Cause, hosts and risk
Leptospirosis is caused by pathogenic spirochetes of the genus Leptospira. According to the ACVIM statement, most canine disease comes from the high-virulence species L. interrogans, L. borgpetersenii and L. kirschneri [1]. Dogs are infected when mucous membranes or damaged skin contact organisms shed in the urine of reservoir hosts. Rodents, especially Rattus norvegicus, are the most important reservoirs worldwide [1]. The organisms can persist in soil and water for weeks to months [1][4]. Merck notes that survival is brief in dry soil or outside roughly 10 to 34 degrees C [3].
The panel describes cases in small-breed urban dogs, puppies as young as 11 weeks, geriatric dogs and rural dogs. It also notes outbreaks tied to daycare and kennel settings in semi-arid regions [1]. Backyard poultry, composting and hoarding can raise rodent exposure [1]. The CDC adds dog parks and boarding or training facilities as higher-risk contact settings [4].
The sources differ on serovar Canicola. Merck describes the dog as its maintenance host [2]. The 2023 ACVIM panel says dogs "should not be assumed to be the reservoir host" when Canicola is suspected, because rodents, horses and pigs have also been identified as sources [1].
Clinical presentation
The ACVIM statement describes leptospirosis as multisystemic. Acute tubulointerstitial nephritis and cholestatic liver dysfunction are the most prominent features, and they usually occur together [1]. The early febrile phase is often nonspecific, with inappetence, lethargy, vomiting, polyuria and polydipsia, or fever. The panel advises that owners of such dogs be told to return for re-examination and bloodwork if the dog has not improved within 24 hours [1]. Merck describes acute kidney injury as the most common presentation in recent years. It adds that some dogs show polyuria and polydipsia without azotemia [2].
Other manifestations listed in the consensus include leptospiral pulmonary hemorrhage syndrome (LPHS), coagulopathy, vasculitis, pancreatitis, uveitis, myocarditis and myositis [1]. Commonly reported laboratory findings include:
| Finding | Reported frequency in dogs with leptospirosis [1] |
|---|
| Thrombocytopenia | 14%-73% |
| Increased creatinine | 55%-100% |
| Increased ALP | 19%-94% |
| Hyperbilirubinemia | 15%-94% |
| Renal glucosuria | 18%-83% |
| Proteinuria | 28%-81% |
The panel recommends thoracic radiographs for all suspected dogs, even without respiratory signs. Findings consistent with LPHS support the diagnosis and call for careful fluid therapy [1]. The panel also notes that marked radiographic changes can appear before respiratory compromise is recognizable [1].
Diagnostic testing
The panel recommends combining serologic tests with organism detection tests, because every available test has limitations [1].
MAT serology
The ACVIM statement calls acute and convalescent MAT "the reference standard test." It notes that veterinary laboratories typically test only 6 to 7 serovars [1]. Titers are often negative or low in the first week of illness, and early antimicrobials may blunt the antibody response. The sensitivity of a single acute titer above 1:800 has been estimated at only 50% [1]. In the other direction, the panel says a single positive titer lacks specificity unless very high (above 1:3200), because vaccination or subclinical exposure can produce titers [1]. Merck takes a somewhat less strict view. The Merck Veterinary Manual states that in a dog with a compatible history, vaccinated more than 3 months earlier, "a single titer of 1:800 to 1:1,600 is good presumptive evidence of acute leptospirosis" [3]. Both sources agree that MAT results cannot reliably identify the infecting serovar [1][3].
Point-of-care antibody tests
The consensus summarizes field studies of two patient-side assays. Reported sensitivities ranged from 68% to 79% across studies in the northeastern US, Germany, Italy and Switzerland, with sample sizes of 22 to 42 affected dogs [1]. Reported specificity for the WITNESS IgM assay was above 97%. The ACVIM panel states that "a positive IgM result in an unvaccinated dog with signs consistent with leptospirosis strongly supports a diagnosis of leptospirosis" [1]. The panel cautions that performance may vary regionally with circulating strains and that more validation is needed. For that reason, it does not yet recommend these assays over MAT. The ACVIM panel recommends that a negative result "should be followed with a second antibody test, 7 to 14 days later" [1].
NAAT on blood and urine
According to the ACVIM statement, NAAT can detect leptospiral DNA before antibodies appear. The panel recommends submitting both blood (in EDTA, because heparin can interfere) and urine collected before antibiotics [1]. In one cited study, PCR sensitivity in dogs was 25% on blood and 69% on urine, with 100% specificity for both sample types. A separate study, using a different PCR assay applied to blood, urine or both, found 21% sensitivity in 42 dogs [1]. The panel notes that vaccination with inactivated vaccines should not cause positive PCR results [1].
Case definition and severity grading
The ACVIM case definition is modeled on the CDC human definition. A probable case meets the clinical criteria plus at least one supportive laboratory criterion. A confirmed case meets the clinical criteria plus at least one confirmatory criterion [1]. The criteria include:
- Clinical criteria: onset of systemic illness within the past 2 weeks, plus two or more clinicopathologic abnormalities from a defined list [1].
- Supportive laboratory criteria: an MAT titer of 800 or more, IgM detection, or positive urine NAAT [1].
