Leptospirosis is caused by several species of Leptospira organisms found most often in rats but sometimes present in some farm animals and in some cats and dogs (presumably from rat-transmitted infection). Transmission is most often through accidental contact with water contaminated by infected rat urine. Those most at risk are sewer workers and slaughterhouse employees, but farmers and campers sometimes come into contact with contaminated water. There is an incubation period of 4-20 days, then abrupt onset of fever, often accompanied by chills, headache, malaise, and conjunctivitis. Muscle pain is present in 50% of cases. The fever typically lasts 4-9 days. WBC count can be normal or elevated. Urinalysis often contains protein and some WBCs. Serum bilirubin is usually normal, but about 10% of cases have mild elevation. Alanine aminotransferase is elevated in about 50% of cases, usually to less than five times normal. About 50% of patients experience a recurrence of fever about 1 week (range, 2-10 days) after the end of the first febrile period. Patients are more likely to demonstrate signs of hepatitis in this phase and may develop symptoms of meningitis. The most severe form of leptospirosis is called Weil’s disease and occurs in about 5% of infections. The most striking findings are a combination of hepatitis and glomerulonephritis, clinically manifested by jaundice with hematuria. Therefore, the disease is sometimes considered in the differential diagnosis of jaundice of unknown etiology. Symptoms of meningitis occasionally predominate. Laboratory findings include leukocytosis with a shift to the left. A mild normocytic-normochromic anemia usually develops by the second week. Platelet counts are normal. After jaundice develops, liver function test results are similar to those in viral hepatitis. After onset of kidney involvement, the blood urea nitrogen (BUN) level is often elevated, and hematuria is present with proteinuria. CSF examination shows normal glucose levels but increased cell count, which varies according to the severity of the case; initially, these are mainly neutrophils, but later, lymphocytes predominate. Cultures on ordinary bacterial media are negative.

Diagnosis often requires isolating the organisms or demonstrating specific antibodies in the serum. During the first week (days 1-8), spirochetes may be found in the blood by dark-field examination in about 8% of cases and can be cultured from the blood in many more. Instead of ordinary blood cultures, one to three drops of blood are inoculated into a special culture medium (Fletcher’s), since larger quantities of blood inhibit the growth of leptospires. The CSF may be cultured toward the end of the first week. During the second week the blood results quickly become negative. During the third week (days 14-21) the spirochetes may often be recovered from the urine of patients with nephritis. Animal inoculation is the most successful method. Antibodies start to appear at about day 7 and are present in most cases by day 12. Antibodies persist for months and years after cure. A titer of 1:300 is considered diagnostic, although without a rising titer, past infection cannot be ruled out completely. If a significant titer has not developed by day 21, it is very rare for it to do so later. In summary, blood cultures during the first week and serologic tests during the second and third weeks are the diagnostic methods of choice.