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Abstract

Objectives

Although Leptospira species infections can be associated with intraocular inflammation in dogs and horses, there is limited information regarding the role these agents play in feline uveitis. The primary objective of this study was to report the prevalence of antibodies to Leptospira species and the presence of Leptospira species DNA in samples from cats with endogenous uveitis. The secondary objective was to assess for coinfections with Bartonella species, Toxoplasma gondii and eubacteria.

Methods

Serum and aqueous humor (AH) samples from 37 cats diagnosed with endogenous uveitis that had been stored at –80°C were selected for this study based on sample availability. PCR assays for Leptospira species, T gondii, Bartonella species and 16S eubacterial rDNA were performed on AH. Sera were evaluated for antibodies to Leptospira species (microscopic agglutination test [MAT]), Bartonella species (IgG ELISA) and T gondii (IgM and IgG ELISA).

Results

Although sera from 2/37 (5.4%) cats were positive for antibodies to Leptospira interrogans serovar Pomona by MAT (1:100 titers), all AH samples were negative for DNA of Leptospira species. One AH sample was positive for DNA of a Streptococcus species but all were negative for DNA of T gondii and Bartonella species. Serum antibodies to Bartonella species (21/37, 56.8%), T gondii (7/37, 18.9%) or multiple agents (5/37, 13.5%) were common.

Conclusions and relevance

Although the results of this study cannot be used to prove or refute Leptospira species as a cause of endogenous uveitis in cats, the detection of specific antibodies to Leptospira Pomona in the sera of two cats suggests that a larger cohort of cats should be tested to further evaluate the hypothesis. The AH of one cat was positive for the DNA of a Streptococcus species and future studies should investigate if post-streptococcal uveitis syndrome can also be found in cats.

Plain language summary

Evaluation of Leptospira species as a cause of intraocular inflammation (uveitis) in cats: a pilot study

A research team from Colorado State University College of Veterinary Medicine and Biomedical Sciences investigated the possibility of Leptospira species causing intraocular inflammation (known as uveitis) in cats. Leptospira species is a bacterium that lives in our surroundings, such as lake and river water, as well as in areas with rats, swine and deer. Leptospira-associated uveitis is a well-known eye disease in horses, dogs and humans but has never been investigated as a possible underlying cause for uveitis in cats. The research team looked at intraocular fluid (aqueous humor) samples from 37 cats from all over the USA and found only two samples that were positive for Leptospira species. They also found only one sample that was positive for Streptococcus species DNA. The Streptococcus bacterium has been associated with uveitis in humans after systemic Streptococcus infection and is known in humans as post-streptococcal uveitis syndrome. Although results of this study cannot be used to prove or refute Leptospira species as a cause of uveitis in cats, the detection of specific antibodies to Leptospira interrogans serovar Pomona in two cats suggests that a larger cohort of cats should be tested to further evaluate the hypothesis. In addition, continued work to determine if post-streptococcal uveitis syndrome occurs in cats should be considered.

Keywords

Antibody aqueous humor eubacteria inflammation intraocular PCR

Introduction

Uveitis is one of the most common ocular diseases in feline ophthalmology.^1,2 Uveitis is classified as exogenous or endogenous, with endogenous uveitis most often diagnosed. Commonly recognized systemic infectious causes of feline endogenous uveitis include Toxoplasma gondii, feline leukemia virus, feline immunodeficiency virus, feline infectious peritonitis, feline herpes virus 1, mycotic disease and Bartonella henselae, though idiopathic uveitis remains one of the most common diagnoses.^1
–6 Despite the various diagnostic tests (serum antibody tests, molecular diagnostic tools) that are available for each agent, performed in serum, blood or aqueous humor (AH), diagnosing an underlying cause of uveitis remains challenging, particularly if serology alone is used. In addition, a definitive diagnosis is often complicated by serological or PCR evidence of coinfections.^5,6

