The loss of weight and/or condition is something that we encounter relatively frequently, but has a real multitude of potential causes. Until a definite diagnosis has been reached, then weight loss should not be taken lightly, as there are some serious and progressive diseases that can ultimately be fatal. Having said this, the majority of cases are fairly simple to diagnose and treat and carry an excellent prognosis.
What Causes Weight Loss?
There are many potential reasons why a horse can lose weight, but they can be divided into categories for ease of understanding:
- Inadequate Feed
- An absolute lack of food, for example, an over grazed field with no grass left.
- A relative lack of food, for example, a horse with a much higher workload being fed the same amount of food.
- Unavailable food, for example, a horse being bullied away from supplementary feed in the field.
- Unable To Ingest Feed
- The horse must be able to prehend food (get it into their mouth!), chew it and swallow it.
- Prehension problems are uncommon, for example, facial nerve paralysis leading to a loss of control of lips.
- Dental problems are extremely common. Inability to chew properly will affect absorption of food, and a painful mouth will reduce appetite.
- Swallowing problems are uncommon. Horses with long standing cases of choke are unable to swallow due to physical obstruction, but not usually for long enough for significant weight loss. Rare conditions of the upper airways can affect the nerves that control swallowing.
- Unable To Digest Food
- This can be due to enzyme deficiencies, so food cannot be broken down. These are extraordinarily rare in the horse, with one example being Exocrine Pancreatic Insufficiency.
- Rarely, foals can be born with a lactase deficiency and be unable to digest milk.
- Unable To Absorb Food
- The lining of the large and small intestines plays a crucial role in the absorption of nutrients, and damage to these membranes can prevent nutrients being absorbed into the bloodstream.
- The most common reason for intestinal damage is infection with various species of worms.
- Excessive Energy Demands
- Other diseases may place extremely high demands of energy on the body, which leads to a relative energy deficiency and subsequent weight loss.
- Infections are the most common cause, although rare diseases such as cancer and heart failure may also cause this.
- Loss Of Energy
- If the body is losing nutrients, then it will have to work overtime to replace them, leading to relative energy deficiency.
- The most commonly lost substance is proteins (usually albumin), and it is most frequently lost through damaged intestinal walls into the gut, leading to diarrhoea. Kidney disease can also cause loss of protein, in this case into the urine.
- The nutrient need not be lost externally – in horses with peritonitis, protein leaks into the abdominal cavity. Although the protein is still within the horse it cannot be used and must still be replaced.
Approaching A Horse With Weight Loss
The three most common reasons for weight loss, by a considerable margin, are feed, teeth and worms. We would usually try and address any issues in these areas before performing expensive diagnostic testing. Your vet will thoroughly examine the horse to look for any other disease problems, examine your horse’s teeth, and ask questions about feeding and worming. They may take a faecal sample for a worm egg count, and a blood sample to look for evidence of tapeworm infection. Even when worm management is good, they may recommend worming as a precaution.
Further investigation may performed for a couple of reasons. Firstly, if the above steps have been taken and weight loss continues, then it is necessary to perform more tests to diagnose the problem. Secondly, if your horse’s weight loss has been sudden and/or severe, then it may be necessary to perform more tests immediately, rather than wait and see if normal treatments work. The following tests are used in diagnosing weight loss, your vet will discuss which tests are appropriate in specific cases.
- Haematology means looking at the cells present in a blood sample.
- We can assess whether any anaemia is present by looking at red blood cells, and look for signs of infection by assessing white blood cell numbers.
- Platelet numbers are also looked at, although diseases involving platelets are very uncommon.
- Biochemistry means looking at the levels of certain enzymes and chemicals in blood.
- We run a broad spectrum of tests that allow us to see any damage to internal organs, such as the liver and kidneys, the muscles and intestines.
- We can also see the level of different proteins, which can help detect ongoing protein loss, and suggest an infectious process.
- Peritoneal Fluid Analysis
- Peritoneal fluid is the fluid which bathes the outside of the abdominal organs, allowing them to move over each other.
- A sample will be collected by inserting a very fine needle into the abdomen through the belly. A full surgical preparation is done to avoid infection, and this is a procedure that carries very low risks.
- Changes will be seen with certain types of intestinal inflammation, and also in cases of peritonitis.
- Oral Glucose Absorption Test
- A blood sample is taken before, and at various intervals after, the administration of a set amount of glucose orally.
- If the small intestine is not absorbing glucose properly then blood glucose levels will not rise as they should do after a high glucose meal.
- Rectal Biopsy
- A small piece of the lining of the rectum is taken using a special biopsy instrument, and analysed at a specialised laboratory.
- There can be a small amount of rectal bleeding, but the test is very safe.
- In approximately 50% of cases with large intestine problems, changes will be seen in a rectal biopsy.
- Abdominal Ultrasound
- Ultrasound examination of the abdomen allows the vet to see pictures of the major abdominal organs.
- Abdominal organ problems can be seen, most commonly thickening of the gut wall.
- Although gastric ulcers are a fairly uncommon cause of weight loss, they can often occur due to the stress of having a different disease.
- When present they can affect appetite significantly, and will need to be treated along with the primary condition.
- Duodenal Biopsy
- Usually performed at the same time as gastroscopy, this involves taking tiny biopsies of the inside of the small intestine through the stomach.
- This can give very useful information as to any problems with the small intestines.
- In extremely rare cases, surgery is necessary to diagnose weight loss problems.
- While providing extremely good diagnostic information, the risks are considerable, especially so in animals with an unknown underlying disease.
Different combinations of these tests will be used in different horses, and your vet will discuss the relative value of each test to your horse. In all but the most severe and sudden weight loss cases, a staged approach can be used, with the most productive tests being used first, and the most invasive and expensive options not used unless absolutely necessary.
Treatment and Prognosis
Both treatment and prognosis depend entirely upon the exact cause of the weight loss. Bearing in mind that in the region of 90% of horses with weight loss have either feeding, dental or worm problems, which are all extremely treatable, then the prognosis for the vast majority of horses with weight loss is good.