Comparative incidence of acute Q fever in two ethnic groups in Israel.

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Q fever is a worldwide, zoonotic disease caused by the obligate intracellular bacterium Coxiella burnetii. The most common reservoirs of this bacterium are domesticated ruminants, primarily cattle, sheep, and goats. Humans typically acquire Q fever by inhaling aerosols or contaminated dust derived from infected parturient animals or animal products [1, 2, 3, 4]. Due to its aerosol route of transmission, the highly infectious bacterium Coxiella burnetii is a potential agent of bioterrorism [1]. Q fever is usually diagnosed by serological tests due to difficulties in culturing the organism. Although Q fever is considered endemic in Israel, there is a lack of information on the incidence of the disease, and in the mid-1990s the Israeli Ministry of Health removed Q fever from its list of notifiable infectious diseases. In the study presented here we reviewed the incidence of acute Q fever in two major ethnic groups (Jews and Arabs) in Israel over the last 11 years. Since more Arabs than Jews live in rural areas and work in animal husbandry in Israel, we expected the attack rate to be highest in this group. In Israel, serological testing for phase I and phase II immunofluorescent antibodies against Coxiella burnetii is performed in two laboratories (The Israeli National Reference Center for Rickettsiosis at the Israel Institute for Biological Research, and the Laboratory of Clinical Microbiology of the Soroka Medical Center). These laboratories detect acute Q fever using an indirect immunofluorescent assay for phase II antibody. An IgG and/or IgM titer of 1:100 is considered indicative of acute disease. For the purpose of this study, we reviewed the records of the two diagnostic laboratories for the period 1991– 2001 to identify all patients with a confirmed diagnosis of acute Q fever. Demographic data, patient ethnicity, and season of infection were retrieved from the records. Patients with chronic Q fever (i.e., titers for phase I 1:100) were excluded. Incidence rates were computed using the average annual population figures for the two study groups (Jews vs. Arabs) as the population at risk. The population figures were obtained from the Israeli Central Bureau of Statistics. Separate incidence rates for Jews and Arabs were also calculated and compared by computing the ratio of observed to expected events, assuming a Poisson probability distribution [5]. Between 1991 and 2001, a total of 15,816 blood samples were collected from patients clinically suspected of having Q fever and sent to the two diagnostic laboratories. Positive results were obtained for only 221 (1.4%) of these patients. The mean number of samples per year sent to both laboratories was 1,438 (range, 897– 2,015; Fig. 1), and the mean percentage of positive samples per year was 1.4% (range, 0.25–3.25%; Fig. 2). The median age of the 221 patients positive for Q fever was 45 years (range, 1–93 years), with a male predominance of 60% (141 males and 80 females). The annual number of cases ranged from 4 to 46 (mean, 20 cases) with a peak seasonal incidence of 58% (129 cases) occurring between May and August. Arabs accounted for 8.1% (18 patients). Arab patients were younger (median age, 36 years; range, 1–70 years) than the Jewish patients (median age, 48 years; range, 1–90 years). The male to female ratio varied between ethnic groups: 1:1 for the Arab patients and 1.84 for the Jewish patients. Between 1991 and 1997 there was a progressive countrywide decline in the annual incidence of Q fever infection, from 0.63 to 0.07 cases per 100,000 population, with the incidence increasing during the years thereafter, reaching 0.74 cases per 100,000 in the year 2001. Although J. Bishara ()) · S. Pitlik · D. Hershkovitz Department of Internal Medicine C and Infectious Diseases, Rabin Medical Center, Beilinson Campus, Sackler School of Medicine, Tel Aviv University, Petach Tivkah, Israel 49100 e-mail: bishara@netvision.net.il Tel.: +972-3-9376301 Fax: +972-3-9221605