Leptospirosis: Current Insights and Practical Approaches for Veterinarians

Pathophysiology and Multi-Organ Manifestations

Leptospirosis is a zoonotic bacterial infection caused by spirochaetes of the genus Leptospira. Infection occurs via mucous membranes or abraded skin when exposed to contaminated urine, soil, or water. Rodents are a primary reservoir, and all dogs are at risk regardless of location or lifestyle. Leptospires invade host tissues, weaken endothelial barriers, and evade immune responses. Clinical complications include acute kidney injury (AKI) (95%), cholestatic hepatopathy (92%) and leptospiral pulmonary hemorrhage syndrome (LPHS) (58%). Other complications include vasculitis, myocarditis, coagulopathy, uveitis, myositis, enteritis, and pancreatitis. All dogs are at risk regardless of signalment, geographic location, and time of year.

Diagnosis

Clinical signs of leptospirosis in dogs are initially nonspecific, presenting as a febrile illness that can progress to renal and hepatic dysfunction, pulmonary involvement, and coagulopathy. Laboratory findings commonly include neutrophilia, thrombocytopenia, and non-regenerative anemia on a complete blood count. Chemistry panels may reveal azotemia, cholestatic hepatopathy with hyperbilirubinemia, and mild to moderate hypoalbuminemia. Urinalysis findings often show isosthenuria, renal glucosuria, and proteinuria. Coagulation testing may indicate prolonged Prothrombin Time/Partial Thromboplastin Time (PT/PTT) or hypercoagulability.

Imaging plays a crucial role in diagnosis and disease monitoring. Radiographs should be performed even in the absence of respiratory signs, as dogs with leptospirosis may have underlying pulmonary pathology, such as leptospirosis-associated pulmonary hemorrhage syndrome (LPHS), vasculitis, or aspiration pneumonia, in which imaging abnormalities could precede clinical signs. Identifying pulmonary changes aids in diagnosing LPHS, assessing fluid therapy, and monitoring respiratory function.

While no pathognomonic ultrasound findings exist for leptospirosis, a normal ultrasound does not rule out the disease. Furthermore, ultrasound is useful in excluding other causes of azotemia.

Diagnostic testing includes serology, PCR, and rapid tests. In serological testing using the microscopic agglutination test (MAT), infection is suggested when titers reach ≥1:800, and a fourfold increase confirms the diagnosis. Testing for multiple serovars increases sensitivity, but it does not identify the infecting serovar. It should be noted that vaccination can influence MAT results.

PCR testing, which detects both viable and non-viable organisms, is the preferred method for early diagnosis of leptospirosis, and testing both blood and urine enhances sensitivity. Recent antibiotic administration may cause false negatives; however, vaccination does not interfere with results.

Rapid tests, such as SNAP Lepto and WITNESS Lepto, serve as useful screening tools by detecting antibodies. These tests may yield negative results early in the disease process and do require additional diagnostics for confirmation. As mentioned above, a confirmatory diagnosis of leptospirosis is achieved through a fourfold or greater rise in Leptospira agglutination titers at a single laboratory, detection of pathogenic Leptospira via PCR in blood or isolation of Leptospira from a clinical specimen by a reference laboratory, though the latter is not clinically practical.

Antimicrobial Therapy

There is no universally optimal antibiotic regimen for the treatment of canine leptospirosis. For dogs that are eating well, the first-line treatment is doxycycline at a dose of 5 mg/kg orally every 12 hours for two weeks to prevent intra-renal persistence of the bacteria. In dogs not eating, ampicillin is administered intravenously at 20 to 30 mg/kg every six to eight hours to suppress bacteremia. If a urine culture is pending, Unasyn at 30 mg/kg intravenously every eight hours is recommended until the urine culture returns negative. In cases of severe acute kidney injury (AKI) classified by the International Renal Interest Society (IRIS) as Grade IV, where creatinine levels exceed 5 mg/dL, doubling the administration interval of Unasyn is advised. Early and aggressive treatment is crucial to prevent complications and improve outcomes.

