Clinical diagnosis of leptospirosis typically is made subsequent to the development of nephritis and/or hepatitis. While there are subtle differences in clinical signs caused by different serovars, in general, all serovars cause renal and liver disease. Once the organism has spread systemically, a variety of clinical signs can occur, including: fever, vasculitis, uveitis with uveal hemorrhage, myositis, and meningitis. Sub-clinical infection is quite common, and leptospira spp. can cause chronic renal failure.

Leptospirosis titers must be interpreted in light of the patient's clinical signs and vaccination history. For healthy vaccinated dogs, leptospira vaccine titers are generally low (ó 1: 400), and will be against the serovars present in the vaccine used. Titers ò 1: 400 in healthy dogs may be due to recent vaccination (within the previous 2 months); Dr. Savelle from Fort Dodge Animal Health indicated that vaccine titers can be very high [up to 1:6,000] during the weeks subsequent to leptospira booster vaccinations.

In dogs having had previous or current infection with leptospira spp. as well as clinical signs of leptospirosis, titers ò 1:800 not attributable to vaccination are considered presumptive evidence of infection. But, titers may be low during the acute stage of infection.

Dogs with clinical signs of leptospirosis should have titers re-checked after 1 to 2 weeks for evidence of rising titers. If an infected dog is clinically ill before treatment is started, titers will invariably increase even in the face of treatment. On the other hand, if treatment is started before any clinical signs of illness are present, titer increases may be blunted.

With infections from different serovars, certain titer patterns are recognized: for L. grippotyphosa, this serovar generates the highest titer, with lower titers to L.pomona and L.bratislava; for L.pomona infection, this titer is very high, with some cross-reaction to L. grippotyphosa and L. bratislava.

Checking titers to monitor the success of treatment is not recommended. If titers are being rechecked, it's best to wait 6 mo after treatment.