Salmonella in Horses
by: Heather Smith Thomas
May 01 2005, Article # 5686
Salmonellosis affects humans, horses, most mammals, and birds. It can cause debilitating–and even deadly–diarrhea. Salmonella bacteria can affect both foals and adults, and they spread easily by horse-to-horse contact and by fomites (shared tools, water buckets, hands, etc., on which bacteria can “hitch a ride” to the next victim). Seemingly well horses can harbor the bacteria, and when stressed, they can shed it or become ill. “The environment can be contaminated by birds, rodents, or other wild animals shedding the organism in feces, including contaminating feed for horses,” says Simon Peek, DVM, MRCVS, PhD, Dipl. ACVIM, clinical assistant professor of large animal internal medicine, theriogenology, and infectious disease in the department of large animal internal medicine at the University of Wisconsin. Salmonella has received attention lately due to several outbreaks of nosocomial disease (infections picked up at a hospital by an animal that did not have that infection upon entrance) in various teaching hospitals.
Peek says there are more than 2,000 types of Salmonella, including several that affect horses. Most common is S. typhimurium, a type that also infects cattle and people. All types are zoonotic (affect animals and humans) except for one type that only infects humans.
“These Gram-negative bacteria cause a variety of problems–most commonly gastrointestinal disease and diarrhea,” says Peek. “Salmonella can also cause abortion, but not as often in horses as cattle. The infection can cause septicemia; in foals it can cause generalized sepsis–bacteria in the blood spreading to multiple organs. In adult horses, bacteria are more likely to be confined to the GI tract, particularly the colon. It is much rarer in mature horses for bacteria to get out of the GI tract and into the bloodstream.”
A foal with septicemia is dull and depressed, with high fever, and can die within 24 to 48 hours. Acute enteritis is the most common sign in adults, with fever and severe diarrhea. The watery diarrhea has a rotten smell and often contains mucus, and sometimes blood. Severe dehydration and toxemia occur; the animal can become very weak. Salmonella can also cause localized infections.
Shedders
“Some horses shed the organism without showing signs of disease; this is why it can become a problem at an equine hospital or referral institution,” notes Peek. “A horse brought in for some other problem may be shedding bacteria in feces, perhaps intermittently.”
Horses do not exhibit true carrier status, which occurs in cattle and some other animals, says Peek. He notes that certain types of Salmonella that affect species like cattle can cause an animal to be infected for life. “This does not happen in horses with the Salmonella types encountered in the U.S.,” says Peek.
Most horses infected with Salmonella clear the organism from the body within days or weeks, or perhaps a few months, he notes. Yet some “silent but deadly” horses that shed the organism, but don’t show any clinical signs. “They won’t do that, however, for the rest of their lives,” he notes.
“The sicker a horse becomes (with clinical illness), the more likely he’ll be shedding the organism in large numbers,” advises Peek. “If a horse is fairly healthy, he’ll probably be shedding low numbers that might not be enough to cause clinical disease in another healthy horse. The problem in hospitals is that we are not dealing with healthy horses. In horses being treated with antibiotics, or that have an upset in the digestive system, or that had colic surgery, the dose of Salmonella bacteria necessary to cause clinical disease is a lot less.”
The short version: If a horse’s immune system is compromised when he is exposed to Salmonella, he is at higher risk.
“These horses may acquire the infection while in the hospital,” says Peek. “There are also situations in which horses come down with clinical salmonellosis while in a hospital; they may have had the organism before they came in, but their immune system was strong enough to keep it at bay. Then they had colic surgery, or some other stressful procedure that allowed it to become a more rampant infection. Some hospitals and referral institutions take fecal samples for culture when a horse first comes in to identify horses with Salmonella organisms, and to protect themselves.”
Hospitals sample incoming horses to find the ones shedding that could be a source of infection for other animals, and horses that are likely to get sick because they are harboring the organism in the intestines.
“Studies at teaching hospitals in the last decade show that the proportion of horses coming to equine hospitals that are shedding Salmonella in feces may be as high as one in 20,” notes Peek. “In a busy hospital, that’s a lot of horses; 5% of hospitalized horses appear normal, but may be shedding organisms even though they are there for a lameness, a throat surgery, or some other elective process.”
This makes the problem much more complicated, and is one reason some hospitals culture horses as soon as they walk in the door, and intermittently during their stay.
