Antigenically related serovars (belonging to the same serogroup) may belong to different species because of horizontal gene transfer of LPS biosynthetic genes between different species. Currently, the cross agglutination absorption test and DNA-DNA hybridisation are used to classify Leptospira species, but are time consuming. Therefore, total genomic sequencing could potentially replace these two methods as the new gold standard of classifying Leptospira species. Transmission
The bacteria can be found in ponds, rivers, puddles, sewers, agricultural fields and moist soil. Pathogenic Leptospira have been found in the form of aquatic biofilms, which may aid survival in the environment.The number of cases of leptospirosis is directly related to the amount of rainfall, making the disease seasonal in temperate climates and year-round in tropical climates. The risk of contracting leptospirosis depends upon the risk of disease carriage in the community and the frequency of exposure. In rural areas, farming and animal husbandry are the major risk factors for contracting leptospirosis. Poor housing and inadequate sanitation also increase the risk of infection. In tropical and semi-tropical areas, the disease often becomes widespread after heavy rains or after flooding.Leptospira are found mostly in mammals. However, reptiles and cold-blooded animals such as frogs, snakes, turtles, and toads have been shown to have the infection. Whether there are reservoirs of human infection is unknown. Rats, mice, and moles are important primary hosts, but other mammals including dogs, deer, rabbits, hedgehogs, cows, sheep, swine, raccoons, opossums, and skunks can also carry the disease.
The endothelial cells produce cytokines and antimicrobial peptides against the bacteria. These products regulate the coagulation cascade and movements of white blood cells. Macrophages presented in humans are able to engulf Leptospira. However, Leptospira are able to reside and proliferate in the cytoplasmic matrix after being ingested by macrophages. Those with severe leptospirosis can experience a high level of cytokines such as interleukin 6, tumor necrosis factor alpha (TNF-α), and interleukin 10. The high level of cytokines causes sepsis-like symptoms which is life-threatening instead of helping to fight against the infection. Those who have a high risk of sepsis during a leptospirosis infection are found to have the HLA-DQ6 genotype, possibly due to superantigen activation, which damages bodily organs.Leptospira LPS only activates toll-like receptor 2 (TLR2) in monocytes in humans. The lipid A molecule of the bacteria is not recognised by human TLR4 receptors. Therefore, the lack of Leptospira recognition by TLR4 receptors probably contributes to the leptospirosis disease process in humans.Although there are various mechanisms in the human body to fight against the bacteria, Leptospira is well adapted to such an inflammatory condition created by it. In the bloodstream, it can activate host plasminogen to become plasmin that breaks down extracellular matrix, degrades fibrin clots and complemental proteins (C3b and C5) to avoid opsonisation. It can also recruit complement regulators such as Factor H, C4b-binding protein, factor H-like binding protein, and vitronectin to prevent the activation of membrane attack complex on its surface. It also secretes proteases to degrade complement proteins such as C3.