Leptospirosis is a zoonosis of worldwide distribution. It is caused by spirochetes of the genus Leptospira. There is one pathogenic species, Leptospira interrogans, but more than 200 serovars of L. interrogans. These serovars are further organized into over two dozen serogroups. The serogroups are based on shared antigenicity and are primarily for laboratory use.

Morphology and Identification

A. Typical Organisms

Leptospirae are tightly coiled, thin, flexible spirochetes 5–15 μm long, with very fine spirals 0.1–0.2 μm wide; one end is often bent, forming a hook. They are actively motile, which is best seen using a dark-field microscope. Electron micro graphs show a thin axial filament and a delicate membrane. The spirochete is so delicate that in the dark-field view, it may appear only as a chain of minute cocci. It does not stain readily but can be impregnated with silver.

B. Culture

Leptospires grow best under aerobic conditions at 28–30°C in semisolid medium (eg, Ellinghausen-McCullough-Johnson Harris, EMJH) in 10 mL test tubes with 0.1% agar and 5-fluorouracil (see also Diagnostic Laboratory Tests). After 1–2 weeks, the leptospires produce a diffuse zone of growth near the top of the tube and later a ring of growth at a level in the tube corresponding to the level of the optimal oxygen tension for the organisms.

C. Growth Requirements

 Leptospirae derive energy from oxidation of long-chain fatty acids and cannot use amino acids or carbohydrates as major energy sources. Ammonium salts are a main source of nitro gen. Leptospirae can survive for weeks in water, particularly at alkaline pH.

Antigenic Structure

 The main strains (“serovars”) of L. interrogans are all serologically related and exhibit cross-reactivity in serologic tests. This indicates considerable overlapping in antigenic structure, and quantitative tests and antibody absorption studies are necessary for a specific serologic diagnosis. The outer envelope contains large amounts of lipopolysaccharide of antigenic structure that is variable from one strain to another. This variation forms the basis for the serologic classification of the Leptospira species. It also determines the specificity of the human immune response to leptospirae.

Pathogenesis and Clinical Findings

 Human infection usually results from leptospires, often in bodies of water, entering the body through breaks in the skin (cuts and abrasions) and mucous membranes (mouth, nose, conjunctivae). Ingestion is considered to be less important. After an incubation period of 1–2 weeks, there is a variable febrile onset during which spirochetes are present in the bloodstream. They then establish themselves in the parenchymatous organs (particularly liver and kidneys), producing hemorrhage and necrosis of tissue and resulting in dysfunction of those organs (jaundice, hemorrhage, nitrogen retention). The illness is often biphasic. After initial improvement, the second phase develops when the IgM antibody titer rises. It manifests itself often as “aseptic meningitis” with an intense headache, stiff neck, and pleocytosis of the CSF. Nephritis and hepatitis may also recur, and there may be skin, muscle, and eye lesions. The degree and distribution of organ involvement vary in the different diseases produced by different leptospirae in various parts of the world. Many infections are mild or subclinical. Hepatitis is frequent in patients with leptospirosis.

Kidney involvement in many animal species is chronic and results in the shedding of large numbers of leptospirae in the urine; this is probably the main source of environmental contamination resulting in infection of humans. Human urine also may contain spirochetes in the second and third weeks of disease.

Agglutinating, complement-fixing, and lytic antibodies develop during the infection. Serum from convalescent patients protects experimental animals against an otherwise fatal infection. The immunity resulting from infection in humans and animals appears to be serovar specific.

Diagnostic Laboratory Tests

 A. Specimens

 Specimens consist of aseptically collected blood in a heparin tube, CSF, or tissues for microscopic examination and culture. Urine should be collected using great care to avoid contamination. Serum is collected for agglutination tests.

 B. Microscopic Examination

 Dark-field examination or thick smears stained by the Giemsa technique occasionally show leptospirae in fresh blood from early infections. Results of dark-field examination of centrifuged urine may also be positive. Fluorescein-conjugated antibodies or other immunohistochemical techniques can be used also.

C. Culture

Whole fresh blood or urine can be cultured in a semisolid medium. Because of inhibitory substances in blood, only one or two drops should be placed in each of five tubes containing 5 or 10 mL of medium. Up to 0.5 mL of CSF can be used. One drop of undiluted urine can be used followed by one drop each of tenfold serially diluted urine for a total of four tubes. Tissue approximately 5 mm in diameter should be crushed and used as the inoculum. Growth is slow, and cultures should be kept for at least 8 weeks.

D. Serology

 The diagnosis of leptospirosis in most cases is confirmed serologically. Agglutinating antibodies first appear 5–7 days after infection and develop slowly, reaching a peak at 5–8 weeks.

Very high titers may be attained (>1:10,000). The reference laboratory standard for detection of leptospiral antibody uses microscopic agglutination of live organisms, which can be hazardous. The test is highly sensitive, but it is difficult to standardize; the end point is 50% agglutination, which is difficult to determine. Agglutination of the live suspensions is most specific for the serovar of the infecting leptospires. Agglutination tests are generally performed only in reference laboratories. Paired sera that show a significant change in titer or a single serum with high-titer agglutinins plus a compatible clinical illness can be diagnostic. Because of the difficulty in performing the definitive agglutination tests, a variety of other tests have been developed for use primarily as screening tests.

Immunity

Serovar-specific immunity follows infection, but reinfection with different serovars may occur.

 Treatment

 Treatment of mild leptospirosis should be with oral doxycycline, ampicillin, or amoxicillin. Treatment of moder ate or severe disease should be with intravenous penicillin, ampicillin, or ceftriaxone.

 Epidemiology, Prevention, and Control

The leptospiroses are essentially animal infections; human infection is only accidental, occurring after contact with water or other materials contaminated with the excreta of animal hosts. Rats, mice, wild rodents, dogs, swine, and cattle are the principal sources of human infection. They excrete leptospirae in urine both during the active illness and during the asymptomatic carrier state. Leptospirae remain viable in stagnant water for several weeks; drinking, swimming, bathing, or food contamination may lead to human infection. Persons most likely to come in contact with water contaminated by rats (eg, miners, sewer workers, farmers, and fishermen) run the great est risk of infection. Children acquire the infection from dogs more frequently than adults do. Control consists of preventing exposure to potentially contaminated water and reducing contamination by rodent control. Doxycycline, 200 mg orally once weekly during heavy exposure, is effective prophylaxis. Dogs can receive distemper–hepatitis–leptospirosis vaccinations.

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