Leptospirosis is still an important infectious disease of dogs, despite the fact that vaccines have been around for more than 30 years. It has a worldwide distribution and is caused by several distinct serovars (subtypes) of the organism Leptospira interrogans. The organism is a spiral shaped bacterium with hook shaped ends that makes a characteristic writhing and flexing movement while moving. There are many distinct subtypes of the leptospirosis organism, although five in particular are responsible for most cases of disease in dogs. Dogs contract leptospirosis either by direct contact with infected urine, bite wounds, eating infected tissue or during birth.

Wild animals like skunks, raccoons and opossums are a major source of infection, although pigs, rats and other animals may harbor the organism and serve as reservoirs of infection. Direct spread is enhanced by crowding of animals, such as in kennel situations. Indirect transmission can occur if susceptible animals are exposed to contaminated food, soil, water or bedding. Stagnant or slow-moving warm water provides an excellent habitat for the organism. This explains the increase in cases during periods of flooding.

Once the organism penetrates the body, it enters the bloodstream and multiplies rapidly and causes the blood vessels to become inflamed. The organism penetrates through the inflamed vessels and invades other organs like the kidneys, liver, spleen, eyes and reproductive tract. The body makes an immune response and eliminates the organism from most organs. Unfortunately, organisms tend to persist in the kidneys and can be released into the urine for several weeks or months. When the organism invades the kidney, sudden impairment of kidney function may result. Depending on how virulent, or infectious, the particular serovar of the organism is, and how strong an immune response the dog mounts, the damage can be mild or severe. Some serovars can cause sudden hemorrhage, liver damage, and most commonly, kidney damage.

Leptospirosis is a zoonotic disease and can be transmitted to humans. The Centers for Disease Control has reported an average of 50 to 100 cases per year for the past 20 years and a few cases come from contact with rats or dogs. The majority of human infections are among those who engage in water sport activities or who experience occupational exposure to wildlife or domestic animal hosts. It has a very low fatality rate in humans.

Diseases causing similar symptoms in dogs include the following:

Other causes of kidney failure

Acute kidney failure

Chronic kidney failure

Antifreeze poisoning

Other causes of liver failure

Infectious hepatitis

Chronic active hepatitis

Liver cancer