Antibiotic Resistance
Peek says one concern that has recently surfaced in veterinary and human medicine is the number of Salmonella isolates that have developed multiple antibiotic resistance patterns, making them hard to treat with antibiotics. “This has raised considerable public health concerns about antibiotic use in livestock,” he says.
“This has become quite controversial,” Peek continues. “Our human medicine colleagues would prefer us to be extremely judicious about use of antibiotics for treating salmonella and other bacterial infections. The fear is that broad and indiscriminate use of antibiotics in animals will further promote resistance, making it more difficult to effectively treat human patients.”
Spread of the Disease
Salmonella is feco-orally spread between animals by manure that contaminates feed or water. A foal might pick it up when nursing a mare or nuzzling her flank if she has lain on dirty bedding or her tail has flicked feces onto her body (if she’s a shedder). A foal will also eat manure and can pick it up that way.
“Normal adult horses in a field rarely eat manure, but mares and foals often eat one another’s feces,” says Peek. “Thus, salmonella can be a bigger problem to control on a breeding farm. Breeders fear this disease because it can be a potentially lethal infection in foals, due to their naïve immune systems. It can get into the bloodstream and cause multiple organ failure.”
Roberta Dwyer, DVM, MS, Dipl. ACVPM (epidemiology specialty) of the Gluck Equine Research Center at the University of Kentucky, says that when a farm has a salmonella outbreak, it’s often little things that derail a good plan for containment. “One of the first things I ask at a farm is whether they have a rodent problem or if they feed on the ground. Mice droppings in hay or grain can be a significant source of Salmonella. Barn cats, bait, and traps are helpful, but if there’s a serious mouse problem, you should contact professional rodent control people,” she says.
Prevention
The best way a farm can protect against salmonella is to pay strict attention to cleanliness, prevent overcrowding, and limit possible exposure. “Getting rid of feces is important, along with isolation of animals that have diarrhea or are recovering from it,” says Peek.
Dwyer says that if a horse develops diarrhea, isolate that animal immediately. “This means putting it in a stall or pen away from other horses until a diagnosis can be made or the diarrhea clears up,” she says. “The exception is foal heat diarrhea, if you are sure that’s what it is. Buy a box of latex gloves and wear a new pair every time you handle an animal or take its temperature.”
Take care to not touch something another person might touch. “If you’ve been touching the sick horse and then go answer the phone, the phone is contaminated,” Dwyer states. “Another person can get bacteria off the phone and go check capillary refill time on another horse, and salmonella is transmitted to that horse.”
If you aren’t careful about your feet, you may track bacteria (from watery feces on the ground or floor where you might not see it) to a clean area, potentially contaminating feed put on the ground or the hair coat of a horse that lies there. If that horse licks himself or a foal nuzzles a dam with a dirty flank, the disease is transmitted.
“People often move horses that were in contact with the sick one, taking them to different pens or pastures, but this is usually a big mistake,” Dwyer advises. “They can be incubating the disease. There is an incubation time in which the organism is multiplying in the body, but the animal is still normal. If you move that exposed animal, you may put other horses at risk.”
The ideal situation is to move the sick horse to a separate quarantine barn; only sick animals with the same disease go there, notes Dwyer. “Some big farms have a barn on the back 40 that is not used except for the occasional sick, contagious animal–and whoever takes care of the animal doesn’t handle other horses,” says Dwyer. After the animal is moved to the sick barn, the stall or pen it was in should be thoroughly cleaned and disinfected.
If a horse gets salmonellosis, other horses should be closely monitored. “If the one that came up positive is in a stall, get the other horses out of stalls, if possible,” says Dwyer. “Even in some cases of undiagnosed foal diarrhea in Kentucky when we couldn’t pinpoint the cause, we finally recommended all the mares foal outside.”
If a farm has a high concentration of animals (and a lot of foot traffic, muck wagons, and hay/straw wagons going through the barn), this is a highly contaminated area in an outbreak. If other horses are currently healthy and have lived in the same pasture together, the safest place for them is on pasture, not in the barn, says Dwyer.
Keep groups together; don’t mix horses that have not lived together. For a big group, put temporary fencing in the pasture they lived in, segregating them into smaller groups. If you’ve divided them and a horse is incubating the disease, he might potentially infect only three or four others instead of the whole herd, she explains.
“Horses should be grouped (in pasture or barn) according to age and use. All horses that go out for trail rides every weekend should be together, for instance, and not mixed with other horses,” says Dwyer. “They should be in a separate pasture, away from the broodmares.”
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