Canine Leptospirosis: Clinical Presentation, Diagnosis, and Zoonotic Risk

Canine leptospirosis is a globally distributed bacterial zoonosis caused by pathogenic spirochetes of the genus Leptospira. The disease presents a diagnostic challenge due to its variable clinical manifestations and requires a multi-modal laboratory approach for confirmation. Understanding the clinical spectrum, the performance characteristics of diagnostic assays, and the epidemiological context of circulating serovars is essential for effective case management and zoonotic risk mitigation. This article provides a detailed reference on the clinical presentation, diagnostic methodology, serovar prevalence, and One Health implications of canine leptospirosis.

Etiology and Pathogenesis

Pathogenic Leptospira species, primarily L. interrogans sensu stricto and L. kirschneri, are maintained in the environment by chronic renal carriage in reservoir hosts. Dogs are both accidental and maintenance hosts depending on the infecting serovar [1, 2]. The spirochete enters the host through mucous membranes or abraded skin, followed by a leptospiremic phase characterized by widespread endothelial damage and multi-organ tropism. The renal tubules are a primary target, and leptospiruria can persist for weeks to months after clinical resolution. The organism's motility, mediated by periplasmic flagella, facilitates rapid dissemination through connective tissues and penetration of tight junctions in renal epithelial cells. Lipopolysaccharide (LPS) and outer membrane proteins such as Loa22 trigger a potent innate immune response, including activation of the cGAS-STING pathway which drives type I interferon production and influences renal colonization dynamics [3]. The balance between host immune clearance and bacterial immune evasion determines the outcome of infection, ranging from acute systemic disease to subclinical renal carriage.

Clinical Presentation in Dogs

The clinical signs of canine leptospirosis exist on a continuum from peracute shock to chronic kidney disease. The incubation period is typically 5 to 14 days following exposure.

Acute Systemic Disease

Acute leptospirosis presents with rapid onset of fever, lethargy, myalgia, and anorexia. Affected dogs may exhibit shivering, reluctance to move, and marked muscle tenderness. Pyrexia is often biphasic, corresponding to the initial leptospiremic phase and subsequent immune-complex deposition. Vomiting and diarrhea may be present, sometimes with hematochezia. Vascular injury leads to petechiation, ecchymoses, and epistaxis in severe cases. Hepatic involvement manifests as icterus, hepatomegaly, and elevations in liver enzyme activities. Renal involvement produces polyuria, polydipsia, oliguria, or anuria, depending on the severity of acute kidney injury (AKI).

A prospective clinical study of 10 dogs with leptospirosis documented serial changes in pulmonary function and structure, demonstrating that pulmonary involvement is more common than previously recognized [4]. Affected dogs developed cough, tachypnea, and hypoxemia attributable to pulmonary hemorrhage and noncardiogenic pulmonary edema. Thoracic radiography revealed diffuse interstitial to alveolar patterns, and computed tomography identified ground-glass opacities and dependent consolidation.

Chronic and Subclinical Manifestations

Dogs that survive the acute phase may develop chronic tubulointerstitial nephritis characterized by progressive azotemia, proteinuria, and inability to concentrate urine. Subtle clinical signs such as weight loss, poor coat quality, and intermittent vomiting may be attributed to other causes if leptospirosis is not considered. Subclinical renal carriage is a significant concern because infected dogs shed leptospires in urine without overt illness, posing a zoonotic risk to household members [5, 6]. Serologic surveys indicate that a substantial proportion of apparently healthy dogs in endemic regions have antibodies to pathogenic Leptospira, confirming the occurrence of subclinical infections [7, 8, 9].

Diagnostic Methods

Diagnosis of canine leptospirosis relies on a combination of serology and direct pathogen detection. No single test has perfect sensitivity and specificity across all stages of disease, and interpretation requires integration of clinical findings, vaccination history, and laboratory results.

Microscopic Agglutination Test (MAT)

The microscopic agglutination test (MAT) remains the reference standard for serodiagnosis and serovar identification. The assay detects agglutinating antibodies against a panel of live or formalin-killed Leptospira serovars representing the major serogroups. A single acute-phase titer of 1:800 or greater in a clinically compatible case, or a four-fold rise in paired acute and convalescent sera, is considered diagnostic. Limitations include cross-reactivity between serogroups, an inability to distinguish vaccine-induced antibodies from natural infection, and the requirement for specialized laboratory capacity with maintenance of live cultures.

