Why is my horse losing weight? The 5 veterinary steps to determine the cause.

If your horse is too thin, there is always a reason. It isn't always visible from the outside, though, which is why your vet works through the case step by step. First come the simple, common causes: parasites, dental problems, feed quality. Only if those come back clear does the search move deeper, into blood tests and, if needed, a gastroscopy to look inside the stomach. This article walks through how that systematic work-up gets built, test by test, and what each one is actually looking for. Understanding the order helps you work with your vet instead of just waiting for answers.

Step 1: General clinical examination

With a thin horse, your vet starts with a general clinical examination. Your horse gets checked from head to tail, with extra attention to body condition score (BCS), teeth, manure, how the horse moves, and its breathing. Your account of the horse's history matters just as much as the exam itself: how long has the horse been thin, has anything else changed (diarrhoea, coughing, lethargy, altered behaviour, colic), and what does its current diet look like.

Based on that first exam, your vet decides which additional tests make sense. Often that means a faecal examination for parasites, blood tests covering organ function, inflammation, protein levels and hormones, and, if the symptoms point that way, a gastroscopy to look directly into the stomach. Step by step, this builds a complete picture of why the weight loss is happening.

Step 2: Deworming and faecal examination

An important step in investigating a thin horse is taking and analysing a manure sample, alongside checking deworming history. Your vet checks when the horse was last dewormed and which product was used, because some worm populations have become resistant to certain dewormers, making them far less effective than the label promises. That's why the manure sample still gets examined under the microscope, to confirm whether parasites are actually present despite treatment. This faecal examination also tells your vet which types of worms are involved. Four parasites account for most parasite-driven weight loss in horses.

Small Strongyles (Cyathostominae)

Small strongyles are the most common parasites in adult horses. They feed on blood and tissue fluid in the intestinal wall, causing chronic low-grade bleeding. Their larvae can sit dormant in the gut wall for months, then emerge all at once in large numbers, a mass emergence that triggers inflammation, diarrhoea, colic and weight loss. Regular manure checks and a deworming plan built around resistance testing are essential [1].

Large Strongyles

Large strongyles are less common now, thanks to decades of routine deworming, but still serious when present. Their larvae migrate through blood vessels on the way to the intestines, a journey that can trigger blood clots and lasting intestinal damage [1].

Roundworms (Ascarids)

Roundworms mostly affect foals and young horses. Living in the small intestine, a heavy burden can cause blockages and colic. A bloated belly, a dull coat and slow growth are the classic giveaways [2].

Tapeworms

Tapeworms attach at the ileocaecal junction, where the small intestine meets the caecum, and can damage the gut wall and its nerve supply there. The result is digestive trouble and a slow, steady weight loss [3].

Based on the faecal examination, your vet draws up a personalised deworming plan. Horses that graze heavily or mix often with other horses benefit most from a strategic, tailored schedule rather than a fixed calendar rotation.

Step 3: Dental check

A good dental check is an essential part of examining a thin horse. If a horse cannot chew properly, it cannot make good use of the feed in front of it, no matter how good that feed is.

A horse's teeth grow continuously, and it takes a steady intake of fibre to wear them down evenly. Without enough roughage, teeth wear unevenly and sharp edges or hooks can form, making chewing painful enough to drive weight loss on its own. Older horses carry an extra burden here: many already have partially worn or missing molars, so they grind roughage less finely and absorb less of it, a very common reason horses become or stay thin later in life [4].

These are the most common dental problems a vet looks for in thin horses:

• Hooks on the molars: cause pain while chewing and make it difficult to grind roughage properly.
• Wave mouth / step mouth: caused by uneven wear, reducing chewing efficiency.
• Loose or missing teeth: horses have less grip on feed.
• Caps in young horses: baby teeth that stay on too long and hinder chewing.
• Diastemata (spaces between molars): food gets trapped, causing pain and inflammation.
• Fractured or split molars: cause acute pain and refusal of hard feed.
• Root infections: cause severe pain and poor appetite.
• Uneven wear due to EOTRH (especially in older horses): a painful disorder of the incisor and canine teeth that can make biting off feed uncomfortable [5].

Have your horse's teeth checked at least once a year, twice a year if it's thin or older. Regular checks catch dental problems before they cost your horse condition.

Step 4: Blood tests

Once the obvious causes, diet, parasites, dental problems, have been ruled out, your vet moves on to blood tests.

The blood goes to an accredited laboratory. The values that come back can point your vet toward problems with digestion, metabolism or inflammation somewhere in the body, often long before those problems show any outward sign.

A companion article on this site goes deeper into which blood values matter and what they can reveal about your thin horse.

Step 5: Gastroscopy

Sometimes your vet needs to look directly inside the stomach to check for ulcers or other problems. That examination is called a gastroscopy.

This test is often suggested if your (thin) horse also shows one or more of these signs:

• frequent yawning
• grinding its teeth
• sensitivity when tightening the girth
• a dull coat
• regular (mild) colic
• tucked-up abdomen

During a gastroscopy, a thin, flexible tube fitted with a small camera passes through the nostril into the stomach. Light sedation keeps the horse calm and still. On the screen, the vet examines the stomach lining directly, in real time.

Among other things, the vet will look for:

• Stomach ulcers (red or irritated areas, sometimes bleeding)
• Glandular ulcers (ulcers near the exit of the stomach)
• Inflamed or thickened areas of the wall
• Excess mucus or feed residues
• Scars or other abnormalities

Stomach ulcers and intestinal inflammation reduce appetite, make eating uncomfortable, and impair nutrient absorption. Over time that shows up as gradual muscle loss and a duller coat.

Gastroscopy is the only way to see the stomach lining directly, and that direct view, read alongside the horse's clinical signs, guides which treatment actually fits [6].

In a nutshell

If your horse is losing weight, or simply not gaining it despite good feed, there is always a reason. Parasites, dental problems, internal disease and stomach ulcers are the most common causes.

The best approach is a systematic one: ruling out potential causes step by step rather than guessing. With the right diagnosis and the right treatment, most horses regain their condition within a few months. Good nutrition does the rest of the work once the underlying cause is under control.