Observed and expected combined effects of clinical mastitis and low body condition on pregnancy loss in dairy cows

Impact of subclinical mastitis on uterine health, reproductive performances and hormonal profile of Zebu × Friesian crossbred dairy cows in and around Jimma town dairy farms, Ethiopia

A longitudinal observational study was carried out from January 2020 to July 2021 to assess the impact of subclinical mastitis (SCM) on reproductive performance and its association with uterine health of crossbred dairy cows. The California Mastitis Test (CMT) and cytobrush technique were used to screen subclinical mastitis and subclinical endometritis, respectively. Milk samples positive for subclinical mastitis were subjected to bacteriological analysis. Data from 84 clinically healthy cows collected and analyzed. The present study revealed a prevalence of subclinical mastitis of 51.2% (43 of 84). The mean days from calving to first service interval were significantly longer in subclinical mastitis positive cows than negative (control) cows (120.51 ± 24.5 and 85.15 ± 28.3, respectively) (P < 0.05). The mean number of services per conception was significantly higher in positive cows (2.51 ± 0.83) than in negative cows (1.59 ± 0.81) (P < 0.05). Lower conception and pregnancy rates at first services were observed in subclinical mastitis cows. Risk factors analysis revealed that prevalence of subclinical mastitis significantly differed with the parity and body condition score (P < 0.05). The current study revealed that subclinical mastitis was significantly and directly associated with subclinical endometritis (P < 0.05). Subclinical mastitis significantly decreased (P = 0.000) progesterone concentrations and increased (P = 0.001) the cortisol concentrations. Staphylococcus aureus were the most predominant bacterial isolates from subclinical mastitic milk, followed by coagulase negative staphylococci (CNS) and streptococci. This study concludes a high prevalence of subclinical mastitis caused by Staphylococcus aureus could inflict harmful effects on reproductive performance of dairy cows, emphasizing the relevance of mastitis control programs in dairy farms.

Association between body condition score fluctuations and pregnancy loss in Holstein cows

The objective of this study was to characterize the associations between body condition score (BCS) and BCS change (∆BCS), determined by an automated camera system during early lactation and close to artificial insemination (AI), and the subsequent pregnancy loss (PL) in Holstein cows. A secondary objective was to determine the impact of disease on PL, considering multiple time periods relative to AI. Data from 9,430 lactations in 6,884 Holstein cows in a commercial dairy operation located in Colorado, USA were included in this retrospective observational study. Cows were subject to first AI at about 80 DIM (primiparous) and 60 DIM (multiparous), following a double OvSynch protocol. Pregnancy diagnosis was performed via transrectal ultrasonography on day 32 ± 3 after AI and reconfirmed on day 80 ± 3 after AI. Cameras mounted on the sorting gate at each exit (n = 2) of the milking parlor generated BCS on a five-point scale with 0.1 increments. The BCS at calving (BCS1), 21 DIM (BCS21), 56 DIM (BCS56), AI resulting in pregnancy (BCSAI), and 90 d post AI (BCSAI90) were selected for the analyses and subsequently categorized as low (≤ lower quartile), moderate (interquartile range), and high (≥ upper quartile). Changes in BCS were calculated by periods of interest as change from calving to 21 DIM; change from calving to 56 DIM; change from 56 DIM to AI; and change from AI to 90 d post AI and assigned into categories to facilitate the analysis. Data were examined using logistic regression, considering parity category, season at calving and AI, DIM at AI, milk yield up to 60 DIM, and occurrence of disease as covariables. The logistic regression analyses indicated that the odds of PL were greater in cows in the low BCS category relative to cows in the high BCS category at 56 DIM (OR 95% CI = 1.41 [1.12-1.79]), AI (1.31 [1.05-1.65]), and 90 d post AI (1.38 [1.10-1.74]). Likewise, cows with large loss in BCS between calving and 21 DIM (1.46 [1.10-1.94]) and loss in BCS between AI and 90 d post AI (1.44 [1.15-1.81]) had greater odds of PL compared with cows with no loss of BCS within the same period. Occurrence of disease at all the time periods considered in the analysis had a consistent detrimental impact on maintenance of the pregnancy, supporting the concept that pre and postconceptional disease affects embryonic survival. Overall, low BCS, more pronounced reductions in BCS occurring closer to AI, and occurrence of disease resulted in greater PL in this Holstein population.

