Epidemiology and Pathogenesis of Leptospirosis

Leptospirosis is caused by Leptospira sp and is transmitted directly or indirectly from animals to humans, human to-human transmission is very rare. The mammalian species mainly harbor the organism leptospires in their kidneys and it becomes the source of infection to human beings and other animals. Rodents are the first identified carriers of leptospirosis. Pigs and cattles can also excrete very large amounts of leptospires and can serve as a source of human infection. Leptospirosis is mostly observed in tropical and subtropical areas with high rainfall. It has been reported in several countries of the South-East Asia Region. The magnitude of leptospirosis differs from country to country depending upon awareness of the public.

India, Indonesia, Thailand and Sri Lanka have the most reported cases during the rainy season. Majority of the farm workers, veterinarians, pet shop owners, field agricultural workers, abattoir workers, plumbers, meat handlers and slaughterhouse workers, coal miners, workers in the fishing industry, military troops, milkers, and sewer workers are the risk groups of getting infected. Leptospirosis occurs mostly during the rainy season in the tropics and during the late summer or early fall in Western countries. Seasonal outbreaks are also associated with flood conditions and drought conditions.

Leptospires are thin, coiled, gram-negative, aerobic organisms which are motile in nature. The hooked ends and paired axial flagella helps them to burrow into tissue. They move through continual spinning on the long axis and can be isolated on artificial media including contaminated water and food, as well as animal bedding, soil, mud, and aborted tissue as the infected urine of animals serve as the source of human infection. They can be viewed through a scanning electron micrograph.

Leptospires can enter the host through abrasions in healthy skin, animal or rodent bites, sodden and waterlogged skin, mucous membranes or conjunctiva, inhaling through lungs, placenta during pregnancy and others. The incubation period observed in this infection is usually 5-14 days but are described in the range starting from 72 hours to a month or more. The pathological effects of leptospires include inconsistent multiorgan tissue injury due to the presence of these spirochetes.

Further, the endotoxin, haemolysin, and lipase of the organism can also serve as the sources of pathogenicity. Occurrence of capillary vasculitis by endothelial edema, necrosis, and lymphocytic infiltration may also be noticed. Secondary tissue injury might occur due to loss of red blood cells and fluid through enlarged junctions and fenestrae. Other than these, decreased glomerular filtration rate, renal failure due to tubular damage, jaundice due to hepatocellular dysfunction, pulmonary damage, interstitial myocarditis, hemorrhage, focal necrosis, and inflammatory infiltration have been documented. A lot of other complications may also be observed which need proper treatment approaches to cure the disease. As the organism has the ability to reside in different organs without showing any symptoms, it is important to confirm its complete removal from the body.