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

TheHorse.com has just published a readers' poll that asked the question "What is your most important parasite concern?"

Here are the results, with my commentary:

Small strongyles (cyathostomes): 26.08% (103)

• A narrow victory over large strongyles but a worthy winner. Cyathostomiasis is definitely a problem and it can cause serious disease. It seems to be increasingly common, or at least increasingly diagnosed. Resistance to dewormers among this group of parasites is also becoming a concerning problem.  Cyathostomes are probably the most important overall parasite issue, in terms of real impact on horse health, in most areas.

 Large strongyles (bloodworms): 23.04% (91)

• Large strongyles, particularly Strongylus vulgaris, used to be a major problem, before the advent and widespread use of ivermectin. These worms migrate through blood vessels in the abdomen and can cause blood clots to form, compromising the blood supply to the colon. This used to be a leading (if not main) cause of surgical colics and killed a lot of horses. They are a very rare problem now because of their susceptibility to drugs in the ivermectin family and fenbendazole.

Tapeworms: 15.70% (62)

• It's hard to determine the importance of tapeworms. There have been conflicting studies about their relevance, with some studies indicating they play a role in certain types of colic (e.g. gas colic, ileocecal intussusceptions) and others saying they are not involved in disease. The risks probably vary by region and type of colic, and tapeworms probably play a role in some colics, but it is difficult to quantify just how big a role.

Bots: 11.90% (47)

• It's not surprising that this was a popular choice since people are (hyper)aware of the tiny bot eggs that can be deposited on the horse's haircoat, especially on the lower legs. However, while bot eggs are visible and the sight of adult bot worms attached to the stomach during a gastroscopy can freak people out, bots are pretty innocuous. Adult bots can cause small superficial errosions in the stomach wall where they attach, but this isn't of much relevance to the horse. The main issue with bots is how much the adult flies annoy horses when buzzing around and laying their eggs.

Other 10.13% (40)

• I'm not really sure what people meant when they chose "other." I can't think of any other parasites that would be contenders for "most important."

Ascarids (roundworms): 9.37% (37)

• I'm surprised this isn't higher. Ascarids are controllable but can and do cause disease, particularly in foals. The main issue is when foals are not dewormed properly early in life, and accumulate a large burden of worms. When these foals are eventually dewormed, the worms  die all at once and can cause an intestinal obstruction.

Pinworms: 3.80% (15)

• It's good to see that there's not much concern, but no one should be most concerned about pinworms. Pinworms are, at best, a minor annoyance that can cause tail rubbing.

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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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Di-tri-octahedral smectite is a type of clay, but it's not just any old clay - it has some useful properties that may be able to help prevent or treat certain intestinal tract diseases. The main benefit of smectite is it's ability to bind to bacterial toxins. Bacterial toxins in the intestinal tract play an important role in intestinal diseases, particularly diarrhea. We previously showed that smectite effectively binds to toxins produced by Clostridium perfringens and Clostridium difficile, two important causes of diarrhea in horses, as well as endotoxin, another important bacterial toxin. These studies were done in a lab.  While smectite has been widely used for the treatment of diarrhea (or prevention in some situations), we've been using it based on the hope that what happens in the lab also happens in the horse, and on our clinical impression that it seems to do something helpful.

Now, we have a little more actual proof that this is the case. A recent study published in the Veterinary Journal (Hassel et al 2009) looked at the ability of smectite to prevent post-operative diarrhea in horses that underwent colic surgery. Horses were randomly assigned to receive smectite (500 g/500 kg body weight in 4 litres of water, once a day for 3 days) or a placebo (4 litres of water, once a day for 3 days). Treated horses had a significantly lower risk of post-operative diarrhea: 11% versus 41% in controls.

Surprisingly, there was no mention of whether treated horses had a higher survival rate, shorter hospital stay or other important outcomes. It would be useful to know these things, because if smectite just prevents mild diarrhea (i.e. the type that just results in loose feces for a couple days) but doesn't actually improve survival, decrease serious disease, shorten hospitalization or reduce complications, then it's not as useful.

So, we have more evidence indicating that smectite can have a beneficial effect in the horse, not just in the lab. Considering it's safe to use and not excessively expensive, and given that diarrhea is a serious problem in horses, continuing to use it makes sense. Nonetheless, more information about the ability of smectite to have an impact on survival, hospitalization, treatment costs or other factors is still needed.

Image source: www.platinumperformance.com

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