Host-serovar species relationships
One of the interesting and important unknowns about leptospires is why certain strains only seen to infect certain species of host. Leptospirosis is known in almost every mammal, from livestock and humans to rodents, marsupials, even sealions. Of the 200+ serovars of the bacteria known, they are grouped (into serogroups) and a particular species generally seems to only ever show natural infection from a few serogroups. In the lab they can often be infected with many others, so it seems a combination of the chemical makeup of the animal’s blood and the environment, geography or mixing of the species. It’s known for example that some wild marsupials living amongst infected rats and mice will have an entirely different group of infections, and they won’t cross between species. In contradiction to that, many of the serogroups dangerous to humans are also carried by animals (cows, sheep, dogs and rodents all commonly carry strains which can infect humans and cause severe illness). The relationship between host and bacteria is very complex and still being researched, but the fact a species is usually only infected with a few strains makes vaccination quite easy, and this is probably the most important factor in preventing leptospirosis developing into a critical planet-wide epidemic.
One thing to say is that although the grade-school picture of host-serogroup specificity (HSS) is quite simple, with statements such as “dogs get canicola, cows get hardjo”, in reality this is only a statistical effect. Dogs can get infected from a whole lot more than just one group, and over time we’re seeing growth and decline in the common HSS relationships as new strains spread and old strains seem to die back. Local variations can be huge, even on a kilometer scale, which is why long-distance movement of livestock can provoke infections (if their ‘local’ vaccine mix did not include the strain present in their new home). A great deal of money and effort is put into developing more wide-ranging and effective animal vaccines, and new mixes are appearing every year. Human vaccines are still in the very early stages and are not yet considered safe and effective enough for widespread use.
The table below gives an indication of the common and less-common serogroups based on host animal (assuming no vaccination). It is NOT EXHAUSTIVE and many of the listed species can be infected by dozens of other strains in the laboratory, we just have yet to see the effect appear in nature.
| Host species | Common infections | Possible others |
|---|---|---|
| Dogs | canicola, icterohemorrhagiae, grippotyphosa, pomona | bratislava |
| Cats | [rarely infected] | |
| Cattle (and deer) |
hardjobovis, pomona, grippotyphosa, icterohemorrhagiae | australis, autumnalis, canicola, bataviae, hebdomadis, krematosis, tarassovi, sejroe, bratislava |
| Pigs | pomona, bratislava, canicola, tarassovi, icterohemorrhagiae | grippotyphosa, sejroe |
| Sheep | pomona, grippotyphosa, bratislava, hardjo | |
| Horses | pomona, bratislava, canicola, icterohemorrhagiae, sejroe |
Infections in primates mimic those in humans. Cats are rarely infected in nature, even though they can be infected experimentally. Fish have yet to be fully researched but it is known they can carry the bacteria. Reptiles and amphibians seem largely immune and are not considered likely carriers but again data is scarce. Rodents seem rarely to be made ill by leptospirosis, but can be carrier hosts for almost every pathogenic serovar. The commonest rodent serovars vary geographically.