Case Presentation: Chronic Weight Loss
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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Rhodococcus equi is a common pathogen in foals between the ages of 1 and 6 months of age that is most infamous for its ability to cause pneumonia. Classic R. equi infection results in the formation of large abscesses throughout the lungs of young foals (see picture left), which can be especially difficult to treat because the bacteria are able to hide from the body’s immune system by living within white blood cells. However, this organism’s bag of tricks doesn’t end with lung abscesses – it can also travel to other parts of the body and cause all sorts of trouble. These kinds of infections may occur with or without the classic lung infection, and are referred to as extrapulmonary disorders (EPDs).
An abscess is an accumulation of pus. Pus is basically dead bacteria, partly broken-down cellular debris, and white blood cells. Sometimes there can be a thick capsule of tough, fibrous tissue surrounding an abscess, particularly if it has been there for a long time (see picture left). Most abscesses occur as the result of a local infection that the body was able to contain (at least partly), but could not eliminate altogether, and that didn't "drain" naturally to an open space (e.g. outside the body). Over time, some abscesses may continue to grow and become quite large, which can cause many different problems depending on where the abscess is. "Sterile" abscesses can also occur, but are much less common - these lesions are not associated with a bacterial infection, but may occur following a severe trauma or in association with certain tumours.
