T. gondii is recognized as an opportunistic parasite in immunocompromised patients [3, 10]. Results of the present study revealed that T. gondii seropositive rate ranged from 58–100% in patients with different forms of HM. The overall seropositive rate was significantly higher in HM patients compared to healthy individuals. In Egypt, seroprevalence rates as high as 90% and as low as 20% were previously reported among patients with other forms of malignancy [19–21]. Similar studies conducted worldwide reported concordant results. In Iran, Gharavi et al. (2017) detected T. gondii IgG antibodies in 56.4% of leukemia patients and 42.4% of the control group [22]. A meta-analysis incorporating nineteen studies conducted in China concluded that the overall T. gondii seroprevalence was significantly higher in the population with cancer compared to those without (20.59% vs 6.31%) [23]. The variations in Toxoplasma seroprevalence rates among different countries may be attributed to different social and cultural habits, distinct environmental and geographical factors, and different livestock rearing practices [7].
Weakened host immunity is a major sequalae in all types of HM [24–27]. Treatment regimens can further exacerbate immunosuppression in these patients [2]. In the case of immunodeficiency, patients with latent toxoplasmosis are at risk for reactivation whereas recently infected patients are at risk for acute disseminated toxoplasmosis, in which case the diagnosis of infection is an emergency [8, 28]. A positive IgG test merely indicates that the host has been infected at some time in the past. Although serologic detection of IgM and high IgG titers suggests an acute infection, and could, therefore, be valuable in emergency diagnosis, the application is limited in immunocompromised patients due to reduced antibody production [7, 29]. This was confirmed in the present study by the significantly lower IgG concentration observed in seropositive patients with HM compared to controls. Therefore, direct detection of the parasite or its DNA in clinical samples is essential [30].
In the present study, PCR analysis revealed detection of Toxoplasma DNA in 32.5% of patients compared to 2.5% of controls with a statistically significant difference. The PCR positivity was not significantly related to the type of malignancy. Generally, detection of T. gondii in clinical samples confirms the presence of parasites which can be due to primary or reactivated infection [31]. Although the presence of Toxoplasma DNA in blood indicates parasitemia, its clinical significance in immunocompromised patients is unclear. T. gondii DNA was detected by PCR in blood samples of 80% of patients with cerebral toxoplasmosis but it may be also detected in the blood of immunocompromised patients who had no localizing signs and symptoms [32–34]. The detection of T. gondii may precede the onset of disease reactivation, indicating that molecular monitoring of T. gondii in peripheral blood may contribute to early diagnosis [35]. Noteworthy, molecular methods can detect parasites that have been released from tissue cysts into the blood stream, but it cannot differentiate the DNA of dead and viable parasites. The parasite may be rapidly destroyed by the immune system but the DNA remains in the blood for up to 13 weeks [36, 37 ]. Accordingly, diagnosis of Toxoplasma-related illness in immunocompromised patients should be based on molecular, clinical, and radiologic findings [38].
PCR amplification of Toxoplasma AF146527 is extremely sensitive and specific for molecular detection of T. gondii as this 529bp fragment is repeated 200- to 300-fold in the genome [18, 31]. In the present study, most seropositive controls (16 out of 17 subjects) were PCR negative. On the other hand,19 out of the 30 ELISA-positive HM patients were negative by PCR. Failure to detect Toxoplasma DNA in a considerable number of seropositive individuals was reported in different population groups [39–41]. This can be explained by the low amount of DNA in blood samples. In chronic T gondii infection, parasites appear rarely in the blood at intervals during the asymptomatic phases of the disease and may be detected by PCR unless efficiently eliminated by the immune system [42]. Bavand et al., (2019) studied HIV patients in Iran and reported that Toxoplasma DNA was detected in only five out of 69 T. gondii IgG-positive patients. A non-severely depressed immunological status can contribute to the clearance of blood parasitaemia [43]. It has to be noted that the absence of DNA does not exclude the presence of active disease [7, 38, 44]. Mild local reactivation of latent cysts may not be associated with positive PCR results in peripheral blood samples. Molecular diagnosis is more useful in cases associated with severe localized infection or dissemination [42].
Intriguingly, the present study showed that Toxoplasma DNA was detected in two patients for whom the results of serological tests were negative. A possible explanation is that blood samples were drawn very early after infection and before the production of antibodies. Another explanation is that the immune system was unable to produce enough immunoglobulins to be detected by the serological ELISA assay due to immunodeficiency. That is why parasite detection by molecular methods is strongly recommended when serology is negative in immunocompromised patients. Collectively, PCR and serological examination showed slight agreement. Therefore, the combination of both techniques would be more accurate for a definite diagnosis of toxoplasmosis.
In the present study, the seropositivity of T. gondii showed a non-statistically significant association with the age and gender of patients. Similar results were reported in previous studies [17, 21, 45].
Contact with infected cats and consumption of undercooked meat are the main sources of T. gondii infection in man [46]. However, both were not significantly associated with T. gondii infection in HM patients. These findings correlate with the findings of an earlier report among different population groups in Egypt [47]. Oocysts are not usually found on cat fur but are often buried in soil along with cat feces. Sanitation and hygienic habits would play a significant role in limiting the transmission of oocysts to humans [48]. Transmission through undercooked meat is largely determined by the prevalence of the parasite in the animals as well as local methods of meat cooking. [49].
Although blood transfusion is a potential transmission route for Toxoplasma infection [50, 51]. it was not linked to T. gondii transmission in HM in the present study. Toxoplasma parasitemia was previously reported in 10% of blood donors in Egypt which suggests that the rate of Toxoplasma transmission is low [15].
One of the limitations of the study is that the more advanced molecular methods such as real time PCR and sequencing of the PCR product were not performed. However, the conventional PCR protocol used proved to be highly sensitive due to the high copy number of the amplified gene fragment [18]. Moreover, the sequence of the used primers was subjected to a BLAST (Basic Local Alignment Search Tool) analysis to ensure specificity of the assay to T. gondii. In each PCR run, we included a positive and a negative control samples to validate the results.