A Scoping Review of the Global Distribution of Causes and Syndromes Associated with Mid- to Late-Term Pregnancy Loss in Horses between 1960 and 2020

Abstract

Equine pregnancy loss is frustrating and costly for horse breeders. The reproductive efficiency of mares has significant implications for a breeding operation’s economic success, and widespread losses can have a trickle-down effect on those communities that rely on equine breeding operations. Understanding the causes and risks of equine pregnancy loss is essential for developing prevention and management strategies to reduce the occurrence and impact on the horse breeding industry. This PRISMA-guided scoping review identified 514 records on equine pregnancy loss and described the global spatiotemporal distribution of reported causes and syndromes. The multiple correspondence analysis identified seven clusters that grouped causes, syndromes, locations and pathology. Reasons for clustering should be the focus of future research as they might indicate undescribed risk factors associated with equine pregnancy loss. People engaged in the equine breeding industry work closely with horses and encounter equine bodily fluids, placental membranes, aborted foetuses, and stillborn foals. This close contact increases the risk of zoonotic disease transmission. Based on this review, research is required on equine abortion caused by zoonotic bacteria, including Chlamydia psittaci, Coxiella burnetii and Leptospira spp., because of the severe illness that can occur in people who become infected.

Equine abortion and stillbirth in California: a review of 1,774 cases received at a diagnostic laboratory, 1990–2022

“…were the most prevalent, which agrees with previous reports. 14,15,17,19,24 Streptococcus spp. are part of the normal microbiota of the genital canal, but they can invade the uterus and cause endometritis, ascending placentitis, and fetal death opportunistically.…”

“…Additionally, several infectious agents known to cause reproductive losses in horses are zoonotic and/or have a major impact on international trade. 15…”

Lethal variants of equine pregnancy: is it the placenta or foetus leading the conceptus in the wrong direction?

“…Induction of ovulation has been found to reduce the risk of EPL (de Mestre et al 2019), possibly acting via the quality of the ovulated oocyte or early luteal function (Köhne et al 2014). Whilst not epidemiological studies are lacking, it is still clear that the presence or absence of pathogens is key to pregnancy loss (Macleay et al 2022) as demonstrated by the marked geographic variation in reporting of certain pathogens.…”

“…Broadly, losses fall into two categories, infectious and noninfectious. A recent scoping review of mid and late gestational losses indicates the intensive interest of the research community in infectious causes of pregnancy loss with studies of placentitis, EHV-1, Chlamydia, Leptospirosis and Nocardioform placentitis dominating the literature of the last three decades (Macleay et al 2022). Equally, once we drill down to individual populations, the picture can look quite different.…”

“…Whilst the endometrial and endocrine environment are clearly important, it is estimated that 50-80% of early pregnancy losses (Hamstead et al 2012) and approximately half of mid to late gestation losses (Roach et al 2021;Macleay et al 2022) have no attributed cause. There are a number of well described genetic variants of human foetuses associated with a phenotype of foetal lethality that account for well over 50% of miscarriages (Chen et al 2021).…”