Asymptomatic Visceral Leishmania infantum Infection in US Soldiers Deployed to Iraq

Abstract

Background: Visceral leishmaniasis (VL), due to Leishmania infantum, is a persistent intracellular parasitic infection transmitted by the bite of infected sand flies. Symptomatic VL has been reported in U.S. soldiers with Iraq deployment. Untreated symptomatic VL can be fatal; asymptomatic VL (AVL) may establish a lifelong risk of reactivation. We report prevalence and AVL risk factors in Operation Iraqi Freedom (OIF) deployers during 2002-11.

Methods: Healthy soldiers exposed to VL endemic areas in Iraq and 50 controls who never traveled to endemic regions were recruited through military healthcare facilities (2015-17). Responses to a risk factor survey and blood samples were obtained. Leishmania research diagnostics utilized included enzyme-linked immunosorbent assay (ELISA), rk39 test strips, quantitative polymerase chain reaction (PCR), and interferon gamma release (IGRA) assays. Statistical analyses included Fisher exact test, Pearson χ2 test, Mann-Whitney U test, and logistic regression.

Results: 200 deployed subjects were enrolled, mostly males (84.0%), of white ethnicity (79.0%), and median age 41 (range 24-61) years. 64% were seropositive for Phlebotomus alexandri saliva antibodies. Prevalence of AVL (any positive test result) was 39/200 (19.5%, 95% confidence interval 14.4%-25.8%). Two (1.0%) PCR, 10 (5%) ELISA, and 28 (14%) IGRA samples were positive. Travel to Ninewa governorate increased risk for AVL (P = .01).

Conclusion: AVL was identified in 19.5% of OIF deployers; travel to northwest Iraq correlated with infection. Further studies are needed to inform risk for reactivation VL in US veterans and to target additional blood safety and surveillance measures.

Keywords: Iraq; US soldiers; asymptomatic; deployed; visceral leishmaniasis.

Figure 1.

Figure 1 shows distribution of reported Iraqi Visceral leishmaniasis cases in 2004 by goverante (A); major cities in Iraq (B); distribution of deployed locations (as % time spent) for US infected (n = 39) (C); and US not infected (n = 161) (D). * P = .01, Mann-Whitney U test comparing person time fractions spent in Ninewa governate (15.3 vs. 5.1).

Figure 2.

IFN-γ levels in SLA proliferated PBMC and whole blood. IFN-γ (pg/mL) production by (A) PBMC or (B) blood stimulated with L. infantum SLA in Iraq deployers (n = 200) and not-exposed control subjects (n  =  46). The assay cutoff is defined as the mean OD + 2 SDs for the values obtained from control samples (180 pg/mL for PBMC and 40 pg/mL for blood). The solid lines represent the mean. The significance of differences between groups was evaluated by the Mann-Whitney U test. P values <.05 were considered significant. Abbreviations: IFN- γ, interferon γ; PBMC, peripheral blood mononuclear cell; OD, optical density; OIF, Operation Iraqi Freedom; SD, standard deviation; SLA, soluble Leishmania antigen.

Figure 3.

Serum IgG measured using L. infantum SLA ELISA in Iraq deployers and control subjects. The dotted line shows the cutoff value, which is defined as the mean OD + 3 SDs for the values obtained with sera from nonexposed controls (0.109). Solid lines indicate mean OD values. The significance of differences between groups was evaluated by the Mann-Whitney U test. P values <.05 were considered significant. Abbreviations: ELISA, enzyme-linked immunosorbent assay; IgG, immunoglobulin G; OD, optical density; OIF, Operation Iraqi Freedom; SD, standard deviation; SLA, soluble Leishmania antigen.