Enzyme-Linked Immunosorbent Assay (ELISA) and Lateral Flow Assays

ELISA platforms targeting IgM and IgG antibodies offer higher throughput and do not require live cultures. An IgM-positive result in a clinically appropriate setting supports recent or active infection. A recombinant Loa22-gold nanoparticle based lateral flow assay has been evaluated for serodiagnosis in canine and bovine samples, showing promise as a rapid point-of-care test [10]. However, the sensitivity of such assays relative to MAT varies across serovars and disease stages. Acute-phase serum samples may yield false-negative results due to the lag between infection and seroconversion. Commercial ELISA kits are widely available and are often used as screening tests prior to MAT confirmation. For a detailed description of ELISA methodology applied to serologic diagnostics in companion animals, the reader is referred to the article on Enzyme-Linked Immunosorbent Assay (ELISA) for Feline Leukemia Virus. Although that article addresses FeLV, the core immunodiagnostic principles and interpretive considerations are directly applicable to leptospiral serology.

Polymerase Chain Reaction (PCR)

Real-time PCR targeting the lipL32 or secY genes provides direct detection of pathogenic Leptospira DNA in blood, urine, and tissues. PCR is most sensitive during the leptospiremic phase (first 7 to 10 days) when organisms are present in the bloodstream. Urine PCR is valuable for identifying renal shedders, including subclinically infected dogs. Advantages of PCR over MAT include:

• Detection of infection before seroconversion
• No confounding by vaccination
• Ability to differentiate pathogenic from saprophytic species
• Quantification of bacterial load via cycle threshold values

Limitations include the short window for blood PCR positivity in acute disease and intermittent shedding in urine. A positive PCR result confirms infection but does not distinguish between active disease and asymptomatic carriage. Multiplex PCR panels that simultaneously detect leptospires and other canine pathogens are increasingly used for syndromic diagnosis of febrile or kidney disease cases.

Additional Biomarkers

Serum sialic acid has been investigated as a broad biomarker of inflammation and infection in veterinary medicine [11]. Elevated sialic acid concentrations may support the diagnosis of leptospirosis when combined with more specific tests, although the diagnostic utility is not sufficiently discriminatory for standalone use. Other indirect markers such as C-reactive protein, symmetric dimethylarginine (SDMA), and urinalysis findings (proteinuria, granular casts, isosthenuria) provide supportive but nonspecific evidence of leptospiral disease.

Diagnostic Algorithm

flowchart TD
    A["Clinical suspicion: fever, jaundice, AKI, PU/PD, vomiting"], > B["Perform blood: CBC, biochemistry, urinalysis"]
    B, > C{"Serology: IgM ELISA or MAT"}
    B, > D{"Blood PCR (acute phase)"}
    C, > E["Positive IgM or MAT ≥1:800"]
    D, > F["Positive blood PCR"]
    E, > G["Confirm with paired MAT if available"]
    F, > H["Urine PCR to rule out carriage"]
    G, > I["Diagnosis: acute leptospirosis"]
    H, > I
    C, > J["Negative or low titer (non-vaccinated)"]
    D, > K["Negative blood PCR"]
    J, > K
    K, > L["Repeat serology and urine PCR in 7-14 days"]
    L, > M["Seroconversion or positive urine PCR?"]
    M, >|"Yes"| I
    M, >|"No"| N["Consider alternative diagnosis"]
    I, > O["Initiate supportive care and antimicrobial therapy"]
    O, > P["Urine PCR at 4 weeks post-treatment"]
    P, > Q["Negative urine PCR: clearance confirmed"]
    P, > R["Positive urine PCR: extend treatment and recheck"]

Serovar Prevalence and Geographic Variation

The distribution of Leptospira serovars in canine populations varies considerably by geographic region, climate, and urbanization. Knowledge of local serovar prevalence informs vaccine selection and diagnostic antigen panels.

North America

An outbreak investigation in Los Angeles County, California identified serovars Icterohaemorrhagiae, Pomona, and Grippotyphosa in clinically affected dogs [1]. The study highlighted the role of urban wildlife, including rats and opossums, as reservoir hosts. In the same geographic region, serovars previously considered rare have been detected with increasing frequency, suggesting shifting epidemiologic patterns.

