Lack of correlation between exposure to lyme disease (Borrelia burgdorferi) and pregnancy loss in mares

Abstract

Exposure to Borrelia burgdorferi, the agent of Lyme disease, has been incriminated as causing an increased incidence of reproductive failure in mares. To test this hypothesis, B. burgdorferi (Lyme) antibody concentrations and incidence of early embryonic death (loss of embryo <45 days gestation, EED) and abortion (loss of fetus >45 days gestation) were determined in mares on two farms, one with a high (66% in November, 1990) incidence of Lyme exposure (Farm A, n=78 mares) and one with a low (<3%) incidence of positive Lyme results (Farm B, n=47 mares). Broodmare management practices and nutrition were similar on the two farms. All mares were bred by artificial insemination and checked by ultrasound for pregnancy at 12-20 days and again at 34-45 days after breeding. Blood samples for ELISA determination of B. burgdorferi antibody concentrations were taken prior to thebreeding season (November, 1990) and at the 34-45 day pegnancy check (February - June, 1991). Farm A had a high incidence of Lyme exposure with 23 seroconversions recorded at the time of the second pregnancy check. Only one of the mares exhibited clinical signs of Lyme disease. There was no difference in the incidence of EED between the two farms (Farm A: 12.8%; Farm B: 7.3%; P=.25) though abortions after 45 days tended to be higher on Farm A (10%) than on Farm B (2%, P=.09). There was no correlation (P=.5) with the Lyme status of mares and occurrence of EED or abortion, however, on Farm A, Early embryonic loss (but not abortion) tended to be correlated with longer residence on Farm A(EED: 31±3 mo; no EED: 17±3 mo, P=.10) though length of residence did not affect Lyme status of the mares at the time of breeding (P=.85) nor was age of the mare correlated in the incidence of EED (P=.85). Exposure to B. burgdorferi does not significantly increase the risk of EED in mares. The effect of chronic exposure to B. burgdorferi on risk of abortion and resistance to clinical signs of the disease warrants further investigation.