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    Figure 1—

    Map of the lower peninsula of Michigan illustrating the geographic regions in a study of the prevalence of antibodies against 6 Leptospira serovars in 1,241 healthy dogs in 1999. Values indicate overall percentages and numbers of dogs with positive titers against any of the 6 serovars.

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Prevalence of serum antibodies against six Leptospira serovars in healthy dogs

Jennifer E. StokesDepartment of Small Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824-1314

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John B. KaneeneCenter for Comparative Epidemiology, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824-1314

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William D. SchallDepartment of Small Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824-1314

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John M. KrugerDepartment of Small Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824-1314

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RoseAnn MillerCenter for Comparative Epidemiology, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824-1314

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Lana KaiserDepartment of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824-1314

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Carole A. BolinDepartment of Pathobiology and Diagnostic Investigation, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824-1314

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Abstract

Objective—To determine the prevalence of antibodies against 6 Leptospira serovars and determine risk factors associated with positive Leptospira titers in healthy client-owned dogs in Michigan.

Design—Cross-sectional study.

Animals—1,241 healthy dogs at least 4 months of age.

Procedures—Dogs were examined by veterinarians at private practices. Vaccinated and unvaccinated dogs were enrolled in the study, which occurred prior to the availability of a 4-serovar (Canicola, Grippotyphosa, Icterohaemorrhagiae, and Pomona) Leptospira vaccine. Sera were tested by use of the microscopic agglutination test to determine antibody titers against Leptospira serovars Bratislava, Canicola, Grippotyphosa, Hardjo, Icterohaemorrhagiae, and Pomona. A questionnaire was used to collect demographic information about each dog to identify risk factors associated with seropositive status.

Results—309 of 1,241 (24.9%) dogs had antibody titers against at least 1 of the 6 Leptospira serovars, which suggested exposure to Leptospira spp. Prevalence of antibodies was highest to serovar Grippotyphosa, followed by Bratislava, Canicola, Icterohaemorrhagiae, and Pomona. Age, travel outside Michigan, exercise outside fenced yards, and exposure to livestock and wildlife were significant risk factors for positive titers.

Conclusions and Clinical Relevance—Among healthy dogs from the lower peninsula of Michigan, > 20% have antibodies against leptospiral serovars historically considered uncommon but more recently incriminated as causing clinical canine leptospirosis. Wildlife and livestock may be of increasing importance as reservoirs for canine leptospirosis as urbanization continues to occur. Expanded vaccination strategies may partially mitigate these trends.

Abstract

Objective—To determine the prevalence of antibodies against 6 Leptospira serovars and determine risk factors associated with positive Leptospira titers in healthy client-owned dogs in Michigan.

Design—Cross-sectional study.

Animals—1,241 healthy dogs at least 4 months of age.

Procedures—Dogs were examined by veterinarians at private practices. Vaccinated and unvaccinated dogs were enrolled in the study, which occurred prior to the availability of a 4-serovar (Canicola, Grippotyphosa, Icterohaemorrhagiae, and Pomona) Leptospira vaccine. Sera were tested by use of the microscopic agglutination test to determine antibody titers against Leptospira serovars Bratislava, Canicola, Grippotyphosa, Hardjo, Icterohaemorrhagiae, and Pomona. A questionnaire was used to collect demographic information about each dog to identify risk factors associated with seropositive status.

Results—309 of 1,241 (24.9%) dogs had antibody titers against at least 1 of the 6 Leptospira serovars, which suggested exposure to Leptospira spp. Prevalence of antibodies was highest to serovar Grippotyphosa, followed by Bratislava, Canicola, Icterohaemorrhagiae, and Pomona. Age, travel outside Michigan, exercise outside fenced yards, and exposure to livestock and wildlife were significant risk factors for positive titers.

Conclusions and Clinical Relevance—Among healthy dogs from the lower peninsula of Michigan, > 20% have antibodies against leptospiral serovars historically considered uncommon but more recently incriminated as causing clinical canine leptospirosis. Wildlife and livestock may be of increasing importance as reservoirs for canine leptospirosis as urbanization continues to occur. Expanded vaccination strategies may partially mitigate these trends.

