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

A new study was recently published in the Journal of Veterinary Internal Medicine (Frederick et al. 2009) which looked at infectious agents found in the feces of foals with diarrhea. Specifically, they looked for rotavirus, Clostridium perfringens, C. difficile, Salmonella, parasite eggs and Cryptosporidium oocysts.

They found at least one infectious agent in the feces of 122 (55%) of the 233 foals in the study. That means, despite testing for a wide range of pathogens, they could not identify an infectious agent in 45% of the diarrheic foals. This is very similar to the situation typically found in adult horses with diarrhea. This could have happened for a number of reasons:

1. No test is perfect. It’s possible that in some of the cases one of the test results was a “false negative,” meaning it did not detect the infectious agent even though it was there.
2. A few cases may have been caused by other infectious agents that were not included in the diagnostic panel.  For example, Rhodococcus equi is a common cause of respiratory disease in foals, but it has also been associated with diarrhea in some cases.
3. The diarrhea was caused by an agent of which we are unaware, and for which we have no test. Researchers are constantly looking for other bacteria, viruses or parasites that may be capable of causing or contributing to diarrhea in foals and adult horses.
4. The diarrhea was not caused by an infectious agent. For example, the authors failed to discuss foal heat diarrhea as a cause of clinical diarrhea in very young foals. This is a well recognized cause of foal diarrhea, but no infectious agents are involved.

The most commonly identified pathogens were rotavirus (20% of cases), Clostridium perfringens (18%), Salmonella (12%) and C. difficile (5%). Overall 191 (87%%) of the foals survived, and survival was not associated with any pathogen identified in the feces (i.e. in this study, foals were not more likely to die if they had one particular pathogen in their feces than another). This must be interpreted very cautiously, however, because the study does not account for other kinds of illness in these foals, or even whether diarrhea was the primary problem for which they were referred to the hospital. Diarrhea, especially in very young foals, can be very serious because they can dehydrate very quickly and are very susceptible to shock of various kinds.

There were a few other issues with this study that are important to keep in mind as well:

1. The group of foals they looked at ranged in age from newborns to 10-months old. The digestive tract of a foal undergoes drastic changes in the first year of life, and it is well known that certain infectious agents only cause disease in foals of particular ages. For example, different parasites may take weeks to months to develop within the intestine of a foal, so even if a foal is infected as soon as it’s born, these parasites cannot cause disease for quite some time. For this reason, it would have perhaps been more useful to look at the data separately for different age groups.
2. There was no control group in this study. Most of the time, if an animal has diarrhea and the test you perform tells you there is a known diarrhea-causing pathogen in the feces , you assume the diarrhea is due to that agent. This is not necessarily always the case. Some pathogens are carried around by totally normal animals, who may get diarrhea for a completely different reason. So what we really need to know now is: if the authors tested 233 foals with normal feces (and the same ages), how common would each of these pathogens be?

While the data may be interesting to look at, this paper doesn't really tell us anything new that will change the way we treat or manage foals with diarrhea in general.  Nonetheless, the information may still be useful for helping to design and interpret future research studies about these pathogens and diarrheal disease in foals.

Image source: www.bbc.co.uk (credit: George Ring)

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Clostridial myonecrosis (also called clostridial myositis or gas gangrene) is a very serious infection caused by growth of Clostridium bacteria in muscle. Clostridia are anaerobic spore-forming bacteria, meaning they only grow in environments without oxygen and they can stay dormant in the form of very tough (e.g. difficult to kill) spores. Clostridal spores can be found in healthy muscle. Presumably they enter the bloodstream periodically from the intestinal tract, but aren't able to grow because of the presence of oxygen in healthy muscle. They lie dormant, waiting for the right conditions to start growing - which in most horses never occurs. Clostridial spores can also be introduced into muscle during injections (e.g. vaccines, medications), surgery or trauma.

If clostridial spores are in a muscle and an anaerobic (oxygen-free) environment is created, they can start to grow. An anaerobic environment may form if the muscle is significantly damaged, such as from intramuscular injection of an irritating drug, or serious trauma. When the spores start to grow, they can cause very serious infection. Various clostridial species can be involved, but Clostridium perfringens is most common. Regardless of the particular species, infection is characterized by a very rapidly progression of soft tissue swelling, pain and systemic signs like fever and toxemia. The affected area can expand very quickly. Clostridia often produce gas when they grow and crepitus (a crackling feeling) can be felt over the infected area from the presence of gas in the tissues.

Clostridial myonecrosis is a life-threatening disease. It requires immediate and aggressive care, but is often fatal even with treatment. One of the most important (and dramatic) aspects of treatment is opening up the infected area to provide drainage and allow air (and oxygen) to enter the tissues. Large incisions need to be made into the muscle (this procedure is known as a fasciotomy). While this can look horrific, it's a critical aspect of treatment. High doses of antibiotics are also needed, along with other supportive care.

Fortunately, clostridial myonecrosis is relatively uncommon. Avoiding this disease involves decreasing the chances of creating the anaerobic environment in which clostridial spores can grow:

• Intramuscular injection of irritating substances should be avoided. Flunixin meglumine (also called Banamine and a few other trade names) is commonly associated with this disease and should not be injected intramuscularly. Other irritating drugs include phenylbutazone (bute) and ivermectin.
• Any muscle soreness or swelling that develops after an intramuscular injection should be taken seriously. A veterinarian should be contacted promptly, especially if the size of the affected area is rapidly expanding.
• There is no evidence that any pre-injection techniques (e.g. swabbing the area with alcohol) have any influence on the occurence of this disease. Clostridial spores are resistant to alcohol, and most other disinfectants.
• If your horse has clostridial myonecrosis, it should be treated at an equine hospital (if that's an option) because of the intensive care that is required. Aggressive and expensive treatment is needed. The prognosis is already guarded, but if optimal treatment isn't started promptly, the prognosis is very poor.

More information about clostridial myonecrosis is available on the equIDblog Resources page.

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