 nm) which was comprised of four curved plates. This inner layer was formed under the outer layer and, although no specific cytoplasmic organelle disappeared with its formation, some unit membranes were observed close to the plasmalemma during its formation. Each curved plate has a marginal swelling and an interposing strip of material is present between the margins of adjacent plates. The plates are joined to the interposing strip by a thin band of osmiophilic material. In oblique and tangential sections through the plates two types of cross banding were observed which differed in periodicity.

Prevalence of Toxoplasma gondii in meat animals, cats and dogs in central Scotland.

Meat animals, cats and dogs in central Scotland were surveyed for the presence of Toxoplasma gondii antibodies using the Sabin-Feldman dye test. Of 251 cattle, 253 pigs, 321 sheep and 2 goats examined, 7·6%, 4·0%, 24·9% and 50% had titres of 1: ⩾ 10. Of 158 cats examined 19·0% were found to have antibodies. There was no significant difference between male and female cats, nor between those aged under six months and those aged over six months. No significant difference was apparent between stray and domiciled cats when all ages were included. However, for those aged over six months, there were significantly more (x2=4·110; p < 0·05) stray cats infected (27·2%) than domiciled cats (10·3%). Of 153 dogs examined 19·6% were found to have antibodies; and though there was a significant difference between the sexes, there were significantly more (x2=14·113; p < 0·001) dogs aged over six months infected (27·9%), than those aged under six months (2·0%).