Salmonella: Beware The Bacteria

We tend to hear about Salmonella, a bacteria, in relation to lectures on kitchen hygiene; it's one of the main reasons why we're warned to wash our hands vigorously after handling raw chicken. But you might recall being forbidden to have a pet turtle as a kid because of the risk of it carrying Salmonella. Truth is, not only might that turtle or chicken be infected with this organism, your horse might be, too.

According to Roberta Dwyer, DVM, MS, Dipl. ACVPM (preventive medicine), Associate Professor at the University of Kentucky's Gluck Equine Research Center in Lexington, Ky., there are more than 2,200 serotypes of Salmonella present in the environment. Some are specific to reptiles and amphibians, others to birds, and still others cross many species barriers. They're in the environment, the soil, and your horse's gut. They're extraordinarily resilient, sometimes lingering in the environment for years (one Scandinavian study turned up viable Salmonella in six-year-old cattle manure in an abandoned barn). All but a few types are transmissible to humans.

Although it's ever-present, under normal conditions Salmonella will have little influence on your horse's heath. But, if he's stressed and his immune system is operating at less than full capacity, the bacteria can sneak in and strike.

Its most common mode of attack in adult horses is the gastrointestinal tract, where it causes acute or chronic (long lasting) diarrhea. In an acute case, the horse might have manure that's loose, liquid, foul-smelling, and sometimes bloody--and he'll also show signs of fever, dehydration, and a hypermotile bowel that emits characteristic "crackling" or "tinkling" sounds. Those are tip-offs for instant action for most horseowners. Some horses demonstrate milder symptoms such as slight depression and "cow patty" manure, which might not be obvious danger signs. But those signs should be taken seriously. Left untreated, what appears to be a simple case of "the runs" might quickly lead to serious dehydration and electrolyte imbalances, which are potentially fatal. Weight loss, laminitis, and kidney shutdown are other risks of a severe episode of Salmonella-triggered dehydration.

To further complicate matters, the liquid manure is extremely infectious, and unless closely contained, it can trigger a farm-wide outbreak of disease in your horses, or in your own intestinal tract.

"When it causes problems, it causes big ones," says Dwyer.

In foals, Salmonella is an even bigger threat. Not only can it trigger bacterial diarrhea (very dangerous, because a foal doesn't have the fluid reserves of an adult horse and can dehydrate in as little as eight hours), but it also has a nasty habit of entering the system through the umbilical stump and causing a bacterial infection we call "navel ill" or "joint ill." In joint ill, a foal (generally a young one, 30 days of age or less) suddenly will exhibit lameness associated with a warm, painful enlargement of one or more of his leg joints. He might become depressed, lose his appetite (look for a distended udder on the mare), and run a fever, which is indicative of infection. Joint fluid drawn from the affected joint capsule might be orange-brown instead of the usual light straw color; less viscous than normal; and cloudy rather than translucent. White blood cell counts from the fluid often are elevated, and it sometimes is possible to isolate the causative organism (or organisms--Salmonella is only one of a number of bacteria that can cause joint ill, and it sometimes acts in concert with other pathogens). Left unchecked, joint ill can cause destructive bony changes in the joint(s) and lifelong arthritis, with a poor prognosis for long term soundness.

What's designated "joint ill" usually is localized to one or two joints, but there's also the possibility of Salmonella spreading throughout the body, causing generalized septicemia. It can also lodge in the respiratory system, where it can trigger bacterial pneumonia. Either situation might sneak up suddenly on an unsuspecting foal; Dwyer has known cases where a foal was apparently robust and healthy one day, and dead the next. Even eye infections can be caused by opportunistic Salmonella.

Any way you slice it, Salmonella can mean a critically ill foal.

Adult horses sometimes can suffer from "sub-clinical" cases of Salmonella. Mysterious bone and joint infections, subcutaneous abscesses, and pneumonia all can be triggered by Salmonella that has entered the horse's system through an open wound or through inhalation. In such cases, your veterinarian will need to isolate the causative organism, for Salmonella is only one of a host of bacterial pathogens that can produce similar symptoms.

A Slippery Suspect

We need to be vigilant about Salmonella not only because it is so ubiquitous in the environment and so contagious, but also because it's such an adaptable organism. Many of its serotypes have developed resistance to certain antibiotics, which can make it difficult to treat. If you're faced with an outbreak on your farm, it can even adapt itself from horse to horse. That means the antibiotic you successfully used to combat the infection in the first horse might do nothing for the fifth sick horse several weeks later.

