Equid Blog : Horse Health & Diseases : University of Guelph : Ontario Veterinary College : Equine Infectious Diseases

A three-year-old Standardbred gelding was presented to the hospital for evaluation and treatment of diarrhea (colitis). The previous week the horse had developed a mild hind-limb lameness which seemed to come and go, and it was decided to treat the horse with antibiotics “just in case” it had something to do with an infection. The horse was treated with ceftiofur (an antibiotic, often sold under the brand name Excenel or Naxcel) for five days. On the fifth day, the gelding developed moderate to severe diarrhea. The next morning the horse also had a fever. He was treated with anti-inflammatories and quickly referred to the hospital for intensive care.

On presentation, the gelding was very quiet. He had a very high heart rate, reddish gums and he was significantly dehydrated. Intestinal sounds could not be heard over the abdomen, indicating that the horse’s intestines were not moving normally, and there was a “ping” on the right side of the abdomen, indicating that there was gas accumulating in the cecum (part of the large intestine). Treatment with intravenous (IV) fluids was started right away to try to correct the dehydration and keep up with the amount of fluid the horse was losing in its diarrhea.

By the next morning the horse’s attitude was improved, but his gums were still an abnormal colour (“toxic mucous membranes”, see picture), indicating that there were inflammatory cytokines (substances released by cells when they’re in distress) and likely bacterial toxins in horse’s bloodstream. Also, despite the IV fluids, the gelding was still dehydrated, likely because he was pooling fluid from his body tissues in his intestine, as well as the more obvious loss of fluid in his ongoing diarrhea. This went on for another two days, despite intensive treatment in the hospital. On the fourth day, the gelding developed severe signs of colic. His large colon became progressively more distended with gas, and the contents of his small intestine started to back-up into his stomach. His heart rate became extremely high, and his pain could not be controlled with sedatives or anti-inflammatories. A belly-tap yielded a red-tinged fluid (normally belly fluid is light yellow), and the concern at that point was that the intestines had become twisted (which can happen in horses with diarrhea as a result of their abnormal intestinal motility). Despite the risks, it was decided to take the horse to surgery - but there was no twist in the bowel. The cause of the colic was that the large colon was severely distended with gas and fluid, and it was barely moving at all. The appearance of the large colon was consistent with extreme inflammation, and the tip of the cecum looked so bad that the surgeons decided to remove it because it was likely dead or dying.

The horse recovered from anesthesia, and IV fluid therapy was continued. Later that day, when the horse was offered some pellets, some intestinal sounds were detectable. The horse soon started to pass diarrhea again, but overall his attitude was much brighter, and his hydration status and (remarkably) blood protein levels remained stable.

Unfortunately the day after surgery the gelding became reluctant to move around the stall. Increased digital pulses were detected on the front feet, and the horse was sensitive to hoof testers – the gelding was developing laminitis. Despite additional treatment, the signs of laminitis became worse and worse. In the end the horse was euthanized, less than a week after being admitted to the hospital.

On necropsy, the entire large colon was severely thickened, filled with green-yellow fluid, and the mucosa (inside surface of the intestine) was ulcerated. Signs of severe acute laminitis were present in all four feet. A specific causative agent of the colitis could not be identified – tests for Salmonella and clostridial toxins were all negative. This is not too surprising as no agent is identified in over half of all adult horse colitis/diarrhea cases. But there is no doubt what set this terrible chain of events in motion – treatment with antibiotics, for a condition that may or may not have ever required antibiotic treatment in the first place.

We talk a lot about antibiotic-induced colitis/diarrhea in horses, but until you’ve seen it for yourself, it can be hard to believe that drugs used every day in both people and animals can have such a devastating effect on a horse. Antibiotics certainly do save lives, but unfortunately there are no “miracle cures” that are entirely without drawbacks. This case clearly demonstrates one of the most important reasons why we so strongly advocate prudent use of antibiotics in horses – their use should never be employed lightly. Although this is a “worst case scenario” that overall occurs uncommonly, the potential is there and should always be taken into consideration.

Photo credit: M. Anderson 2007

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An eight-year-old Quarter Horse gelding was presented for examination due to chronic weight loss over several months. Lately he’d also been lying down frequently and exhibiting increased breathing effort, so he was treated for what was suspected to be mild signs of colic, but failed to improve. He’d also collapsed once during mild exercise.