Leptospirosis is a worldwide zoonotic disease caused by spirochetal bacteria of the genus Leptospira. All mammals may be susceptible to leptospirosis, and exposure is often from contaminated wildlife urine.^7,8 Systemic leptospirosis has the ability to trigger immune mediated uveitis, such as seen in equine recurrent uveitis (ERU)^9
–13 and, to a lesser extent, canine infectious uveitis.^12,14 Nearly all mammals can serve as carriers of leptospirosis, shedding organisms from the proximal tubules of the kidneys. The most common means of transmission to humans is through abrasions on the skin and mucous membranes that come into contact with contaminated water sources infected with the urine of infected animals.¹⁵ Clinical disease in humans can range from mild and self-limiting febrile illness to severe multiple organ dysfunction.¹⁵ The pathogenesis of leptospirosis-associated uveitis is poorly understood in both humans and animals. There is often a prolonged symptom-free period between leptospiral infection and onset of ocular signs. During the acute phase of infection, the most common clinical symptom involving the human eye is non-specific ocular congestion.¹⁶ As the adaptive immune system clears the organism from circulation, leptospires continue to persist in immune-sequestered sites like the eye.^16,17 Leptospiral-associated uveitis is also believed to be associated with the migration of antibodies into the anterior chamber from the systemic circulation with a breakdown of the blood-ocular barrier.¹⁸ In horses with ERU, there is growing evidence to suggest that autoimmune responses to ocular tissue components play a significant role in pathogenesis.^16,17

Recently, leptospirosis has been implicated more commonly as a cause of feline systemic disease but it is currently unknown whether the agents are a common cause of feline uveitis.^8,19
–22 In these studies, the prevalence of Leptospira species DNA in feline urine is in the range of 0–68%, with no clear association with clinical disease. In one study, 1/3 cats with clinical signs of leptospirosis with renal insufficiency also had anterior uveitis.²³ This finding supports our hypothesis that leptospirosis can be the underlying cause for uveitis in cats.²³

Because of the small volume of AH collected during aqueous paracentesis for cytology and other tests, results of microbiological culture are low yield and rarely reported in studies of feline uveitis.^3,24 However, it is possible that some cases could have other bacterial triggers of inflammation. PCR assays to amplify the DNA of eubacteria can be performed on small sample volumes and have been used to assess for bacterial causes of ocular inflammation in humans.^25
–27

The primary objective of this study was to report the prevalence of Leptospira species antibodies and DNA in samples from cats with endogenous uveitis. The secondary objective was to assess for coinfections by Bartonella species, T gondii and eubacteria.

Materials and methods

Study design and samples

This was an exploratory descriptive study performed using sera and AH samples from cats diagnosed with endogenous uveitis that were originally submitted between 2002 and 2013 to the Colorado State University (CSU) Veterinary Diagnostic Laboratory by outside veterinary ophthalmologists for infectious disease testing. Details of age, sex, breed and the state of origin were required when feline sera and AH samples were submitted for investigation. The samples originated from 10 American states (Table 1) and had been collected in a range of veterinary settings, including specialty ophthalmology practices and teaching universities, by board-certified veterinary ophthalmologists who made the diagnosis of endogenous anterior uveitis. Any case in which it was suggested that the uveitis could have been due to exogenous causes (ie, corneal ulceration, trauma, toxin) was excluded. The included cases represent a convenience sampling based merely on the presence of an adequate volume of AH and sera to be used to complete the additional diagnostic testing described here. The laboratory instructed outside veterinarians to ship the samples on cold packs. After the original assays were performed, the samples were stored at –80°C until processed for this study.

Table 1

The American states from where samples of aqueous humor (AH) and serum were originally mailed to Colorado State University for infectious disease testing (of the 37 AH and serum samples submitted, 28 were accompanied by information regarding state of origin)

State	Number of samples
California*	1
Colorado	5
Florida*	1
Idaho	3
Ohio*	1
Tennessee*	1
Texas*	10
Virginia*	1
Washington*	2
Wyoming	3

High flea-risk states

Molecular assays

AH samples were thawed at room temperature and centrifuged at 20,000 g for 10 mins. The supernatant was removed and stored, and the AH pellet was reconstituted in 200 µl sterile phosphate-buffered saline. Total DNA was extracted from the AH pellet with a final elution of 100 µl using a commercially available kit, following the manufacturer’s instructions (Blood Mini Kit; Qiagen). All samples were assessed by a spectrophotometer (Nanodrop 1000; Thermo Fisher Scientific) to confirm the presence of DNA. PCR assays for the DNA of Leptospira species,²⁸ T gondii²⁹ and Bartonella species³⁰ and 16S rDNA assay for eubacterial DNA³¹ were performed on all AH samples as described by the manufacturer (VetMAX-Plus qPCR Master Mix; Thermo Fisher Scientific) and as previously described.