Leptospirosis is a widespread, potentially fatal zoonotic bacterial disease that affects most mammals.1 At least 8 recognized Leptospira spp, comprising approximately 230 serovars, are pathogenic for a number of mammals, including humans. The serovar is the basic taxon of Leptospira organisms, and serovars are classically defined on the basis of surface antigens, although molecular methods to differentiate serovars are also available.2 Detection of antibodies against the surface antigens of the various leptospiral serovars by use of an MAT3 is the most common diagnostic test for leptospirosis.

Pathogenic serovars of Leptospira are typically associated with 1 or more maintenance hosts that serve as reservoirs of infection for incidental hosts. Maintenance hosts may be wildlife or domestic animals including livestock. Within maintenance-host populations, infection rate is high and transmission is efficient but clinical disease is uncommon. Transmission from maintenance hosts to incidental hosts typically occurs when an incidental host contacts an infected maintenance host or its urine. Leptospira infection in an incidental host may be subclinical but can cause profound multisystemic disease involving the hepatic, renal, and coagulation systems.1

Twelve pathogenic Leptospira serovars have been associated with dogs, of which Canicola, Grippotyphosa, Icterohaemorrhagiae, Pomona, and Bratislava are most commonly encountered in clinical cases.4 Although each serovar is antigenically distinct, agglutinating antibodies that are produced against a specific serovar may cross-react with other serovars. The serovar with the highest titer is often interpreted as the infecting serovar, and lower titers are interpreted as antibody cross-reactivity between serovars.1,5 In contrast, protective antibodies are more serovar specific, and antibodies against 1 serovar generally do not confer crossprotection against other serovars. Therefore, a dog that has been vaccinated with a typical commercially available bivalent or quatravalent vaccine may be protected against disease caused by the vaccine serovars but is likely susceptible to infection by other serovars.4,6–8

Historically, Icterohaemorrhagiae and Canicola have been the most common serovars associated with clinical leptospirosis in dogs in North America, and dogs are considered the maintenance hosts for serovar Canicola.9–11 However, during the past 20 years, the prevalence of clinical disease associated with these 2 serovars has decreased, and other pathogenic serovars, particularly Grippotyphosa, Pomona, and Bratislava, have been implicated as important causes of canine leptospirosis in North America and elsewhere.12–18 The shift in relative importance of serovars associated with clinical leptospirosis has been attributed to widespread use of bivalent Leptospira vaccines introduced in the 1960s, as well as increased contact between dogs and wildlife reservoirs in expanding suburban environments.

Most canine Leptospira infections are subclinical, and therefore, the prevalence of seropositivity in dogs with clinical disease may not accurately reflect the epidemiologic features of leptospirosis in the general canine population.12,19–22 Furthermore, interpretation of Leptospira agglutinating antibody titers may be complicated by prior vaccination with Leptospira bacterins.5 Because previous reports12–19,23–28 investigating the epidemiologic features of canine leptospirosisis have been largely restricted to studies of populations of dogs with clinical disease or to random-source dogs with unknown health and vaccination histories, the purpose of the study reported here was to investigate the epidemiologic features of Leptospira exposure in a population of healthy client-owned dogs with known vaccination histories in a time frame prior to the introduction of quatravalent Leptospira vaccines.

Material and Methods

Study design—A cross-sectional study design was used to determine the prevalence of antibodies against various Leptospira serovars and risk factors associated with positive Leptospira titers in healthy client-owned dogs in Michigan. A structured questionnaire was mailed to 767 private small animal veterinary practices in the lower peninsula of Michigan to solicit participation in the study. The state was divided into 4 regions (northern, central, southeastern, and south central-western; Figure 1), and a proportional random sample of 150 practices was selected from the practices interested in participation, to ensure that the number of practices in the study would be proportional to the actual number of practices found in each geographic region. Participating practices were asked to submit serum samples from up to 20 healthy client-owned dogs between August and December 1999, when the highest antibody concentrations in dogs exposed to Leptospira species would be expected to occur, on the basis of the seasonal nature of canine leptospirosis in the United States.1,11,13

Figure 1—
Figure 1—

Map of the lower peninsula of Michigan illustrating the geographic regions in a study of the prevalence of antibodies against 6 Leptospira serovars in 1,241 healthy dogs in 1999. Values indicate overall percentages and numbers of dogs with positive titers against any of the 6 serovars.

