Promising lines of research into common breeding problem in mares – review

Endometritis - inflammation of the lining of the uterus - is one of the most commonly encountered challenges in equine breeding.
Endometritis – inflammation of the lining of the uterus – is one of the most commonly encountered challenges in equine breeding. File image.

Several lines of research into treating or preventing endometritis in horses show promise, according to the authors of a just-published review.

Endometritis – an inflammation or irritation of the lining of the uterus – is one of the most commonly encountered challenges in equine breeding. It can be difficult to resolve and causes considerable economic losses to the industry.

Jane Morrell and Antonio Rocha, writing in the journal Frontiers in Veterinary Science, describe it as a multifactorial condition, which develops as an exaggerated form of the normal physiological response to breeding.

Seminal plasma proteins, spermatozoa, bacteria and debris trigger an inflammatory response. Normally, the resulting fluid and neutrophils are then cleared from the uterus along with the debris.

However, in some mares, the response is prolonged or exaggerated, with much fluid formation and neutrophil infiltration leading to acute endometritis.

A bacterial cause has been implicated, although in some cases no pathogenic organisms can be isolated on culture. It has been suggested that any one of a variety of bacteria could be involved, or disruption of the uterine microbiome could be responsible.

The pair said repeated episodes of acute endometritis may lead to greater problems arising from the chronic form of the condition, including deterioration of uterine blood vessels.

In their review, Morrell and Rocha observed that, despite decades of research, the exact cause of both acute and chronic endometritis in mares remains unknown.

While the cause in some cases may be bacterial in origin, culture of microorganisms may be uninformative. “Either no organisms are isolated, or the organisms present are not known to be pathogenic.”

It is not known whether the bacteria isolated are responsible for the condition from the start, or are secondary to the underlying cause.

“The fertility issues associated with chronic endometritis may be resolved, at least in some cases, by treatment with antibiotics, but such treatment in the absence of a positive identification of a pathogen goes against current guidance on the prudent use of antibiotics,” they said.

A better understanding of persistent endometritis is required in the hope of finding a means of prevention and/or better methods of treatment than are currently available, they said.

Conventional treatment methods involve uterine lavage, with or without agents that cause contractions, to aid uterine clearance. There is also the option of antibiotics, and possible surgery to correct structural issues with the vulva.

“These treatments have been available for decades … but the persistence of the problem in equine breeding suggests that new approaches might be necessary, perhaps in combination with some of the conventional therapies.”

The authors noted that several non-traditional therapies have been proposed, although some of them are based on studies involving a small number of mares.

These therapies include the use of antimicrobial peptides, immunotherapy, platelet-rich plasma, and stem cells.

Of these, the most promising appears to be platelet-rich plasma, with its high concentration of growth factors having a cell-regenerative and anti-inflammatory effect. Its use in one study was associated with an increased conception rate in mares. In a larger study in Arabian mares, the length of the oestrous cycle was shortened, the thickness of the uterine lining was increased, and pregnancy rates improved in the treated mares.

The researchers proposed several ways forward to identify potential solutions to endometritis. One is complete identification of the uterine microbiome, which could help identify a specific microbial cause in persistent cases. They noted that while the uterine microbiome of healthy mares was investigated in a 2017 thesis, a similar study on susceptible mares in the same environment is needed.

The authors also proposed the development of alternatives to proteins in semen extenders. “Almost all of the semen extenders used in equine breeding contain protein of some description,” they said.

In other species it has been possible to replace proteins of animal origin with synthetic substances, they noted. “It would be interesting to investigate whether similar formulations would support stallion spermatozoa whilst potentially not stimulating an exaggerated response from the uterine epithelium.”

They said alternative ways of processing semen should also be investigated, which might prevent the exaggerated response to breeding seen in some mares.

The options described could be tried in different combinations, they added. The pair also stressed the importance of strict hygienic precautions when inseminating mares or carrying out reproductive examinations.

Morrell and Rocha said future research into methods of treating biofilms could prove to be rewarding, but it could be beneficial to avoid the problem altogether.

“Novel semen extenders that do not contain protein could be useful but probably the most effective way of avoiding the problem would be to remove all seminal plasma (and most of its load of bacteria) from semen doses for artificial insemination by colloid centrifugation.”

Morrell is with the Swedish University of Agricultural Sciences Uppsala, Sweden; and Rocha is with the University of Porto in Portugal.

Morrell JM and Rocha A (2022) A Novel Approach to Minimising Acute Equine Endometritis That May Help to Prevent the Development of the Chronic State. Front. Vet. Sci. 8:799619. doi: 10.3389/fvets.2021.799619

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

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