Description and Distribution
Leptospirosis is a group of diseases caused by antigenically distinct members of the bacteria Leptospira interrogans that infect humans, domestic animals and wild animals. The diseases vary in severity from unapparent to fatal depending on the host involved and the infective serological group (serovar). The disease is responsible for significant economic losses to the livestock industry, primarily due to abortion, reduction of milk and weight gain, and secondarily due to death. Incidence of leptospirosis in domestic animals is highest in cattle and lowest in sheep and goats. Leptospirosis has little affect on the health of wildlife. Certain species of wildlife, however, may serve as a source (reservoir) of infection for domestic animals and humans.
There are 184 distinct serovars of L. interrogans belonging to 20 serogroups identified worldwide. In North America, the most common serovars associated with disease of domestic animals and humans are icterohaemorrhagiae, autumnalis, canicola, pomona, grippotyphosa and hardjo. The first 3 are the most important serovars from the veterinary and public health standpoints. Twenty-five pathogenic serovars have been isolated from wildlife in the United States including 2 (grippotyphosa and pomona) from white-tailed deer.
The Leptospira organism was first isolated in 1916 by a team of Japanese researchers. They named it Spirochaeta icterohaemorrhagiae, and it was renamed Leptospira in 1917. The Leptospira organisms are small, filamentous, motile, spirochetes coiled around an axial filament ranging in size from 0.1 to 0.3 µm in width and 6 to 20 µm in length. They are easily killed by disinfectants, heat, drying, and pH values below 6 or above 8.
Leptospirosis has a cosmopolitan distribution, and can be found in domestic and wild animals across the U.S. and worldwide. In Michigan, where white-tailed deer have been serologically tested, 50 of 190 (26.3%) were found to be reactive to pomona which suggests a low rate of exposure to the organism.
Transmission and Development
Leptospires are ubiquitous and abundant due to their abilities to infect a variety of animals and to persist outside the host. Transmission occurs through contact with contaminated water or contaminated food, during coitus, or through transplacental transmission. The leptospire bacteria have a predilection for the kidneys and the urinary tract where they may produce a persistent infection. The leptospire organisms may be excreted with the urine resulting in transmission of the bacteria. Infective urine constitutes the major sources of infection for humans, domestic animals and wild animals. Transmission usually occurs when there is direct contact between urine droplets or urine contaminated water and the mucous membranes of the eye, nose and mouth or through abraded skin. Transmission of the leptospire organisms may also occur through the food chain by the ingestion of leptospiral-infected carcasses by carnivorous species. This is the most significant mode of spread of leptospires among carnivores.
Humans usually are exposed by swimming in urine-contaminated water, or occupationally through exposure to an infected animal's urine.
The leptospire organisms can survive outside the body if environmental conditions are favorable. The bacteria prefer moist, slightly alkaline soil, stagnant ponds, and low-flow, slow-moving, slightly alkaline streams. In these conditions the organism can survive for several weeks.
Once infection has occurred, the leptospires multiply in the kidneys, lungs, reproductive organs and brain. Uterine penetration of pregnant animals results in infection of the fetuses.
Clinical Signs and Pathology
Clinical signs observed with leptospirosis vary from inapparent to fatal depending on the host and the serovar. Acute leptospirosis is evident initially by a fever that may persist from a few hours to several days. In humans the disease is characterized by fever, headaches, muscle aches, vomiting, conjunctivitis, weakness, meningitis, and in severe cases, icterus and kidney failure. In domestic species clinical signs consist of fever, weight loss, anorexia (loss of appetite), abnormal milk secretion, anemia, hematuria (blood in the urine), hemoglobinuria (hemoglobin in the urine), muscular weakness, pulmonary congestion, icterus, abortion (a few days to several weeks following infection of the fetus), stillbirths or persistent infection in newborns, and death. Naturally occurring leptospiral infection in wildlife are apparently self-limiting and asymptomatic with the host recovering from the initial stages of the disease while remaining as a reservoir host. Experimental infections in white-tailed deer resulted in clinical signs ranging from an unapparent infection to a febrile response, abortion, and death. Clinical signs observed in the experimental deer were a transient fever, anorexia, weakness, anemia, hemoglobinuria and icterus.
Pathological findings vary as much as the clinical signs of disease. In humans, pathological changes occur in the kidney parenchyma. In domestic species, the leptospires cause hepatitis (cellular necrosis and separation of liver cord cells). Encephalitis (inflammation of the brain) and nephritis (inflammation of the kidney) occur.