Changes in body condition score from calving to first insemination and milk yield, pregnancy per AI, and pregnancy loss in lactating dairy cows: A meta-analysis

We determined the association of body condition score (BCS) at calving, at first postpartum artificial insemination (AI), and change in BCS between calving and first AI on pregnancy per AI (P/AI) at 30-45 d, pregnancy loss to 60-85 d, and milk yield in lactating dairy cows. Outcome data were included from 15 studies and 47 herd-year combinations. Additional variables included season of AI, herd, days in milk at first AI, parity, and of mean daily milk yield within 2 wk of first AI. The BCS scale employed was a standard 1-5 scale (1 = severe under conditioning or emaciated and 5 = severe over conditioning) with 0.25 cut points. Presynchronization treatments that included PGF_2α and GnRH increased (P < 0.05) the proportion of cows with luteal function before AI compared with PGF_2α alone. Compared with no presynchronization treatment those that included PGF_2α or PGF_2α and GnRH increased (P < 0.05) first P/AI. Cows having BCS ≥2.75 at AI had greater (P < 0.01) first P/AI than cows with BCS <2.75. As BCS at first AI increased, P/AI increased in a linear (P = 0.04) fashion and was greater in cows expressing estrus when BCS at AI was <2.50. Presynchronization had no association with P/AI for cows with BCS at calving <3.00 compared with those with BCS ≥3.00. In contrast, multiparous cows tended (P = 0.06) to have greater P/AI when they calved with BCS ≥3.00 compared with <3.00. Increasing BCS at AI was associated with decreased (P = 0.01) pregnancy loss. Pregnancy per AI did not differ among cows according to the magnitude of prebreeding BCS loss, but more multiparous cows losing more than 0.5 units of BCS tended to have greater pregnancy losses in second-parity cows (P = 0.09) and in cows of third or greater (P < 0.001) parity. Daily milk yields at first AI differed among parities as expected, but a parity by BCS at calving interaction was detected (P = 0.008). Daily milk yield at first AI decreased (P < 0.001) linearly as BCS at AI increased, with an exacerbated greater negative effect during summer. More prebreeding loss in BCS was associated with more (P < 0.05) milk yield in first- and second-parity cows. We concluded that greater BCS at first AI was associated with improved P/AI, but magnitude of prebreeding BCS loss was not associated with P/AI. In contrast, more pregnancy loss was associated with more prebreeding BCS loss in multiparous cows. Cow having lesser BCS at AI and greater prebreeding loss in BCS produced more milk than their herd mates of greater BCS and lesser prebreeding loss in BCS, respectively.

Mini-Review: The high fertility cycle

The development of fertility programs and their adoption by the dairy industry over the past decade is a major driving factor underlying the dramatic increase in reproductive performance in lactating dairy cows during the past 20 years. Another major driving factor underlying this increase in reproductive performance is what we describe in this minireview as the "high fertility cycle." We now know that reproductive performance and the incidence of certain periparturient health events are interrelated. The high fertility cycle describes the relationship between body condition score (BCS) change during the periparturient period and postpartum health events and subsequent reproductive performance in which lactating dairy cows that establish pregnancy by 130 d in milk have shorter calving intervals and thereby gain less BCS during the current lactation and dry off and calve at a lower BCS (2.75 to 3.0) than cows with a longer lactation. After calving, these cows undergo less BCS loss, experience fewer health issues, have greater fertility at first insemination, and have reduced early pregnancy losses after establishment of pregnancy and thereby become pregnant before 130 d in milk. This minireview overviews these relationships and highlights the key concepts underlying the high fertility cycle. Future randomized, controlled experiments are needed to causally link these relationships between BCS change and fertility in lactating dairy cows.

Subclinical Mastitis Detected during the Last Gestation Period Can Increase the Risk of Stillbirth in Dairy Calves

Simple Summary

The aim was to investigate the relation of subclinical mastitis detected during the last gestation period and its pathogens with stillborn calves, considering that parity and herd size may also affect this result. This study shows that the late gestation period is challenging for stillbirth in next lactation. Collectively, these results suggest that decreasing incidence of subclinical mastitis during the last gestation period (from the 210th day of pregnancy) can decrease the risk of stillbirth in dairy calves. Further, it is important to identify the pathogen because the highest risk of stillbirth was found in cows with mastitis caused by Escherichia coli, Staphylococcus aureus, Streptococcus agalactiae, pathogenic Staphylococci and other Streptococci. Cows at the first calving were 1.38–1.65-times higher risk for the stillbirth of calves than in cows of parity ≥ 2.