Citation: Journal of the American Veterinary Medical Association 230, 11; 10.2460/javma.230.11.1657

Selection of participants—Dogs judged as healthy on the basis of results of a physical examination and history were candidates for the study. Samples were acquired while veterinarians were collecting blood for clinical diagnostic tests (eg, routine preanesthetic testing). Dogs with a history of leptospirosis or current liver, renal, or other systemic disease were excluded from the study. Because maternal antibodies may interfere with interpretation of results, all participants were at least 4 months of age.

Sample collection and testing—Veterinarians collected 3 mL of blood in a serum separator tube. Serum was harvested, transferred to a 3-mL clot tube, and stored at −20°C. Samples were stored at Michigan State University until Leptospira antibody titers were measured by use of an MAT. Serum samples were analyzed for agglutinating antibodies against 6 Leptospira serovars: Bratislava, Canicola, Grippotyphosa, Hardjo, Icterohaemorrhagiae, and Pomona. Two-fold dilutions of serum from 1:50 to 1:6,400 were tested with the MAT,3 and the titer was recorded as the reciprocal of the highest dilution of serum that agglutinated ≥ 50% of the leptospires.

Definition of a positive titer—A positive titer in vaccinated dogs was defined as a positive reaction at a dilution of ≥ 200 for serovars Bratislava, Grippotyphosa, Hardjo, and Pomona; ≥ 1,600 for Icterohaemorrhagiae; and ≥ 3,200 for Canicola. A positive titer in unvaccinated dogs was defined as a positive reaction at a dilution of ≥ 200 for serovars Bratislava, Canicola, Grippotyphosa, Hardjo, Icterohaemorrhagiae, and Pomona. A dog could be classified as having a positive titer to > 1 serovar. Among seropositive dogs, the serovar with the highest titer was considered to be the serovar to which the dog was likely exposed. Dogs in which the highest titer was to more than 1 serovar were classified as likely having been exposed to each of those serovars.

Risk factors for positive Leptospira titers—To identify risk factors associated with a positive antibody titer to 1 or more Leptospira serovars, participating owners completed a demographic questionnaire about their dog. Information collected included sex (sexually intact female, spayed female, sexually intact male, or castrated male), age, breed, location of residence (northern, central, southeastern, or south central-western), housing (indoors, outdoors, or both), degree of restraint when outdoors (leashed, fenced, or free roaming), travel outside of Michigan, exposure to other animals (dogs, cats, wildlife, or domestic livestock), exposure to natural water sources (creeks, ponds, or lakes), swimming, and vaccination status (date and type of vaccines and whether Leptospira bacterin was in the vaccine).

Statistical analysis—All statistical analyses were performed on data from the 309 seropositive dogs and the serovars to which they were likely exposed. Test prevalence of seropositive dogs and the serovar or serovars to which they were likely exposed was computed for each serovar of interest for different subject groups (all dogs combined, dogs vaccinated with Leptospira bacterin, and unvaccinated dogs) by geographic region by use of the following equation:

article image

Univariable and multivariable analyses were conducted to assess the association between risk factors of interest and the serovars of likely exposure. The Mantel-Haenszel χ2 test was used to assess the association of categoric risk factors (eg, geographic region and sex) with positive titers, and ANOVA was used to test associations between serovar of likely exposure and the continuous risk factor of age. Multivariable logistic regression models were used to simultaneously assess the associations of various risk factors on the probability of a dog being exposed to a specific leptospiral serovar. The multivariable models were generated by use of a hierarchical backwards model building procedure. Risk factors with P b 0.25 in univariable analysis were considered for inclusion in the full multivariable model. Interaction terms were developed on the basis of biological plausibility and the results of descriptive statistics. Risk factors were selected for elimination by removal of the risk factor with the highest P value. The reduced model was executed, and confounding was assessed by computing odds ratios for risk factors that remained in the model. If odds ratios of any of the remaining risk factors changed > 10%, the removed variable was considered to be a confounder for the remaining risk factors and was retained in the model. The model with the best combination of lowest model deviance (as assessed by comparing model Akaike information criterion before and after removing a variable) and covariate P values (P < 0.05) was considered to be the final reduced form of the multivariable model.

