In order to become infected, a quantity of the leptospira bacteria must enter your body. The main routes for any bacteria to enter a human are through ingestion (eating or drinking), inhalation (breathing) or through broken skin. Aquiring a leptospiral infection is not as easy as other infections, and a great deal harder than contracting some viral infections such as a common cold. The critical factor is that the bacteria themselves cannot survive for very long unless immersed in fresh water or urine. Whilst they are often highly infectious, the chances of them surviving in the environment long enough to be transferred into a human are often miniscule.
Unlike viruses, bacteria are rarely airborne. The must exist immersed in water, and so except in very high vapor situations (waterfalls, spraying operations, air conditioning mist towers, etc) airborne infection is not a risk.
Being in close proximity to an infected animal is perfectly safe provided there is no fluid contact.
There are of course several high-risk situations where such transmission is possible, and we detail them below. Firstly let us dispel a few rumors:
- Human-to-human transfer is almost unheard of.
- Saltwater, or freshwater treated with chlorine or UV-sterilised, is generally safe.
- Any surface or object that has been dessicated (dried out completely for over an hour) will be safe, even if it gets wet once more. Once the bacteria are dead, they remain dead.
- rats are the most common carriers, but all mammals are susceptible to the illness. Domestic dogs are easily infected but cats are remarkably immune and very rarely become infected.
- Almost every ‘routine’ disinfection method will kill the bacteria (bleach, acids, alkalis, chlorines, domestic disinfectants, UV filtration, steam-cleaning, irradiation and dessication).
- There is no human vaccine licenced for use in Europe, Asia or the USA.
Risk situations
Thousands of sportsmen and underground/waterway workers are exposed to contaminated areas every day of the year and the incidences of serious infection are low, however in areas of the world where the disease is not endemic, humans do not have immunity to the bacteria. Any exposure can transfer the infection, so the only barrier is precautions and common sense.
Clearly the majority of high-risk sites will have rat populations and an area of fresh water. Given this, there are several obvious things that present an extremely high risk:
- Swimming and scuba diving
- Kayaking and canoeing
- Sailing, windsurfing, fishing and skiing
- Caving and underground exploration
- Cleaning and jetting operations (due to the high vapor generation)
- Animal handling, control and management (domestic, wild and farmed)
There are many lower-risk activities (such as hiking, horse-riding, etc) where you may pass through contaminated areas but are unlikely to come into direct contact with rodents or water, plus several specific activities where risks are high but only in the event of an accident (such as working at height over water).
Anyone engaged in work-related activities in contaminated sites is at risk, and as well as common activities that are known to be high-risk (sewer inspection, rodent control, garbage collection) workers must also consider the risks from general cuts and scrapes. Construction, horticulture, farming and mining all present a hazard of skin damage where there may also be contaminated water.
Domestic pets and farm animals
It is rare in the developed world for domestic pets to be infected by leptospirosis. Many dogs are vaccinated, cats are often immune and captive-bred rodents are assumed to have never been exposed to the bacteria. However, it is often the case that a domestic pet contracts the infection after exposure to wild rodents or their environment (swimming in lakes, catching rats, etc). The disease is often very serious in domestic mammals, and without treatment is usually fatal. Any animal is itself a source of infection for the first few weeks of the illness, so their body fluids carry the bacteria. Pet owners should exercise personal hygiene when dealing with any infected animal (or when removing wild rodents brought into the house by a pet), however it is very rare for a human to become infected from their pet.
Farm animals are equally susceptible, and most are vaccinated. However farm workers are often at direct risk from their working environment where a rodent population is established. Open water, drinking trays, slurry and wet mud can all sustain the bacteria for long enough to risk cross-exposure to those on the site.
Areas of high risk
As we have already said, the Leptospira bacterium is predominantly spread by the urine of animals. Most cases result from rats, though farm animals can also carry the infection and wild or domestic livestock can also present a risk. The bacteria can survive for up to one month if the urine is transferred into water immediately, though dies rapidly if dried out. For this reason farm animals and wild/domestic livestock presents less of a risk than rats as the exposure must be to the urine directly. Rat populations exist near water and the urine from the colony easily passes into the environment. The bacterium can only survive for this length of time in fresh water. In saltwater the organism is killed within a few hours. Also the flow and volume of water are critical – the bacteria do not multiply beyond the host and so a very large or fast-flowing river is less of a risk than a small pool. Underground water sources present a unique risk in that they are often protected from ever drying out, have clean pH-neutral water and can be used as nest sites by rodents.
High risk
- Small slow-flowing rivers, ditches and pools
- Underground caves, sewers and drains
- Marshes and permanently wet soil
- Fountains, water features, ponds and moats without disinfection equipment
Low risk
- Fast-flowing rivers
- Tidal estuaries (saltwater exposure)
- Very large lakes and rivers
- Hard surfaces which regularly dry out
- Polluted and contaminated water where heavy metals are present and the pH is non-neutral
- Water that is regularly above 70 degrees centigrade
The bacteria is endemic to rat populations of numerous species across the world and it is a sensible policy to assume all rats, wherever they are found, harbor the infection unless proved otherwise.