Our results show that Toxoplasma infection increased the risk of traffic accidents in military drivers. The data also indicate that RhD positivity protected infected subjects against the increase in the rate of traffic accidents. In RhD-negative subjects, the probability of traffic accidents increased with titer of anti-Toxoplasma antibodies suggesting that the risk of traffic accidents decreased with duration of Toxoplasma infection. The probability of traffic accidents increased across the years of testing. This phenomenon was, probably, caused by the observed downward trend in overall quality, e.g. in intelligence, of conscripts (data not shown), related to the forthcoming abolition of compulsory military service in the Czech Republic in 2004.

Our results are in good agreement with the results of laboratory testing of reaction times in Toxoplasma-infected and Toxoplasma-free, RhD-positive and RhD-negative subjects [6, 9, 10]. Latent toxoplasmosis had a strong negative effect on RhD-negative subjects and no effect on RhD-positive subjects. The strength of the negative effect of toxoplasmosis on reaction times increased with duration of infection (with decreasing anti-Toxoplasma antibody titer) [6]. In contrast, in the current study the risk of traffic accidents decreased with duration of infection. It should be noted, however, that the decrease of the risk of traffic accidents with duration of infection was already observed in a previous case-control study [7]. The most plausible explanation for such conflicting results is that, on the one hand, reaction times increase with duration of infection and, on the other hand, the infected drivers are able to gradually adapt to their impaired reaction times. Such effect has already been described in aging drivers [19].

It was previously speculated that personality differences between Toxoplasma-infected and Toxoplasma-free subjects could result in lower willingness of Toxoplasma-infected subjects to participate in serological surveys and therefore also in their underrepresentation in a control population [7]. The design of the current study, i.e. a prospective cohort study, made it possible to confirm that the increase of the risk of being involved in a traffic accident rather than a decreased chance of being enrolled in epidemiological surveys is responsible for the higher prevalence of Toxoplasma-infected subjects among victims of traffic accidents compared to the general population of the same area.

At face value, our results in some respect contradict those of the previous traffic accident studies. In the present study, the effect of latent toxoplasmosis on traffic accidents was not apparent in an unsorted population of RhD-positive and RhD-negative subjects. When infected with Toxoplasma, the RhD-negative subjects had a significantly higher and RhD-positive subjects a nonsignificantly lower incidence rate of traffic accidents than the corresponding Toxoplasma-free subjects. On the other hand, the present results are in agreement with the unpublished results of our retrospective case-control study performed in Prague between 2002–2006 using the same method as in the 1997–2000 study [7]. In that study, similarly as in the present prospective study, the effect of latent toxoplasmosis on the incidence rate of traffic accidents in the RhD unsorted population was not significant. In January 2000, a dramatic change in Czech traffic rules came into action. For example, pedestrians did not have priority at zebra crossings before the year 2000. We suppose that changes in behaviour of Czech drivers and pedestrians could be responsible for differences between results from 1997–2000 and 2001–2006.

It must also be noted that the effect of latent toxoplasmosis on reaction times varies across testing minutes [6, 9, 10]. The effect of toxoplasmosis on reaction time is always stronger in RhD-negative subjects than in RhD-positive subjects. However, RhD-negative Toxoplasma-infected men expressed nonsignificantly shorter reaction times (p = 0.37) than RhD-negative Toxoplasma-free subjects in the first minute of the three minute-test [10]. The authors speculated that two effects of infection might interplay in Toxoplasma-infected men. Latent toxoplasmosis is known 1) to impair reaction times in infected men [6] and animals [20] and also 2) to increase the concentration of testosterone in infected men [21–23]. Increased concentration of testosterone positively influences the level of personality trait competitiveness in men [24], which, in turn, could enhance performance under certain circumstances. Therefore, the negative effects of latent toxoplasmosis on psychomotor performance in men could influence the rate of traffic accidents under certain conditions only, which could explain some differences in results between the published case-control studies and present cohort study.

Our study has several limitations. Despite the fact that nearly 80% of draftees agreed to participate in our study, this population cannot be considered fully representative of the Czech general population. For example, our anecdotal observation of increased frequency of traffic accidents in 20% of drivers who did not agree to participate, suggested that this subpopulation differed in both psychological profile and probability of traffic accident from subpopulation under the study.

The subjects were screened for latent toxoplasmosis in the beginning of the military service only. Some drivers probably acquired toxoplasmosis during their 0.5–1.5 year military service. It must be stressed, however, that the presence of Toxoplasma-infected drivers in our set of Toxoplasma-free subjects (as well as possible presence of Toxoplasma-free drivers in our set of Toxoplasma-infected subjects) can only increase risk of false negative but not false positive results of our statistical tests.

According to military legislation all traffic accidents including material damage and/or injuries shall be reported to the military police. Of course, some less serious accidents might remain unreported; however, according to the experience of military traffic experts completeness of military records is much higher in comparison with the civic sphere (personal communication).

A major limitation of the present study is the absence of RHD genotype data. Previous case-control study on a large sample of blood donors has shown that RhD-positive heterozygotes are resistant to pathological effects of toxoplasmosis while RhD-positive homozygotes are only temporarily resistant: their psychomotor performance decreases with length of infection. The psychomotor performance of RhD-negative homozygotes decreases immediately after infection. Our population of RhD-positive drivers includes both RhD-positive heterozygotes and RhD-positive homozygotes (some of them with relatively low concentration of anti-Toxoplasma antibodies and therefore relatively old infection). Possible contamination of the accident protected RhD population by an unknown number of non-protected RhD-homozygotes might decrease the power of our tests and undervalue the strength of the observed effects.

The absence of data on RHD genotype of subjects makes any speculation on the mechanism of RhD positivity protection very difficult. However, the results of previous studies [6, 9, 10] and current knowledge about localization and probable function of RH proteins [11, 12] suggest that RhD and RhCE proteins that act as ion pumps, coded for at the RH locus and localized on the erythrocyte membrane, are involved in the regulation of ion balance in some critical compartment of nerve or muscle tissue. Such regulation could be important especially in the subjects handicapped by the presence of Toxoplasma cysts in nerve and muscle tissues.