Risk of reactivated toxoplasmosis in haematopoietic stem cell transplant recipients: a prospective cohort study in a setting withholding prophylaxis.

Abstract Objectives Reactivation of latent toxoplasmosis may be life-threatening in haematopoietic stem cell transplant (HSCT) recipients. We conducted an eight-year-long prospective study on the diagnosis and monitoring of reactivated toxoplasmosis in paediatric HSCT recipients. The primary objective was to determine the incidence of reactivated toxoplasmosis in a setting that withholds prophylaxis until engraftment. The second objective was to identify the subgroups of HSCT recipients particularly prone to reactivation who may benefit the most from regular PCR follow-up. Methods Serological and qPCR screening targeting the Toxoplasma 529 bp gene was performed pre-HSCT, and continued by weekly monitoring post-HSCT for a median time of 104 days. Results Reactivated toxoplasmosis was diagnosed in 21/104 (20.2%), predominantly in allo- (19/75) and rarely in auto-HSCT (2/29) recipients. Over 50% (14/21) of cases were diagnosed during the first month post-HSCT, while awaiting engraftment without prophylaxis. Toxoplasma disease evolved in only three (14.3%, 3/21) patients, all treated by allo-HSCT. Reactivation was more frequent in patients treated for acute lymphoblastic leukaemia (3/27, p=0.03) and especially, in recipients of haploidentical stem cells (10/20, p=0.005). Seronegative status of the donor (where was known) contributed to 75% (12/16) cases of reactivated toxoplasmosis after allo-HSCT. Conclusions The presented results show that peripheral blood-based qPCR, both pre- and post-HSCT, is a valuable asset for the diagnosis of reactivated toxoplasmosis, whereas the results of serology in recipients should be interpreted with caution. Weekly qPCR monitoring, at least until successful engraftment and administration of prophylaxis, allows for prompt introduction of specific treatment.