Asia

A molecular characterization of Leptospira in dogs from Thailand identified L. interrogans serovars Australis and Bataviae as predominant, with L. kirschneri also detected [2]. In the Yangtze River region of China, a seroprevalence study reported that 24.5% of dogs had MAT titers against at least one serovar, with Canicola and Icterohaemorrhagiae being the most common [9]. A systematic review and meta-analysis of canine leptospirosis in China further emphasized the association between canine seroprevalence and human case incidence, particularly in rural agricultural settings [5].

South America

A study in the Fulni-o Indigenous community in Pernambuco, Brazil, found a canine seroprevalence of 15.2%, with Icterohaemorrhagiae and Cynopteri being the dominant serogroups [7]. In Northern Colombia, molecular surveillance using PCR detected Leptospira DNA in blood and urine from 12.3% of domestic dogs, many of which were clinically asymptomatic [8]. These findings underscore the role of dogs as sentinels for environmental contamination.

Europe and Australia

Environmental and meteorological factors, including rainfall and temperature, are major drivers of leptospirosis incidence in domestic dog populations [12]. A study of contact dogs exposed to clinical canine leptospirosis cases in Australia found that seroconversion occurred in 30% of in-contact dogs within the same household, highlighting the importance of within-home transmission and shared environmental exposure [6].

Comparative Genomics and Host Range

Genomic comparison of L. interrogans isolates from humans, dogs, and wild animals in Japan revealed high genetic conservation across hosts, supporting the absence of strict host adaptation and the potential for cross-species transmission [13]. The presence of shared core genome sequences and conserved virulence factors across isolates from different species reinforces the One Health relevance of canine leptospirosis as a sentinel event for human risk.

One Health Implications and Zoonotic Risk

Dogs infected with pathogenic Leptospira serve as both sentinels and potential sources of infection for humans. The zoonotic risk is greatest when dogs are subclinical shedders, as owners and veterinary personnel may handle urine- contaminated materials without appropriate barrier precautions.

Transmission Pathways

Leptospires are shed in the urine of infected dogs and can survive for weeks in moist environments such as standing water, mud, and soil. Humans typically acquire infection through contact with contaminated water or soil, with the organism entering via mucous membranes or breaks in the skin. Occupational and recreational activities that involve exposure to stagnant water, including farming, gardening, and water sports, increase the risk of infection in areas where canine leptospirosis is endemic.

One Health Surveillance

Integration of canine serosurveillance data with human case reporting allows for early detection of environmental contamination and emerging serovars. The ecological study of leptospiral interaction in bovine farms in rural Colombia, which included canine sampling, demonstrated that dogs are effective sentinels for identifying high-risk areas [14]. Similarly, the systematic review of human and canine leptospirosis in China found a positive correlation between canine seroprevalence and human incidence at the provincial level, supporting the utility of canine data for public health risk mapping [5].

Prevention and Veterinary Public Health

Vaccination against leptospirosis reduces the severity of disease and limits renal colonization, thereby decreasing the risk of zoonotic transmission. Currently available bacterin vaccines cover a limited number of serovars, and breakthrough infections with non-vaccine serovars occur [1]. Strict hygiene protocols, including the use of gloves when handling urine-contaminated materials and disinfection of kennel surfaces with bleach or quaternary ammonium compounds, are essential for reducing occupational exposure in veterinary clinics and shelters.

Conclusions

Canine leptospirosis remains a challenging infectious disease due to its protean clinical manifestations, the limitations of available diagnostic tests, and the potential for zoonotic transmission. Acute disease is characterized by fever, AKI, hepatic injury, and pulmonary hemorrhage. Chronic infection may be clinically silent but perpetuates environmental contamination. Diagnosis requires a strategic combination of serology (MAT or ELISA) and PCR on blood and urine, interpreted in light of vaccination history and local serovar prevalence. The geographic distribution of serovars is dynamic, with urban and peri-urban wildlife serving as important reservoir hosts. A One Health approach that integrates veterinary diagnostic surveillance with public health monitoring is essential for reducing the dual burden of disease in dogs and humans.

References

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