The University Hospital 12 de Octubre in Madrid, Spain, acts as the reference hospital for SOT in South Madrid. We performed a retrospective study of all consecutive adult patients who underwent kidney, liver, or heart transplantation during January 1, 2005–January 1, 2013, and lived in the outbreak area. Patients who underwent SOT before January 1, 2005, were excluded because of the difficulty of ensuing long-term follow-up and the potential of heterogeneity in posttransplant practices. Patients who died or had moved to a different place of residence before outbreak onset were excluded (Technical Appendix Figure 1).

The primary study outcome was the occurrence of VL, the diagnosis of which required confirmation of parasitemia (online Technical Appendix) (10). We recorded pretransplant, peritransplant and posttransplant variables and collected various environmental factors prospectively by unblinded, direct interview with the patients. Patients were considered to have frequent contact with dogs if patients reported having dogs at home or taking care of dogs and to have the habit of visiting the park if they reported visiting once a year. The distance between the place of residence and the park was obtained by locating the patient’s home address and measuring the shortest linear distance to the nearest border of the parkland by means of an online mapping tool (Google Maps; Google Inc., Mountain View, CA, USA).

The beginning of the exposure period was set as July 2009 (outbreak onset) for patients who underwent SOT before the outbreak and as the transplant date for those who underwent SOT after outbreak onset. In both cases, the exposure period extended to the date of diagnosis of VL, death, or December 2013. We chose to end the study in December 2013 because the incidence of leishmaniasis decreased thereafter because of the implementation of control measures. The clinical research ethics committee of the University Hospital 12 de Octubre approved the study, and participants provided informed consent.

We analyzed 68 patients (Table 1) for a median follow-up of 4.4 (interquartile range 2.39–6.95) years. VL was diagnosed in 7 patients, yielding a cumulative incidence of 10.3% (95% CI 3.1%–17.5%) and an annual incidence of 2,997 (95% CI 1,213–6,161) cases per 100,000 population. Details on disease pathology and therapy were recorded (Table 2). The mean interval between transplant and diagnosis was 1.34 ± 0.89 years. No patients had visited highly VL-endemic countries.

Figure 1

Figure 1. Distribution of VL among solid organ transplant recipients, Madrid, Spain, January 1, 2005–January 1, 2013. Columns represent the number of solid organ transplant procedures performed each year at the University Hospital...

Black sub-Saharan African SOT recipients were more likely than other recipients to become affected by VL (relative risk 6.40, 95% CI 1.76–23.29, p = 0.049) (Table 1). All 7 episodes of VL occurred in patients who underwent transplantation during the outbreak period (Figure 1).

Figure 2

Figure 2. Spatial distribution of solid organ transplant recipients in the southwest area of Madrid, Spain, in relation to park that was focus of visceral leishmaniasis (VL) outbreak, January 1, 2005–January 1, 2013....

The median distance between the place of residence and the park was significantly shorter for recipients with VL (399 m) than for those without (1,370 m; p = 0.001) (Figure 2; Technical Appendix Figure 2). We explored the predictive accuracy of this variable by establishing the optimal cutoff value with the area under the receiving operating characteristic curve analysis. Recipients living <1,000 m from the park (26.1%, 6/23) had a higher incidence of VL than recipients living >1,000 m away (2.2%, 1/45; relative risk, 11.74, 95% CI 1.50–91.78; p = 0.005). At 4 years, a lower percentage of the SOT recipients living <1,000 m from the park were free from VL than those living >1,000 m away (61.0% vs 98%; p = 0.001 by log-rank test) (Technical Appendix Figure 3).

Our study suggests that the incidence of VL in SOT recipients is notably higher than that in the general population (11). Acquisition of the parasite most likely occurred posttransplant because all but 1 recipient affected with VL (from whom serum samples could be recovered) were seronegative for Leishmania spp. before transplantation.

Our findings suggest that environmental factors might be crucial in modulating the incidence of VL in immunocompromised hosts, such as SOT recipients; the distance from the patient’s residence to the focus of the outbreak (6,7) emerged as a key risk factor. The median distance between the park and the homes of recipients with posttransplant VL was <500 m; the maximum flight distance of female sand flies is 600 m (12,13). Therefore, persons living within this radius had a higher chance of being bitten by the VL vector. A similar association was described for the general population during this outbreak (6).

Undergoing transplantation during the outbreak period was another risk factor for VL. This finding suggests that, in the case of an outbreak in a country with low baseline incidence, pretransplant screening of patients listed for SOT for VL-specific antibodies should be considered and repeated during the posttransplant period for the prompt detection of de novo infection. Recipients should also receive specific counseling to reduce the risk of being bitten by sand flies. In addition, treating physicians must maintain a low threshold of suspicion for VL for persons on immunosuppressive therapy during a VL outbreak.

We found that 28% of posttransplant VL cases occurred in black recipients born in sub-Saharan Africa, even though this subgroup only represented 2.4% of the overall population in the affected area (14). An association between sub-Saharan African ethnicity and VL has also been reported in the general population (4). No apparent relationship was found between the race of the patient and the frequency of parkland visits. Both black recipients in question came from Equatorial Guinea, a country not considered endemic for leishmaniasis by the World Health Organization (15). Therefore, the potential association between genetic susceptibility and posttransplant VL warrants further investigation.

Limitations of this study include the small sample size and that interviewers were not blinded to the diagnosis of VL. However, the objective nature of the questionnaire minimized the potential risk for bias. When assessing degree of exposure to sand flies, we used only indirect variables (i.e., distance between the patient’s residence and park, habit of visiting the park) as surrogate measures. Regarding the distance from the park, only linear distances were assessed without considering the potential presence of physical obstacles in the sand fly flight trajectory. Because of these limitations, our findings must be interpreted with caution.

Our study indicates several risk factors (being black and from sub-Saharan Africa, having an SOT during the outbreak, and living <1,000 m from the outbreak focus) useful for helping physicians treat SOT recipients during a VL outbreak. Doctors should select the patients with these risk factors for counseling to minimize their exposure to vectors and active monitoring for prompt diagnosis.