AAEP Reproduction Coverage: Fungal Endometritis in Mares

A bay mare walks forward, facing the camera, in a grassy paddock. Mares that have been repeatedly bred might be susceptible to fungal endometritis.
Fungal endometritis in mares appears to be opportunistic, originating from the environment, feces or the semen of a carrier stallion. Nicole Troost/iStock via Getty Images

Endometritis caused by molds or yeast is an uncommon cause of infection, accounting for less than 7% of diagnosed endometritis cases. However, compared to bacterial infections, fungal endometritis in mares has a poor prognosis. 

“Fungal infections appear less common, but are we really checking?” questioned Tom Stout, MA, VetMB, PhD, DECAR, from the Department of Clinical Sciences, Faculty of Veterinary Medicine, Utrecht University, the Netherlands. 

Stout shared his knowledge on fungal endometritis at the 2022 American Association of Equine Practitioners Convention. 

He advised: “Don’t group all causes of fungal endometritis together. You can look for hyphal fungi (molds) such as Aspergillus spp. and yeasts such as Candida spp. But a wide range of organisms reportedly cause fungal endometritis.” 

These infections appear to be opportunistic. They originate from the environment, feces or even the semen of a carrier stallion, for example. Mares with compromised uterine/vaginal defenses (due, for example to pneumovagina or recurrent persistent breeding-induced endometritis) are predisposed, as well as those with systemic issues (e.g., pituitary pars intermedia dysfunction).

“Repeated intrauterine antibiotic therapies may also contribute to these opportunistic infections by disturbing the normal commensal flora of the vaginal and uterine microbiomes,” Stout added. 

Signs of Fungal Endometritis in Mares

Signs of fungal infections are similar to typical endometritis cases—inflammation with uterine fluid and yellowy/grey vulvar discharge.

“But sometimes, affected mares initially look normal,” warned Stout. 

Swabs from mares suspected of fungal infection must be cultured on Sabarauds’ agar/chromogenic agar for up to five days before concluding that there is no growth. Cytology might reveal neutrophils as well as the fungal organisms or hyphae themselves and, therefore, yield a more rapid diagnosis. 

Typing and Identification of Fungi

Typing and identification of fungi can help practitioners decide on the appropriate treatment. However, that can take a few weeks and requires specialized tests.

“Sometimes the uterine swab and cytology are clear and indicate infection with the intracellular yeast Candida albicans,” said Stout. “You will need an endometrial biopsy to confirm the degree of damage to the endometrium. That may make you question whether it is really worthwhile forging on with this mare.”

Treatment for Fungal Endometritis

Stout warned that these infections can be hard to treat. They require an extended period of local treatment (more than six days). That is due to potential colonization deep into the endometrium. He said there also is “a reservoir of infection in the caudal vagina.” Plus, if there are predisposing factors, then they need to be identified and treated. 

When treating, owners should be aware of the poor prognosis and the expense—about $600-700 per cycle. 

“If the fungal organisms are successfully cleared, then nearly half of treated mares will have Steptococcus zooepidemicus infection in the next cycle,” he warned. “This shows the fungi are gone, but you now need to treat the bacteria.”

Stout then described his current treatment strategy: flushing one time with 250 mL acetic acid 2% then flushing out with 3 L LRS. This is followed by six consecutive days of clotrimazole (500 mg in a liquid formulation administered intrauterine). Clotrimazole treatment is extended to both the vagina and clitoris as well. 

Alternatively, practitioners can consider substituting the acetic acid for 1-3% hydrogen peroxide, and the clotrimazole with either nystatin or polymyxin B. 

Final Thoughts

In summary, Stout said that—thankfully—fungal endometritis is uncommon. 

“These infections have a guarded prognosis, are expensive to treat and require a long duration of treatment with antifungals,” Stout concluded.

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