Survival of leptospires in the environment
Leptospira exist in two groups, the pathogenic parasitic types, and the free-living saprophytes. All require the same basics to survive (water, oxygen, stable pH and temperature) but their life cycles and food requirements are different. It was once thought that saprophytic leptospires could ‘transform’ into pathogens after they entered a host, but this is now known to be untrue.
These live as free-swimming bacteria in bodies of water, and do not use a host animal for their life cycle. They feed from microscopic decaying organic matter in the water, and will survive and reproduce anywhere with suitable water conditions and a supply of organic material. All leptospires are relatively slow-growing compared to the ‘common’ bacteria, and so colony densities in a body of water tend to be uniform and constant. They cannot cause significant infections and their only interest, apart from being some of the most numerous creatures on the planet, is that they can cause positive reactions in some laboratory tests, making analysis of the ‘safety’ of water samples more difficult. The presence of saphrophytic leptospires in a body of water is not a sign that it is unclean or contaminated, in fact the bacteria prefer clean water without chemical pollution. It also has no bearing on the likely presence of pathogenic strains, however there is a theory that their widespread presence could explain why many fish and amphibians are immune to infection from the pathogenic strains – these species may have developed antibodies because of constant exposure to the saphrophytes even though they were not made ill by them.
These require a host in order to complete their life cycle, and whilst they can be cultivated in prepared serum they do not support stable colonies outside of the host. Survival and reproduction within the host animal is of course determined by the host’s immune response and species, what is of concern here is the survivability of the bacteria outside the host. Pathogenic leptospira cause illness in their host, although of course they do not intend to do so – in fact many researchers argue that illness is a bad idea for leptospira, since a host that stays alive will be able to shed more of the bacteria in their urine. However the bacteria clearly haven’t worked that out yet.
Outside a host animal, or the lab, it is believed that pathogenic leptospires have a close to zero reproductive output – in some cases limited binary fission has been seen, possibly caused by long-term absence of a host species, but in general terms the bacteria do not multiply. Individuals can however survive for extended periods if the conditions are suitable, so whilst a colony in a pond or lake may not increase over time, it is also unlikely to decrease particularly fast either. Where carrier hosts regularly add fresh urine to an environment the pathogenic colonies can maintain an almost continuous presence.
Pathogenic leptospires reproduce best at body temperature, but can survive over a wide range. They appear to be unable to tolerate temperatures over about 42°C, which can explain why bacterial growth is reduced or reversed in patients with very high fevers, with no reports of survivial when heated to over 55°C. Cold is easier to tolerate, and they can be frozen (in ice or liquid nitrogen) and subsequently revived. Leptospires have been isolated from kidneys that have been deep-frozen as part of the food distribution chain. Their levels of activity and ability to reproduce drops when cooled below 10°C.
As leptospires do not have a waterproof membrane they must remain immersed in water to survive, and so are killed immediately their environment dries out unless they are intentionally prepared (they can for example be freeze-dried). The inability to survive out of water is the single most important control factor in the natural environment, as it means they are unable to create infection risks from dry surfaces.
Pathogenis leptospires are extremely sensitive to chemicals of all kinds, and so are quite easy to kill – the links below detail many ways to disinfect. Detergents, acids and heavy metals are all lethal at very low concentrations and this means that pathogenic colonies find it difficult to survive in very polluted water – so the worst industrial effluent is (in terms of leptospirosis at least) safer than a pure mountain lake!