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

I've had a run of questions about testing for Helicobacter in horses with gastric (i.e. stomach) ulcers. Helicobacter pylori is unquestionably an important problem and clear cause of gastric ulcers in people. However, people aren't horses. The human stomach differs greatly from the equine stomach, and gastric ulcers in people are much different than gastric ulcers in horses. There is no (I repeat, NO) evidence that Helicobacter is a cause of ulcers in horses. Despite this some unscrupulous molecular diagnostic companies offer unvalidated tests for Helicobacter in horses, for reasons that make absolutely no sense.

Don't waste your money on Helicobacter testing if your horse has ulcers. Spend it on treatment and management changes to reduce the risks of ulcers recurring.

(click image for source)

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Another horse farm in Queensland is under quarantine for Hendra virus. This follows the high-profile outbreak on another Queensland farm this past summer that resulted in the death of a veterinarian. Ongoing identification of affected farms is quite concerning for a disease that is typically very rare and sporadic. It's hard to say what this means.

Does it mean that Hendra virus is spreading and becoming more common? It's certainly a possibility. Even though we've known about this virus for a while and it's only caused rare outbreaks until now, established infectious diseases sometimes change their patterns for no clear reason.

Does this mean that Hendra virus is just being recognized more now? This is certainly a possibility too. Not all apparent changes in disease patterns are truly changes in disease patterns. Improved awareness, diagnostic tests or increased efforts to diagnose a disease can all lead to the impression that a disease is becoming more common when in fact it's just the same. I don't know whether that's the case in Queensland, but Hendra is certainly getting a lot more attention now.  I suspect that many horses that would not have been tested in the past are now being tested.

How do we answer these (and other) questions? Research and surveillance. We need to know a lot more about this virus. We need to make sure that horses with signs that might be indicative of Hendra virus get tested. Since veterinary medicine is a user-pay system and diagnostic testing often gets skipped (especially when a horse has already died), some form of subsidized testing might be required. Unfortunately, a lot of it comes down to money. Money is needed to do research, provide diagnostic testing, perform surveillance and educate everyone involved. Money for these types of efforts is getting harder to find, although it's usually much easier during a high-profile outbreak.

Image: Grey-headed Flying-fox (Pteropus poliocephalus) (source: www.environment.gov.au, photographer: Andrew Smith)

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During a strangles outbreak, people often ask how (and when) they can say their farm is "strangles-free." They usually don't like the answer. Declaring a facility to be free of strangles, a highly contagious disease caused by the bacterium Streptococcus equi, is not cheap, easy or quick. It is, however, important.

A major source of the strangles bacterium is horses that have been infected but either didn't get sick or have recovered, yet are still shedding S. equi in their nasal secretions. In some horses, S. equi can persist for long periods of time (i.e. months) in their nasopharynx (throat area) or guttural pouches. Identification of these carriers is crucial for strangles control and must be done to have confidence that strangles has really been eradicated from a farm.

Identification of carriers involves taking samples from the nasopharynx to test for the bacterium. Nasopharyngeal washes are preferred but nasopharyngeal swabs (note: this is different from a nasal swab) are also acceptable. These should be collected by a veterinarian. They can be tested for S. equi by culture or PCR (a molecular test), or both. Any positive horses need to be investigated further, including endoscopy and culture of the guttural pouches.

Three negative samples from the nasopharynx and/or guttural pouches, collected weekly, are required to declare a horse free of strangles. If a positive horse is identified during the process, the clock starts again on its testing, plus all other horses with which it has had contact. Even though a horse may have had three negative samples, if it has been in contact with a positive horse, you have to assume there's a chance it was infected after testing started, so you need three more samples after its last contact with any positive horse.

While this process is not cheap, easy or quick, it is strongly recommended.

At the same time, it's also a good idea to review why the strangles outbreak occurred and how it can be prevented in the future. This is a step that's often overlooked.

More information about strangles can be found on the equIDblog Resources page and in our archives.

Image: A horse with draining tracts from classic strangles abscesses between the lower jaw bones

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To steal a line from Winston Churchill, equine protozoal myeloencephalitis (EPM) is a riddle, wrapped in a mystery, inside an enigma. It's a frustrating disease from many standpoints, and myths abound. A recent article from TheHorse.com highlights some of these myths.

