P. P. Aitken

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.

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.

Toxoplasma infection and response to novelty in mice.

Three groups of mice were infected withToxoplasma and used for behavioral testing using a Y-maze. One group was infected when adult and two groups congenitally, one of these born to dams infected during gestation, the other to dams chronically infected prior to mating. In an initial habituation period each mouse was exposed to a black arm and stem of the maze, entrance to a white arm being blocked by a transparent door. In a subsequent free-choice trial both arms were black and the mouse was free to explore all parts of the maze. During both periods infected mice were more active than controls. Infected mice engaged in less grooming behaviour indicative of less approach-avoidance conflict than controls prior to entry into a choice arm at the beginning of the free-choice trial. Infected mice spent more time in the familiar than in the novel (previously blocked) arm during the free-choice trial; conversely, uninfected mice spent more time in the novel than in the familiar arm. It is suggested that the reported behavioural changes would lead to dissemination of the infection in the environment by ultimately making infected mouse intermediate hosts more susceptible to predation by domestic cats, the definitive hosts ofToxoplasma.

The influence of congenital Toxoplasma infection on the spontaneous running activity of mice

Home-cage running-wheel activity of mice congentally infected withToxoplasma was recorded over 24 days. Infected mice were consistently more active than uninfected controls over the entire testing period. This finding extends previous studies and indicates that such increased activity levels occur not only in novel but also in familiar environments, and suggests that congenital toxoplasmosis tends to render mice ‘hyperactive’. If such behavioural alterations occur in wild mice, it is likely that infected mouse intermediate hosts would be more susceptible to predation by cats, the definitive hosts ofToxoplasma.

Experimental toxocariasis in mice and its effect on their behaviour

When compared to uninfected controls, mice infected with second-stage larvae of Toxocara canis exhibited: (1) impaired motor performance, (2) increased ambulatory activity, (3) a greater relative preference for more exposed areas and (4) a greater overall preference for novel environments. The infected mice also appeared to show less approach-avoidance conflict and seemed less cautious when presented with novel stimuli. These behavioural differences were independent of measures of general health. Possible explanations for the observed behavioural deficits are discussed. Such alterations in behaviour may be implicated in the continuation of the life-cycle of this parasite by making mouse paratenic hosts more liable to capture and subsequent ingestion by canids, the definitive hosts of T. canis.

Experimental toxocariasis and hyperactivity in mice

An observational study using videorecordings and computerassisted data analysis was undertaken in order to investigate the behaviour of mice infected with larvae ofToxocara canis. The findings indicated that the infection had a marked effect on five readily and reliably differentiable categories of murine behaviour. A marked increase in the number of shorter bouts of each of the five behaviours was also associated with the infection. These results support previous findings and further suggest thatT. canis infection affects the way in which mice respond to their environment. In particular the infection appears to be associated with hyperactivity in mice. Possible causes of such behavioural abnormalities as well as implications of these findings for clinical studies concerned with relationships betweenT. canis infection and hyperactivity in children are discussed.

Toxocara canis infection and hyperactivity

An observational study using video recordings and computer assisted data analysis showed that infection with Toxocara canis larvae had a marked effect on five readily and reliably differentiable categories of murine behaviour. The infection was also associated with an increase in the number of shorter bouts of each behavior. These results indicate that infection with T. canis renders mice hyperactive, and would appear to justify a complete reappraisal of the role of this neurotropic parasite as a cause of behavioural abnormalities such as hyperactivity in children.