Detection of Salmonella dublin mammary gland infection in carrier cows, using an enzyme-linked immunosorbent assay for antibody in milk or serum

Abstract

SUMMARY An elisa has been developed for measurement of milk and serum IgG concentrations directed against Salmonella dublin . Four groups of cows were studied: group A—7 experimentally challenge-exposed cows (infected, recovered group); group B—6 normal uninfected randomly selected control cows; group C—7 naturally occurring S dublin carrier cows; and group D—6 normal uninfected S dublin negative cows from the same herd as group C. Group-A cows were inoculated orally, or inoculated orally and then iv , but none became a S dublin carrier. As expected, all 7 group-A cows responded with a marked increase in elisa titer after oral exposure to virulent S dublin , starting with a mean serum titer of 17.7% and reaching a peak mean serum titer of 79.3% approximately 76 days after initial exposure. As determined by necropsy and organ culturing of the remaining cows, none of the group-A cows became carriers. The mean serum elisa titer for group-B uninfected control cows was 14.1% ( sd ± 12.8%). The mean milk elisa titer was -1.0% ( sd ± 5.5%). Colostrum and then milk gave false-positive results for up to 2 weeks after onset of lactation. Group-B cows were culture negative for S dublin in feces and milk during lactation, and when tissues were cultured after euthanasia. Milk and serum samples for elisa , and milk and fecal samples for culturing were taken from all group-A and -B cows twice a week for 6 months. Statistical correlation ( P < 0.05) was found between serum and milk elisa titers. A highly significant ( P < 0.001) difference in serum elisa titers was demonstrated between control (group B) and infected cows (group A). Milk and feces from group-C carrier cows were cultured for S dublin 5 days a week for 11 to 13 months. Six of the 7 cows calved during this period. Fecal shedding was sporadic in 7 cows. Milk shedding was frequent in certain quarters of 4 of the cows and was sporadic or absent in other quarters of these cows and it was sporadic in 2 cows, and 1 cow had culture-positive milk only twice. The overall milk-shedding rate was 46% (792 positives/1,733 samples), whereas the overall fecal-shedding rate was 4% (65 positives/1,733 samples). Shedding in the 4 weeks after parturition was 28% in milk and 5% in feces. Six group-C cows had strongly positive elisa titers in serum and milk, whereas 1 cow (the cow that had only 2 positive milk cultures) had relatively low elisa titers. Group-C cows had a mean serum titer of 85.2% ( sd ± 19%) and mean milk titer of 70.6% ( sd ± 35.5%). These results indicate that IgG elisa may be useful in detection of S dublin milk shedding (mammary gland infection) carrier cows. Milk shedding in the 4 persistent shedders ranged from 10 1 to 10 5 organisms/ml, and was associated with evidence of chronic active mastitis. Group-D cows, culture-negative herd mates of group-C carrier cows, were monitored in a manner identical to that used for group-C cows. All cows remained culture-negative for S dublin in feces and milk and results of organ culturing were negative for S dublin after euthanasia. The elisa titers remained negative, with a mean group-D titer of 8 ± 7.7% on serum, and 0.6 ± 5.5% on milk. A highly significant difference in serum ( P < 0.0001) and milk ( P < 0.0001) elisa titers was demonstrated between group-C carrier cows and group-D uninfected herd mates.