Some epidemiological features of the Mediterranean spotted fever re-emerging in Bulgaria

The Mediterranean spotted fever (MSF), caused by Rickettsia conorii conorii strain Malish, is transmitted by the brown dog tick Rhipicephalus sanguineus. In Bulgaria, cases of MSF occurred in two epidemic waves, the first in 1948-1970, (after there were no report of new cases more than for 20 years) and the second started in 1993 and is still going on. The AIM of the study was to investigate the epidemiological characteristics of the re-emerging MSF in Plovdiv city and its suburbs, which is the largest endemic region in the country.

The MSF patients treated between 1993 and 2011 were 1254. MSF was confirmed by immunofluorescent assay (IFA) in the Reference Rickettsioses Laboratory. Descriptive and analytic epidemiological methods were used to determine the routes and ways of infection, the epidemiological locus, seasonality, patients' age, gender and social structure, and the clinical severity of the cases.

We established that MSF spread in the region in two distinct phases: from 1993 to 2003 during which the disease was increasingly spreading and the second phase taking place from 2004 till 2011 during which the disease was gradually decreasing. The incidence is between 0.13 and 25.62, mean 10.91 per 100 000 populations (11.88 and 9.56 per 100 000 populations for phases I and II, respectively); between 1.14% and 6.25% of the infected people died (mean 2.07%, 3.46% and 1.25% for phases I and II, respectively). The age distribution of patients shows predominance of 40-59-year-olds (31.66%), reaching a peak in patients older than 60 years (37.5%). Children and adolescents under 19 years are less affected (18.12%), while the least affected are the young adults between 20-39 (12.7%). Urban population is almost twice as affected as rural population regardless of the gender. The disease has summer seasonality, peaking in August. Eschar (tache noire) was found in 77.91% of the patients. Despite the decline and predominance of mild forms (43.12%), the re-emerging MSF still presents with lots of severe forms (11.45%) and malignant forms (8.54%), which makes almost one fifth of all patients to be at serious risk of getting ill with a severe disease with complications and possible fatal outcome.

Almost two decades after it re-emerged in Bulgaria, MSF is still potentially active despite the decreasing incidence and mortality rate. During the phase of decline, the re-emerging MSF kept the basic epidemiologic characteristics it had in the first phase of rapid increase. The epidemiologic characteristics of MSF in the region are in support of its prevention and control.