Abstract

We hypothesized that subclinical mastitis detected during the last gestation period can increase the risk of stillbirth in dairy calves. The aim was to investigate the relation of subclinical mastitis detected during the last gestation period and its pathogens with the stillbirth of calves. Cows from the 210th day of pregnancy were selected for the study. They were divided into two groups: the first group—subclinical mastitis was confirmed on the farm by the California mastitis test (CMT); the second group of cows—mastitis was not confirmed by the CMT test. Groups of cows were compared according to the results of their calving—the number of stillborn calves. A stillborn calf was defined as a calf that dies at birth or within the first 24 h after calving, following a gestation period of 260 days. Our results suggest that decreasing the incidence of subclinical mastitis during the last gestation period (from the 210th day of pregnancy) can decrease the risk of stillbirth in dairy calves. Further, it is important to identify the pathogen because the highest risk of stillbirth was found in cows with mastitis caused by Escherichia coli, Staphylococcus aureus, Streptococcus agalactiae, pathogenic Staphylococci and other Streptococci. Cows at the first calving had a 1.38–1.65-times higher risk of having stillborn calves than cows of parity ≥ 2. From a practical point, veterinarians and farmers can consider the effect of subclinical mastitis during late gestation on the risk of stillbirth and it could help for strategies of optimizing reproductive performance in dairy cows.

Symposium review: The role of adipose tissue in transition dairy cows: Current knowledge and future opportunities

Adipose tissue (AT) is a central reservoir of energy stored in the form of lipids. In addition, AT has been recognized as an immunologically and endocrinologically active tissue of dairy cattle. The recent literature on AT biology of transition dairy cows has often focused on the possible negative effects that originate from excessive body fat. However, the highly efficient energy-storage capability of this tissue is also vital to the adaptability of dairy cattle to the change in nutrient availability, and to support lactation and reproduction. An excessive degree of mobilization of this tissue, however, is associated with high circulating fatty acid concentrations, and this may have direct and indirect negative effects on reproductive health, productivity, and disease risk. Furthermore, rapid lipolysis may be associated with postpartum inflammation. Research on the role of AT is complicated by the greater difficulty of accessing and measuring visceral AT compared with subcutaneous AT. The objective of this review is to provide a transition cow-centric summary of AT biology with a focus on reviewing methods of measuring AT mass as well as to describe the importance for production, health, and reproductive success.

Effect of body condition change and health status during early lactation on performance and survival of Holstein cows

Body condition score (BCS) and disease records are commonly available in dairy operations. However, the effect of BCS changes (ΔBCS) considering specific health profiles has not been investigated extensively. The objective of this study was to assess the effects of different levels of ΔBCS on fertility, milk yield, and survival of Holstein cows diagnosed with reproductive disorders (REP; dystocia, twins, retained fetal membranes, metritis, and clinical endometritis), other health disorders (OTH; subclinical ketosis, left displaced abomasum, lameness, clinical mastitis, and respiratory disease), or with no disease events (HLT) within 40 days in milk (DIM). Data included lactation information from 11,733 cows calving between November 2012 and October 2014 in 16 herds across 4 geographical regions in the United States (Northeast, Midwest, Southwest, Southeast). Cows were evaluated for BCS at 5 ± 3 DIM (BCS5) and at 40 ± 3 DIM (BCS40) and the difference between BCS40 and BCS5 was classified as excessive loss of BCS (EL; ΔBCS ≤-0.75), moderate loss (ML; ΔBCS = -0.5 to -0.25), no change (NC; ΔBCS = 0), or gain of BCS (GN; ΔBCS ≥0.25). Multivariable logistic regression was used for assessing potential associations between the outcomes of interest and ΔBCS and health. The effect of the interaction term ΔBCS by health group was not statistically significant for any of the study outcomes. The odds of resumption of ovarian cyclicity (ROC), in GN, NC, and ML cows were 1.94 (95% CI: 1.57-2.40), 1.59 (1.28-1.97), and 1.27 (1.10-1.47) times greater than the odds of ROC in EL cows, respectively. The odds of pregnancy at 150 DIM (P150) in GN cows were 1.61 (1.20-2.17) times greater than the odds of P150 in EL cows. Cows with REP or OTH disorders had smaller odds of ROC compared with HLT cows [REP: OR = 0.65 (0.56-0.76) and OTH: OR = 0.79 (0.68-0.92)]. For pregnancy outcomes, REP cows had smaller odds of pregnancy at the first artificial insemination compared with HLT cows [0.70 (0.58-0.84)]. Similarly, REP cows had smaller odds of being diagnosed pregnant by 150 and 305 DIM compared with HLT cows [P150: 0.73 (0.59-0.87), P305: 0.58 (0.49-0.69)]. Overall, average daily milk within the first 90 DIM was greater in EL (39.5 ± 1.13 kg/d) and ML (38.9 ± 1.11 kg/d) cows than in NC (37.8 ± 1.12 kg/d) and GN (36.2 ± 1.12 kg/d) cows. On the other hand, average daily milk within the first 90 DIM was lower in REP (37.0 ± 1.11 kg/d) cows compared with OTH (38.7 ± 1.12 kg/d) and HLT cows (38.6 ± 1.11 kg/d). The magnitude of ΔBCS and the health status of early lactation cows should be considered when assessing subsequent cow performance and survival.

