Diagnostic performance of two serologic tests and fecal culture for subclinical paratuberculosis, and associations with production

Abstract

A sample of three dairy herds served by the same private veterinary practice was selected on the basis of similarrity of size, calf-rearing practices and owner cooperation. All animals were tested for paratuberculosis three or four times at 6-month intervals by fecal culture, enzyme-linked immunosorbent assay (ELISA), agar-gel immunodiffusion (AGID) serology. Fecal-culture results were reported to herd owners through their veterinary practitioner. Production data and somatic-cell counts were collected from two of the herds using DHI (Dairy Herd Improvement, Powell, OH) records. Sensitivity and specificity relative to fecal culture of single test results were 71% and 83% for ELISA and 38% and 100% for AGID, respectively. Repeated tests gave sensitivity and specificity of 73% and 61% for ELISA and 33% and 100% for AGID when tests were interpreted in parallel testing. A maximum-likelihood procedure was used to generate estimates of sensitivity and specificity of single (91% and 82%) and repeated (73% and 66%) ELISA testing. There were no significant differences by sign test between fecal culture and ELISA in the time period which infection was first identified ( P=0.38). Fecal culture detected infection earlier than AGID. Mean milk production (controlling for differences in somatic-cell counts) was 3275 lbs. per lactation or 18.8% higher in fecal culture-negative cows than in fecal culture-positive cows. No associations were found between milk production and ELISA results.