One of the biggest problems with EPM is diagnosis. In Ontario, a large number of horses are "diagnosed" with EPM, but very few actually have the disease. Probably hundreds of horses (particularly race horses) are treated for every single horse that actually has EPM. Many horses get treated without any diagnostic testing at all, but merely based on vague (or no) signs that could be caused by numerous other conditions.

A key point in the article is the need for diagnostic testing and a reasonable suspicion of EPM before considering the diagnosis (and treatment). Unfortunately, none of the tests we have available are very good by themselves, which complicates matters. When you have a marginal quality test and you use it in horses that probably don't have disease, you end up with a lot of false positive results, which often results in unnecessary treatment.

According to experts in this article (with whom I absolutely agree), EPM should only be diagnosed if:

• The horse has clinical signs consistent with the disease (this does not include vague, non-neurological signs like not running as fast as the trainer or owner wants)

        AND

• A veterinarian has ruled out all other neurologic diseases that are similar to EPM

        AND

• You have a positive EPM test result.

There are several testing options, each with good and bad points. There's not currently one "right" test that everyone should use. They key is SOME test should be used and it MUST be used in conjunction with a proper examination and other testing to rule out diseases like Wobbler's syndrome. Only when you are left with no other diagnoses and a positive EPM test should you really consider EPM likely.

There's a good statement at end of the article: "If it looks like EPM and smells like EPM ... it probably isn't. EPM remains a rare cause of neurologic disease in horses." EPM certainly does exist and can be a serious disease, but make sure you are not wasting time, effort and money treating a horse that doesn't really have it.

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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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This post originally appeared (in modified form) on www.wormsandgermsblog.com on January 1, 2009.

An important step in diagnosing infectious diseases and determining the optimum approach to treatment and management is rapid and accurate diagnostic testing. Many different testing methods are used, particularly bacterial culture (at least for bacterial diseases). Molecular testing has revolutionized the field of microbiology, and is making inroads into the field of diagnostic testing. Polymerase chain reaction (PCR) testing is a very powerful tool that can be used to detect DNA or RNA from specific microorganisms.  This technique can be very useful, but it can also be easily misused or misinterpreted.

The potential PROS of molecular diagnostic testing include:

• Rapid turnaround time: Testing can take as little as a few hours versus a few days for other tests like bacterial culture.
• Sensitivity: Organisms that are difficult or impossible to grow in a lab can be detected, and they can often be detected at lower levels than with other diagnostic methods.

The potential CONS of molecular diagnostic testing include:

• Sample contamination: This is a common concern with highly sensitive molecular tests - even a minute amount of contamination in the sample can cause a false positive result.
• Test inhibition: Samples from complex biological sites (e.g. stool) can contain substances that interfere with the many complex molecular reactions upon which the tests rely. Without good (and proven) methods to prepare the sample, this can result in a false negative result.
• Biologically irrelevant results: Some bacteria that cause disease are also commonly found as part of the normal microflora in healthy animals - simply finding it does not tell you that it is necessarily relevant to the problem. For example, Clostridium difficile can be found in the intestine of normal, healthy horses, but the diagnosis of C. difficile diarrhea requires detection of the bacterial toxins in stool samples, not just the bacterium itself.  A molecular test that simply identifies the presence of C. difficile, even if it identifies strains that possess the genes to produce toxins, tells you nothing about whether the bacterium was actually producing toxins in the animal.
• Lack of validation: This is a common problem with many (if not most) molecular tests. Some companies, especially those that just run molecular tests, offer a huge array of completely unvalidated and sometimes illogical tests.  It is also important to remember that tests must be validated for each species in which they are used - a test that works well in people will not necessarily work on a sample from a horse or a dog.

Molecular testing can be useful in some situations. If you are unsure, here are some things to ask the lab:

• Do they have a validated test that provides relevant results?  If they don't have good data (ideally published data) that their test is useful, accurate and reproducible, I'd avoid it.
• Do they have a quality control program, which includes running positive and negative control samples with each test batch?

Finally, as with any test that we use in veterinary (or human) medicine, it's important to evaluate all  results in the context of what is happening with the animal - treat the patient, not the test result.

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