Experimental model of toxin-induced subclinical mastitis and its effect on disruption of follicular function in cows

This study establishes an experimental model for subclinical mastitis induced by Gram-positive (G+) exosecretions of Staphylococcus aureus origin or Gram-negative (G-) endotoxin of Escherichia coli origin to examine its effects on follicular growth and steroid concentrations in Holstein dairy cows. Cows were synchronized with the Ovsynch protocol followed by a series of follicular cycles that included GnRH and PGF_2α doses administered every 8days. Cows received small intramammary doses of either G+ (10μg, n=10) or G- (0.5μg, n=6) toxin, or saline (n=6; uninfected control) every 48hours for 20days. Follicular fluids were aspirated from preovulatory follicles before (aspiration one: control), at the end of (aspiration two: immediate effect), and 16days after the end of (aspiration three: carryover effect) toxin exposure. During the 3weeks of subclinical mastitis induced by G+ or G-, no local inflammatory signs were detected in the mammary gland and no systemic symptoms were noted: body temperatures of the treated cows did not differ from controls; plasma cortisol and haptoglobin concentrations were not elevated and did not differ among groups. Somatic cell count was higher in the treated groups than in controls, and higher in the G- versus G+ group. For analysis of reproductive responses, cows were further classified as nonaffected or affected based on an more than 20% decline in follicular androstenedione concentration in aspiration two or three relative to the first, control aspiration. Most G- (5/6) and 40% of G+ (4/10) cows were defined as affected by induced mastitis. An immediate decrease in the number of medium-size follicles was recorded on Day 4 of the induced cycle, toward the end of the 20-day mastitis induction, in the affected G+ compared with uninfected control group (1.0±0.5 vs. 3.0±0.4 follicles; P<0.05); the affected G- and nonaffected G+ subgroups exhibited a similar numerical decline in the number of follicles. A carryover (but not immediate) decrease to 51% and 62% in follicular estradiol concentrations in G- affected group and G+ affected group was detected relative to controls (P<0.05). The nonaffected G+ subgroup did not differ from its control counterparts. Based on the current experimental model, subclinical IMI induced by G+ or G- toxin disrupts follicular functions, and it seems that the ovarian pool of early antral follicles is susceptible to subclinical mastitis.