Overview of Leptospirosis

Leptospira are aerobic, gram-negative spirochetes that are fastidious, slow growing, and have characteristic corkscrew-like motility. The taxonomy of Leptospira is complex and can be confusing. Traditionally, Leptospira were divided into two groups; the pathogenic Leptospira were all classified as members of L interrogans, and the saprophytic Leptospira were classified as L biflexa. Within each of these species, leptospiral serovars were recognized, with >250 different serovars of pathogenic Leptospira identified (based on surface antigens) throughout the world. The serovars are often grouped into antigenically related serogroups. With the increased use of genomic information for the classification of bacteria, the genus Leptospira was reorganized. There are currently 21 recognized genomospecies of leptospires, including both pathogenic, intermediate, and nonpathogenic organisms. Pathogenic leptospires are now identified in 9 species of Leptospira, with 6 species being regarded as intermediate in pathogenicity, and 6 being nonpathogenic. Some of the common leptospiral pathogens of domestic animals now have different species names. For example, L interrogans serovar Grippotyphosa is now L kirschneri serovar Grippotyphosa. The two types of serovar Hardjo have been formally split into two species: serovar Hardjo type hardjo-bovis (found in the USA and much of the world) is now L borgpetersenii serovar Hardjo and the less common serovar Hardjo type hardjo-prajitno (found primarily in the UK) is now L interrogans serovar Hardjo. The revised nomenclature is now reflected in the scientific literature but not on labels for vaccines and pharmaceutical products. Fortunately for clinicians, the serovar and serogroup names remain in common use and are useful when discussing the epidemiology, serology, clinical features, treatment, and prevention of leptospirosis.

Essentially all mammals are susceptible to infection with pathogenic Leptospira, although some species are more resistant to disease. Among common companion animals and livestock, leptospirosis is most frequently recognized in cattle, swine, dogs, and horses. Cats have historically been considered to be resistant to disease but have been shown to seroconvert on exposure to leptospires. Recent evidence suggests that the role of leptospires in the pathogenesis of feline renal disease should be reexamined. Leptospirosis in wildlife is common, although the disease is most often noticed only when the wildlife serve as a source of infection for domestic animals or people.

Leptospirosis is found throughout the world. The infection (and disease) is more prevalent in warm, moist climates and is endemic in much of the tropics. In temperate climates, the disease is more seasonal, with the highest incidence after periods of rainfall.

Although >250 serovars of pathogenic Leptospira are recognized, a subset of leptospiral serovars are prevalent within a particular region or ecosystem and are associated with one or more maintenance hosts, which serve as reservoirs of infection (see Common Maintenance Hosts of the Pathogenic Leptospires Associated with Disease in Domestic Animals in the USA and Canada). Maintenance hosts are often wildlife species and, sometimes, domestic animals and livestock. Each serovar behaves differently within its maintenance host species than it does in other, incidental host species. In maintenance hosts, leptospirosis is generally characterized by a high prevalence of infection, relatively mild acute clinical signs, and persistent infection in the kidneys and sometimes the genital tract.

Diagnosis of maintenance host infections is difficult because of relatively low antibody responses and the presence of few organisms in the tissues of infected animals. Examples of this type of infection are serovar Bratislava infection in swine and serovar Hardjo infection in cattle. In incidental hosts, leptospirosis is characterized by a low prevalence of infection, severe clinical signs, and a short renal phase of infection. Diagnosis of incidental host infections is less problematic because of a marked antibody response to infection and the presence of large numbers of organisms in tissues of infected animals. Examples of this type of infection are serovar Grippotyphosa infection in dogs or serovar Icterohaemorrhagiae infection in cattle and swine.

Characterization of a host/serovar interaction as a maintenance or incidental host infection is not absolute. For example, swine and cattle infected with serovar Pomona behave as a host intermediate between the two forms, with the organism persisting in the kidneys but the host showing a marked antibody response to infection.

Transmission among maintenance hosts is often direct and involves contact with infected urine, placental fluids, or milk. In addition, the infection can be transmitted venereally or transplacentally with some host/serovar combinations. Infection of incidental hosts is more commonly indirect, by contact with areas contaminated with urine of asymptomatic maintenance hosts that shed leptospires in their urine. Environmental conditions are critical in determining the frequency of indirect transmission. Survival of leptospires is favored by moisture and moderately warm temperatures; survival is brief in dry soil or at temperatures <10°C or >34°C. The organisms are killed by freezing, dehydration, or direct sunlight.

