Increased risk of traffic accidents in subjects with latent toxoplasmosis: a retrospective case-control study

The parasitic protozoon Toxoplasma gondii (phylum Apicomplexa) infects various warm-blooded vertebrates, including 30–60% of humans in most countries [1]. After a short phase of acute toxoplasmosis the infection proceeds into its latent phase when cysts are formed and these survive for the rest of the host's life, mainly in neural and muscular tissues. In immunocompetent human subjects the latent phase of infection is usually considered asymptomatic and harmless [2].

Toxoplasma is known to induce specific behavioural changes in infected rodents. Observed decreases in motor performance [3], learning capacity [4], neophobia [5], and fear [6], and increases in activity [7, 8], and reaction times [9] are usually considered evolutionary adaptations of the parasite that facilitate the transmission from intermediate to definitive host (typically from rodents to cat) by predation [10]. Human latent toxoplasmosis leads to prolongation of reaction times [11] and changes in personality profiles [12, 13]. These changes are probably side effects either of the rodent-aimed manipulative activity of Toxoplasma or of some pathogenic activity of the parasites in the brain. The changes cannot influence the risk of predation in modern humans; nevertheless, prolongation of reaction times could increase the risk of other incidents such as traffic accidents. If this is true then the prevalence of toxoplasmosis in participants in traffic accidents should be higher than in the general population living in the same area.

Here we report the results of retrospective case-control study that compares the seroprevalence of toxoplasmosis in persons injured in traffic accidents with the seroprevalence of toxoplasmosis in general population living in the same area.

The seroprevalence of latent toxoplasmosis in subjects involved in traffic accidents and in the general population was compared by a Mantel-Haenszel test for age-stratified data (Table 1, Fig. 1). The result suggests a significantly higher seroprevalence in the traffic accident set (Chi² _MH = 21.45, p < 0.0001). The value of the odds ratio (OR) suggests that subjects with latent toxoplasmosis had a 2.65 (C.I.₉₅= 1.76–4.01) times higher risk of a traffic accident than the toxoplasmosis-negative subjects. The ORs varied between particular age strata; however, the test of homogeneity of odds ratios shows that these differences are not significant (Zelen statistics = 0.0076, p = 0.88). Analyses of the traffic accident sample found no significant differences in ORs between drivers and pedestrians or between men and women. To check the robustness of our results, we repeated all analyses with six different age strata (15–19, 20–29, 30–39, 40–49, 50–59, and 60–70 years). The results were very similar to those reported in Table 1. However, due to the low number of observations in some strata, the relative risks in particular age strata fluctuated. (The total OR = 2.48, C.I.₉₅= 1.64–3.77; age 15–19, OR = 1.87, C.I.₉₅ = 0.36–9.73; age 20–29, OR = 2.79, C.I.₉₅ = 1.35–5.78; age 30–39, OR = 1.06, C.I.₉₅ = 0.28–3.96; age 40–49, OR = 4.04, C.I.₉₅ = 1.32–12.39; age 50–59, OR = 2.12, C.I.₉₅ = 0.83–5.43; and age 60–70, OR = 3.16, C.I.₉₅ = 1.05–9.52.) Again, there was no significant difference in age of the accident and control groups within these age strata (p = 0.25, Pitman's exact test).

Positive correlation between the subjects' age and the probability of being latent toxoplasmosis-positive in the Czech republic was reported [14] and some correlation between the probability of traffic accident and the subjects' age could also exist. Therefore, increased risk of traffic accident of subject with latent toxoplasmosis can be theoretically caused by higher average age of traffic accident victims than of the controls within particular age-strata. To test this hypothesis, we checked the data for increased age of traffic accident victims in comparison with controls within particular age strata using a one-tailed Pitman's test. The results suggest that the age of the accident group was only slightly higher than the age of controls (p = 0.15, exact test). The Pitman's tests for particular age strata show that only for the first stratum (15–29 years) was the mean age of the accident group (22.7 years) significantly higher than the average age of controls (20.2 years,) (p = 0.028, Pitman's exact test). Therefore, this factor cannot be responsible for the higher prevalence of latent toxoplasmosis in the accident group in three other strata.

