Salmonella spp. cause a multitude of diseases in horses, including diarrhea, abscesses, septicemia, and other ailments. Over 2,200 serotypes of salmonellae are known and can be identified at the National Veterinary Services Laboratories (NVSL) in Ames, Iowa. With the exception of S. typhi, which only affects humans, all other salmonellae are zoonotic, posing possible transmission from animals to people.
In a NVSL report for isolates serotyped from July 1, 2000 - June 30, 2001, the most frequently isolated serotypes from the clinical cases, herd and flock monitoring, and meat inspection of all species were S. typhimurium, heidelberg, newport, agona, and kentucky; all have also been isolated from horses. From equine clinical cases reporting a primary or secondary Salmonella infection during this same time period, the most frequent serotypes recorded were S. agona (213), typhimurium (207), typhimurium var copenhagen (52), newport (211), and newington (54). Another 287 samples accounted for 44 serotypes.
The vast majority of horses that have died in Central Kentucky are submitted for necropsy at the University of Kentucky Livestock Disease Diagnostic Center. Any salmonellae isolates cultured are forwarded to NVSL for serotyping.
This surveillance has been ongoing since 1985, with the last report in the October 1996 Equine Disease Quarterly. From January 1990-June 1996, S. typhimurium was the predominant postmortem isolate, newport and thompson were the next most common serotypes, and 29 other serotypes were cultured.
Figure 1 illustrates the trend in serotypes isolated from 1996-2000. There was a leveling off of S. typhimurium from previous years (a high of 34 post mortem cases in 1994) and a steady increase in S. typhimurium var copenhagen. The most prominent feature is the significant increase in the isolation of S. agona, which emerged in 1999 and is ongoing to date. These isolates are from both primary and secondary infections. Previous to 1999, S. agona had not been a common serotype isolated from post mortem or clinical cases since 1984.
During the period 1997-2000, 21 other serotypes were identified, and, as in previous years, the occasional new serotype appears that has not been seen in Central Kentucky. For example, in 2000, S. othmarschen was isolated from a newborn foal with a primary infection.
The emergence of a new serotype of significant equine importance emphasizes the need for veterinarians and horse owners to be aware of the problem and take preventive measures. Animals coming onto a farm should be isolated for a minimum of two weeks to prevent the introduction of infections. Routine, rigorous disinfection of stalls with chemicals known to be effective against salmonellae in the presence of organic matter is essential, both in hospitals and on farms. Since no commercially available vaccine exists against salmonellae, disinfection and biosecurity are the primary preventive measures that must be undertaken. Horse owners need to be aware of the zoonotic potential of any Salmonella-positive horse and take proper precautions (isolation techniques, use of protective clothing, washing hands, etc.).
These measures are critical because of the zoonotic potential for any salmonellae, which can be deadly in immunocompromised individuals and dangerous to pregnant women. Of considerable concern to the human and veterinary medical professions is the emergence of multidrug-resistant strains of bacteria. In a report from the Centers for Disease Control and Prevention (MMWR, June 28, 2002), multidrug-resistant S. newport isolates were reported from New York, Michigan, Pennsylvania, Ohio, and Connecticut. The implicated cause of the outbreak was exposure to raw or undercooked ground beef, but the report also emphasizes the need for Salmonella surveillance, the judicious use of antibiotics by the veterinary and medical professions, and the continuous education of veterinarians and horse owners concerning biosecurity issues.
Dr. Roberta M. Dwyer, (859) 257-4285, firstname.lastname@example.org, Maxwell H. Gluck Equine Research Center, University of Kentucky;
Dr. Mike Donahue (859) 253-0571, email@example.com, Livestock Disease Diagnostic Center, University of Kentucky.