Improving the laboratory diagnosis of leptospiral uveitis in horses by using an indirect ELISA for the detection of antibodies against Leptospira spp. in intraocular samples

Equine recurrent uveitis (ERU) is a frequently occurring disease, at least in horses in Germany. A vitrectomy is predominantly the course of action in cases where a chronic intraocular leptospiral infection is present. In some cases, the clinical and ophthalmological diagnosis is not conclusive and in these cases a preoperative laboratory test using anterior chamber fluid is indicated. However, false negative results occur when only MAT is used. The aim of this study was to investigate intraocular samples from horses with recurrent uveitis by using ELISA to detect anti-leptospiral antibodies and to compare the results to those of more established laboratory tests, i.e. MAT and PCR. Preliminary investigations indicated a high sensitivity of ELISA in the detection of leptospiral uveitis in horses. 80 eyes of 72 diseased horses were included in the study. The patients' history was taken and a thorough ophthalmologic examination was performed. Intraocular fluids were obtained by diagnostic anterior chamber paracentesis (n = 29) or therapeutic vitrectomy (n = 80) and submitted for MAT, ELISA and PCR to detect leptospiral antibodies and/or DNA. The ELISA allowed the separate determination of the immunoglobulin classes M, G and A. For statistical evaluation Cohen's kappa and the chi-square tests were applied and a p-value of 0.05 or less was considered to be significant. 22 healthy horses (n = 42 eyes) served as controls. 78% of vitreous samples from horses with ERU and 10% of healthy controls reacted positively in the MAT, whereas in the ELISA it was 85% and 0%, and in the PCR it was 61% and 0%, respectively. Specific IgA was detected in 84% of the vitreous samples from horses with ERU, and in 11% of cases, IgA was solely detectable. Differences were highly significant (p<0.001). None of the aqueous samples in the control group contained leptospiral antibodies. Up to 64% of aqueous humour specimens that were obtained for diagnostic purposes yielded a positive result in the MAT. For the ELISA, it was even 89%. Anti-leptospiral IgA was the most frequently detected immunoglobulin class. There was a high level of agreement in the MAT and ELISA results. However, ELISA provided a greater number of positive results. The detection of IgA against leptospires proved to be especially sensitive for an intraocular leptospiral infection and, thus, important for the laboratory diagnosis of leptospiral uveitis in horses. Looking at the cost involved in laboratory tests, it would be reasonable to start with an IgA ELISA. Only if there are negative results, additional tests could be performed. In single IgA negative cases there might be positive reactions using MAT, PCR or ELISA for IgG or IgM.