The Development of a Streptoccocus uberis mastitis Challenge Protocol
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Date
2007-03-11
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Abstract
A series of experiments was conducted using a S. uberis bacterial isolate in order to produce a predictable and effective protocol for experimental induction of mastitis. A bacterial isolate was obtained from a clinical mastitis infection in a local, North Carolina dairy herd. In vitro growth of the bacterial isolate produced a predicted lag, log, stationary, and death phase. Comparison of optical absorbance values to bacterial concentrations resulted in a Pearson's correlation coefficient of 0.71. It was determined that time of growth would be a better indicator of bacterial concentration. S. uberis isolates were incubated for 3 hours of growth and placed on ice to observe temporal changes in bacterial concentration. Bacterial concentration did not differ over time (P=0.96), indicating that ice storage maintained S. uberis concentrations for 3 hours. S. uberis bacterial concentrations deviated from predicted values in growth curves, so an alteration to the serial dilution protocol was performed in order to reach the desired concentration.
Four Holstein cows were challenged with 5, 400 cfu of S. uberis in 2 quarters that contained no mastitis pathogens and were compared to 4 Holstein control cows. The inoculum concentration produced a 100% incidence of clinical infection in all quarters challenged. The challenged group exhibited elevated quarter milk scores, quarter pain scores, attitude scores, and quarter size scores, with first clinical signs occurring at 36 hours. Rectal temperatures were highest at 36 hours (40.5 ± .3 o C vs. 38.5 ± .3 o C; P<0 .0001) (mean ± SEM) compared to control. Challenged quarter somatic cell counts (SCC) were elevated at 24 hours post-infusion in comparison to control quarter SCC at 0 (Challenge 24: 3.5 X 106 ± 5.6 X 106 vs. Control 0: 4.3 X 104 ± 3.1 X 104; P < 0.0001) and 24 hours (Challenge 24: 3.5 X 106 ± 5.6 X 106 vs. Control 24: 6.7 X 104 ± 5.0 X 104; P = 0.003). Challenge quarter SCC at 24 hours also differed from challenge SCC values at time of challenge (Challenge 24: 3.5 X 106 ± 5.6 X 106 vs. Challenge 0: 1.4 X 105 ± 2.1 X 105; P < 0.0001).
While a predictable mastitis model was acquired using 5,400 cfu, the response in challenged cows was more severe than preferred. In the attempt to obtain a clinical infection that was physiologically more benign (low-grade pyrexia, less severe quarter abnormalities, and faster response to treatment), it was determined that lower doses might still produce a high incidence of clinical infection while presenting less severe clinical signs. In this trial, 500 cfu (low dose; n=6 quarters in 3 cows), 1,950 cfu (medium dose; n=5 quarters in 3 cows), and 4,000 cfu (high dose; n=6 quarters in 3 cows) were inoculated into quarters that contained no intramammary pathogens. All cows (n=9) and 13⁄17 (76.5%) quarters developed a clinical infection. Regardless of dose, an overall elevation in SCC from baseline values was reported following challenge (P < 0.0001). SCC was found to be elevated in the high dose group on day 2, and in both the medium and high dose group on days 3 and 4 (P < .05). The low dose tended to be elevated 2 days post challenge (P < 0.1). Regardless of dose, peak rectal temperature after challenge (39.8 ± .2 ◦C) was higher (P = 0.03) than initial rectal temperature (39.1 ± .1 ◦C). Milk score, quarter pain score, quarter size score, attitude score, and appetite score were elevated following challenge for each dose. Udder temperature was elevated only in the low group (P < .05). Serum levels of IL-1β, TNF-α, and IL-8 were not elevated during the sampling period. Five hundred cfu of this S. uberis strain produced a predictable clinical infection in challenged cows. Therefore, higher doses would not be needed to produce mastitis.
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Keywords
mastitis, uberis, Streptococcus, experimental challenge
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Degree
MS
Discipline
Animal Science
