Seroepidemiology of infection with Toxoplasma gondii in healthy blood donors of Durango, Mexico

Estimates indicate that up to one third of the world's population is infected by T. gondii [1, 2]. Most infections in immunocompetent humans are asymptomatic and in up to 10% of infected individuals cervical lymphadenopathy or ocular disease occur [2]. Primary infection acquired during pregnancy could result in congenital toxoplasmosis [2, 3]. In immunocompromised individuals, T. gondii infections could cause central nervous system disease as encephalitis or brain abscess [2, 4]. Routes of parasite transmission in humans include 1) ingesting food or water that is contaminated with oocysts shed by cats; 2) eating undercooked or raw meat containing tissue cysts [2, 5, 6] and; 3) transplantation and blood transfusion [7‐10].

The epidemiology of T. gondii infection in general, and in blood donors in particular has been poorly studied in Mexico. There is not any surveillance study or T. gondii screening program in blood donation, women in child bearing age or immunosuppressed patients. Studying a blood donor population is a valuable approach to determine epidemiological characteristics in adults of a community and might provide findings that could be found in the general adult population of the same community. There is a lack of information about the epidemiology of T. gondii infection in blood donors in northern Mexico. Therefore, we performed a cross-sectional study to determine the prevalence of T. gondii infection in blood donors of Durango City, Mexico and to identify characteristics of blood donors associated with seropositivity.

In the present study, we found a 7.4% seroprevalence of T. gondii infection in blood donors of Durango, Mexico. This prevalence could be considered low and questions on the sensitivity of the test might rise. However, we think that the sensitivity of the tests was very good and the results were reliable. The sensitivity of the IgG test is 94%. Quality control of both IgG and IgM test runs showed valid results. The low prevalence of seropositivity for T. gondii found is comparable with that detected in other healthy population of Durango City using a different detection method [13]. This prevalence is much lower than that reported in the southern Mexican State of Yucatan [14], and the central Mexican State of Jalisco [15] where researchers found that 69% and 29% of blood donors were positive for anti-T. gondii antibodies, respectively. Our prevalence found in Durango City is also much lower than those reported in blood donors from other Latin American countries, as Brazil [16], Cuba [17], and Chile [18]. Similarly, our prevalence is much lower than that found in blood donors in Malaysia [19], Saudi Arabia [20], Czech Republic [21], and Mali [22] were prevalences varied from 21% to 52.1%. In contrast, our prevalence was comparable with the 9% prevalence found in blood donors of Loei Province, Thailand [23]. It is possible that differences in the characteristics of the blood donors and differences in the environments might contribute to explain the lower prevalence of T. gondii infection found in our blood donor population than those reported in blood donors from southern and central Mexico or other countries. Most of our blood donors belonged to a low socio-economic level and eating meat is not a frequent practice among them. In addition, consumption of undercooked or raw meat is rarely found among our blood donors. Environmental characteristics of Durango City as a dry climate, and a high altitude may also contribute to explain the low frequency of infection. This explanation is supported by previous observations that prevalence of T. gondii infection in populations living in dry climates was lower than those living in other climates [2, 24, 25], and lower in populations living in high altitudes than those in low altitudes either in humans [26, 27] or in animals [28].

With respect to the demographic characteristics of the blood donors, we observed that the frequency of seropositivity increased with age, and this observation agrees with those reported in other studies [16, 20]. In addition, we observed that seropositivity to T. gondii decreased as educational level in donors increased (p = 0.04). To the best of our knowledge, a similar observation in blood donors had not been reported. This finding deserves further study in which additional results might confirm or challenge our finding. We can not state that water influenced our result since untreated water consumption or other water variables were not associated with seropositivity in our study. Socio-economic status does not explain our finding since most donors belonged to a low income population. We speculate that high education is linked to good hygienic sanitary practices thereby reducing the transmission of the parasite. Male and female blood donors showed comparable prevalences of T. gondii infection, and this result does not support previous observations that showed a higher prevalence of infection in male than female blood donors [16, 23]. The number of female donors in this study was much lower than that of male donors. Therefore, further studies are needed to elucidate risk factors associated with seropositivity in female donors. The knowledge of factors associated with seroconversion in women is particularly important during their reproductive age in order to design preventive measures and avoid acute infections during pregnancy. The fact that in this study the variable "cats at home" was associated with infection indicates that T. gondii infection in our infected population might have occurred by ingesting parasite oocysts in contaminated food or water. Contact with cats has not always been associated with T. gondii seropositivity in epidemiology studies, as shown in a previous study in blood donors and HIV patients [19] or in pregnant women [13]. In contrast, in our study we did not observe an association of seropositivity with consumption of any meat explored indicating that meat consumption was not relevant in parasite transmission in the blood donors studied. Meat consumption has been found to be an important factor in parasite transmission in several studies [13, 29]. However, in some studies no association between T. gondii infection and meat consumption has been found [12, 19]. The very low number of donors with anti-T. gondii antibodies found in this study reduces the statistical power to find further associations between seropositivity and the epidemiological characteristics in blood donors. Certainly by increasing the sample size the statistical power increases and some risk factors with borderline significance might turn out to become significant. This is especially interesting for risk factors including meat consumption, [13, 29], and soil floors [13].

Interestingly, nearly 2% of our blood donors had also IgM antibodies against T. gondii. The absence of this infection marker in subjects with anti-T. gondii IgG antibodies indicates a chronic infection but its presence does not necessarily indicate an acute infection. We performed detection of IgM antibodies only in IgG positive samples because the presence of IgM antibodies alone is rarely seen. Anti-T. gondii IgG antibodies appears very early after infection [30], therefore, the window period between the appearance of IgM and the appearance of IgG is extremely short and the probability to find an IgM positive/IgG negative infected subject seems to be quite low to financially justify a systematic IgM screening in a cross sectional study. In addition, seropositivity to IgM alone is not considered an acceptable diagnostic criterion for acute infection. Anti-T. gondii-specific IgM antibodies are detectable early after infection and can persist for prolonged times after infection [2, 31]. IgM-positive donors with parasitemia may hold a potential for parasite transmission by blood transfusion. We were unable to judge whether a fraction of our blood donors might represent a risk group for parasite transmission by blood transfusion as reported previously [8, 15].

We concluded that the prevalence of T. gondii infection in blood donors of Durango City, Mexico is low as compared with those reported in central and south Mexico and the majority of other countries. T. gondii infection in our blood donors was most likely acquired by contact with cats. Infection increased with age and decreased with educational level.