The OR significantly increased with level of anti-Toxoplasma antibody titre (p < 0.0001, exact Cochran-Armitage trend test; Fig. 2), being low (OR = 1.86, C.I.₉₅ = 1.14–3.03) for the 99 subjects with low antibody titres (8 and 16), higher (OR = 4.78, C.I.₉₅ = 2.39–9.59) for the 37 subjects with moderate titres (32 and 64), and very high (OR = 16.03, C.I.₉₅ = 1.89–135.66) for the 6 subjects with titres higher than 64. In fact, of these six subjects with high titres, i.e., subjects with supposedly relatively recent or massive T. gondii infection, five were found among the 146 traffic accident participants and only one among the 446 controls.

The increased seroprevalence of toxoplasmosis in victims of traffic accidents suggested that the subjects with latent toxoplasmosis had significantly higher risk of traffic accidents than non-infected subjects. Relative risk of traffic accidents was highest in subjects with supposedly relatively recent or massive T. gondii infection and decreased with the decrease of anti-toxoplasma immunity, i.e. with duration of Toxoplasma infection.

Retrospective case-control study cannot reveal why a higher seroprevalence of latent toxoplasmosis occurs among victims of traffic accidents. Theoretically, toxoplasmosis can either increase the probability of traffic accident or decrease the probability that the subject visits a physician and therefore enters the control set. Results of independent recent studies on prevalence of toxoplasmosis in Prague, however, suggest that the former possibility is more probable. After taking into account the differences in age and gender composition of particular sets, the differences between seroprevalence measured in the multipurpose immunological surveys and in the set of 857 conscripts or in the set of 723 pregnant women were not significant (conscripts: p = 0.12, women: p = 0.78) [16]. It is also possible that some unknown confounding factor, such as the socioeconomic status of a person, can influence the probability of both Toxoplasma infection and a traffic accident. The most parsimonious explanation, however, seems to be that the Toxoplasma-infected individuals have increased risk of traffic accidents due to their worse psychomotor performance. Previously published results suggest that the effect of latent toxoplasmosis on human reaction times is rather moderate, accounting for only 8% of the total variance [11]. However, the comparison of psychomotor performance in the first, second, and third minutes of the test indicates that Toxoplasma-infected subjects have reasonably good reaction times in the first minute of the test but lose concentration more quickly than the controls. It has been reported in literature that older drivers are able to compensate for the prolongation of their reaction times [17]. It suggests that other behavioural factors, like the capacity for long-term concentration, are more critical with respect to the risk of a traffic accident [19]. It is difficult to quantify capacity for long-term concentration under natural conditions using laboratory test. However, it is possible that the risk of traffic accident is in fact more strongly influenced by the toxoplasmosis-associated decrease of capacity of concentration than by the moderate increase of reaction time.

The existence of positive correlation between concentration of antibodies and relative risk of traffic accident contrasts with previously reported negative correlation between concentration of antibodies and increase of reaction times of infected subjects [11]. Possibly, the positive correlation between the concentration of antibody and the risk of traffic accident observed in the present study reflects a positive correlation between intensity of infection and its behavioural effect rather than a negative correlation between duration of infection and its behavioural effects. It is also possible that the long-term infected subjects with low antibody concentration already successfully reduced the risk of accident by adjusting their behaviour to the decrease of their psychomotor performance. In both studies, we eliminated all subjects with acute toxoplasmosis from the data set by screening the subjects for IgM antibodies. Moreover, in the present study, a positive correlation existed even within the subset of subjects with low titres (8–32) (p = 0.032). Therefore, the effect of latent toxoplasmosis, rather than transient effect of acute toxoplasmosis disease seems to be more probable candidate for explanation of increased risk of traffic accident in Toxoplasma-positive subjects.

The dormant stages of Toxoplasma, the tissue cysts, are located mainly in brain and muscular tissues of infected animals [18]. In rodents, these cysts usually survive in an infectious form until the death of the host. The low rate of decrease of specific antibodies in an infected individual [19], extremely low frequency of seroconversion (loss of specific antibodies) among seropositive subjects [14], and high frequency of reactivation of toxoplasmosis in seropositive AIDS patients [20, 21] suggest the existence of life-long Toxoplasma infections in humans as well. The slowing of reaction times and other behavioural changes that can increase the risk of predation in Toxoplasma-infected rodents are probably mediated by production or induction of production of a neurotransmitter, possibly dopamine, [22] in the brain of infected animals. In humans, such manipulative activity is non-productive for Toxoplasma. It must be considered here "an evolutionary constraint," as it arises as a side effect of selection in different host species, with no apparent direct adaptive significance in humans [23]. Indeed, the difference in behavioural alterations induced by Toxoplasma in various animal species might help to reveal the neurophysiological mechanism(s) of the manipulation activity of the parasite.