Leptospirosis is caused by infection with serovars of Leptospira interrogans sensu lato. Organisms are transmitted by direct contact with infected urine, bite wounds or ingestion of infected tissues, or indirectly,
through contact with infected water, soil, food or bedding. Survival of leptospires is promoted by stagnant warm water, a
neutral or slightly alkaline pH, and temperatures between 0 and 25 degrees celsius. In California, the incidence of leptospirosis
is highest in the late fall and winter.
There are over 150 antigenically-distinct pathogenic serovars, which are grouped into serogroups. Serovars known to affect
dogs include Canicola, Icterohemorrhagiae, Grippotyphosa, Pomona, Ballum, Bratislava, Autumnalis, Bataviae, Australis, and
Hardjo. Each serovar is adapted to a one or more mammalian host species (maintenance hosts). Other hosts act as incidental
hosts. Disease in incidental hosts tends to be more severe and the duration of shedding is generally shorter. Maintenance
hosts include dogs (Canicola); rats (Icterohemorrhagiae); raccoons, skunks, voles and opossums (Grippotyphosa); cattle and
pigs (Pomona); pigs (Bratislava); cattle (Hardjo); and mice (Ballum). The prevalence of infection with a serovar in dogs depends
on the degree of contact between the dog population and the maintenance host for that serovar.
The most common species thought to infect dogs before the introduction of the Leptospira vaccines were Icterohemorrhagiae and Canicola. Vaccines containing only serovars Icterohemorrhagiae and Canicola do not protect
against other serovars. Since introduction of the bivalent bacterins containing these serovars, there have been decreasing
reports of disease due to Canicola and Icterohemorrhagiae, and increasing reports of disease caused by serovars Pomona, Grippotyphosa
and Bratislava. Vaccine pressure, and increasing contact between dogs and skunks, raccoons and opossums, have been suggested
as reasons for this change.
Pathogenic leptospires penetrate abraded skin or mucus membranes and multiply in the bloodstream and tissues, causing hepatic
and/or renal failure and vasculitis. A number of studies have suggested an association between clinical manifestations and
the infecting serovar, although this is not clear cut. It seems likely that infections with serogroup Pomona are associated
with more severe renal failure. Clinical manifestations may also depend on age, outbreak, and geographical location.
Most infections are subclinical. Younger, large breed, outdoor adult dogs are commonly affected. Younger animals tend to be
more severely affected. Males may be predisposed.
Lethargy, anorexia, vomiting, pyrexia, dehydration, abdominal pain and increased thirst are common signs of acute leptospirosis.
Reluctance to move, icterus, and petechial and ecchymotic hemorrhages may be noted.
Leukocytosis, thrombocytopenia, azotemia, hypoalbuminemia and elevated liver enzymes are common. Urinalysis may reveal isosthenuria,
proteinuria, glucosuria and casts. Thoracic radiography may reveal a focal or diffuse interstitial to bronchointerstitial
pattern; alveolar patterns may represent pulmonary hemorrhage. Hepatomegaly, splenomegaly, renomegaly and/or peritoneal effusion
may be evident from abdominal radiography. Hyperechoic renal cortices and mild renal pelvis dilation are occasionally seen
with abdominal ultrasound.
Diagnosis is generally based on serology using the microscopic agglutination test. Respective titers are provided for each of several different serovars, and the results are serogroup specific. The organism
with the highest titer is identified as the infecting one; lower titers represent cross reactions between serogroups. Titers
are usually negative in the first week of illness, and demonstration of a fourfold rise in titer is required over a 1-4 week
interval. Antibiotic therapy early in disease may blunt the rise in titer. Postvaccinal titers against Icterohemorrhagiae,
Canicola, Grippotyphosa and Pomona occasionally rise as high as 1:3200 for a few months after vaccination, and these can interfere
with interpretation. It is important to note that results can vary between laboratories. Use of a laboratory with a high level
of quality control is recommended.
Darkfield microscopy of the urine is not recommended as sole test for diagnosis because of the large number of false positives and false negatives. Silver staining and fluorescent antibody or immunoperoxidase staining of tissue specimens can also yield false negatives, and does not help identify the infecting serovar. Culture is difficult because of the fastidious growth requirements of leptospires. Cultures must be incubated for several weeks.
Multiple sampling may be required due to intermittent shedding. PCR assays are becoming more widely available, and have potential to be rapid, sensitive and specific, but will not provide information
about the infecting serovar.