- Confirmatory laboratory criteria: a fourfold or greater titer rise at a single laboratory, positive blood NAAT, or isolation of the organism by a reference laboratory [1].
The panel uses the International Renal Interest Society (IRIS) acute kidney injury grades to guide some decisions. At AKI Grade 4 and above, defined in the statement as serum creatinine above 5 mg/dL, the panel recommends doubling penicillin dosing intervals and considering early referral for extracorporeal kidney support such as hemodialysis [1].
Treatment, monitoring and prognosis
The panel recommends antimicrobials for suspected or probable cases. It acknowledges that the optimal drug and duration are unknown and that clear benefit has not been shown in humans [1]. If vomiting or other adverse effects prevent oral doxycycline, the ACVIM panel lists three initial IV options: ampicillin ("20-30 mg/kg IV q6-8h"), amoxicillin ("20-30 mg/kg IV q6-8h") or penicillin G ("25 000-40 000 U/kg IV q6-8h") [1]. Merck states that a penicillin course should be followed by 2 weeks of doxycycline to clear the renal carrier phase. Merck also lists enrofloxacin or clarithromycin as possible carrier-phase options [2]. The ACVIM panel, by contrast, describes other antimicrobials as either less effective at clearing kidney tissue in rodent models or reserved as critical drugs in some countries [1].
For supportive care, the consensus stresses avoiding iatrogenic fluid overload, especially with pulmonary involvement. The panel notes that markedly polyuric dogs may need fluids above 20 mL/kg/h [1]. It describes opioids as usually appropriate for pain and does not recommend NSAIDs [1]. It also recommends a highly digestible diet with normal-to-high protein rather than a renal diet [1]. The ACVIM panel states that "more evidence is needed before supplements such as S-adenosyl methionine, silymarin, vitamin E, or ursodeoxycholic acid can be recommended for treatment of leptospirosis" [1].
The panel suggests considering a serum chemistry panel every 24 hours during hospitalization, PCV every 24 hours and a CBC every 48 hours. It recommends re-examination no later than 1 week after discharge [1]. According to the statement, azotemia typically returns to reference range within 10 to 14 days, bilirubin falls more slowly than ALT and ALP, and a delay in C-reactive protein normalization justifies re-evaluation for complications [1].
On prognosis, the panel names hyperbilirubinemia and hypocoagulability as negative factors in dogs. It cites a study of 254 dogs in which a serum bilirubin of at least 0.6 mg/dL was strongly associated with death or euthanasia (odds ratio 16.4) [1]. LPHS has been associated with mortality of up to 70%, although the panel notes that milder forms exist [1]. The panel cites a study of 36 dogs that failed conservative management and then received extracorporeal therapy, in which more than 80% recovered. The statement does not give further design details for that study [1].
Vaccination and prevention
According to the ACVIM statement, North American bacterins cover serovars Icterohaemorrhagiae, Canicola, Grippotyphosa and Pomona. European quadrivalent vaccines substitute Australis for Pomona [1]. Immunity is serogroup-specific, and the panel notes that disease has occurred in fully vaccinated dogs but appears uncommon [1]. A cited meta-analysis of 21 vaccines found more than 80% protection. The panel cautions that the trials used high-dose intraperitoneal challenge, which does not mimic natural exposure [1].
The panel recommends that vaccines "be administered annually to all dogs starting at 12 weeks of age, regardless of breed" [1]. It further recommends:
- Two initial injections 4 weeks apart [1].
- Initial and booster doses 3 weeks apart for shelter dogs at intake [1].
- Required vaccination at boarding and daycare facilities [1].
- The broadest serovar coverage available when regional culture data are lacking [1].
- Vaccination as soon as possible after recovery from the disease [1].
The panel cites more recent research indicating that severe adverse event rates are not higher for leptospirosis vaccines than for distemper-parvovirus or rabies vaccines [1]. Merck likewise describes the vaccine as safe in most cases [2]. No leptospiral vaccine is currently approved for cats [1], and the CDC notes that no vaccine for cats is available in the United States [4].
Zoonotic risk and client communication
The ACVIM panel judges the risk of transmission from sick dogs to people to be low when basic precautions are taken [1]. During the first 48 hours of treatment, the panel advises minimizing patient movement, labeling cages and keeping pregnant staff away from these patients [1]. For dogs that need urinary catheterization, it recommends indwelling rather than intermittent catheters to reduce staff exposure to urine [1]. It states that an isolation ward is not required and could compromise care [1]. The panel describes inactivating large urine volumes with a "1:1 dilution with 10% bleach (5000 ppm sodium hypochlorite)" made fresh daily [1]. For household surfaces, the CDC's general guidance is "1 part household bleach in 10 parts water" [4].
For clients, the panel recommends that owners avoid contact with the dog's urine, and wear gloves and eye protection when cleaning it up, until 48 hours of doxycycline have been given. Owners should also be told to contact their own physician with questions [1]. The CDC notes that an animal that is not fully treated may shed bacteria in urine for up to 3 months [4]. After a risk assessment, the panel suggests considering prophylactic doxycycline at 5 mg/kg q12h PO for 14 days for other dogs in the household [1].