Results

A total of 736 small animal practices responded to enrollment letters, and 150 practices were randomly selected for sampling. Serum samples from 1,379 dogs were collected, for which 1,241 had complete demographic data and were included in the study. Serum samples from dogs with complete risk factor data were tested. The distribution of dogs with various titers against the 6 serovars was determined (Table 1). Three hundred nine dogs had 463 positive titers (certain dogs had 2 or more positive titers).

Table 1—

Distribution (No. of dogs with titer) of serovar-specific antibody titers among 1,241 healthy Michigan dogs that were vaccinated (n = 950) or not vaccinated (291) with a leptospiral bacterin.

SerovarVaccinatedTiter
0501002004008001,6003,2006,400
BratislavaYes659134593130141256
No2322116771520
CanicolaYes201189220164101521823
No145683523155000
GrippotyphosaYes385212178973619788
No14460432466512
HardjoYes94512020000
No28830000000
IcterohaemorrhagiaeYes2622462501284613500
No17467331322000
PomonaYes6971654822122121
No21751111020000

     Dogs may have > 1 positive titer.

Results of univariable analyses—The serovars that were most frequently associated with likely exposure of seropositive dogs, Bratislava and Grippotyphosa, were serovars that were not present in canine vaccines during the study period (Table 2). Vaccination was associated with greater prevalence of likely exposure to serovars Bratislava and Grippotyphosa (P ≤ 0.05). Of positive dogs, only 9.1% of unvaccinated dogs and 9.5% of vaccinated dogs had likely exposure to > 1 serovar. Older age was associated with likely exposure to serovars Grippotyphosa and Icterohaemorrhagiae, although the number of dogs with likely exposure to serovar Icterohaemorrhagiae was small (Table 3). Significantly fewer dogs in the northern region had likely exposure to serovar Bratislava, compared with other regions (Table 4). No other geographic associations were significant. No significant associations were found between likely exposure to Leptospira spp and breed. Although associations were not significant, the breed groups with the highest likelihood of exposure were terriers for all serovars and for serovars Bratislava and Icterohaemorrhagiae, toy breeds for serovar Canicola, and working breeds for Grippotyphosa and Pomona (Table 5).

Table 2—

Prevalence of likely exposure to specific leptospiral serovars among seropositive dogs.*

SerovarAll dogs (n = 309)Unvaccinated dogs (n = 88)Vaccinated dogs (n = 221)
No.%No.%No.% 
Bratislava only655.2113.8545.7
Canicola only282.3248.240.4
Grippotyphosa only16713.53712.713013.7
Hardjo only10.10010.1
Icterohaemorrhagiae only90.762.130.3
Pomona only100.820.780.8
Bratislava and Canicola10.10010.1
Bratislava and Grippotyphosa141.110.3131.4
Bratislava, Grippotyphosa, and Pomona10.10010.1
Bratislava and Pomona10.110.300
Canicola and Icterohaemorrhagiae50.451 700
Grippotyphosa and Icterohaemorrhagiae30.210.320.2
Grippotyphosa and Pomona30.20030.3
Hardjo and Pomona10.10010.1
Any serovar30924.98830.222123.3

Exposure assessed as the serovar or serovars with the highest titer in a given sample.

Table 3—

Comparison between ages of dogs with and without likely exposure to specific leptospiral serovars.

Serovar (No. of dogs)*Mean age (y)Pvalue
Likely exposedUnlikely exposedANOVAWilcoxon rank sum
Bratislava (82)5.25.30.96810.8589
Canicola (34)5.95.20.23840.0844
Grippotyphosa (188)5.95.10.00520.0018
Hardjo (2)7.55.20.36090.2620
Icterohaemorrhagiae (17)7.25.20.01950.0111
Pomona (16)5.85.30.57280.5615
Any serovar(309)5.85.10.00180.0002

Seropositive dogs may be classified as likely exposed to > 1 serovar.