Despite the many serovars of Leptospira and host species, the key steps in pathogenesis of the disease are similar in all host/serovar combinations. Leptospires invade the body after penetrating exposed mucous membranes or damaged skin. After a variable incubation period (4–20 days), leptospires circulate in the blood and replicate in many tissues including the liver, kidneys, lungs, genital tract, and CNS for 7–10 days. During the period of bacteremia and tissue colonization, the clinical signs of acute leptospirosis, which vary by serovar and host, occur. Agglutinating antibodies can be detected in serum soon after leptospiremia occurs and coincide with clearance of the leptospires from blood and most organs. As the organisms are cleared, the clinical signs of acute leptospirosis begin to resolve, although damaged organs may take some time to return to normal function. In some cases, severely damaged organs may not recover, leading to chronic disease or death.

At this point, the disease in incidental and maintenance hosts diverges. Leptospires remain in the tubules of the kidneys of incidental hosts for a short period of time and are shed in the urine for a few days to several weeks. In maintenance hosts, however, leptospires often remain in the renal tubules, genital tract, and less commonly, the eyes, despite the presence of high levels of serum antibody. Leptospires are shed in the urine and genital secretions of persistently infected animals for months to years after initial infection, and these animals become an important reservoir of infection, with the potential to transmit infection to other reservoir hosts or to incidental hosts at risk of developing clinical disease.

The clinical signs of leptospirosis depend on the host species, the pathogenicity of the strain and serovar of Leptospira, and the age and physiologic state of the animal. Subclinical infections are common, particularly in the maintenance host. In incidental hosts, leptospirosis is an acute, systemic, often febrile illness characterized by renal and/or hepatic damage. In addition, there may be effects on other body systems resulting in clinical problems such as uveitis, pancreatitis, bleeding, hemolytic anemia, muscle pain, or respiratory disease.

In both incidental and maintenance hosts that are pregnant at the time of infection, localization and persistence of the organism in the uterus may result in fetal infection, with subsequent abortion, stillbirth, birth of weak neonates, or birth of healthy but infected offspring. In general, incidental hosts abort acutely, whereas in maintenance hosts, abortions or other reproductive sequelae may be delayed by several weeks or months.

Diagnosis of leptospirosis depends on a good clinical and vaccination history and laboratory testing. Diagnostic tests for leptospirosis include those designed to detect antibodies against the organism and those designed to detect the organism in tissues or body fluids. Serologic testing is recommended in each case, combined with one or more techniques to identify the organism in tissue or body fluids.

Serologic assays are the most commonly used technique to diagnose leptospirosis in animals. The microscopic agglutination test (MAT) is most frequently used. It involves mixing appropriate dilutions of serum with live leptospires of serovars prevalent within the region. The presence of antibodies is indicated by the agglutination of the leptospires, with the reported titer being the highest dilution of serum that results in 50% agglutination. The MAT is a complex test to perform and interpret, and it requires the maintenance of live leptospiral cultures. An ELISA test to diagnose canine leptospirosis is offered by a commercial laboratory in the USA. This test detects antibodies to LipL32, a membrane protein found on pathogenic leptospires. The currently available assay provides a qualitative negative or positive result and will also detect antibodies induced by vaccination. A comparison of this test to the MAT has not been reported, and it is likely that the numerical titers provided by the MAT will provide more diagnostically useful information than a qualitative ELISA.

Interpretation of serologic results from the MAT is complicated by a number of factors, including cross-reactivity of antibodies, antibody titers induced by vaccination, and lack of consensus about the level of antibody titer that indicates infection. Antibodies produced in an animal in response to infection with a given serovar of Leptospira often cross-react with other serovars. In some cases, these patterns of cross-reactivity are predictable based on the antigenic relatedness of the various serovars of Leptospira, but the patterns of cross-reactive antibodies vary between host species. Paradoxical reactions may occur with the MAT early in the course of an acute infection, with a marked agglutinating antibody response to a serovar other than the infecting serovar. In addition, there is evidence of lack of consistency between diagnostic laboratories. For these reasons, the infecting serovar in an individual animal cannot be reliably identified as the serovar to which the animal develops the highest titer. The real value of the MAT is in providing a numerical titer to allow comparison of acute and convalescent values.

