Of course the most important advice is to go and see your doctor if you start to feel unwell. Early symptoms resemble a cold or flu, with a fever. Remember also that water contaminated with leptospires is likely to be contaminated with other harmful bacteria and chemicals. You may not have been infected with leptospira, but there are a whole host of other things you could have caught.
When you visit your doctor, explain what you were doing and what you think you have been exposed to. If you say ‘Doctor, I went swimming and now I have a fever and a headache‘ you will most likely be given penicillin and sent home with a suspected case of the flu, or sinusitis. If you say ‘Doctor, i went swimming and there were rats about, or a funny smell. I’ve heard of people getting a nasty illness from rats and I’m worried I’ve got it‘ then you’ll prompt your doctor into ordering the right blood tests without putting their nose out of joint. Many doctors have never seen a case of leptospirosis and may not associate it with your symptoms. However, they also may not like you arriving and quoting medical terms at them like you’re a consultant bacteriologist. If you’re not getting anywhere, then you can resort to helping them towards a diagnosis a bit more forcibly.
If you know someone else who may have been exposed, and they won’t visit a doctor, then watch them for the classic symptoms of the infection – fever, a short-lived rash, headaches, nausea. Most important is to watch for anyone getting a mild illness, recovering and then about a week later feeling very ill again.