On examination, the gelding was quiet, alert and otherwise physically normal except for his poor body condition. However, on rectal palpation there was a large, firm, non-painful mass within the caudo-dorsal (i.e. upper rear) abdomen. revealed a large (42 centimeter) firm mass in the caudo-dorsal abdomen. The mass was further evaluated using ultrasound via the rectum. the mass was multi-lobulated (i.e. made up of many pockets on the inside) and had a large blood supply. It did not appear that the mass was directly attached to or growing in any abdominal organs (e.g. kidneys, intestine). Blood work showed a high white blood cell count (mature neutrophilia), moderate anemia, and high protein levels due to an abnormal increase in globulins (hyperglobulinemia).

These findings were highly suggestive of a large abdominal abscess, but a cancerous mass could not yet be ruled out. It was decided to take the horse to surgery the next day in order to better evaluate the mass, collect samples and remove it if possible. In surgery, the mass was found to be right at the root of the intestinal mesentery (the large membrane that carries the blood supply to the intestines), and there were a large number of adhesions between the mass and the base of the cecum, as well as to some loops of the small intestine. A needle and syringe were used to remove a sample of the mass’s contents in a sterile manner. The fluid retrieved had the appearance of thick pus, which further supported the tentative diagnosis of an abscess. Unfortunately, due to the location and size of the mass, as well as the number and size of adhesions, it could not be safely removed. The horse was therefore euthanized while still under anesthesia.

Necropsy examination confirmed that the mass was an abscess. The capsule of the abscess was extremely thick and tough, indicating that it had been developing over a very long period of time. A long-standing abscess such as this explained all of the gelding’s clinical signs – weight loss and moderate anemia due to chronic disease, high globulin levels due to constant stimulation of immune cells by the infectious focus, and recumbency and collapse due to discomfort caused by entanglement of the intestines in the adhesions associated with the abscess.

Culture of the fluid sample retrieved at surgery yielded a heavy, pure culture of Streptococcus equi subsp. equi – the bacterium that causes strangles. This horse had what’s known as “bastard strangles,” which is a recognized complication that occurs occasionally in horses that have had the classic upper respiratory infection. In these cases the S. equi invade beyond the respiratory tract and can end up anywhere in the body. Then, just as the bacterium does in the lymph nodes around the head and throat in classic cases, the S. equi can form abscesses. The abscesses may form in internal lymph nodes (which is likely what happened with this gelding) or in organs like the kidneys or even the brain. These abscesses tend to develop slowly and insidiously. Even if they can be identified, they are typically extremely difficult to treat effectively, and unfortunately euthanasia is often the end result. Other bacteria that can cause similar abscesses include Rhodococcus equi, Corynebacterium spp. and Arcanobacterium pyogenes.

Strangles is endemic in the horse population – whenever horses are mixed or brought together in large groups there is a risk of strangles transmission. We cannot eliminate the risk, but we can try to reduce it as much as possible using basic infectious disease control measures. More information about strangles is available on the equIDblog Resources page and in our archives.

Image: A Standardbred in poor body condition due to chronic debilitation as a result of large abdominal abscess, similar to the case described here (photo credit: M. Anderson).

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A five-year-old Quarter Horse mare was found down in the field in the mid-afternoon.  The horse was seen moving around the field normally less than eight hours earlier.  She was found near a fence, but there were no external signs of trauma on the mare's body.  Some green feed material was present at the nostrils.  With encouragement the mare was able to stand, but she was very unsteady and uncoordinated, particularly in the hind limbs. Upon examination by the veterinarian in the field, it was also noted that the horse could not open her mouth normally (lockjaw - which is often a sign of tetanus in horses), and the mare seemed "dazed".  There were three other horses kept in the same field, all of which appeared completely normal.  All the horses were fed the same round-bale hay (a new bale was just put out the previous day).  The mare was vaccinated for rabies in 2008 and West Nile in 2009 (spring).  There was no movement of horses on or off the farm (i.e. the horses were not taken to shows/competitions or off-site rides).

When the horse arrived at the hospital, she was down on the trailer.  By that time she had a fever, high heart and respiratory rates, and she was dehydrated.  Although she was still aware of things going on around her, she was very depressed.  She was still able to see, and she could move all four legs and her tail.  However, it was even difficult for her to lie on her chest (e.g. sternal recumbency), so she would roll to one side instead (e.g. lateral recumbency).  Her jaw remained rigid, but she could still move her tongue a little.

With a lot of encouragement and help the mare was able to stand up and stumble off the trailer, but she was so weak and uncoordinated in all four legs that she fell down again before she could even walk the 10 metres to her stall.  Eventually she made it to her stall, and she was able to stand for about an hour before she lied down (or fell down) again.  A urinary catheter had to be used to drain the mare's bladder because she did not seem to be able to urinate on her own.

