Salmonella Antimicrobial Susceptibility

In the October 1996 issue of the Equine Disease Quarterly, Salmonella isolates from equine cases over a six-year period at the Livestock Disease Diagnostic Center (LDDC) were discussed. From June 1994 to June 1996, 3,340 equine necropsies on adults, neonates, and fetuses were performed at the LDDC. Salmonella isolates from necropsy cases during the two years have been closely examined.

Dr. Marie Petrites-Murphey of the LDDC reviewed all cases to determine whether the Salmonella isolated was the primary cause of disease, as in cases of enterocolitis or septicemia, or the isolate was considered secondary or not a cause of disease. Salmonella can be found in the gut flora of normal horses, and was isolated in horses which died of head injury, neonatal isoerythrolysis, uterine hemorrhage, and poisonings.

In assessing the 39 primary salmonellosis cases, typhimurium was the most frequently isolated serotype (16), followed by newport (8), oraienburg (3), and eight other serotypes with one to two isolates each. The age distribution for the horses with S. typhimurium cases ranged from a 10-month fetus to a 16-year-old horse, with all animals coming from different farms. Ten animals were six months of age or less; two animals were 7-12 months of age, and four horses were older than 12 months. The antibiotic susceptibilities of these 16 S. typhimurium isolates were examined; 15 were susceptible or moderately susceptible to 14 or 15 antimicrobial disks, and one from an adult horse with acute colitis was susceptible only to polymyxin B, cephalothin, amikacin, and ceftiofur.

Occasionally a single serotype predominated among clinical and necropsy cases for a short time, then disappeared. Of Salmonella isolates from central Kentucky clinical and necropsy cases during January 1990 to June 1996, S. oranienburg was cultured only in 1995 on 13 occasions. Seven isolates were from clinical cases (six feces, one joint) and six were isolated at necropsy.

Post mortem findings indicated that three horses died or were euthanatized due to primary Salmonella infections (one severe osteomyelitis in a foal, one enterocolitis/septicemia in a foal, and one enteritis from a 20-year-old horse). Seven isolates came from horses on one farm during 3-month period (four feces, two intestinal contents, and one joint). One foal tested S. oranienburg positive on fecal culture, was euthanatized due to suppurative arthritis, and oranienburg was cultured from a draining tract in the leg. Both isolates had identical antibiotic susceptibility patterns with resistance to neomycin, kanamycin, sulfa drugs, tetracycline, and tribrissen. Of the other five isolates from this farm's horses, four were susceptible to virtually all 15 antibiotics tested, and one had antibiotic resistance to six antibiotics which differed from the resistance pattern of the isolate from the first foal. Even in animals from the same farm over a short period of time, one Salmonella serotype can have various antibiotic susceptibility patterns.

Antibiotic resistance can be a significant problem in veterinary hospital salmonellosis outbreaks in the United States. One equine hospital occurrence involved horses infected with S. typhimurium which was susceptible only to ceftiofur and amikacin. Although this outbreak resulted in several equine deaths, no human cases of disease were reported. In another outbreak, Salmonella infantis was identified in a veterinary hospital causing high morbidity, but low mortality in large animals, primarily horses. Infections occurred from January through September 1996. At the onset of the problem the organism was resistant to five antimicrobials in a susceptibility panel. After May 1996 the majority of the isolates were resistant to eight antimicrobials and only susceptible to amikacin and enrofloxacin.

All salmonellae which affect horses can potentially cause disease in humans and other exposed animals. Strict isolation of clinically ill horses, proper disposal of feces and bedding, thorough disinfection of facilities and equipment, use of protective clothing, and frequent handwashing can help prevent the spread of salmonellosis to other horses, animals, and people.

Equine Disease Quarterly, funded by Lloyd's of London

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