Drug resistance and virulence-associated genes screening in Salmonella enterica isolated from Caspian pony, Iran.

Researchers tested fecal samples from healthy Caspian ponies in northern Iran and found a low prevalence of Salmonella enterica Typhimurium (2 of 143), but the isolates carried key virulence genes and showed multidrug resistance. This combination indicates a potential zoonotic risk and supports implementing strict hygiene and prudent antibiotic use.

What the study set out to investigate

• Determine how often healthy Caspian ponies shed Salmonella in their feces (asymptomatic carriage).
• Identify which Salmonella serotypes were present.
• Screen isolates for two virulence-associated genes (fimA and sseL) using PCR.
• Assess antibiotic susceptibility profiles of the isolates via disc diffusion to detect drug resistance.

Why this matters

• Salmonellosis is a leading zoonotic and veterinary health concern; horses can shed Salmonella without signs and contaminate environments, handlers, and other animals.
• Serotype Typhimurium is a common cause of non-typhoidal salmonellosis in animals and humans and is frequently linked to multidrug resistance (MDR).
• Detecting virulence genes together with MDR in isolates from healthy animals signals potential for harder-to-treat infections and environmental persistence.

How the study was done

• Population and sampling: 143 fecal samples from healthy Caspian ponies of differing ages and sexes in northern Iran.
• Isolation and identification: Bacteriological culture followed by standard biochemical identification; confirmation and serotyping support by PCR methods.
• Virulence gene screening: PCR detection of fimA (fimbrial adhesion) and sseL (type III secretion effector) in Salmonella isolates.
• Antibiotic susceptibility testing: Disc diffusion method to profile resistance across multiple antibiotic classes.

Key findings

• Prevalence: 2 of 143 fecal samples (1.39%) were positive for Salmonella.
• Serotype: Both isolates were Salmonella enterica serotype Typhimurium.
• Host factors: Both positives came from females younger than 6 years (note: small numbers limit inference about risk factors).
• Virulence: Both isolates carried fimA and sseL genes.
• Resistance: Isolates exhibited multidrug resistance by disc diffusion (specific resistance patterns not detailed in the abstract).

What the detected virulence genes imply

• fimA
  • Encodes a major subunit of type 1 fimbriae, surface structures that mediate adhesion to host cells and biofilm formation.
  • Association: Aids colonization of the gut, which can enhance shedding and transmission.
• sseL
  • Encodes a secreted effector delivered by the Salmonella Pathogenicity Island-2 (SPI-2) type III secretion system.
  • Function: A deubiquitinase that helps intracellular survival and modulates host immune responses, potentially contributing to persistence and severity.

Antibiotic resistance and its significance

• Multidrug resistance (MDR) in Salmonella Typhimurium is a well-recognized clinical and public health issue because it narrows effective treatment options.
• Detection of MDR in isolates from healthy ponies indicates a possible reservoir of resistant strains in the equine population and environment.
• Without specific drug-by-drug data provided, the exact clinical implications are unknown, but the general risk is greater treatment difficulty and potential for spread to other animals and people.

How to interpret the low prevalence

• A 1.39% detection rate indicates relatively infrequent shedding among the sampled healthy ponies at the time of sampling.
• Because shedding can be intermittent and influenced by stress, diet, transport, and season, a single sampling may underestimate true carriage over time.
• With only two positives, estimates are imprecise; prevalence could differ by region, management practices, and sampling season.

Strengths and limitations

• Strengths
  • Focus on healthy animals provides insight into asymptomatic reservoirs.
  • Use of both culture and PCR increases confidence in identification.
  • Concurrent assessment of virulence genes and resistance offers a more complete risk profile.
• Limitations
  • Small number of positives limits statistical inference about risk factors (age, sex) and generalizability.
  • Geographic scope restricted to northern Iran; results may not represent other regions or management systems.
  • Single time-point sampling misses intermittent shedders and seasonal variation.
  • Abstract does not report the full antibiotic panel, specific resistance phenotypes, or minimal inhibitory concentrations.
  • Only two virulence genes were assessed; Salmonella virulence is multifactorial and involves many loci and islands.

Implications for veterinary practice and public health

• Biosecurity and hygiene
  • Implement consistent manure management, hand hygiene, and equipment disinfection to reduce fecal-oral transmission.
  • Isolate and manage animals with diarrhea or after stressful events that can trigger shedding.
• Antimicrobial stewardship
  • Use antibiotics judiciously in equine practice, guided by susceptibility testing when treatment is needed.
  • Avoid unnecessary prophylactic use to limit selection pressure for resistance.
• One Health perspective
  • Coordinate surveillance across animal, human, and environmental health sectors to monitor resistant Salmonella.
  • Educate handlers and farm workers on zoonotic risk mitigation.

What would strengthen the evidence base next

• Larger, multi-regional cohort studies of Caspian ponies and other equids with repeated (longitudinal) sampling to capture intermittent shedding.
• Detailed antimicrobial resistance profiling (including classes and specific agents) and genotypic resistance determinants.
• Broader virulence gene and genomic analyses (e.g., whole-genome sequencing) to map lineages, mobile elements, and transmission routes.
• Risk factor analysis linking management practices, age, sex, transport, and seasonality to shedding and resistance.
• Environmental sampling (water, feed, bedding) to identify reservoirs and contamination pathways on farms.

Bottom line

• In this sample of healthy Caspian ponies, Salmonella shedding was uncommon, but the detected S. Typhimurium isolates carried important virulence genes and showed multidrug resistance.
• Even low-prevalence carriage can pose a meaningful One Health risk, justifying robust hygiene practices and careful antibiotic use in equine settings.