Posted on February 21, 2009 by Scott Weese

Diarrhea: Why Culture?

Diarrhea is a potentially life-threatening condition in horses. It's also frustrating from a diagnostic standpoint because, even with the most complete/comprehensive testing, a cause is only identified in a minority of cases. This is true for other species too, including people, and is a reflection of the complex nature of the intestinal tract and the numerous possible causes of intestinal disease. Diagnostic testing obviously costs money, so if it gives us an answer less than half the time anyway, it begs the question - why bother? Well, here are some points to consider:

  • In some situations, you may find a cause that requires a different treatment, so diagnosis has a direct impact on patient care and probably the outcome.
  • Many causes of diarrhea are infectious and it's important to know if other horses may have been exposed to a transmissible pathogen.
  • Some causes of diarrhea, especially Salmonella, can also infect people, and it's important to know to what people have been exposed.
  • It's also important to know whether a horse might still be shedding a transmissible pathogen after its diarrhea resolves. For example, horses with salmonellosis can shed Salmonella for a while (often a few weeks, sometimes longer) after they have recovered from their diarrhea. This could be a source of infection for other horses and people.
  • Outbreaks of diarrhea can occur. It's much better to know early on what you are dealing with, rather than waiting until mulitple horses have been infected.

I consider diagnostic testing money well spent in cases of diarrhea. Some people don't want to test because they don't want to know, particularly about Salmonella. However, it's been proven again and again that the "head-in-the-sand" approach will backfire when it comes to infectious diseases.

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Posted on January 12, 2009 by Maureen Anderson

Potomac Horse Fever

Potomac Horse Fever (PHF), a disease named for the river near Washington DC where it was first identified, is caused by Neoricketsia risticii.  The technical name for PHF is equine monocytic ehrlichiosis (EME).  This causative organism is normally found in a trematode parasite with a somewhat complex life cycle involving bats, snails and certain aquatic insects. It is therefore associated with swampy/marshy areas in certain regions.  Horses are aberrant hosts of N. risticii, in which infections can cause a range of disease from very severe colitis and laminitis to mild non-specific illness.  The organism is typically very sensitive to treatment with tetracyclines, but the disease can progress very quickly, so treatment early in the course of disease (if PHF is strongly suspected) is crucial.

In general, PHF is an uncommon disease in horses, but there are certain areas of Ontario where cases occur with some regularity. For example, cases have been reported in Eastern Ontario (Brighton to Kemptville), Keswick area and Ancaster. The incidence is usually highest in late summer. Transmission is normally associated with horses grazing pastures containing infected flies (caddisflies).

Although vaccines for PHF are available and appear to be safe, their efficacy is very questionable. At least in part, this may be due to the fact that multiple strains of N. risticii occur naturally, but only a single strain (from the 1980s) is included in the vaccines.  Vaccination is not considered necessary for horses in areas where the disease has not been previously reported. Given the peak incidence of the disease in late summer and the nature of the vector, if someone wants to vaccinate, they should do so in late spring. Preventing access of horses to areas near swamps and other insect breeding grounds may be as (or more) effective than vaccination alone.

Photo Credit: Buck Cash, Great Falls, Potomac River, VA

 

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