Leptospirosis: A Worldwide Zoonotic Disease

By Ian Veitch

Introduction

Leptospires are Gram-negative spirochetes that all share a distinctive cell structure consisting of a long, tight spiral that hooks at both ends (1, figure 1). Leptospires are a well-known spirochete mainly because they cause leptospirosis, the most widespread zoonotic disease in the world that has been found in almost all species of mammals examined (2). Chronic carriers of leptospirosis are usually wild or domestic animals, such as rats, dogs, cattle, and pigs (3). Humans can contract the disease; however, leptospirosis in humans is always acquired from an animal source. Pathogenic leptospires reside in the proximal renal tubules of the kidneys of carriers and are subsequently excreted in urine. Urine tainted with leptospires then can contaminate soils, surface water, streams and rivers (3). It is pertinent to note that not all leptospira species are pathogenic. There are twenty documented species in the genus Leptospira (figure 2). These twenty species are classified as pathogenic, non-pathogenic, or intermediate/opportunistic. While only nine of twenty species are labeled as pathogenic, more than 500,000 cases of severe leptospirosis are reported each year, with mortality rates exceeding 10% (4). Furthermore, the incidence of leptospirosis is higher in the tropics than in temperature regions because of longer survival of leptospires in the environment in warm and humid conditions (5). Preventing and diagnosing leptospirosis can be a complicated process, partly due to the sheer diversity of bacteria within the genus Leptospira .

Leptospira Classification Systems

From a distance, all leptospira appear as spiral-shaped bacteria with distinctive end hooks. If examined closely, through various laboratory methods, the vast diversity of all leptospira bacteria can be appreciated. Two main classification systems have been implemented in an attempt to bring order to the sheer diversity found in the Leptospira genus.

DNA-based Classification

This form of classification is based on DNA-DNA relatedness established through hybridization (2). DNA-Based classification yielded twenty different species in the Leptospira genus. Nine species are pathogenic, five species are intermediates, and six species are saprophytic.

Serological Classification

Serological classification organizes individual bacterial strains of a species into smaller groups based on their cell surface antigens. Antigens are foreign substances that induce an immune system response from the body, such as the heightened production of antibodies. Currently, there are more than two hundred serovars within the Leptospira genus. The basis for this method of classification lies in the cellular structure of leptospires. Like other Gram-negative bacteria, leptospires have a triple-layered cell envelope, which consists of an outer membrane, a peptidoglycan cell wall, and an inner membrane. The outer membrane lipids contain long sugar polymer extensions called lipopolysaccharides, or LPS (1, figure 4). LPS is a major antigen recognized by the sera of infected humans and animals (3). Serovar classification does not always match the genetic classification of leptospira species. For instance, DNA-similar strains can show up in different serogroups, and two serovars in the same serogroup can have drastically different DNA marker variations (6). Serological classification has shown that specific serovars are host-adapted to specific reservoir species; therefore, those serovars don’t cause disease in those hosts (7). ). For instance, L. interrogans serovar Canicola is host-adapted to dogs, L. interrogans serovar Bratslavia is host-adapted to horses, L. interrogans serovar Pomona is host-adapted to pigs, and L. interrogans serovar Icterohaemorrhagiae is host-adapted to rats. Furthermore, serological classification is helpful epidemiologically as the production of leptospira vaccines is directly related to antigen diversity.

Prevalence of Leptospirosis and Preventative Measures

Leptospirosis is one of the major zoonotic diseases worldwide and it has a large impact on both human and veterinary public health (8). ). Leptospirosis is prevalent in both developed and developing countries; however, the disease is primarily associated with poor communities that lack proper sanitation facilities and flood-prone regions (4). Pro-active prevention methods for reducing the spread of leptospirosis can be difficult due to the sheer diversity of leptospirosis-inducing bacterial strains.

Prevalence

Pathogenic leptospires live in the proximal renal tubules of the kidneys of carriers and are spread through the excretion of urine. Infections of animals or humans occur from direct contact with urine or indirectly from contaminated water (3). There is a worldwide occupational association with human contraction of leptospirosis (3). Direct occupational exposure occurs with many farmers, veterinarians, meat inspectors, and rodent control workers, while indirect exposure occurs with many sewer workers, miners, soldiers, septic tank cleaners, fish farmers, and game keepers (9).

Tropical, developing countries are also constantly battling outbreaks of leptospirosis. Many cases of leptospirosis in humans come about from activities of daily life, such as walking barefoot in damp conditions or by contamination of drinking water (9). Furthermore, there are many stray dogs in most developing countries and they have been classified as a significant reservoir for human infection (10). Another obstacle developing countries face is the spread of contaminated water during flooding. Many studies have documented instances where leptospirosis infected individuals increased soon after flooding (9). This phenomenon is likely due to the increased exposure to contaminated water with flooded regions.

Faine (1994) developed three epidemiological patterns of leptospirosis. The first pattern occurs in temperate climates where few serovars are involved and human infection almost always occurs through direct contact with infected animals through farming of cattle and pigs (11). The second pattern occurs in wet tropical areas that have many more serovars infecting humans and animals and larger numbers of reservoir species (11). The last pattern occurs in urban environments and involves rodent-borne infection (11). ). Because of heightened rodent control measures, this last pattern is rarely seen in developed countries; however, the pattern is still seen in outbreaks occurring in slum areas in developing countries (11).