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Alarming levels of multidrug-resistant strains of a common pathogen were detected in samples taken from the wombs of mares with fertility problems.

Streptococcus equi subsp. zooepidemicus (S. zooepidemicus), is a rare human pathogen, but is frequently associated with endometritis – inflammation of the uterus lining – in horses.

Researchers with the University of Naples in Italy set out to isolate S. zooepidemicus strains associated with bacterial endometritis in mares and to define their antimicrobial resistance profile.

S. zooepidemicus is one of the main causes of infectious endometritis in horses. Being an opportunistic pathogen, it can also be isolated in horses suffering from respiratory infections, which generally occur in the presence of stressful conditions, such as transport, high temperatures, or viral infections.

Francesca Paola Nocera and her fellow researchers, writing in the journal Antibiotics, said S. zooepidemicus is usually responsible for a dormant subclinical infection, persisting in uterine tissue and resisting the immune system and drug treatment.

This, they said, is related to the ability of S. zooepidemicus to survive in epithelial cells in the form of “persister” cells. “Persister” bacterial cells form a dormant subpopulation of cells that are tolerant to antibiotic treatment. They are capable of resuming normal growth.

The study team noted that, besides these “persister” cells’ ability to resist antibiotics, the circulation of multidrug-resistant streptococci, including S. zooepidemicus, has increased over the years because of antibiotic misuse.

The authors noted that previous research had indicated the presence of a genetic subpopulation of S. zooepidemicus strains associated with infectious endometritis. “Therefore, it appears clear that some strains adapt better than others to survive and colonize the endometrial environment.”

In their study, the Italian researchers took uterine swabs from 96 mares with fertility issues, with 23 S. zooepidemicus isolates obtained, representing a prevalence of 11.7%.

The antibiotic resistance profiles of the isolates revealed a high percentage of resistance to amikacin (95.6%), ampicillin (73.9%) and tetracycline (69.6%), while ceftiofur and ceftriaxone were found to be highly effective.

“An intriguing value of resistance to penicillin (34.8%), which represents the first-choice antibiotic in equine S. zooepidemicus infections, was observed,” the study team wrote.

“An alarming result was represented by the high prevalence of multidrug-resistant strains, with 82.6% of the total isolates showing resistance to more than three classes of antimicrobials,” they reported.

“Our result agrees with many other studies, which report an increasing trend of multidrug resistance associated with S. zooepidemicus strains over the years.”

The researchers said the increasing spread of multidrug-resistant S. zooepidemicus has become a relevant veterinary issue, highlighting the need for continuous surveillance of this pathogen.

“A proper diagnosis and an effective therapeutic approach are needed to allow a rapid and effective antimicrobial treatment.”

To this end, an appropriate antimicrobial stewardship program, including the promotion of the correct use of antimicrobial drugs after a proper diagnosis, is needed to allow an effective therapy, they said.

The study team comprised Nocera, Elena D’Eletto, Monica Ambrosio, Filomena Fiorito, Ugo Pagnini and Luisa De Martino, all with the University of Naples.

Nocera, F.P.; D’Eletto, E.; Ambrosio, M.; Fiorito, F.; Pagnini, U.; De Martino, L. Occurrence and Antimicrobial Susceptibility Profiles of Streptococcus equi subsp. zooepidemicus Strains Isolated from Mares with Fertility Problems. Antibiotics 2022, 11, 25. https://doi.org/10.3390/antibiotics11010025

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

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