Scientists explore microbial communities in mares in placentitis study

"We believe there is rationale to challenge the dogma of the placenta being sterile in horses," the study team said.
“We believe there is rationale to challenge the dogma of the placenta being sterile in horses,” the study team said. ©

Researchers have identified different microbial communities in mares early in their pregnancy, including in the placenta, in a step toward understanding more about the development of placentitis.

Placentitis is the leading cause of infectious abortion in horses. It can also result in weak or growth-restricted offspring, Kalie Beckers and her fellow researchers noted in the journal Animals.

Ascending placentitis is the most common form, caused by bacteria ascending from the lower reproductive tract, and vaginal and fecal microbe contamination.

However, less is known about the development and progression of other types, such as nocardioform placentitis.

“We believe there is rationale to challenge the dogma of the placenta being sterile in horses,” the study team said.

The researchers, with the University of Louisiana, set out to identify the microbial communities in different body sites of the pregnant mare in early gestation to establish a core microbiome that may be upset in pathologic pregnancies such as placentitis.

The researchers theorized that the equine placenta harbors a distinct resident microbiome in early pregnancy, and that there would be a disparity in bacterial communities from the oral, vaginal, and fecal microbiome.

Samples were collected from the mouth, vagina, anus, and placenta of five pregnant ponies between 96 and 120 days of gestation.

DNA sequencing was used to examine core bacterial communities in the samples.

The bacterial community in the placentas was found to be significantly different from those of the feces, mouth and vagina. Feces had the greatest species diversity, while the oral cavity and placenta similarly had the least.

The placenta showed similarities in its microbial communities to the oral cavity.

At the genus level, some similarities were shown between body sites, even though the placenta did harbor its own unique microbiome in relation to diversity, relative abundance, and bacterial richness.

The placenta was dominated by Gemella, Rikenellaceae RC9, Porphyromonas, and Streptococcus.

“The predicted placental bacterial sources identified the oral cavity as a major contributor in one horse in this study, while the remaining samples’ sources were of unknown origin,” the study team said.

The researchers, discussing their findings, said diagnosing placental infections during equine pregnancy is difficult since outward clinical signs are often not present.

“The ability to predict positively mares that will develop any type of placentitis would be valuable in equine practice.”

Understanding the relationship between the equine placenta and resident bacterial populations during healthy and diseased pregnancies could provide the opportunity to use bacteria in the mouth, feces and/or vagina as biomarkers for predicting placentitis and associated adverse outcomes.

It has been shown that there are differences in the uterine microbiome between regions within the United States and between continents, they noted.

“Therefore, we chose to characterize only pregnant mares from the same location to reduce variables such as a climate, forage, and feed, etc. that may influence the pregnant microbiome.”

The authors said further investigations are required to evaluate clinical cases of equine placentitis as well as the mare’s uterine microbiome before pregnancy.

Rapid screening of microbial biomarkers from sites other than the placenta that can be sampled non-invasively may improve the ability to detect pregnant mares that will develop pathology before clinical signs are present, they said.

“Identification of bacterial targets in extra-placental body sites as causative in adverse pregnancy outcomes would revolutionize the way we manage pregnancy in the mare.”

They said further research needs to be completed to investigate how bacteria are translocated to the placenta from other body sites and how they contribute to the development of placentitis.

“Continuation of this study would involve the development of a screening method using next-generation sequencing to identify microbial community dysbiosis rapidly to monitor mares that could potentially develop placentitis.”

The study team comprised Beckers, Viviane Gomes, Kassandra Crissman, Chin-Chi Liu, Gary Childers and Jenny Sones, all with Lousiana State University in Baton Rouge; and Christopher Schulz, with Southeastern Louisiana University in Hammond.

Beckers, K.F.; Gomes, V.C.L.; Crissman, K.R.; Liu, C.-C.; Schulz, C.J.; Childers, G.W.; Sones, J.L. Metagenetic Analysis of the Pregnant Microbiome in Horses. Animals 2023, 13, 1999.

The study, published under a Creative Commons License, can be read here

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