Outbreak of tularemia: a case-control study and environmental investigation in Turkey

The aim of this study was to identify the potential factors associated with infection sources and modes of transmission during a recent outbreak (October 2004) of tularemia in Suluova, Turkey.

Following the diagnosis of five patients with tularemia in October 2004, active surveillance was initiated to identify further cases. This was a matched case-control study with analysis based on the first 43 cases of tularemia (probable or suspected) and 43 matched controls. A probable case was defined as a patient, resident in Suluova, who had signs and symptoms (regional lymphadenopathy and fever) compatible with tularemia and a positive serology or PCR for Francisella tularensis during the period October 21 to November 31, 2004. A suspected case was defined as a patient with compatible signs and symptoms who did not meet the laboratory criteria for a probable case, who also had no laboratory evidence of infection by other microorganisms, and who was resident in Suluova between the same dates. The microagglutination test was used for serological diagnosis. A standardized questionnaire was used to collect information on general demographics, exposure to all known sources of tularemia infection, potential risk factors related to water and animals (i.e., fishing, farming, hunting, and other activities), and the environmental conditions of the house. PCR was used to screen for evidence of the tularemia agents in clinical samples from patients and water samples.

The overall attack rate was 2.3 per 1000 population (86/38000). Twenty-eight suspected cases and 15 probable cases of tularemia were included in the study. The most common presenting symptom was lymphadenopathy present in 95.3%, followed by fever (83.7%) and sore throat (79.1%). Twenty-eight out of 43 were reported to have painful lymph nodes. F. tularensis was detected by PCR in samples obtained from the ulcerated lesions of two patients. In the multivariate logistic regression model, keeping a domestic animal in the garden was associated with an increased risk of contracting the disease (OR=10.87; 95% CI: 1.26-93.65; p=0.03). F. tularensis was detected by PCR in the water sample obtained from the rivulet that passes through Suluova.

The results of this study show that case-control studies may be useful for analyzing epidemics and for identifying the source of infection. In order to prevent water-related zoonotic infections, water and sewerage systems should be improved.