Leptospirosis is caused by the spirochete bacteria Leptospira interrogans sensu lato. More than 150 distinct serovars have been identified, and eight are known to infect dogs. Leptospira serovars infect 160
mammalian species. Maintenance hosts are carrier animals that provide a reservoir for the bacteria. Incidental hosts are other
animals that have a more severe clinical illness and a briefer period of shedding. Maintenance hosts for the different serovars
include dogs (canicola); rats (icterohemorrhagiae); raccoons, skunks and opossums (grippotyphosa); cattle and pigs (pomona);
pigs (bratislava); cattle (hardjo); and mice (ballum).
The most common serovars infecting dogs in the United States used to be icterohemorrhagiae and canicola. After widespread
use of vaccines containing these serovars, the incidence of classic leptospirosis caused by these serovars decreased. There
is no cross-protection against other serovars, and in recent years grippotyphosa, pomona, autumnalis and bratislava have emerged
as more common pathogens causing leptospirosis in dogs.
Animals become infected with leptospirosis through direct contact with infected urine, ingestion of infected tissues or exposure
from animal bites. Indirect transmission occurs through contact with contaminated water, soil, bedding or food. Most cases
of leptospirosis occur in late summer and early fall, especially after heavy rains or flooding. Exposure to stagnant water
contaminated by wildlife urine appears to be a major source of infection for dogs.
The spirochete bacteria penetrate abraded skin or mucous membranes and multiply in the blood and other tissues, causing hepatic
and/or renal disease and vasculitis. Herding dogs, hounds, working dogs and male dogs are at increased risk. Serovar icterohemorrhagiae
causes icterus, liver failure, uremia and pyrexia. The canicola serovar causes acute nephritis with less hepatic involvement,
and serovars pomona, bratislava, autumnalis and grippotyphosa cause acute renal failure without hepatic insufficiency.
Clinical signs of leptospirosis include lethargy, depression, anorexia, vomiting, dehydration and fever. Renomegaly and abdominal
pain also are common. Some dogs have CNS involvement, including neck pain, ataxia or seizures. Icterus may be present in dogs
with hepatic involvement.
Leukocytosis and thrombocytopenia are the most common abnormalities on the hemogram. Abnormalities in serum chemistries include
azotemia, hyperphosphatemia, metabolic acidosis and elevated liver enzymes. Vomiting often causes hyponatremia, hypochloremia
and hypokalemia, but hyperkalemia may be present in severe cases with acute renal failure. Urinalysis reveals casts, renal
tubular epithelial cells, blood cells, glucosuria, proteinuria and bilirubinuria. Specific gravity is usually <1.020. Darkfield
microscopy of the urine may reveal spirochetes, but these cannot be differentiated from Borrelia spirochetes. Because of periodic
shedding, a negative darkfield microscopy does not rule out leptospirosis.
Thoracic radiographs may reveal a reticulonodular pattern with a caudo-dorsal distribution. These pulmonary changes are thought
to be secondary to vasculitis and/or pulmonary hemorrhage. Abdominal ultrasound study often reveals bilateral renomegaly with
hyperechogenicity of the renal cortices. There may be a medullary band of hyperechogenicity and dilation of the renal pelvises.
The microscopic agglutination test (MAT) is the most common serologic test employed for diagnosis. Multiple serovars are tested,
and a single titer >1:800 associated with clinical signs is considered diagnostic. There is some cross reaction between serovars,
and when several are positive, the one with the highest titer is considered to be the infective agent. Vaccination titers
usually are low, ranging from 1:100 to 1:400. In acute disease, titers may be negative for seven to 10 days, and retesting
in two to four weeks is recommended. Antibiotics will lower the magnitude of the titer.
An ELISA test is available for IgM and IgG antibodies, but does not delineate serovars or vaccine titers. The IgM titer is
increased seven to 14 days post-infection, and the IgG titer is elevated two to three weeks post-infection.
A PCR test is available through some laboratories. It is especially useful for identifying early infections and carriers,
and is considered sensitive and specific. Other methods of diagnosis include silver stains or immunohistochemistry of renal
or hepatic biopsies with identification of the spirochetes on histopathologic sampling.