Table 4—

Prevalence (%) of dogs with likely exposure to any Leptospira serovar and to specific Leptospira serovars amoung 1.241 dogs, by geographic region, with odds ratios (OR)and 95% confidence intervals (CI) for likely exposure in each region compared with all other regions in Michigan.

Region (No. of dogs)Any serovar (n = 309 dogs)BratislavaCanicolaGrippotyphosaIcterohaemorrhagiaePomona
Northern (161)
Prevalence21.11.9*5.011.81.91.2
OR0.940.941.030.961.011.0
95% CI0.87–1.030.92–0.970.99–1.070.90–1.020.98–1.030.98–1.02
Central (219)
Prevalence21.46.82.313.20.51.4
OR0.951.00.990.970.991.0
95% CI0.88–1.020.96–1.040.97–1.020.92–1.030.98–1.00.98–1.02
Southeastern (471)
Prevalence25.77.62.115.91.31.1
OR1.021.020.991.011.01.0
95% CI0.95–1.090.99–1.050.97–1.010.97–1.070.99–1.010.98–1.01
South central-western (390)
Prevalence27.47.22.816.71.81.5
OR1.051.01.01.031.011.0
95% CI0.98–1.130.98–1.040.98–1.020.97–1.080.99–1.020.99–1.02

Significantly (P < 0.05) different from all other regions.

Table 5—

Prevalence (%) pf dogs with likely exposure to any Leptospira serovar and to specific Leptospira serovars among 1,241 dogs of various breed groups.

Breed groupNo.Any serovarBratislavaCanicolaGrippotyphosaIcterohaemorrhagiaePomona
Herding12527.28.02.417.601.6
Hound7324.78.22.713.701.4
Mixed26825.05.61.915.71.51.5
Nonsporting6126.24.94.914.73.31.6
Sporting42921.27.01.414.00.90.7
Terrier7930.412.73.813.93.81.3
Toy5928.93.411.913.61.70
Working14727.24.13.417.72.02.7
Overall1,24124.96.62.715.11.41.3

Results of multivariable analyses—Because of the low numbers of positive results for serovar Hardjo (n = 2), multivariable logistic regression models were only attempted and successfully developed for the remaining 5 serovars (Table 6). Dogs (all and vaccinated) living in the northern region of the state were at decreased risk for likely exposure to serovar Bratislava, and unvaccinated dogs with high livestock exposure were at higher risk, as were all dogs with wildlife exposure. The risk factor for likely exposure to serovar Grippotyphosa was age (all dogs and unvaccinated). Unvaccinated dogs living in the south central-western region were at decreased risk for likely exposure to serovar Grippotyphosa. Exposure to livestock and travel outside Michigan were associated with increased risk for likely exposure to serovar Pomona for all dogs and vaccinated dogs.

Table 6—

Significant (P ⩽ 0.05) risk factors associated with likely exposure to Leptospira serovars, from multivariable logistic regression models for all dogs (n = 1,235) and vaccinated (945) and unvaccinated (290) dogs.

Risk factorStudy populationBratislavaCanicolaGrippotyphosaIcterohaemorrhagiaePomona
OR95% CIOR95% CIOR95% CIOR95% CIOR95% CI
NorthernAll dogs0.250.08–0.79NSNSNSNSNSNSNSNS
CentralVaccinated0.120.02–0.85NSNSNSNSNSNSNSNS
South central-westernVaccinatedNSNS9.31.0–86.2NSNSNSNSNSNS
UnvaccinatedNSNSNSNS0.210.05–0.90NSNSNSNS 
AgeAll dogsNSNSNSNS1.11.02–1.111.151.01–1.32NSNS
UnvaccinatedNSNSNSNS1.111.01–1.221.181.01–1.39NSNS
Fenced yardAll dogsNSNS0.290.13–0.66NSNS0.200.06–0.71NSNS
LeashedUnvaccinatedNSNSNSNSNSNS7.21.6–31.6NSNS
FreeaUnvaccinatedNSNSNSNSNSNS4.61.1–20.2NSNS
TravelbVaccinatedNSNSNSNSNSNSNSNS3.71.1–12.0
LivestockAll dogsNSNSNSNSNSNSNSNS3.21.0–9.7
VaccinatedNSNSNSNSNSNSNSNS4.01.2–13.0
Unvaccinated4.01.2–13.4NSNSNSNSNSNSNSNS
WildlifeAll dogs1.61.0–2.5NSNSNSNSNSNSNSNS
SurfaceAll dogsNSNS0.480.23–0.99NSNSNSNSNSNS
WatercVaccinatedNSNS0.100.01–0.91NSNSNSNSNSNS
VaccinedAll dogsNSNS0.050.02–0.13NSNS0.130.04–0.37NSNS

