Human-to-human transmission

An infected human will shed leptospires in their urine for a period both during and after the illness, and so can present a risk of infection to others but only in a specific way. General social interations are prefectly safe, as the bacteria are not airborne. Saliva is not considered a high risk, as the bacteria cannot tolerate the acidity of the human mouth for very long, so although we advise against it the risks from sharing food, cutlery or cups is very small. Items that can dry between uses, such as towels, are also of extremely low risk once they are dry – but handling very wet bedding, blood-soaked clothing or similar can present a risk.

The presence of bacteria in the urine means that leptospirosis is a sexually-transmitted infection, since during intercourse there will be the opportunity for small volumes of urine to exchange between partners. There are several documented cases of human infection via sex, but insufficient data to show if a particular sexual practice is more or less risky (though any event where urine can come into contact with damaged skin is hazardous).

In humans, viable bacteria will be present in the urine after about 2 days from the exposure (day E2), and they will usually remain present for a few weeks after illness, but there are recorded cases of humans shedding the bacteria for up to 11 months. Treatment with antibiotics can reduce the symptoms and also reduce this shedding time, but in most patients their urine contains detectable bacteria for smany weeks after clinical recovery. Our advice is to assume some risk from urine for up to 12 months after the acute illness has faded.

The risks from direct blood transfer are actually less of an issue, as the bacteria are present in the blood for only a short time (typically they appear in noticeable levels at day E1 (first day after exposure), and reduce on or before day S10 (10th day of symptoms). It does remain a concern for medical staff handling samples and treating injuries, but due to the rapid death of leptospira when dried the usual blood-related hazards such as needlestick injuries are far less important than when dealing with viruses.

Pregnant mothers may pass the infection on to their fetus, see below for information.