Serological assays

All sera were thawed and evaluated for antibodies to Leptospira species (microscopic agglutination test [MAT]),³² Bartonella species (IgG ELISA)³³ and T gondii (IgM and IgG ELISA)³⁴ using assays provided to veterinarians by the CSU Veterinary Diagnostic Laboratory (accredited by the American Association of Veterinary Laboratory Diagnosticians). For Leptospira species antibodies, the serovars assessed were Canicola, Grippotyphosa, Hardjo, Icterohaemorrhagiae and Pomona. As no study has optimized the MAT for use with feline sera, a titer of >1:50 was chosen based on published experience with the assay in cats and the recommendation of the diagnostic laboratory performing the test.³²

Statistical analysis

The mean age and age range, as well as descriptive data for sex and breed were reported. Estimated prevalence rates, reported as percentage (%) and 95% confidence intervals (CIs; modified Wald method), were calculated for Leptospira species, T gondii and Bartonella species using Prism (GraphPad Software).

Results

Details of the age, sex, breed and eye(s) of cats diagnosed with uveitis and tested for select pathogens in this study are shown in Table 2. The sample submission sheet was the only history available for the cats; therefore, no additional information on other diagnostic tests or treatments was available.

Table 2

Characteristics of enrolled cats

Characteristic	Value
Age (years) (n = 25/37)	6.51 (0.29–13)
Sex (n = 27/37)
Male intact	1
Male castrated	14
Female intact	1
Female spayed	11
Breed (n = 27/37)
DSH	20
DLH	3
Maine Coon	2
Siamese	1
American Shorthair	1
Eyes (n = 17/37)
OU	7
OD	6
OS	4

Data are n or mean (range)

DLH = domestic longhair; DSH = domestic shorthair; OD = oculus dexter (right eye); OS = oculus sinister (left eye); OU = oculus unitas (both eyes)

Molecular assay results

Intact DNA was documented in all AH samples by spectrophotometry. Only one AH sample was positive for DNA of a targeted infectious disease agent and DNA sequencing confirmed this to be a Streptococcus species.

Estimated seroprevalence rates

A summary of all combinations of serum antibody test results with estimated prevalence rates and 95% CIs is presented in Table 3. Out of 37 cats, two (5.4%) were positive for antibodies to Leptospira Pomona by MAT (1:100 titers). Antibodies to at least one target organism were present in the sera of 25/37 (67.6%) cats (Table 3). Antibodies were detected to a single agent in some cats and to a combination of agents in others (Table 3). Antibodies to Bartonella species were most common (21/37, 56.8%) followed by antibodies to T gondii (7/37, 18.9%). Of the 28 cats where information on the state of origin was provided, the 17 (60.7%) that were seropositive for Bartonella species were from high flea-risk states. Of the two cats with antibodies to Leptospira Pomona one also had antibodies to Bartonella species. The two cats seropositive for Leptospira Pomona were located in the states of Washington and Virginia. Positive antibody titers identified coinfection in 5/37 (13.5%) cases. All coinfection cases were associated with Bartonella species infection (Table 3). There were no cases of T gondii and Leptospira species coinfection. A total of 20/37 (54.1%) cats had positive antibody titers for a single agent only.

Table 3

Seroprevalence of Bartonella henselae, Toxoplasma gondii and Leptospira interrogans in 37 cats diagnosed with endogenous uveitis

Antibodies to each agent	n	Positive (%)	95% CI
Any Bartonella	21	56.8	40.90–71.34
Bartonella only	16	43.2	28.66–59.10
Any Toxoplasma	7	18.9	9.17–34.51
Toxoplasma only	3	8.1	2.06–22.03
Any Leptospira	2	5.4	0.57–18.63
Leptospira only	1	2.7	<0.01–15.05
Bartonella and Toxoplasma	4	10.8	3.70–25.29
Bartonella and Leptospira	1	2.7	<0.01–15.05
Antibody to any agent	25	67.6	51.38–80.45
All antibody assays negative	12	32.4	19.55–48.62

Detection of antibodies to more than one agent is ‘Bartonella and Toxoplasma’ and ‘Bartonella and Leptospira’

CI = confidence interval in % (CI × 100)

Discussion

To the authors’ knowledge, this is the first study evaluating the prevalence of Leptospira species antibodies in a population of cats diagnosed with endogenous uveitis. The 5% seroprevalence for Leptospira in our study is within previously reported prevalence rates for cats of 4–48.8%.^35
–38 Because of the convenience sampling nature of the current study, convalescent titers could not be evaluated to determine if the two positive titers were evidence of true infection. The magnitude of the titers observed was lower than in studies of vaccinated healthy cats and cats diagnosed with kidney disease (documenting a maximum titer of ⩾1:12,800).³⁹ However, it has been documented that cats can produce Leptospira antibodies at lower titers in cases diagnosed with systemic disease.^36,40
–42 A study by Shropshire et al³² found positive titers in two vaccinated cats but only one of these cats maintained a positive titer at day 42. There is no approved vaccine for Leptospira species in cats and so it is unlikely any of the cats were vaccinated for Leptospira before AH collection. Therefore, we believe their positive Leptospira titers are a true finding and indicate exposure to Leptospira species. However, infection was not confirmed by positive Leptospira species PCR results.