Widespread vaccination of dogs and livestock with leptospiral vaccines also complicates interpretation of leptospiral serology. In general, vaccinated animals develop relatively low agglutinating antibody titers (1:100 to 1:400) in response to vaccination, and these titers persist for 1–4 mo after vaccination. However, some animals develop high titers after vaccination which persist for ≥6 mo.

Consensus is lacking as to what constitutes a diagnostic titer for leptospiral infection. A low antibody titer does not necessarily exclude a diagnosis of leptospirosis, because titers are often low in acute disease and in maintenance host infections. In cases of acute leptospirosis, a 4-fold rise in antibody titer is often observed in paired serum samples collected 7–10 days apart. Diagnosis of leptospirosis based on a single serum sample should be made with caution and with full consideration of the clinical picture and vaccination history of the animal. In general, with a compatible clinical history and vaccination >3 mo ago, a titer of 1:800 to 1:1,600 is good presumptive evidence of leptospiral infection. The use of paired acute and convalescent titers is strongly recommended whenever possible. Antibody titers can persist for several months after infection and recovery, although there is usually a gradual decline with time.

Immunofluorescence can be used to identify leptospires in tissues, blood, or urine sediment. The test is rapid and has reasonable sensitivity, but interpretation requires a skilled laboratory technician. Immunohistochemistry is useful to identify leptospires in formalin-fixed tissue but, because there may be small numbers of organisms present in some tissues, the sensitivity of this technique is variable. A number of PCR procedures are available, and each laboratory may select a slightly different procedure. Unfortunately, few publications have confirmed the validity of all the commercially available PCRs, which likely vary considerably in their performance. PCR techniques allow detection of pathogenic leptospires in blood, urine, or tissue samples but do not determine the infecting serovar. Culture of blood, urine, or tissue specimens is the only method to definitively identify the infecting serovar. Blood may be cultured early in the clinical course; urine is more likely to be positive 7–10 days after clinical signs appear. Culture is rarely positive after antibiotic therapy has begun. Culture of leptospires requires specialized culture medium, the organisms are fastidious and slow-growing, and diagnostic laboratories rarely culture specimens for the presence of leptospires. Thus, culture is of little value to clinicians.

Avoidance of exposure to free-ranging wildlife and domestic animals that may be maintenance hosts for Leptospira is difficult because rodents, raccoons, opossums, and skunks are frequently found in rural and urban environments. The cornerstone of leptospirosis prevention is vaccination with polyvalent inactivated vaccines. Immunity to leptospirosis is believed to be serovar specific and, therefore, vaccines are formulated for various species to include the relevant serovars. There are currently no leptospiral vaccines for horses. Leptospiral vaccines are generally designed and evaluated for the ability to prevent clinical signs of disease, although some vaccines have also been shown to significantly reduce renal colonization and urine shedding.

People are susceptible to infection with most of the pathogenic serovars of Leptospira but are incidental hosts and, therefore, not important reservoirs of infection. Occupational exposure is a rick factor, and veterinarians, veterinary staff, livestock producers, and dairy workers are at increased risk. In addition, recreational exposure to waters contaminated with urine of domestic animals or wildlife presents a risk. Animal owners have contracted leptospirosis via contact with infected companion animals and livestock.

The principal route of infection is contact with infectious body fluids (blood in acute cases or urine) via mucous membranes. In people, the disease varies from subclinical to severe and can be fatal when renal or hepatic failure occurs. The most common signs are fever, headaches, rash, ocular pain, myalgia, and malaise. Transplacental infection, abortion, and infection of infants via breast feeding have been described, making exposure of pregnant women of particular concern. Laboratory techniques are necessary for a definitive diagnosis. Because diagnosis of leptospirosis in animals is difficult based on clinical signs, veterinarians may wish to implement an infection control program in which animal body fluids are handled only with gloved hands and hand washing is routine. It is also essential for staff to take precautions when handling or nursing animals suspected or confirmed to have leptospirosis. Appropriate precautions include wearing gowns, shoe covers, and gloves to avoid contaminating exposed skin or spreading organisms. Face shields should be worn when handling wet bedding or cleaning cages, stalls, or runs to avoid contact of aerosolized organisms with mucous membranes.