"Some Salmonella strains are susceptible to several different antibiotics," says Dwyer, "while others are resistant to all but a couple. Your veterinarian may need to do repeated sensitivity tests over time on the cultures obtained from cases to determine which drug is the best choice."

It's next to impossible to predict how virulent a strain of Salmonella is likely to be. Many horses contract the organism, go through a few days of being under the weather, and shake it off; others might find themselves with a life-threatening condition, the onset of which can be gradual or sudden. Horses even can become "carriers," shedding the bacteria from their systems into the environment in their manure without demonstrating any symptoms of the disease.

"That's the thing about this infection," says Dwyer. "Horses have a highly variable response to it. It can be asymptomatic, mild, or seriously nasty."

Because of the possible vast number of serotypes of Salmonella in the soil, horse's environment, and air, it's next to impossible to formulate an effective vaccine against the bacterium. You might be able to protect against one, or even a couple of dozen serotypes, but you can't protect against 2,200 of them. (A vaccine is used in Australia on broodmares; veterinarians there feel it is of some help against certain serotypes, athough it's unlikely to provide 100% protection against any of them.)

Halt! Who Goes There?

Faced with a mystery infection on your farm (whether it manifests itself as diarrhea in your foals or mature horses, septicemia, joint ill, or pneumonia), you'll want your veterinarian to do a culture as soon as possible to identify the culprit and determine the most effective treatment. Cultures can be taken from a fecal sample, blood, or joint fluid, depending on the nature of the disease.

Because lab cultures can take a couple of days, however, your vet might want to take action even before the results come back. In the case of a horse with diarrhea, replacement fluid therapy (usually by IV, for more rapid absorption) is the first line of defense. Anti-inflammatories, designed to help bring down the fever, also might be recommended. However, administering antibiotics might not be a good idea.


Antibiotics tend to kill bacteria indiscriminately--the good along with the bad. It's suspected that one of the main ways in which Salmonella takes hold is when populations of "good" bacteria in the equine hindgut are compromised as a result of a high-carbohydrate, low-fiber diet (resulting in cecal acidosis from carbohydrates being fermented in the gut), from the stress of hard training or shipping, from a sudden change in the diet, a heavy parasite burden, or any number of other factors. Although undetectable, when populations of beneficial bacteria die, opportunistic pathogens such as Salmonella quickly can gain a foothold. They are more tolerant of acid gut conditions than the beneficial populations and sometimes encounter little opposition. Often in cases of bacterial diarrhea, administering antibiotics only exacerbates the problem!

"Antibiotics can be a double-edged sword," agrees Dwyer.

The case for using antibiotics is clearer when you're treating a foal with joint ill or septicemia. But Dwyer emphasizes, "It must be your vet's decision. The wrong antibiotic can worsen the condition. I'm dead set against lay people just grabbing a leftover bottle of penicillin and treating a horse."

If your horse's condition warrants immediate action, your veterinarian might choose to make an educated guess and start him on one antibiotic while waiting for the results of lab cultures to definitively diagnose Salmonella. (Because horses might shed the organism sporadically, it sometimes takes more than one culture to come up with an answer; just one more frustrating detail of dealing with this pathogen.) Once Salmonella has been identified as the cause, sensitivity tests can help define which antibiotics are most likely to have an effect. The results might prompt your vet to change his or her choice of drugs to treat your horse.

Because dehydration is such a serious concern, horses with bacterial diarrhea almost always benefit from replacement fluid therapy.

"IV fluids are the quickest way to see results," says Dwyer. "You can see them perk up. You can administer fluids by nasogastric tube, too, but it's slower, and you can only give them so much. A foal's stomach can get distended, and even an adult horse's stomach is quite small and can't absorb a lot."

The electrolytes sodium, potassium, chloride, and calcium might be out of balance with a dehydrated horse as well. "You need to balance these out, but not over-correct," Dwyer says. "Bloodwork can give you a lot of information here. Your veterinarian will also probably want to check for anemia, levels of glucose and proteins, liver enzymes, and BUN--which can indicate problems with kidney function, thanks to a buildup of toxins in the body."

Anti-diarrheal medications, such as Pepto Bismol or activated charcoal (which absorb toxins), sometimes are prescribed for horses with bacterial diarrhea, as are probiotics, which can help re-populate the gut with beneficial bacteria. In severe cases, plasma transfusions can be used to help re-establish normal plasma protein values in the blood.

