Host-serovar species relationships
One of the interesting and important unknowns about leptospirosis is why certain serovars only seen to naturally infect certain species of host. In the lab animals can often be infected with many others, so there is a complex series of interations, both inside the animal and in the environment, that limits the infection in some way as yet not fully understood. 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 easier.
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.
Leptospirosis is famously a ‘disease of rats’ but in reality rodents are remarkably immune. Rodents can become carriers of a selected number of strains, but are impossible to infect with any other. They do not become noticeably ill from the infection, and so leptospirosis is not a ‘rat disease’. The reason it becomes important is that the strains rodents can carry are the same strains that cause severe illness in other animals, and in humans. It’s simply a coincidence, but it’s what makes leptospirosis such a serious issue.
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 routinely in nature.
| 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 | |
Domestic cats are an interesting case – although they often come into contact with feral rodents they rarely contract the infection, although they can be given it in a lab. We believe this is an evolutionary development.
Although fish (both temperate and tropical) are not easy to infect and rarely carry the bacteria, there is an issue in some cases of the water used to transport them being the problem – international commercial fish transport for the pet market uses regular water changes and is unlikely to cause a problem, but fish captured by amateurs may permit the bacteria a free ride back to the tank. They will then survive in the water for several weeks or months and could potentially risk infection to pets that drank from the water, or humans handling it with injured skin.
The carrier state and reservoir hosts
Some animals will recover from their first infection and then continue to shed bacteria in their urine, often for the rest of their lives. This ‘carrier state’ is found in most rodents and some mammals, but not in humans. Carriers are critical to the life cycle of leptospires because the carrier spreads bacteria into the environment, where they can hopefully infect a new individual. Without carriers, the infections would die with each animal. Rats are the most widespread and famous carrier, but in some areas it’s another species that dominates (such as mice, marsupials or raccoons). Carriers are different from acute cases in recovery (where the urine may contain bacteria for several months) as in carriers there is no sign of any reduction over time.
The species that acts as the main carrier in a region is called the reservoir host, as they maintain the level of bacteria in a region despite changes in the local environment and climate. Luckily the reservoir hosts for most countries are species that remain territorial and do not engage in large-distance migration.