Equine Salmonella in Argentina: Genetic relatedness with humans and other animal species and first detection of serovar Freetown.

Overview

• This study examined the presence and genetic characteristics of Salmonella bacteria in horses in Argentina to understand their potential to infect humans and other animals.
• It reported a very low rate of Salmonella carriage in healthy horses but found multiple strains in sick horses, including the first-ever identification of the Salmonella Freetown serovar in horses in South America.

Background

• Salmonellosis in horses causes serious gastrointestinal illness, especially dangerous for foals, hospitalized, or immunocompromised animals.
• Horses can intermittently shed Salmonella, acting as carriers and potential sources of infection for other animals and humans.
• Understanding which Salmonella strains infect horses and their genetic relationship with strains from other species is important for managing public health risks.

Objectives

• To determine the frequency of Salmonella presence in clinically healthy horses in Argentina.
• To analyze isolates from horses showing clinical signs of salmonellosis between 2006-2017.
• To genetically subtype Salmonella strains via pulsed-field gel electrophoresis (PFGE) and compare them with strains from other species.
• To assess the zoonotic potential and epidemiological connections between equine Salmonella and those from humans and other animals.

Methods

• Fecal samples were collected from 249 healthy horses between 2018 and 2022 to screen for Salmonella.
• 13 isolates from horses with salmonellosis symptoms from 2006 to 2017 were included for analysis.
• Salmonella serovars were identified, including S. Typhimurium, S. Newport, S. Oranienburg, and S. Freetown.
• PFGE was used to determine the genetic profiles and possible clonal relationships between strains.
• Patterns were compared with a national PulseNet database containing strains from humans, cattle, pigs, goats, and birds.

Key Findings

• The carrier rate among healthy horses was very low—only 0.4% (1/249 samples positive for S. Newport).
• Among clinically ill horses, multiple Salmonella serovars were detected, predominantly S. Typhimurium (9 isolates), along with others including the first detection of S. Freetown in South American horses.
• PFGE showed genetic diversity among serovars but also revealed clonal outbreaks, particularly of S. Typhimurium in a hospital setting.
• Several equine Salmonella isolates shared identical genetic patterns with strains from diverse species (humans, cattle, pigs, goats, birds), indicating cross-species transmission or common sources of infection.
• This genetic relatedness supports the idea that horses may serve as reservoirs for Salmonella that could spread to humans and other animals, underscoring the importance of surveillance within a One Health approach.

Significance and Implications

• The first report of S. Freetown in horses in South America highlights the emergence of new or previously undetected serovars in this region, raising concerns about surveillance gaps.
• Low detection rates in healthy horses suggest either low prevalence or potential underdiagnosis—possible due to intermittent shedding or limitations in sampling/testing.
• Shared PFGE patterns among different species indicate that Salmonella can circulate widely across animal populations and humans, increasing zoonotic risk.
• These findings advocate for including equine populations in epidemiological monitoring programs to better detect, understand, and control Salmonella transmission.
• Improved veterinary and public health surveillance can support One Health efforts to manage zoonotic infections emerging at the human-animal interface.

Limitations

• Relatively small sample size, especially for healthy animals, may impact the accuracy of carriage rate estimations.
• Underdiagnosis is possible because of intermittent shedding patterns or lack of routine testing in veterinary hospitals.
• The retrospective inclusion of clinical isolates from 2006-2017 might not fully represent current epidemiology.

Conclusions

• Salmonella infection in Argentine horses is rare among healthy individuals but present in clinical cases with various serovars, some genetically linked to strains infecting other animals and humans.
• Equine populations could act as reservoirs in the transmission of zoonotic Salmonella, demanding integrated surveillance efforts.
• Detection of S. Freetown expands the known geographic and host range of this serovar, underscoring the need to enhance pathogen monitoring.
• The study supports a One Health approach to controlling Salmonella, involving coordinated tracking across veterinary and public health domains.