Firstly, the bacteria is water-borne. You cannot catch it from shaking hands, talking to someone, petting an animal, sharing clothes, etc. If you suspect something or someone to be infected, you should avoid contact with water and fluids, including damp soil, toothbrushes, facecloths, etc. For pets and animals, avoid any contact with excreta and wash your hands after grooming them. It is extremely unlikely that saliva from a pet could transfer infection, though letting your dog lick your face or share food is a bad idea, for many reasons. Unlike a cold or flu infection, the bacteria is not easily aerosolized, so living next door to a human or animal patient will present no risk in itself.
Classically leptospirosis was known as a disease of dogs, and they are very susceptible to infection, usually by the canicola strain. Cats, on the other hand, are very rarely infected. All domestic rodents (rats, mice, hamsters, etc) are capable of being infected.
Humans will only fall ill if infected with a small sub-group of the 200 or so leptospira strains. Animals are similarly specific to the strains that can infect them and become resident as a host. For example, humans can be infected directly from rats, and directly from domestic dogs, however it is rare for infection to pass from dogs to rats or vice versa. There is no guaranteed method of predicting what species of animal could be infectious to humans, and almost any mammal, from rats to dolphins, can carry the bacteria and transmit it.
Caring for an infected domestic or farm animal is not in itself a high risk, provided common precautions are taken in personal hygiene.
Human to human infection can take two routes – direct and indirect:
- Direct transmission occurs when body fluids of an infected person are transferred to a target individual. This can occur via exchange of blood, semen, milk, urine, faeces or through the placenta to an unborn infant. There is of course a risk from handling body tissues, which is relvent to medical staff.
The bacteria must enter the target body, via ingestion, sexual contact or direct blood contact. It is not currently considered that saliva exchange (via kissing) carries a significant risk. Sexual intercourse does however pose a risk and infection has been reported via this route.
- Indirect transmission occurs when body fluids (usually blood or urine) are transferred via the environment to a target. This could occur in the handling or washing of urine- or blood- soaked clothes or bedding, but is rare. Some isolated cases of urine contamination of drinking water have been reported from isolated locations.