Dogs allowed to roam free

Dogs traveled outside Michigan.

Ponds, lakes, creeks, rivers, and streams.

Dogs vaccinated against serovars. Canicola and Icterohemorrhagiae.

NS = Not significant.

Discussion

The study population was chosen to minimize bias towards clinically affected dogs that were more likely to be seropositive for leptospiral antibodies. The fact that the participating dogs were chosen randomly and that all participating dogs were healthy minimized sampling bias. Because leptospiral vaccination status was known and all samples were collected prior to use of vaccines containing serovars Grippotyphosa and Pomona, confounding associated with vaccination was also minimized. Because dogs were assessed for positive titers independent of disease, positive titers likely represented exposure to Leptospira spp.

The MAT is the most commonly used diagnostic test for detecting leptospiral antibodies and is relatively serovar specific, although dogs with positive titers generally have sera that cross-react to some extent with other serovars.1,5,6 The serovar with the highest titer is interpreted as the infecting serovar, and lower titers are interpreted as antibody cross-reactivity between serovars. The MAT does not differentiate between antibodies induced by vaccination and those produced because of natural exposure. Antibody titers associated with vaccination are typically < 400 and measurable for < 4 months after vaccination, although there is considerable variation in the titers detected in individual vaccinated dogs.5,8

The definition of positive titers in leptospiral serosurveys varies widely and ranges from 10 to 800, depending on the nature of the population surveyed and the purposes of the study.18,19,24,26–28 Many surveys have been done in dogs with unknown vaccination status, which further complicates interpretation of titers. In this study, we had definitive information regarding the vaccination history of the dogs, which afforded the opportunity to use different cutoff titers for dogs that had been vaccinated and those that had not. A cutoff titer of ≥ 100 is commonly used for nonvaccinal serovars. In this study, a more conservative cutoff of ≥ 200 was used to minimize the effect of cross-reacting antibodies. However, in seroprevalence surveys among vaccinated dogs, a higher cutoff for serovars contained in the vaccines is warranted to minimize the overestimation of seroprevalence in the vaccinated population. Therefore, in this study, the cutoff titers for vaccinal serovars (Canicola and Ictero-haemorrhagiae) were different for vaccinated and nonvaccinated dogs and were chosen in consideration of the ranges seen in healthy vaccinated dogs at the Diagnostic Center for Population and Animal Health, where > 15,000 dog sera are tested for leptospiral antibody annually. The distribution of positive titers among vaccinated and nonvaccinated dogs in the present study supports the contention that these cutoff titers were sufficiently high to minimize the effect of vaccination on the seroprevalence of Canicola and Icterohaemorrhagiae reported here. A cutoff titer of ≥ 200 was used for serovars Canicola and Icterohaemorrhagiae in nonvaccinated dogs. Because the titer cutoffs chosen for this study were more conservative than are typically used when assessing for Leptospira exposure, interpreting vaccine response as actual exposure was minimized. The overall prevalence of 24.9% for seropositive dogs determined in this study was more likely an underestimate than an overestimate of actual exposure.

There is some controversy regarding the role of serovar Bratislava as an important pathogen for dogs in the United States. An experimental study22 involving exposure of young Beagles to an isolate of serovar Bratislava did not result in infection of the dogs. However, the authors of that study concluded that the particular isolate they used may have lost virulence during in vitro culture and that the results of the study with a single isolate should not be interpreted to indicate that serovar Bratislava is not a canine pathogen. Evidence to support the potential of serovar Bratislava as a pathogen of dogs includes evidence that serovar Bratislava is present in animal populations in the United States29 and clinical reports from the United States16,30 and United Kingdom19,31 that indicate that serovar Bratislava is infectious for dogs and can be associated with clinical disease.