When your vet suspects Salmonella is at work on your farm, he or she likely will recommend you quarantine all affected horses. If you have access to a large animal veterinary hospital, placing them in isolation units there might be the best choice, but that might not be a viable option if you live hundreds of miles away from such a facility. Quarantine can be done at home, but it requires dedication and vigilance. Because Salmonella has the potential to spread not only from horse to horse, but also to you and your family, you'll need to designate only one person to handle the affected animals.

"Wear latex gloves, put disposable covers on your boots, and isolate all of the equipment you use around the sick horses," Dwyer advises. "Don't allow someone whose immune system is compromised--a young child or someone who has lupus or who's recently had chemotherapy, for example--to be exposed. Handle the affected horses only after you've handled the healthy ones, and don't go back to the healthy ones till you've been thoroughly disinfected, washed your hands, and changed clothes.

"Treat all cases of diarrhea as contagious, and think about where that manure is going. You can track it throughout the barn very easily, or expose your healthy horses to it if you spread it through their fields or living environment."

Dwyer points out that even horses which suffer from joint ill or septicemia can be shedding Salmonella in their manure; it doesn't need to be liquid to be contagious!

Presuming your horse fights off a Salmonella infection (up to 90% do, with prompt veterinary intervention), he might continue to shed the organism from for some time. Therefore, it's a mistake to turn an "apparently" recovered horse out with the herd. Instead, Dwyer recommends maintaining him in quarantine for at least 30 days, culturing fecal samples for evidence of Salmonella a few times a week, until the horse has three to five consecutive "clean" samples.

"There used to be serious concerns that some horses would be permanent carriers," she says. "Twenty years ago, horses were sometimes euthanized, for fear they'd be chronic contaminants to the rest of their population. I don't think that stigma is there so much anymore, because we can run the cultures and demonstrate when he's no longer shedding the organism in significant amounts."

Fighting The Spread

Naturally we'd all prefer to ward off Salmonella infections in our barns than have to struggle with an actual outbreak. But with these microscopic invaders always poised to make another raid, what can the average horseowner do to keep Salmonella (and other bacterial infections) out of his or her barn? There are no guarantees, of course, but here are some simple strategies, suggested by Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, Professor of Equine Medicine at Colorado State University's Veterinary Teaching Hospital and Biomedical Sciences Department in Fort Collins, Colo.

"It's important to understand that Salmonella primarily is spread by the animal ingesting the bacteria," says Traub-Dargatz. "It can be spread by inhalation as well, but it's rare to get an infectious dose that way. The main mode of transmission is by ingesting fecal material--from rodents and birds in your horse's barn. Protect your grain from rodents and birds by storing it in closed, chew-proof containers, and eliminate roosting areas for birds in your barn as much as possible."

Dwyer agrees. "Here's a take-home point I use to illustrate the situation to clients. It takes less than 100 Salmonella bacteria to infect a mouse, and one fecal pellet from such a mouse can be a great little incubator for the bacteria. If that fecal pellet gets into your feed or bedding, it can get into your horse. So anything you can do to affect rodent control in your barn can only be beneficial."

Controlling birds in your barn might seem like an impossible task, but Dwyer has seen barns where the owners seem to be begging for infection to take over. In one such barn, she says, there was a bird's nest situated directly above one horse's feed tub, and waste was being splattered every day on that tub! You might not be able to eliminate birds from your loft or hay storage areas entirely, but you can make it more difficult for them to roost in your barn (try placing fine wire mesh along your rafters to keep them out of the upper regions above your horses' stalls, suggests Traub-Dargatz) and eliminate nests in places where they might contaminate your horses' grain. Needless to say, it's probably not a good idea to have free-ranging chicken, ducks, or geese wandering through the same areas your horses inhabit, either.

Think about your water sources as well. If you keep poultry on your farm, keep ducks and geese out of any ponds to which your horses have access. Have a stream or spring-fed pond in your pasture? Know what's upstream. Potential sources of contamination include a livestock operation seeping manure into your water supply, a leaky septic tank, or a sewage treatment plant.

Whether it's drawn from a well or from a standing pond, testing your water for fecal coliforms is an excellent idea. While high levels aren't a direct indicator of disease, says Traub-Dargatz, chances are if coliforms are high, Salmonella levels could be too.