A number of factors may have influenced Leptospira species antibodies and PCR results in this sample set. The use of samples submitted by multiple veterinary ophthalmologists did not allow for standardization of the AH collection method or presample collection treatments, which may have affected results.⁴³ Another reason for possible false AH Leptospira DNA negative results may include a low leptospiral load, which could be the case in this study based on the low titers observed. It is also possible that an antibiotic with activity against Leptospira species may have been recently administered, which can lead to negative PCR assay results. An alternate explanation is that leptospirosis is an uncommon cause of feline endogenous uveitis.

As all available diagnostic tests have limitations, if clinical suspicion of leptospirosis is present, a combination of serologic assays and organism detection tests is recommended.⁴⁴ Reliance on the use of serology as a means of diagnosis is uncertain, and opinions differ as to what constitutes a significant leptospiral titer. In addition, in horses, a positive serological reaction may not indicate current or recent infection, as titers can persist for up to 7 years.⁴⁵ For most infectious diseases like leptospirosis, serum antibodies are most beneficial for determining whether the patient has been exposed to the pathogen but do not document active or current infection. However, if no other evidence of causation is found during the diagnostic workup, a positive titer combined with appropriate clinical manifestations often supports a presumptive diagnosis leading to a therapeutic trial.⁴⁴

The two cats with positive titers for Leptospira Pomona were from the states of Virginia and Washington. The indoor/outdoor status of these cats was unknown. A case report by Harkness et al⁴⁶ identified a domestic farm cat positive for Leptospira Pomona in New Zealand. It was noted that this cat was previously brought from another farm where an outbreak of bovine abortion had been attributed to serovar Pomona. Previous studies have reported various host-serovar antibody associations, with serovar Pomona more commonly reported in swine and cattle.⁴⁷ Serovar Pomona has been detected in cats in various European countries,^47,48 and is considered one of the more commonly reported serovars in cats in the USA.⁸ Many feline studies have confirmed renal carriage of Leptospira species by PCR, mainly serovars Pomona, Autumnalis, Australis, Icterohaemorrhagiae, Grippotyphosa and Sejroe.⁸ This supports cats as a chronic reservoir host and a possible source of zoonotic exposure. More specifically, serovars Pomona, Grippotyphosa and Bratislava are commonly reported in studies of ERU in Europe and the USA.^9,10,49 In addition, a case report describes three cats diagnosed with leptospirosis, in which a single cat was diagnosed with concurrent uveitis. This cat showed a titer of 1:1600 for Leptospira Pomona (along with 1:1600 for Leptospira Bratislava and 1:800 for Leptospira Grippotyphosa).²³

This study did not find any feline AH samples that were positive for Leptospira species by qPCR, indicating the two positive serologic results in this study were not predictive of AH PCR results. Leptospira species DNA has been identified in the AH of horses with naturally occurring ERU.^10,50 Similar to the present study, an equine report also showed a poor correlation between serologic results and the presence of leptospiral DNA.¹⁰ Human studies have discussed difficulty in obtaining positive culture or molecular assay results but supported DNA sequencing as the diagnostic method of choice for identifying leptospiral organisms in AH.⁵¹

As a result of the low percentage of cats seropositive for Leptospira species, no statistical analysis was performed for risk of coinfection in this study. A single cat that was positive for Leptospira species on MAT was also positive for B henselae serum IgG. Additional studies with a larger sample size are required to further investigate the role of coinfection and leptospirosis.

Results of 16S rRNA qPCR assay revealed a single positive eubacterial isolate confirmed as a Streptococcus species. Post-streptococcal uveitis syndrome (PSUS) is an uncommon manifestation of post-streptococcal syndrome in humans, and there have been only a few reports in the human literature.⁵² The pathophysiology of post-streptococcal syndrome is not well understood, although it is thought that an immune-mediated reaction is triggered by the production of tissue-specific antibodies, leading to complications such as reactive arthritis, glomerulonephritis and erythema nodosum. The onset of immune-mediated sequelae has been described at 7–35 days after infection.⁵³ The first documented case of PSUS in humans was reported as bilateral non-granulomatous uveitis after streptococcal pharyngitis.⁵⁴ All reported cases of PSUS were treated with corticosteroids without antibiotics.^52
–54 PSUS has not been reported in veterinary medicine. Although the single case positive for Streptococcus species is not definitive evidence for a similar syndrome in cats, this information provides a new area of future research into other possible causes of feline uveitis. However, the possibility of bacterial contamination during aqueous paracentesis cannot be excluded in this case.