Effect of peripartal administration of mycobacterium cell wall fraction on health and fertility of Holstein cows under organic-certified management

The main objective of this study was to evaluate the effect of peripartal administration of a commercially available nonspecific immune stimulant (mycobacterium cell wall fraction; MCWF [Amplimune, NovaVive Inc., Napanee, ON, Canada]) on the incidence of disease during early lactation and subsequent fertility of dairy cows. A second objective was to characterize the dynamics of circulating white blood cells (WBC) and metabolic markers following treatment administration. Cows in an United States Department of Agriculture (USDA) organic-certified dairy herd were blocked by parity and, based on sequential calving dates, randomly assigned to receive two injections (5 mL s.c.) of either a placebo (saline solution) as a control (CON; n = 71) or MCWF (n = 65) at enrollment (7 d before expected calving) and within 24 h after calving. Blood samples were collected from a subsample of the study population (MCWF = 16; CON = 18) for WBC count at enrollment, at day 2 post enrollment, and at days 1, 3, 7, and 14 after calving. Serum fatty acids, beta-hydroxybutyrate, and Ca concentrations were determined at days 1 and 7 postpartum (MCWF = 21; CON = 21). Main outcome variables included incidence risk of peripartal and early lactation health disorders and pregnancy at first artificial insemination (AI), at 100, and at 150 days in milk (DIM). In addition, the average daily milk yield up to 90 DIM and death and live culling before 305 DIM were compared. Treatment effects were assessed using multivariable logistic regression, time-to-event analyses, and repeated measures analysis of variance (ANOVA). A treatment effect on the incidence risk of some of the health disorders in the study was established. Incidence risk of metritis and clinical mastitis <28 DIM was smaller in MCWF than in CON cows (36.9% vs. 50.7% and 6.3% vs. 19.7%, respectively). On the contrary, the incidence risk of respiratory disease <28 DIM was smaller in CON (0%) than in MCWF (7.7%). Reproductive performance of multiparous cows was affected by MCWF administration: pregnancy at first AI and pregnancy at 100 and 150 DIM were greater in MCWF than in CON (35.6% vs. 19.2%; 51.1% vs. 25.0%; and 64.4% vs. 40.4%, respectively). Overall, median intervals from calving to pregnancy were 90 vs. 121 d in MCWF and CON cows, respectively. No treatment effects on the dynamics of circulating WBC or in postpartum metabolic status were established. No differences for milk yield or for the proportion of cows that survived up to 305 DIM were determined, although cows in MCWF left the herd earlier than cows in CON. In conclusion, incidence risks of metritis and mastitis in early lactation were smaller in cows receiving MCWF, whereas the incidence risk of respiratory disease was smaller in CON. Fertility significantly improved in MCWF compared with CON cows. As this study was performed in an organic-certified dairy, specific health and reproductive management practices may affect the external validity of the current findings.

Early-lactation diseases and fertility in 2 seasons of calving across US dairy herds

