Abstract:Leptospirosis is worldwide re-emerging zoonosis with protean clinical manifestations. The more common mild, anicteric form is characterized by nonspecific symptoms such as fever, headache, chills, myalgia, nausea and abdominal pain. The severe, potentially fatal form (Weil's disease) typically presenting as triad of jaundice, acute renal failure (ARF), and hemorrhage is estimated to occur in 5-15% of all human infections with mortality rate up to 40%. Elder age is one of poor prognostic factors in leptospirosis. There are few studies about leptospirosis in children and no one comparative study of pediatric and adult leptospirosis in regions with temperate climate. Our aims were to describe retrospectively the disease spectrum and outcome differences in children and adults with leptospirosis over a period of 41 years and to compare severity in different age groups. The study material comprised the data from 105 consecutive leptospirosis cases from Central Northern Bulgaria treated in Clinic of Infectious Diseases at University Hospital -Pleven (1976-2016) (95 male, age 37±18 years, lethal outcome in 12.38%), followed by comparative analysis of group of children (age ≤17-years-old; N 1 =13) versus group of adults (age ≥18 years-old; N 2 =92). Results: Fever (100%), hepatomegaly (92%), conjunctival suffusions (88%), myalgia (87%), nausea and vomiting (84%), shivering (79%), splenomegaly (74%), headache (65%), jaundice (64%), oliguria (58%), tachycardia (55%), hypotension (50%), abdominal pain (41%), and hemorrhagic diathesis (37%) were the most characteristic manifestations in whole series. Only headache and meningitis had had a significantly higher prevalence in the group of children. Leucocytosis, blood urea nitrogen, serum creatinine, serum bilirubin, aminotransferases, gamma glutamyl tranferase (GGT) and serum amylase levels were significantly higher in group of adults. Potassium level, total protein and albumins were significantly lower in the same group. The distribution of cases in whole series according to severity was as follows: mild cases -27, moderate -44 cases and severe -34 cases. In group of children seven mild and six moderate cases had been observed versus 20 mild, 38 moderate and 34 severe cases in the group of adults. All thirteen deceased patients were adults. We concluded that in Pleven region, leptospirosis in children usually is mild and anicteric. The leptospiral meningitis is not unusual manifestation for pediatric leptospirosis. The major factors leading to death in adults with leptospirosis were lung edema and brain edema due to severe ARF.