In wildlife species, pathological changes due to naturally occurring infections result infrequently. Gross lesions consistent with an infection are not evident in white-tailed deer, striped skunks, raccoons, opossums, red fox, gray fox, rats and mice. In experimental infections with L. interrogans pomona in white-tailed deer, pathological changes consisting of nephritis, hepatitis, and widespread hemorrhages have been observed.
A combination of clinical signs and serologic test results provides the most effective diagnostic technique. There are 3 methods used to diagnose leptospirosis in mammals: demonstration methods, serologic methods, and bacteriologic methods. The demonstration methods involve examining body fluids and tissues under dark field microscopy, and histologic examination of biopsy material with special silver stains (Warthin-Starry) or Giemsa stain. The serologic methods consist of fluorescent antibody, and microscopic and macroscopic agglutination tests. Depending on the severity or stage of infection, an animal may react serologically to one or all antigens used to determine the serological profile. Using bacteriologic methods, the isolation of leptospires and their subsequent identification provides the most conclusive proof of infection. Culturing these organisms, however, is difficult. The choice of tissue or body fluid to be cultured depends on the stage of the disease. Isolation of leptospires from domestic animals is most successful during or immediately after the acute stage of the disease. Isolation is best accomplished by direct culture of blood, cerebrospinal fluid, urine or tissues (acute fatal cases-liver and spleen, chronic cases-kidney). Leptospires are fragile and do not survive the changes of postmortem autolysis.
Treatment and Control
In humans, a number of antibiotics (streptomycin, penicillin and tetracycline) have been administered to treat acute leptospirosis. Recovery from leptospirosis results in an immunity for the particular serovar that caused the infection. In domestic animals, antibiotic therapy has been effective in reducing leptospiremia and the severity of clinical signs if administered during the acute stage of the infection. A single dose of 25 mg/kg of dihydrostreptomycin has proven to be an effective procedure. Tetracycline administered in the feed is an effective prophylactic (disease preventing) procedure. Administration of leptospiral bacterins can result in the development of protection for 6-12 months. Revaccination of cattle should be conducted every 6-12 months. Repeated vaccinations may result in serologic titers equivalent to those caused by an actual infection.
Because cases of naturally occurring disease have not been documented, no wild animals have been treated for leptospirosis. There is very little known about the immunological features of leptospirosis in wild mammals. Antibodies to several serovars have been found in various wildlife species. These animals are capable of being infected with one or several Leptospira serovars and can then serve as reservoir hosts. They will have an unapparent and transient infection with no clinical signs of the disease.
In order to control leptospirosis in humans, swimming in stock ponds or slow-moving streams frequented by domestic and wild animals should be avoided. If a worker is in a high-risk occupation, protective gloves and boots should be worn. In domestic animals, strict adherence to disease preventative management procedures will significantly reduce the dissemination of leptospiral infections. Domestic animals should be fenced away from ponds, marshes and streams that may be contaminated by leptospires. Control in wildlife is not feasible due to the multiplicity of serotypes, the broad range of susceptible hosts, and the extended carrier state of the disease. It is highly unlikely that leptospirosis can be eradicated from wild mammals.
The significance of leptospirosis in wildlife species that appear to be susceptible to Leptospira infections (white-tailed deer, raccoon, striped skunk, red fox, gray fox, opossums, rats and mice) is that they serve as reservoir hosts only. Even this status is not clear because very few leptospirosis outbreaks in humans and domestic animals have implicated wildlife species other than rodents (rats and mice). In Michigan, we are concerned primarily with the likelihood of transmission from white-tailed deer to cattle and humans. Investigations have revealed lower prevalences of leptospiral antibodies in deer than cattle; much lower rates of organism isolation from deer than cattle; very few deer with clinical disease; and differences in the frequencies of similar serovars between deer and cattle. While these results indicate that white-tailed deer are exposed to Leptospira organisms, the deer play an insignificant role in the transmission of bovine leptospirosis.
Human cases are more common in summer and early autumn (probably due to contact with contaminated water) and in individuals that, either by choice or occupation, have frequent contact with infected animals or contaminated environments. No human cases of leptospirosis have been traced to contact with deer, and the risk of infection from free-ranging, wild animals is virtually non-existent.
For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.