Salmonellosis in a group of ponies: failure to identify a chronic active carrier.

This research study investigates an outbreak of salmonellosis that resulted in the deaths of three ponies. Despite identifying the Salmonella typhimurium strain in some surviving ponies, no chronic active carriers (shedders) were found that could have contributed to the spread of the disease.

Research Context

• The research contextualizes an episode where three out of thirty-three ponies died post relocation of the herd in a new environment. This sparked the need for a comprehensive analysis to explore the cause of these concerning deaths, including a possible explanation of a chronic active carrier.

The Initial Findings

• Autopsies and bacterial cultures identified the infectious agent as Salmonella typhimurium in two of the deceased ponies, a 5-day-old foal, and a 2-year-old gelding. No culture was performed on one mare that displayed no premonitory signs of illness before death.

Post-mortem Lesions

• In-depth examinations revealed distinct clinical manifestations of the infection in the deceased foal and gelding. Lesions in the foal were mainly hemorrhages in the colon and spleen, with visible adhesions to the parietal peritoneum. The gelding exhibited signs of severe typhlitis and colitis.

Discovery of Other Infected Ponies

• In the subsequent investigation, fecal samples were collected from all remaining ponies over five consecutive days. It was found that five ponies, including the mother of the deceased foal and the young foal of the dead mare, were shedding the Salmonella typhimurium strain. However, all strains identified were of the same type, suggesting the outbreak resulted from a single source of infection.

Follow-up Investigation

• Weekly fecal cultures were conducted for the next nine weeks and showed that no pony was shedding Salmonella typhimurium a month after the initial tests. This testing regimen continued on a monthly basis for the subsequent year, with no Salmonella growth identified, indicating there was no active carrier in the herd.
• The initial source of Salmonella that triggered the outbreak still remained undetermined, despite extensive environmental culture testing.

Research Conclusions

• While the investigators did not pinpoint the original source of the Salmonella infection, they were able to rule out the presence of a chronic active carrier in the herd. The study provides valuable insights into the transmissibility of Salmonella in equine populations and demonstrates the importance of a comprehensive and long-term monitoring strategy in controlling an outbreak.