When you introduce a new horse to your operation, inquire about the history of infectious disease in his previous environment. It's a common-sense question that almost no one asks! Keep any new additions to your herd isolated from other horses for at least a couple of weeks (Traub-Dargatz says 21 days is ideal). During that time, allow no direct contact between the new horse and the established herd, and don't allow your other horses to come into contact with the new arrival's manure, either.

"People don't think about things like this--but if your new horse leaves manure in your wash rack or trailer, and you hose it into your pasture, that can contaminate the whole field," says Traub-Dargatz. "Remember that animals can shed Salmonella without showing any outward signs of disease."

Foals are particularly vulnerable to Salmonella because their immune systems are not yet at full strength. In high-risk areas such as foaling stalls, it's best to build with materials that can be disinfected (i.e., concrete walls and floors, as opposed to wood and dirt). Then be diligent about actually doing the disinfecting on a regular basis.

Although they're not commonly added to horse feeds, it's wise to watch out for animal sources of fat (tallow), or protein (bone, fish, or feather meal) in your feed. Any of these has the potential to transmit Salmonella to your stock. Instead of buying cheaper feeds, stick to those containing only good-quality vegetable sources of fat (such as corn or soy oil) and protein (soy, cottonseed, or linseed meal, for example). And of course, store your feed in rodent- and bird-proof containers.

"I think water and feed are logical things we have some control over," says Traub-Dargatz.

Pasture management can play a role in controlling the spread of Salmonella on a farm. Traub-Dargatz recommends cleaning manure from your pastures whenever possible--or at least dragging the fields with a chain harrow to break up manure piles and expose them to sunlight (the UV rays help kill bacteria). Keeping your manure pile isolated from areas where horses graze is important, too. Horses are unlikely to contract Salmonella as long as their exposure rate is low; you don't want the concentrations of bacteria to build to the point where they overwhelm their defenses.

"Think about the ways manure gets spread around," says Traub-Dargatz. "There are lots of ways Salmonella can get moved around your farm. Contain the feces, and you'll help contain the organism."

Hospital Outbreak: A Learning Experience

In 1996, Colorado State University's large animal hospital found itself battling an alarmingly stubborn outbreak of the infection, which eventually affected some 59 animals and caused three fatalities (two horses and a llama). Horses in veterinary hospitals are especially vulnerable to bacterial contagions because they're weak, often on antibiotics, recently have been transported, and come from many different herds. Traub-Dargatz--who was on the front lines battling the outbreak--says colic cases are at particular risk.

"They're the 'sentinels' of an outbreak, because they're the most likely to shed the organism, and they're also the most vulnerable to succumbing to the bacteria."

The CSU outbreak was detailed in a 1996 article in the Journal of the American Veterinary Medical Association. "It was a very hard time for us, because we were trying to help horses, and instead we inadvertently exposed many of them to disease," says Traub-Dargatz. "We traced the outbreak back to an initial case that arrived in the hospital in January of 1996, but it took us a while to realize that the incidence of nosocomial Salmonella infections was on the rise." (Nosocomial means an infection which originates in a hospital setting.)

"We shut down the entire hospital and disinfected the facility in a very systematic way. For two weeks, we turned horses away and accepted no new cases. Then we re-opened, and the first two animals admitted cultured positive for Salmonella after six days. We had to rethink the entire thing and figure out where the infection was coming from."

Eventually the staff at CSU zeroed in on the rubber mats used in the stalls. "They turned out to be slightly porous, and they were harboring the bacteria even though we'd scrubbed them many times. We replaced them with big, solid-sheet rubber mats, which were very expensive and hard to work with, but less likely to have that problem--and we were meticulous about sealing the edges against the walls so fecal material couldn't linger there. It was a vast improvement.

"We also focused on strict biocontrol issues, teaching the staff and the students to be more diligent about handling one horse, then another without washing their hands in-between. And we instituted a very active monitoring system that cultures all of the animals three times a week. We're very happy with how it's gone, and we learned a lot from the situation."

Traub-Dargatz notes that while there's always a risk of an outbreak like that in a hospital setting, it's important for owners to remember that veterinary hospitals also go to greater lengths to prevent the spread of disease, and are built with sealed walls and cleanable surfaces, which make the facility easier to disinfect than the average barn. There have been no further outbreaks at CSU since that fateful spring of 1996, a testament to the hard work and attention to detail of its faculty, staff, and students.

About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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