Catching it early is important, I don't have the exact statistics.
This article also discusses prevention:
From:
Leptospirosis in Dogs - Signs - Diagnosis - Treating Leptospirosis Signs and Symptoms of Leptospirosis
The severity of symptoms varies, and depends on the dog (age, immune response, vaccination status), the strain of
Leptospira, and other factors. Some dogs may have mild symptoms or no symptoms at all, but severe cases can be fatal. Signs and symptoms may include:
fever
joint or muscle pain - this may manifest as a reluctance to move
decreased appetite
weakness
vomiting and diarrhea
discharge from nose and eyes
frequent urination - may be followed by lack of urination
yellowing of the gums, membranes around the eyes, and skin (
jaundice)