COMPARISON OF METHODS FOR ESTIMATION OF TOXOPLASMA GONDII-SPECIFIC ANTIBODY-PRODUCTION IN THE AQUEOUS-HUMOR OF CATS

Intraocular production of Toxoplasma gondii-specific antibody in cats has been estimated by comparing the ratio of T gondii-specific antibod y in aqueous humor and serum with the ratio of total immunoglobulins i n serum and aqueous humor (Goldmann-Witmer coefficient; aqueous antibo dy coefficient; C value). It has been proposed that in human beings, c omparison of the ratio of T gondii-specific antibody in aqueous humor and serum with the ratio of antibodies against a nonocular pathogen in serum and aqueous humor is more accurate than methods using total imm unoglobulin quantification. We developed an ELISA for detection of cal icivirus-specific antibodies in the serum and aqueous humor of cats. B y evaluating calicivirus-specific antibody concentrations in the aqueo us humor of healthy and diseased cats, calicivirus was assessed as a n onintraocular pathogen. The ratio of T gondii-specific antibodies in t he aqueous humor and serum and the ratio of calicivirus-specific antib odies in serum and aqueous humor were evaluated as a means of estimati ng intraocular T gondii-specific antibody production. A field strain o f feline calicivirus was isolated, cultured, and purified. A calicivir us-specific IgG ELISA was developed for detection of feline caliciviru s-specific IgG in serum and aqueous humor. Calicivirus-specific IgG wa s measured in the serum and aqueous humor from 3 groups of control cat s. Results suggested that calicivirus is a nonintraocular pathogen in cats and that calicivirus IgG detected in aqueous humor is attributabl e to leakage across a damaged blood-ocular barrier. Intraocular produc tion of T gondii-specific antibodies was estimated, using 2 formulas. The C value was calculated by multiplying the ratio of T gondii-specif ic IgM or IgG in aqueous humor and serum by the ratio of total immunog lobulins (using the corresponding IgM or IgG class) in serum and aqueo us humor. The C-tc value (Toxcoplasma-calicivirus Goldmann-Witmer coef ficient) was calculated by multiplying the ratio of T gondii-specific IgM or IgG in aqueous humor and serum by the ratio of calicivirus-spec ific IgG in serum and aqueous humor. Serum and aqueous humor samples w ere obtained from 41 client-owned cats with uveitis, and T gondii-spec ific C values and C-tc values were calculated. Toxoplasma gondii-speci fic IgM or IgG C values of 10 or greater or T gondii-specific IgM or I gG C-tc values of 1 or greater were considered to be suggestive of int raocular T gondii-specific antibody production. Of the 41 cats, 20 (48 .7%) had evidence of intraocular production of T gondii-specific antib ody on the basis of either an IgM or IgG C value of 10 or greater. A C -tc value could not be calculated in 3 cats because calicivirus-specif ic IgG was not present in aqueous humor. Of the 38 eats for which C-tc values could be calculated, 25 (65.8%) had evidence of intraocular pr oduction of T gondii-specific antibody on the basis of either an IgM o r IgG C-tc value of 1 or greater. The C values and C-tc values were in agreement for 75.9% of IgM containing samples and 75% of IgG containi ng samples. Sensitivity, specificity, predictive value of a positive t est result, and predictive value of a negative test result for an IgM or IgG C value, when compared with the corresponding IgM or IgG C-tc v alue were determined. The results indicate that use of the C value for estimation of intraocular T gondii-specific antibody production will result in 28.6 (IgM) to 50% (IgG) false-negative results and 12.5% (Ig M and IgG) false-positive results, when compared with the C-tc value.