Supportive Care

Supportive care for canine leptospirosis includes fluid therapy tailored to the dog’s hydration status, urine output, and respiratory function. Dogs that are anuric, oliguric, have vasculitis or have severe hypoalbuminemia warrant extreme caution with fluid therapy due to the concern for possible fluid overload, and they should be referred to a tertiary facility. Dogs with leptospirosis can become extremely polyuric and require large fluid volumes to maintain hydration.

Supportive care involves gastrointestinal support, including antiemetics and gastroprotectants. If necessary, nutritional support should be provided via enteral feeding to maintain adequate caloric intake. Intensive monitoring of respiratory rate, blood pressure, electrolytes, complete blood count, and chemistry panels is crucial for the early detection of complications. In severe cases, advanced therapies, such as mechanical ventilation and dialysis, may be required to manage respiratory distress and acute kidney injury effectively.

When to Refer

Early dialytic intervention is recommended to prevent morbidity associated with acute kidney injury (AKI) rather than being used as a delayed salvage measure. Dialysis does not directly repair the kidneys but helps manage the clinical manifestations of uremia while allowing time for renal recovery after antibiotic therapy. Some dogs require multiple dialysis treatments, and a subset will not recover despite intervention. Referral for hemodialysis is indicated in dogs with IRIS AKI Grade 4, which is characterized by a serum creatinine level greater than 5 mg/dL. Traditionally, other indications for dialysis include severe acidosis, electrolyte disturbances, intoxication, fluid overload, and severe uremia. If there is a concern for declining respiratory status, severe hyperbilirubinemia, or progressive multisystemic disease, these patients should also be referred.

Prognosis

The literature documents several survival rates for dogs with leptospirosis. As seen in a 2017 evaluation by Knöpfler et al., dogs treated conservatively had a 60% survival rate to discharge.⁵ However, according to the 2024 study by Ioannou et al., dogs receiving dialysis had a higher survival rate of 73%.⁴ It is possible that if the dogs treated conservatively had had access to renal replacement therapies, their prognosis may have been different.

The Ioannou et al. study also found that among dialysis survivors, 75% of dogs that survived to discharge were alive at long-term follow-up (≥6 months). However, 50% of those long-term survivors developed chronic kidney disease (CKD), requiring ongoing monitoring. Notably, the degree of azotemia was not associated with survival to discharge, but it was seen that the presence of multisystemic disease and severe hyperbilirubinemia may indicate a worse prognosis. Owners, therefore, should be warned about the possibility of CKD in these patients.

Vaccination

Leptospirosis vaccination is now a core vaccine. Annual vaccination is recommended for all dogs starting at 12 weeks of age, with priority given to those in endemic areas. If a dog was previously vaccinated but is no longer up to date, then the dog will need to have an initial vaccine followed by a booster. Dogs recovering from leptospirosis should be vaccinated as soon as possible after recovery as there are multiple infecting serovars, and it is unknown how long immunity lasts following natural infection.

While vaccination significantly reduces the risk of disease, infection can still occur in vaccinated dogs. Notably, in the study by Ioannou et al. on dogs with leptospirosis requiring hemodialysis, none of the affected dogs were fully vaccinated against the disease.

Conclusion

Canine leptospirosis remains a significant veterinary and public health concern. Early diagnosis, aggressive treatment, and comprehensive prevention strategies are critical in improving patient outcomes and reducing zoonotic risk. Early dialytic intervention is now recommended to reduce mortality. The leptospirosis vaccine is now a core vaccine. Ongoing research and updated American College of Veterinary Internal Medicine (ACVIM) guidelines provide valuable insights for managing this complex disease.

References

1. Daroz BB, Fernandes LGV, Cavenague MF, et al. “A review on host-Leptospira interactions: what we know and future expectations.” Front Cell Infect Microbiol. 2021; 11:777709.
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4. Ioannou ADF, Tai C, Labato MA, Butty EM. “Retrospective evaluation of 22 dogs with leptospirosis treated with extracorporeal renal replacement therapies (2018-2021).” J Vet Intern Med. 2024 Mar-Apr;38(2):1051-1059. doi: 10.1111/jvim.16998.
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9. Rimer D, Chen H, Bar-Nathan M, et al. “Acute kidney injury in dogs: etiology, clinical and clinicopathologic findings, prognostic markers, and outcome.” J Vet Intern Med. 2022; 36(2): 609-618.