Serovar Autumnalis was not included in this study, although positive canine titers have been reported.23 Interpretation of serovar Autumnalis titers is not fully understood at this time because of erroneous or paradoxical high reactivity.6,15,23 In 1 study,6 dogs vaccinated with only serovars Grippotyphosa and Pomona developed titers to serovar Autumnalis that were significantly higher than the titers to either vaccinal serovar. The sera collected for the present study, which were collected prior to the availability of vaccines containing serovar Grippotyphosa and Pomona, will be useful in determining the prevalence of titers to serovar Autumnalis in a nonvaccinated population.

Canine Leptospira serologic surveys from areas corresponding to the southeastern and south central-western regions in Michigan have been reported. In association with 75 cases of human leptospirosis in Detroit reported in 1939, 16% of examined rats were seropositive for leptospiral infections and 10 of 13 dogs had positive results of MAT against serovar Icterohaemorrhagiae but lacked titers against serovar Canicola.32 Results of a substantial leptospiral serologic survey33 of Detroit-area dogs were also reported in 1980. Dog sera were tested by use of the MAT for antibodies against serovars Canicola, Grippotyphosa, Icterohaemorrhagiae, and Pomona. Of 433 stray dogs, 23.8% were seropositive against serovar Icterohaemorrhagiae, followed by serovars Canicola (8.3%), Grippotyphosa (6.9%), and Pomona (6.7%), whereas seropositvity among suburban dogs was 16.3% for Icterohaemorrhagiae, 4.0% for Canicola, and 1.6% for Pomona. No dogs were seropositive against serovar Grippotyphosa. In contrast, in the present study, 15.9% of dogs in the southeastern region had likely exposure to serovar Grippotyphosa, with no dogs likely exposed to serovar Icterohaemorrhagiae, but overall likely exposure in the southeastern region (25.7%) was similar to that reported in 1980. The marked decrease in likely exposure to serovar Icterohaemorrhagiae may reflect use of Icterohaemorrhagiae bacterins and, probably more importantly, better Norway rat control as a result of curbside garbage pickup.a The increased likely exposure to serovar Grippotyphosa may reflect increased exposure to maintenance hosts for Grippotyphosa (raccoons, opossums, and skunks) as suburban expansion occurs.

Of 500 healthy dogs from Lansing, Mich, tested for antibodies against 2 serovars in 1950, 26.8% were seropositive against serovar Canicola and 2.6% against serovar Icterohaemorrhagiae.34 The author noted that dogs in the study were unlikely to have had exposure to rats. Lansing is located in the south central-western region of the present study where likely exposure to serovar Canicola was 2.8% and likely exposure to serovar Icterohaemorrhagiae was 1.8%, but 16.7% of dogs were seropositive against Grippotyphosa. The decrease in prevalence of likely exposure to serovar Canicola may reflect use of Canicola bacterins, whereas the nearly equivalent overall exposure rate was a reflection of increased exposure to serovars Grippotyphosa and Bratislava, perhaps as a result of suburban encroachment on wildlife.

Serovar prevalence results for all dogs from the 4 Michigan study regions can be compared with published serovar reports for dogs with leptospirosis and with reports of dog sera submitted to diagnostic laboratories presumably because clinical leptospirosis was a diagnostic consideration. In most leptospirosis case report series published in the preceding 15 years, Grippotyphosa was the predominant serovar.12–15,17,18 An exception is the only report16 originating from western North America. In that report of California dogs with clinical leptospirosis, serovars Bratislava and Pomona predominated and only 2 dogs had any titer to serovar Grippotyphosa. In contrast, published reports from midwestern and eastern North America all found grippotyphosa to be the predominant or nearly predominant infecting serovar. It is possible that this difference reflects geographic maintenance-host differences as supported by a report35 of low serovar Grippotyphosa seropositivity among California raccoons, in contrast to high Grippotyphosa seropositivity among raccoons east of the Mississippi River.36–38 Similarly, recently published seropositivity reports24,25 based on submissions to diagnostic laboratories indicate that serovar Grippotyphosa titers predominate.

