Characteristics of Leptospira and its antigens

Leptospirosis is a collective term used to denote all infections of humans and animals by members of the genus Leptospira. Leptospires are helical-shaped, thin, flexible, motile spirochetes with semicircular hooked ends (occasionally one or both may be straight). They are 0.1 p.m in diameter and 6-20 |xm in length and are surrounded by an outer envelope or external sheath which covers the cylindrical body. Two periplasmic flagella are inserted subterminally at opposite ends of the cell, with the free ends overlapping in the center of the cell. The flagella are located between the outer envelope or sheath and the protoplasmic cylinder. Inside the protoplasmic cylinder are the cytoplasmic contents.

Various protective and genus-specific antigens are located in the Leptospira outer envelope. Lipopoly-saccharide material appears to be an important antigenic component of the outer envelope. Antigenic proteins have been visualized by immunoblotting, although only those proteins whose epitope groups remain recognizable by antibodies are detected. Numerous protein bands are found in the outer envelope or on the surface of intact cells. Major antigenic proteins of 22-26, 31-34, 38, 40-44, 50, 60-64 and 95 kDa are found in the outer envelope. Additionally, antigenic proteins of 18, 24, 35, 42, 66 and 77 kDa vary quantitatively between the serovars studied. Leptospira interrogans-specific nonagglutinating, nonopsonic antigen (35 kDa) has also been found in outer envelope preparations, but there is speculation that this may have originated from the inner surface of the outer envelope. A 33 kDa protein found in serovar grippotypbosa is presumed to be associated with avirulent cultures, while the 41 and 44 kDa proteins appear to be associated with hamster-virulent isolates. Proteins of molecular weights 31, 34 and 37 kDa make up the flagella. The heat shock protein (hsp) is 64 kDa and shares common antigenic determinants with a wide variety of bacteria. Seven new proteins were produced by serovar bardjo when it was cultivated at 37°C.

Two species of Leptospira are recognized on the basis of infectivity of laboratory animals: Leptospira biflexa, which comprises the nonpathogenic leptospires commonly found in natural waters, and Leptospira interrogans, which includes all the parasitic and pathogenic serovars (serotypes). Typing has traditionally been performed by means of agglutination tests with high-titered antisera. The traditional classification methodologies are being augmented by the use of monoclonal antibodies, and the interrogans species is being further subdivided into 'geno-species' by restriction endonuclease analysis and DNA probes. The leptospire genome is approximately 4500 kb, and a circular plasmid of approximately 350 kb has been reported.

Leptospires are aerobic and most are readily cultivated in artificial media containing 10% rabbit serum or 1% bovine serum albumin plus long-chain fatty acids (Tween 80) supplemented with ammonium chloride, ferric sulfate, calcium, magnesium, zinc, thiamine and cyanocobalamin. Optimum cultivation temperature is 30°C at a pH of 7.2-7.5. The optimum incubation time for growth ranges from a few days to 4 or more weeks. Fresh isolates tend to require longer incubation. Generally maximum cell densities are attained in 4-10 days, depending on the inoculum and organism being cultivated. Leptospires are visualized by dark-field or phase-contrast microscopy.

Leptospirosis is a zoonosis of worldwide distribution, affecting a wide variety of domestic and wild mammals. Many serovars occur, predominantly in select animal hosts, but distribution of these serovars occurs readily. One mammalian species may be the primary reservoir for several different serovars. Parasitic leptospires often reside in the mammalian nephritic tubule, from which they are shed into the urine. The duration and intensity of shedding is often dependent on the host and infecting scrovar. Following infection, rodents may shed high numbers of some Leptospira serovars for the remainder of their life, while shedding other serovars for a relatively short period of time. Domestic mammals generally shed leptospires for several months, with decreasing intensity up to 6 months.

Infections occur as a result of direct contact with the urine of chronically infected animals or indirectly by contact with the Leptospira-contaminated water (streams or ponds) or moist soil. Pathogenic leptospires are able to survive more than 3 months in neutral or slightly alkaline soil or water. Animals and humans acquire leptospires either by direct contact with urine or tissues from an infected animal or indirectly by contact with Leptospira-contaminated soil or water. Once the host is exposed, leptospires may enter through mucous membranes of the eye, nose, mouth, genital tract or abraded skin.

Most human cases of leptospirosis occur in young adult men, with a peak incidence of summer and early fall. Indirect contact with an infected animal via contaminated water or soil is a very common cause of human infection other than those individuals whose occupation (veterinarian, abattoir workers, dairy farmers, etc.) places them in dircct contact with numerous animals. However, the hazards of swimming in ponds or streams frequented by livestock has been demonstrated repeatedly. In the human, disease transmission generally ends, as human-to-human transmission is very rare, only occurring via transplacental infection of the fetus. Worldwide, rodents are the most common source of human infection. In the USA the most common source of human infection is dogs, livestock, rodents and wild mammals. The most common serovars infecting humans are canicola, pomona, ballum, icterohaemorrhagiae, grippotyphosa, australis and hebdomidis.

Domestic animals generally become infected in a manner similar to humans, i.e. via contact with ].ep-tospira-contaminated urine, water (used by livestock for drinking), soil, infected tissues or uterine discharge. Leptospires have been demonstrated in semen from cattle and pigs and may be transmitted by natural and artificial insemination. Leptospires have been isolated from cows' milk, however natural transmission via this medium has never been demonstrated.

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