In the current study, B henselae and T gondii were the most identified infectious agents involved in endogenous uveitis. Bartonella species seroprevalence in this study was 56.8%, which is similar to that of a previous study.⁵⁵ The first case report of uveitis in a cat suggested Bartonella species as the cause, which responded readily to doxycycline. In this study, there were notable intraocular species-specific antibodies, although culture and PCR were not performed.⁵⁶ Subsequent epidemiologic studies have found that Bartonella antibodies are commonly identified in cats with uveitis, and intraocular antibody and DNA presence is more common in cats with uveitis than in those without uveitis.³⁰ Other studies have found more healthy cats to have positive Bartonella species titers compared with cats diagnosed with uveitis.^5,52 Much of the discrepancy in the results is likely related to the increased exposure to fleas in shelter cats considered as controls. Likewise, 60.7% (17/28) cats with information on state of origin in the present study were seropositive for Bartonella species and were from high flea-risk states.⁵⁷ Thus, a positive titer may in fact be due to increased risk for flea exposure. As it is unknown whether an antibiotic with activity against Bartonella species had been recently administered, it is possible that the prevalence based on the PCR assay results may be underestimated.

T gondii is also a commonly implicated infectious agent associated with endogenous uveitis in cats. Toxoplasma species seroprevalence in this study was 18.9%. T gondii has been studied extensively, making a clear association between infection and uveitis in cats, especially those that are systemically ill.^3,58
–60 However, the role of T gondii in older, otherwise healthy cats with uveitis remains unclear, as there are many systemically healthy cats seropositive for the organism.^5,61 It is important to remember that neither the presence of a positive titer nor titer magnitude in serum is necessarily diagnostic of clinical toxoplasmosis. However, a positive titer can aid in making the association between an infectious agent and a syndrome. As it is unknown whether an antibiotic with activity against T gondii had been recently administered, it is possible that the prevalence based on the PCR assay results may be underestimated. A larger study including a control cohort of geographically and housing matched (indoor/outdoor status) cats without uveitis would be required to further strengthen this presumed association.

Conclusions

Although the results of this study cannot be used to prove or refute Leptospira species as a cause of endogenous uveitis in cats, the detection of specific antibodies to LeptospiraPomona in the sera of two cats suggests that testing of a larger cohort of cats is required to investigate further. In addition, continued work to determine if PSUS occurs in cats should be pursued.

Footnotes

Acknowledgements

We would like to acknowledge all clinics/veterinarians who provided AH and serum samples along with background data of the cats.

Accepted: 30 November 2025

Conflict of interest

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

This research was supported by an educational grant from the Kenneth W Smith Professorship in the Center for Companion Animal Studies, Colorado State University, College of Veterinary Medicine and Biomedical Sciences, Fort Collins, Colorado.

Ethical approval

The work described in this manuscript involved the use of non-experimental (owned or unowned) animals. Established internationally recognized high standards (‘best practice’) of veterinary clinical care for the individual patient were always followed and/or this work involved the use of cadavers. Ethical approval from a committee was therefore not specifically required for publication in JFMS. Although not required, where ethical approval was still obtained, it is stated in the manuscript.

Informed consent

Informed consent (verbal or written) was obtained from the owner or legal custodian of all animal(s) described in this work (experimental or non-experimental animals, including cadavers, tissues and samples) for all procedure(s) undertaken (prospective or retrospective studies). For any animals or people individually identifiable within this publication, informed consent (verbal or written) for their use in the publication was obtained from the people involved.

ORCID iD

Michala de Linde Henriksen

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Evaluation of Leptospira species as a cause of endogenous uveitis in cats: a pilot study

Abstract

Objectives

Methods

Results

Conclusions and relevance

Plain language summary

Keywords

Introduction

Materials and methods

Study design and samples

Molecular assays

Serological assays

Statistical analysis

Results

Molecular assay results

Estimated seroprevalence rates

Discussion

Conclusions

Footnotes

Acknowledgements

Conflict of interest

Funding

Ethical approval

Informed consent

ORCID iD

References