Salmonella infection in the spotlight in effort to prevent and control disease

Epidemiological and bacteriological analysis is helping to develop better prevention and control strategies for industry

Key messages

• Epidemiological studies are helping determine the prevalence and predictive factors for Salmonella shedding and the impact of Salmonella-associated disease in the Australian thoroughbred breeding industry.
• Understanding the factors associated with Salmonella shedding and the characteristics of the bacteria will help to develop better prevention and control strategies.
• Veterinary hospitals have been found to have no effect on the risk of Salmonella shedding; the problem is a broader issue within the equine industry.

Salmonella enterica is the most important contagious enteropathogen of horses; it is associated with intestinal dysfunction and damage, including colic, diarrhoea and systemic inflammatory response syndrome. Intensive treatment is often required, and horses are at risk of death.

Owners, breeders and managers will soon have a better understanding of the impact of Salmonella infection in the Australian thoroughbred industry thanks to research being undertaken at Charles Sturt University.

Prof. Kris Hughes is leading the study investigating factors associated with the likelihood of Salmonella infection in horses and the characteristics of Salmonella strains.

Compounding the major health and welfare risks to horses, and the disruption to the sustainability and functionality of studs and equine veterinary hospitals, Salmonella isolates can be resistant to multiple types of antibiotics, undermining treatment efforts.

Kris said antimicrobial drugs (AMDs) were often administered to treat confirmed or suspected bacterial infections, but systemic AMD treatment disrupted the intestinal microbiota and predisposed to antimicrobial resistance (AMR).

“With the threats and impact of AMR, there is increasing scrutiny of AMD use in animal industries and expectations of appropriate AMD stewardship by the veterinary industry,” he said.

“We urgently need strategies to treat and prevent microbial-associated diseases that don’t rely on the use of AMDs, and that contribute to sound stewardship practices.”

Studies unveil factors associated with shedding

The research team is undertaking retrospective and prospective epidemiological investigations to determine the prevalence and predictive factors for Salmonella shedding, and the impact of Salmonella-associated disease in two key industry settings — equine veterinary hospitals and thoroughbred stud farms.

“Both settings are integral in horse health management and are at the forefront of infection control and biosecurity, so it makes sense to undertake our investigations in these settings,” Kris said.

The study into the prevalence of Salmonella shedding in horses admitted to equine hospitals found it is a common issue. Out of 1,600 horses involved in the study to date, 23.3% were found to have shed Salmonella, meaning it is an important concern for veterinarians and other equine industry stakeholders.

An additional finding is that the hospital itself does not play a role in Salmonella shedding. “There was no effect of the hospital on the risk of shedding, so the problem is not specific to certain hospitals, but rather a broader issue within the equine industry,” Kris explained.

“We also found there is a significant association between horses presenting with colic and Salmonella shedding. However, there was no association between horses presenting with diarrhoea and Salmonella shedding. This challenges the previous belief that Salmonella is mainly associated with diarrhoea in horses.

“Another important finding is that horses that have recently had anaesthesia and surgery, specifically for colic, are at the highest risk of shedding Salmonella.”

Withholding feed is also associated with Salmonella shedding. This is likely due to the stress and changes in the intestinal microbiota and function that occur when feed is withheld. These factors can lead to an increased risk of Salmonella colonisation.

The researchers also reviewed the hospital records of horses with Salmonella infection to determine the serotypes and antibiotic sensitivity patterns of Salmonella isolates from horses. They found that the most common serotype in three regions over the past five years was Muenster, with lower prevalences of other serotypes, such as Typhimurium. However, two regions had no predominant serotype, recording numerous serotypes.

“These findings indicate a significant change in prevalent Salmonella serotypes in horses in Australia compared to previous reports,” Kris noted.

“It is clear that Salmonella shedding is common and a significant concern in veterinary practice. By understanding the factors associated with Salmonella shedding and the characteristics of the bacteria, we can work towards better prevention and control strategies.”

“We also found no association between serotypes Muenster and Typhimurium and the outcome of the infection. But horses infected with Typhimurium were more likely to die than those infected with other serotypes.”

Additionally, 62% of isolates were found to be resistant to multiple antibiotics. Concerningly, this resistance was to several critically important drugs, including enrofloxacin, ciprofloxacin, third-generation cephalosporins and macrolides. The percentage of resistance varied for each drug, with erythromycin and clarithromycin showing the highest levels of resistance.

“In the context of One Health — Salmonella can be passed from animals to humans — the presence of multidrug resistance has potential implications for human health. Our findings are relevant to everyone involved in the equine industry, as well as healthcare professionals,” Kris said.

“It is clear that Salmonella shedding is common and a significant concern in veterinary practice. By understanding the factors associated with Salmonella shedding and the characteristics of the bacteria, we can work towards better prevention and control strategies.”

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Epidemiological and bacteriological analyses of Salmonella infection in the Australian thoroughbred breeding industry

Project number: PRO-015534

Principal investigator: Prof. Kris Hughes

Research organisation: Charles Sturt University