Reduced conception rates associated with bovine mastitis during a ‘window of opportunity’

MASTITIS is a common disease of dairy cows and an important cause of economic loss to the dairy industry as a consequence of compromised milk quality and lower milk yields. Mastitis also affects fertility, and has been associated with longer intervals from calving to first insemination or conception, and lower conception rates (Barker and others 1998, Santos and others 2004). However, the precise timing of the effects of mastitis on fertility are not clear. This short communication describes a study to test the hypothesis that clinical mastitis is associated with lower conception rates in cattle and to determine when mastitis exerts its effects on conception. Information was collected retrospectively from seven Holstein dairy herds. The animals were housed in cubicles over the winter months and turned out to pasture in the summer. The cows were milked twice daily and fed total mixed rations, tailored to meet the requirements of their stage of lactation. All inseminations were by artificial insemination, and accurate service dates were recorded. The average conception rate for the seven herds around the time of the study was 44 per cent (range 36 to 54 per cent). Each herd was selected for its consistent and complete high-quality health recording, and had a long history of fortnightly routine fertility visits by the same experienced veterinary surgeon (R. W. B.). At these visits, pregnancy determination was performed by transrectal ultrasonography in cows at least 30 days after the last recorded insemination. Data for cases of clinical mastitis were obtained by inspection of the farm records for instances of intramammary antibiotic treatment. The data from each farm were collected and recorded in the Interherd computer recording system (NMR) at each veterinary visit by a technician. The data included each cow’s identity, lactation number, calving date, insemination and conception dates. Data were transferred from the Interherd system to an Excel database (Microsoft), and the accuracy of data transfer was confirmed by an independent technician. The data were then examined, and cows affected with mastitis were paired with an unaffected cow on the same farm, in the same lactation, within seven days of the time between calving and first insemination, and within two months of the month of calving. The pairs of animals were grouped into groups in which mastitis was diagnosed and treated during the three weeks (48 animals) or six to three weeks (34 animals) before insemination; or those in which mastitis was diagnosed during the three weeks (44 animals) or three to six weeks (34 animals) after insemination. The grouping of animals with respect to the time of first insemination was chosen to determine whether mastitis exerted an effect before fertilisation or on the embryo. Within each group, the proportion of mastitic and normal cows that conceived were compared using Pearson chi-squared analysis. The calving to conception interval and the calving to first service interval were explored using Kaplan-Meier survival analysis, and comparisons between groups were made using the log rank test; conceptions more than 200 days after calving were censored. Statistical tests were performed using SPSS version 13.0 (SPSS) and significance was attributed when P<0·05; data are quoted as mean (se). More unaffected cows became pregnant to first insemination than cows with mastitis that was diagnosed and treated within the three weeks before insemination (15 of 24 [63 per cent] v eight of 24 [30 per cent]) (P<0·05), but conception rates were the same in unaffected animals and in paired animals that had been diagnosed with mastitis in the period three weeks after first insemination (nine of 22 in both groups). The conception rates did not differ significantly between the unaffected cows and cows that had mastitis between six and three weeks before insemination (six of 17 v eight of 17) (P=0·49) or between three and six weeks after insemination (four of 17 v eight of 17) (P=0·15). The normal cows and cows that had mastitis during the three weeks before insemination had similar calving to first insemination intervals (83·2 [6·1] v 81·4 [3·4] days) and similar numbers of services per conception (1·7 [0·23] v 2·04 [0·22]). The calving to conception interval did not differ significantly between the normal and mastitic cows (102 [11] v 119 [eight] days). However, the effect of the lower conception rate to first insemination was clearly illustrated by a shift to the right of the survival curve for cows with mastitis (Fig 1). In the present study, reduced conception rates were observed in cows that suffered mastitis within the three weeks before insemination, but not at other times. This difference was particularly evident, as the unaffected cows had a notably high conception rate. However, the strength of the present study was the pairing of affected and unaffected animals. In addition, the animals that had mastitis before insemination were likely to be as fertile as the normal animals, because they had a similar number of inseminations per conception. Thus, it would appear that the mastitis exerted an effect on the developing oocyte, ovarian antral follicle growth or ovulation, rather than on the survival of the embryo. The mechanism for this effect is not known, but it may involve the effects of bacterial products or the associated inflammatory response from the mammary gland. Inflammatory responses associated with mastitis can blunt the preovulatory luteinising hormone surge (Hockett and others 2000), and cytokines produced during mastitis can also interfere with oocyte maturation (Soto and others 2003). Similar effects occur with other infections, such as uterine disease, in cattle where ovarian follicle growth is perturbed (Sheldon and others 2002). In conclusion, there appears to be a ‘window of opportunity’ for the infection or the inflammatory response to disrupt the growth or function of the ovarian follicle or oocyte, but further investigation is required to elucidate the mechanisms underlying these observations. Veterinary Record (2007) 161, 61-62