Risk factors identified in the present study of healthy dogs were both similar and dissimilar to some of the risk factors identified in dog populations not known to be healthy. Age was significantly associated with increased likely exposure to serovars Grippotyphosa and Icterohaemorrhagiae regardless of vaccination status, and this was previously reported in a retrospective study25 of dogs with leptospirosis caused by unspecified serovars. Age may have allowed greater cumulative exposure to maintenance hosts for serovars Grippotyphosa and Icterohaemorrhagiae. The finding that unvaccinated healthy dogs living in the south central-western region of Michigan were at reduced risk for likely exposure to serovar Grippotyphosa cannot be easily explained but may be related to complex factors of exposure to maintenance hosts and unidentified environmental influences. For example, serovar Grippotyphosa–infected raccoons may have higher population densities in urban than in suburban areas.39

Risk factors were also identified for serovar Bratislava exposure. Dogs living in the northern and central regions were at reduced risk, possibly because reservoir hosts such as rats, pigs, and horses have lower population densities in those regions. Unvaccinated dogs with high livestock exposure were at highest risk for serovar Bratislava exposure. The overall high risk for serovar Bratislava exposure associated with wildlife exposure remains unexplained.

Travel outside Michigan and exposure to livestock were risk factors for likely exposure to serovar Pomona for all dogs and for vaccinated dogs. It is possible that this finding reflected increased exposure to cattle, although other reservoir hosts such as pigs, skunks, and opossums cannot be ruled out.

Leptospiral vaccination reduced the risk of dogs being seropositive to serovars Canicola and Icterohaemorrhagiae, the 2 serovars included in commercially available bacterins at the time of the study. Because these 2 serovars were in the vaccine that was in use during the period the study was conducted, findings support the notion that the vaccine was effective in protecting dogs from developing positive titers against these serovars. Additionally, findings support the cutoffs used in this study as not classifying titers in a false-positive manner. Exercise in a fenced yard was associated with reduced risk of likely exposure to both serovars, whereas exercising leashed or free was associated with increased risk of likely exposure to serovar Icterohemorrhagiae in unvaccinated dogs. Vaccinated dogs located in the south central-western region were at increased risk for likely exposure to serovar Canicola, a finding not easily explained in light of the lack of similar findings for other serovars. Exposure to surface water (eg, lakes, ponds, rivers, and streams) was not associated with likely exposure to serovar Canicola; exposure to water has been identified both as a risk factor and as not associated with leptospirosis.40,41

In addition to geographic location, degree of restraint when exercised, travel outside Michigan, and vaccination status, other potential risk factors including sex, breed, and housing were evaluated. None of these factors were significantly associated with risk in this population of healthy dogs. In previous reports25 of dogs with leptospirosis or dogs tested because leptospirosis was a diagnostic consideration, hounds, herding dogs, working dogs, and gun dogs were at increased risk.

Results of the present study indicated that > 20% of healthy, client-owned dogs from the lower peninsula of Michigan likely had been exposed to Leptospira serovars that were previously uncommon. In addition, results indicated that exposure to Leptospira was widespread and was not associated with breed as has been suggested. Because serovar Grippotyphosa, a serovar maintained by wildlife, was the predominant serovar identified, certain strategies can be used to either mitigate or minimize risk. Administration of vaccine containing leptospiral serovar Grippotyphosa and limiting exposure to wildlife reservoirs may be appropriate considerations.

ABBREVIATION

MAT

Microscopic agglutination test

a.

Pollard D, Detroit Department of Health and Wellness Promotion, Detroit, Mich: Personal communication, 2006.

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Contributor Notes

Dr. Stokes' present address is Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37901-1071.

Dr. Kaiser's present address is Department of Medicine, College of Human Medicine, Michigan State University, East Lansing, MI 48824.

Supported by the Companion Animal Fund, College of Veterinary Medicine, Michigan State University; and Fort Dodge Animal Health.

Presented in part as an abstract at the American College of Veterinary Internal Medicine Forum, Denver, May 2001.

Address correspondence to Dr. Schall.