Leptospirosis continues to be a significant clinical presence in canine medicine since its resurgence this past decade. In addition to an increased number of cases, more diverse clinical presentations are being recognized. Selection of appropriate vaccines and interpretation of serologic results in the presence of vaccinal titers are emerging issues in clinical practice.

Leptospirosis is caused by a spirochete, a motile spiral-shaped bacteria. Distribution is worldwide. Two species of leptospires are primarily responsible for disease in dogs. Leptospira interrogans includes the serovars bratislava, autumnalis, icterohemorrhagiae, pomona, canicola and hardjo. A second species, Leptospira kirschneri contains the serovar grippoty-phosa. Each serovar has a known reservoir host of one or more species. These reservoir hosts may be subclinically infected and shed organisms for months to years.

When other susceptible species, or incidental hosts, become infected with that serovar they become acutely ill. For example, reservoir hosts for L. kirschneri, serovar grippotyphosa are the vole, raccoon, skunk and opossum. When dogs are infected with this serovar as incidental hosts they become ill. Dogs, however, are the reservoir hosts for L. interrogans, serovar canicola which can cause clinical disease in horses, cows and pigs. Although reservoir hosts may develop illness, disease in incidental hosts is more severe, shedding periods are shorter, and transmission between animals is less likely.

Infection occurs by direct contact with leptospires from infected animals. Infected urine, venereal contact, bite wounds and ingestion of infected tissues are all possible routes of transmission. Indirect spread from contact with contaminated soil and food may also result in infection. The organism may survive for days to months in warm, moist environments.

Seropositive cats have been identified, but little clinical disease has been reported in this species, as cats appear to be relatively resistant to leptospirosis.

Once infected animals become bacteremic, leptospires multiply in the kidney, liver, spleen, central nervous system, ocular tissue and genital tract. In dogs, serovars canicola and grippotyphosa result in more renal dysfunction, whereas serovars icterohemorrhagiae and pomona produce more hepatic damage. Reservoir hosts may be subclinically infected and shed organisms for months to years after recovery. Dogs with protective titers may have mild, inapparent illness or no clinical signs. In susceptible animals, however, the acute phase of illness may result in acute renal failure. Leptospiral toxins result in hepatic necrosis and subsequent cholestasis and hyperbilirubinemia. Vascular endothelial damage during the bacteremic phase can result in disseminated intravascular coagulation and thrombocytopenia. Although hemolysis is uncommon in dogs, a case of refractory hemolytic anemia with positive leptospires serology responded to treatment for leptospirosis. Cattle infected with L. interrogans, serovar pomona frequently develop hemoglobinemia and hemoglobinuria from a hemolytic toxin produced by that serovar.

Although dogs appear to make a full recovery with appropriate treatment, chronic disease can be a sequela of acute leptospirosis. Compensated chronic renal failure may follow acute renal failure. Hepatocellular damage and altered hepatic immune response may lead to chronic hepatitis.

Dogs with leptospirosis commonly present with fever, anorexia, polydipsia, vomiting and dehydration. Hyperesthesia may result from renal, muscle or meningeal pain. Coagulopathies may lead to epistaxis, petechiae, hematemesis or melena. It is important to recognize that leptospirosis may be subclinical, or that it may present as chronic illness. Thus, leptospirosis should be included in the differential diagnosis in dogs with fever of unknown origin or with renal or hepatic disease of unknown etiology. Two dogs presenting for evaluation of polyuria and polydipsia without concurrent signs of illness, one four months old and one middle-aged, tested positive for leptospirosis and responded to treatment. Clinical awareness and appropriate test selection led to a resolution of their disease; but these animals could easily have been misdiagnosed as having congenital renal disease or interstitial nephritis.