The objective of this study was to characterize incidences of health disorders during early lactation in a large population of Holstein cows calving in 2 seasons across multiple US dairy herds. In addition, cumulative effects of combinations of health-related events on fertility and survival by season of calving and parity number were tested. Data were prospectively collected from a total of 11,729 cows in 16 herds located in 2 regions in the United States [north (7,820 cows in 10 herds) and south (3,909 cows in 6 herds)]. Cows were enrolled at parturition and monitored weekly for disease occurrence, reproductive events, and survival. Health-related events were grouped into reproductive disorders (REP; dystocia, twins, retained fetal membranes, metritis, and clinical endometritis) and other disorders (OTH; subclinical ketosis, mastitis, displaced abomasum, and pneumonia). Counts of health events within 50 d postpartum were added into each of the groups and categorized as 0, 1, 2, 3, and ≥4 for REP and 0, 1, 2, and ≥3 for OTH. Multivariable logistic regression was used for testing potential associations between categories of disease occurrence and outcome variables, including resumption of ovarian cyclicity, pregnancy per artificial insemination (AI), pregnancy loss, and survival up to and after 50 DIM. The incidence of disease varied with season of calving and parity, and these 2 variables were associated with the reproductive and survival outcomes. The size of the detrimental effect of disease incidence on reproduction and survival depended on disease group and varied for each specific outcome. Resumption of ovarian cyclicity decreased as incidences of disorders increased in both REP and OTH categories. Pregnancy at first AI also was smaller in greater number of REP categories, but the effect of number of OTH categories on pregnancy at first AI was not consistent. Similarly, pregnancy loss at first AI was not affected consistently by REP or OTH. Survival was reduced by REP and OTH. The magnitude of these negative effects was variable, depending on season of calving and parity, but consistently increased with the number of health events during early lactation.

Incidence and risk factors for reproductive disorders in organic certified dairies

The objective was to determine the lactational incidence risks of reproductive disorders (RD) including dystocia (DYS), retained fetal membranes (RFM), puerperal metritis (PM), clinical endometritis (CE), pyometra (PYO), late pregnancy loss (LPL), and culling due to reproductive reasons (CRR) in large organic certified dairies with predominant use of Holstein genetics. In addition, potential risk factors for RD, including parity category, milk yield in the previous lactation, season of dry-off, access to grazing at dry-off, length of the dry period (DPL), length of the close-up period (CUPL), calculated gestation length (CGL), season of calving, access to grazing at calving, concurrent health conditions, and total number of artificial inseminations were analyzed. A retrospective study was performed using information from 8 organic certified herds located in northern Colorado and west Texas, ranging from 900 to 6,000 milking cows. Cow information consisted of finalized lactations starting with calvings occurring between January 1, 2016, and December 31, 2018. Data were analyzed using time-to-event analyses and multivariable logistic regression. Clinical endometritis had the greatest lactational incidence risk (incidence risk = 10.1%; 95% CI = 9.8-10.3%), followed by DYS (8.8%; 8.5-9.1%), PM (4.9%; 4.7-5.1%), PYO (4.0%; 3.8-4.1%), LPL (3.6%; 3.4-3.7%), and RFM (2.0%; 1.8-2.1%). The incidence risk of CRR was 7.5% (7.3-8.7%), representing 29.1% (28.4-30.0%) of all culling reasons. Median time from calving to diagnosis of PM, CE, PYO, and LPL were 6, 23, 111, and 228 d, respectively. Median time for CRR was 391 d. The main risk factors for RD were DPL, CGL, and CUPL. Season of dry-off was a risk factor for RFM, PM, CE, LPL, and PYO. Short DPL (<30 d) was associated with increased odds (95% CI) of RFM [2.1 (1.6-2.7)], PM [1.4 (1.2-1.7)], and LPL [2.5 (1.5-4.2)]. Shortened CGL (<270 d) increased the odds of DYS [1.5 (1.3-1.8)], RFM [3.2 (2.4-4.1)], and PM [2.4 (1.7-3.3)], whereas extended CGL (>285 d) increased the odds of DYS [1.5 (1.4-1.7)], PM [1.5 (1.2-2.0)], CE [1.5 (1.1-2.0)], and CRR [1.2 (1.10-1.4)]. Shortened CUPL was associated with increased odds of all the RD. Cows spending less than 7 d in the close-up groups had greater odds of DYS [1.4 (1.1-1.8)], RFM [2.1 (1.8-2.4)], PM [1.9 (1.6-2.1)], PYO [1.5 (1.2-1.8)], LPL [2 (1.7-2.3)], and CRR [1.4 (1.04-1.8)] compared with cows spending more than 14 d in the close-up group. Season of calving was associated with the odds of PM [summer = 1.13 (1.01-1.2)], PYO [summer = 1.7 (1.1-2.5)], and LPL ([fall = 0.8 (0.6-0.96)] compared with winter calving. Other risk factors affecting RD were parity number, total number of artificial inseminations, and concurrent health conditions. Overall, lactational incidence risks of reproductive diseases were comparable with those reported in conventional herds. The identified risk factors for RD open opportunities for interventions, especially focused in precalving management, estimation of due dates, and closer monitoring of pregnancies from natural services in large organic certified dairy herds.

