IgG Against Dengue Virus in Healthy Blood Donors, Zanzibar, Tanzania

Francesco Vairo

, Emanuele Nicastri, Salma Masauni Yussuf, Angela Cannas, Silvia Meschi, Mwanakheir AA Mahmoud, Azza H. Mohamed, Paul Mohamed Maiko, Pasquale De Nardo, Nazario Bevilacqua, Concetta Castilletti, Antonino Di Caro, Vincenzo Racalbuto, and Giuseppe Ippolito

Author affiliations: “L.Spallanzani” National Institute for Infectious Diseases, Rome, Italy (F. Vairo, E. Nicastri, A. Cannas, S. Meschi, P. De Nardo, N. Bevilacqua, C. Castilletti, A. Di Caro, G. Ippolito);Directorate General of Development Cooperation, Rome (F. Vairo, V. Racalbuto); Ministry of Health, Unguja, Zanzibar, Tanzania (M.A.A. Mahmoud, A.H. Mohamed, S.M. Yussuf); Mnazi Mmoja Hospital, Unguja, Tanzania (P.M. Maiko)

Abstract

We conducted a seroprevalence survey among 500 healthy adult donors at Zanzibar National Blood Transfusion Services. Dengue virus IgG seroprevalence was 50.6% and independently associated with age and urban residence. These data will aid in building a surveillance, preparedness, and response plan for dengue virus infections in the Zanzibar Archipelago.

In eastern Africa, the available evidence indicates that dengue virus serotypes 1, 2, and 3 (DENV-1, -2, -3) are common causes of acute fever (1). A recent map of DENV transmission has shown that the virus could be transmitted in most eastern African countries, including mainland Tanzania and the Zanzibar Archipelago (2).

In 2010, a PROMED report raised concerns about DENV infections in Tanzania (3). That same year, travelers from Europe and Japan were found to be infected with DENV-3 after they returned from mainland Tanzania or Zanzibar (4–6). In Tanzania, seroprevalence rates for febrile outpatients in Tosomaganga (Iringa Region) and Pemba Island (Zanzibar) in 2007 (7) and in Moshi (Arusha Region) in 2007–2008 (8) were 1.8%, 7.7%, and 10.7%, respectively. To determine DENV circulation in the Zanzibar Archipelago, we assessed the seroprevalence of DENV among adult blood donors at the Zanzibar National Blood Transfusion Services (ZNBTS).

The Study

We conducted a cross-sectional seroprevalence survey at ZNBTS from September 20 to December 10, 2011. ZNBTS is located in Stone Town, the principal city of the Zanzibar Archipelago. Ethics approval was obtained from the Zanzibar Medical Research Ethical Committee. The sample size was calculated by using methods for proportion. The estimated prevalence was set at 50% because data were not available regarding the true prevalence of the infection in the area. Considering a population of ≈1,000,000 inhabitants and a confidence level of 95%, the sample size was set at 384 donors. Sample size was then increased to 500 donors to account for those lost to follow-up.

During the study period, all consecutive adult donors attending ZNBTS, who had been screened and selected for blood donation, were enrolled in the study. Donors were screened by serologic tests for hepatitis B virus, hepatitis C virus, HIV, and Trepomena pallidum; they were selected for blood donation if results of all screening tests were negative. A structured interview was conducted by using a close-ended questionnaire after the donor signed the informed consent form and before the screening.

From each enrolled person, 10 mL of venous blood was collected. After the screening tests, the remaining serum was divided into 2 aliquots: 1 was stored at −20°C at the sample processing site for performance of the IgG ELISA at Mnazi Mmoja Hospital in Unguja, Zanzibar, and 1 was dispatched to the “L. Spallanzani” National Institute for Infectious Diseases in Rome, Italy, for testing by immunofluorescence assay (IFA) for IgG. At the end of the collection phase, samples were tested by Panbio Dengue IgG Indirect ELISA kit (Inverness Medical Innovations Australia Pty Ltd, Sinnamon Park, Queensland, Australia) according to the manufacturer’s instructions. A positive ELISA result was defined as having an index value >1.1. To compensate for the low specificity of the ELISA, we tested samples by IFA with homemade slides and a mix of uninfected and DENV-2 (New Guinea C strain)–infected Vero E6 cells. The diagnostic accuracy of the IFA has been described (9).

Donors were considered positive for IgG against DENV if results of both tests were positive. Discordant results were considered negative. All districts except the urban district were considered rural areas. Univariate association between DENV IgG positivity and donor characteristics was assessed by means of odds ratios (ORs) and 95% CIs, by χ2 for categorical values, and Student t-test for continuous variables. A multiple logistic regression model using a backward procedure was used. All variables were entered in the backward selection model, and a cutoff level of p = 0.10 was used for subsequent selections. Data management and analysis were performed by using STATA version 11 (StataCorp, College Station, TX, USA).

Five hundred persons consecutively attending ZNBTS were selected for blood donation and, therefore, were eligible to be enrolled in the study. Demographic characteristics of the participants are shown in Table 1. The mean age was 32 years; 97.2% of the participants were male. Most donors had a water storage container (71.2%) and/or resting water near their home (88.0%). Bed nets were used by 59.4% of the donors, but only 40% reported insecticide spraying at home.

Of the 500 blood samples, 253 (50.6%) were positive by both tests, 77 (15.4%) were positive by ELISA and negative by IFA, and 170 (34.0%) were negative by both tests. DENV IgG prevalence was 50.6% (95% CI, 46.2–54.9). Considering IFA as the reference standard, ELISA sensitivity and specificity were 100% and 68.8%, respectively. According to univariate analysis, DENV IgG–positive donors were significantly more likely to be older (OR 1.32, 95% CI 1.19–1.47) and live in urban districts (OR 3.18 95% CI 2.21–4.69) compared with DENV IgG-negative donors (Table 1). According to multivariate analysis, older age (adjusted OR [AOR] 1.42, 95% CI 1.27–1.61) and living in an urban district (AOR 4.09, 95% CI 2.72–6.17) were independently associated with DENV IgG positivity (Table 2). Moreover, borderline evidence indicated an association of positivity with the presence of resting water near the home (AOR 1.66, 95% CI 0.99–2.76).