The mare was treated with intravenous fluids (supportive therapy), and anti-inflammatories and steroids to try to reduce the inflammation that was suspected to be going on in her brain and spinal cord.  Despite all this, her condition continued to worsen, and by the next morning the mare could not even sit up and was becoming less aware of her surroundings.  The mare was therefore humanely euthanized.

Post-mortem tests in this mare confirmed there was inflammation in the brain based on a high number of inflammatory cells in her cerebral spinal fluid (CSF).  Because of the clinical presentation, some of the brain tissue also had to be sent away for rabies testing, which was (thankfully) negative.  Once that result was back, samples were also tested for evidence of infection with equine herpes virus (EHV-1), West Nile virus and Sarcocystis neurona (the cause of equine protozoal myeloencphalitis), all of which were also negative.  Botulism was also considered, but this disease is very difficult to test for in horses.  In the end, the final diagnosis, and the cause of the mare's neurological signs, was infection with eastern equine encephalitis (EEE) virus .

This case of EEE was diagnosed in September 2009 in Southwestern Ontario.  The description of the presentation, and how rapidly this mare deteriorated, demonstrates just how devastating and severe this disease can be.    This case also tells us that there are mosquitos in the area that are carrying EEE.  Vaccination of the other horses in the region will not provide protection before the end of this mosquito season, but owners of horses in the same area should seriously consider (and discuss with their veterinarian) vaccination of their horses against EEE in the spring, prior to the next mosquito season.  No one can say for certain if vaccination of this mare would have prevented the infection, or decreased the severity of the infection, but it likely would have helped.  In the meantime, as always, decreasing exposure to mosquitos as much as possible (if there are any left) should be the priority.

Image: TEM of the salivary gland of a mosquito infected with eastern equine encephalitis (EEE) virus (source: CDC Public Health Image Library #7057).

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One (of many) unique anatomical aspects of horses is their guttural pouches. These pouches open into the throat area and are essentially large openings in what is the equivalent of a horse's Eustachian tube. They contain several important structures including nerves and major blood vessels such as the carotid artery (see top picture right). The reason horses have guttural pouches is unclear, but it has been proposed that they are used to help cool the blood going to the brain during intense exercise under hot conditions. Regardless of their purpose, horses can develop a variety of problems with their guttural pouches, some of which can be life-threatening.

One such problem is fungal infection of a guttural pouch, which is a condition known as guttural pouch mycosis. This condition may go completely unnoticed for quite some time, but it becomes very serious if the fungus begins to grow over one of the large blood vessels in the pouch, particularly the carotid artery. The fungal infection can weaken the wall of the artery, ultimately causing it to rupture.  This results in massive blood loss (which comes out of the horse's nose) and it is a potentially life-threatening event.

Early signs of guttural pouch mycosis may include a slight bloody nasal discharge, typically from just one nostril. A few other problems can cause this type of discharge too, like an ethmoidal hematoma. However sometimes there are no warning signs before the horse has a major bleed. I've had cases where there was just a trickle of blood reported by the referring veterinarian, but by the time the horse arrived at the hospital, the blood was gushing from the nose like water from a hose. These massive bleeds can lead rapidly to death due to blood loss. Ideally, cases are identified before a major bleed occurs, but this is not always possible. Here are some points to consider:

• Take all nosebleeds seriously. Don't panic, but have your horse evaluated promptly by your veterinarian in order to identify the source of the blood, even if the cause of the bleeding can't be determined.  The cause of blood coming from a guttural pouch should be considered guttural pouch mycosis until proven otherwise.
• The fungi that cause guttural pouch mycosis are widespread in the environment. The reason that some horses develop this type of infection while others do not is not known. There is nothing that can be done to prevent it.
• If guttural pouch mycosis is diagnosed (or suspected), the horse should be referred to an equine hospital. The best way to prevent a severe bleed is to cut off the blood supply to the affected area of the artery, which is done surgically. In the case of the carotid artery, which is connected to several other major blood vessels at the base of the brain by a structure called the Circle of Willis, the blood supply must be blocked both above and below the level of the fungal plaque in the guttural pouch.  If the artery is only blocked on the side closest to the heart, then the collateral circulation from the Circle of Willis can still result in a severe bleed.  This collateral circulation is also the reason that a major artery, like the carotid, can be blocked on one side (right or left) without harming the horse.
• The fungus in the pouch "feeds" off the blood supply from the affected artery, so when the blood vessel is occluded, the fungal infection becomes much easier to treat.  Treatment of guttural pouch mycosis with anti-fungal medications that are infused into the guttural pouch is typically not effective, and the risk of a fatal bleed is always present, until the blood supply to the affected artery is cut off.

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