Quantifying the effects of mastitis on the reproductive performance of dairy cows: A meta-analysis

The objective of this study was to conduct a meta-analysis to quantitatively characterize the effects of mastitis on the reproductive performance of dairy cows as well as to identify factors that interact with this relationship. A total of 29 publications were identified that contributed 24, 41, 27, 38, and 13 trial results to the meta-analysis of how mastitis is related to time to first service (TFS), days open (DO), services per conception (SPC), pregnancies per insemination at first service (FSP/AI) and pregnancy loss (PL), respectively. The meta-analyses were conducted using multilevel linear mixed-effects models. Overall, high levels of heterogeneity were present and meta-regression models only explained a small amount of heterogeneity. Results suggest that cows with mastitis pre-first insemination experience, on average, an additional 13.29 d to first service [95% confidence interval (CI): 6.64, 19.95] when compared with cows with no mastitis in the same time period. Moreover, in relation to cows with no mastitis in the same time period, 22.34 additional DO (95% CI: 12.89, 31.79) were estimated, on average, for cows with clinical mastitis at pre-insemination leading to conception. Additionally, 32.41 added DO (95% CI: 20.58, 44.25) were estimated, on average, for cows with clinical mastitis at pre- or post-insemination leading to conception compared with cows with no mastitis in the same time period. Finally, 20.03 additional DO (95% CI: 3.11, 36.95) were estimated, on average, for cows with subclinical mastitis pre- or post-insemination leading to conception compared with cows with no mastitis in the same time period. Effect size estimates from the meta-regression models for SPC, in relation to cows with no mastitis in the same time period, suggest that, on average, SPC increases by 0.46 inseminations (95% CI: 0.30, 0.62) for a cow experiencing mastitis pre-insemination leading to conception. When mastitis occurs either pre- or post-insemination leading to conception, SPC is expected to increase, on average, by 0.72 inseminations (95% CI: 0.48, 0.95) compared with cows with no mastitis in the same time period. The estimated effect sizes for FSP/AI suggest a risk ratio of conceiving at first insemination of 0.90 (95% CI: 0.82, 0.99) for cows with mastitis diagnosed pre-first insemination with respect to cows with no mastitis in the same time period, and a risk ratio of 0.79 (95% CI: 0.72, 0.86) for cows diagnosed with mastitis either pre- or post-first insemination with respect to cows with no mastitis in the same time period. Publication bias was identified in 4 of the meta-analysis models (TFS, DO, SPC, and FSCR), but no influential trials were identified in any models; the reliability of the meta-analysis results should be interpreted carefully keeping these limitations in mind. Further meta-regression analysis would be valuable as additional studies are published that report other potential sources of heterogeneity.

Epidemiologic and economic analyses of pregnancy loss attributable to mastitis in primiparous Holstein cows

The main objective of the study reported here was to examine the association between pregnancy loss (PL) and previous exposure to clinical or subclinical mastitis before breeding or during gestation in primiparous Holstein cows. A secondary objective was to estimate the cost of clinical mastitis during gestation, including that of PL attributable to mastitis in study cows. A total of 687 primiparous Holstein cows from 1 dairy farm were included in a matched case-control study. Study cows were declared pregnant via ultrasound on d 33 after timed artificial insemination (TAI). Case cows (n = 78) were those diagnosed as nonpregnant by rectal palpation on d 47 or 75 after TAI. Control cows were those confirmed as pregnant by rectal palpation on d 47 and 75 after TAI. Case cows were matched with eligible controls according to year of calving and calving-to-conception interval ±3 d. Cows were assigned to 1 of 3 groups: (1) cows not affected with clinical or subclinical mastitis; (2) cows affected with subclinical mastitis (Dairy Herd Improvement Association somatic cell score >4.5); and (3) cows affected with clinical mastitis during 2 exposure periods, 1 to 42 d before breeding or during gestation (1 to PL diagnosis day for case cows, and 1 to 75 d for control cows). Conditional logistic regression was used to model the odds of PL as a function of previous exposure to mastitis in study cows. Mastitis before breeding was not associated with PL. The odds of PL were 2.21 times greater in cows affected with clinical mastitis during gestation (95% confidence interval = 1.01, 4.83), compared with cows without mastitis, after controlling for breeding type and lameness. The cost of clinical mastitis during gestation was $149, which includes the cost ($27) of PL attributable to mastitis. In conclusion, this study provides evidence that clinical mastitis during gestation can cause PL in primiparous dairy cows leading to economic losses.