Subclinical Infections with Crimean-Congo Hemorrhagic Fever Virus, Turkey - Vol. 18 No. 4 - April 2012 - Emerging Infectious Disease journal - CDC
Table of Contents
Volume 18, Number 4–April 2012
Volume 18, Number 4—April 2012
Subclinical Infections with Crimean-Congo Hemorrhagic Fever Virus, Turkey
Crimean-Congo hemorrhagic fever virus (CCHFV) infection was first recognized in Turkey in 2002. Since that time, the number of diagnosed cases has increased to the magnitude of an outbreak with major public health consequences (1,2). Although the clinical spectrum may change from asymptomatic infection to severe hemorrhagic disease, most studies to date have included only symptomatic cases (1–6). Therefore, the true incidence of infection with the virus, the full spectrum of severity of disease, and its epidemiologic features are unknown. The purpose of our study was to investigate the seroprevalence of CCHFV infection in a sufficiently large sample representative of the region affected during this outbreak in Turkey and to describe the main epidemiologic features, including the proportion of subclinical cases.
AbstractTo investigate Crimean-Congo hemorrhagic fever virus in Turkey, we conducted a seroepidemiologic survey during January–April 2009. Seroprevalence of infection was 10% in a sample from an outbreak region and increased with patient age, indicating that the virus had been previously present in Turkey. We also estimated that 88% of infections were subclinical.
During January–April 2009, we conducted a survey that included obtaining venous blood samples from participants. The study sample was selected by using a random geographic cluster sampling stratified by age and sex from rural residential areas of Turkey where 99.5% of CCHF cases were reported (Figure 1). Only adults >18 years of age who were living in the study area for >1 year were eligible for the study. The study was approved by the Central Ethics Committee, and informed consent was obtained from all participants. A study questionnaire included questions on demographics, socioeconomic status, behavior characteristics, medical history, known risk factors for CCHFV infection, and participants’ awareness of the outbreak and infection prevention methods.
Serum samples were tested for IgG against CCHFV at the Virology Reference Laboratory of the Refik Saydam National Public Health Agency, (Ankara, Turkey) by using a commercial ELISA kit (Vector-Best, Novosibrisk, Russia). Although the sensitivity and specificity of the kit were not specified by the manufacturer, studies that used this method have reported a sensitivity of 87%–98.3% and a specificity of 99%–100% (7,8). We defined subclincial cases as those in persons who were seropositive although they were not given a diagnosis or had not had severe symptoms compatible with CCHF at any time.
In addition, we compared information in our database with that in the database of reported cases at the Ministry of Health, Turkey. We used the χ2 test, t test, and Mann-Whitney U test for univariate statistical comparisons, as appropriate. Multivariable logistic regression was used to assess independent risk factors for seropositivity. We used the direct standardization method to adjust age-specific seropositivity rates for our study population with the age composition of the entire outbreak region. This adjustment enabled us to calculate expected numbers of infected (seropositive) persons in the outbreak region (Technical Appendix [PDF - 46 KB - 1 page]). Clinically diagnosed cases were compared with expected numbers of infections, and an observed:expected ratio was calculated.
The survey included 3,671 adults of whom 3,557 (97%) provided blood samples for serologic analysis. The mean ± SD age of the study population was 44.3 ± 16.2 years, the female:male ratio was 1.04 (51%:49%), and the most common occupation was farming (52.4%). Only 18.2% had a history of tick bite.