From the CDC"
http://www.cdc.gov/ncidod/dbmd/diseaseinfo/leptospirosis_g.htm

Given it's a bacterium, that's between 5-15 microns in length, your standard water filters should deal with it just fine. If I recall, a lot of "good" filters get down to 0.2 microns, so you're golden.

Anyway, to summarize: symptoms start about a week later: fever, jaundice, "flu-like." Treatment is simple: doxycycine or a penicillin drug. I guess you could ask your doctor about getting the antibiotic before the trip, but personally I wouldn't give it to you (it just helps create resistant organisms). It's transmitted through breaks in the skin and mucous membranes. So, keep untreated water out of your mouth, eyes, nose, and don't pee/poop in the water. Keep your scrapes out of the water. It's carried in the urine of many animals.

Info from "Medical Microbiology, 3rd Ed."