Plant Toxins That Cause Liver Disease in Horses

By Dr Duncan Houston

How toxic weeds, mouldy feed, and contaminated water can damage the liver, and what horse owners should watch for.

Liver disease in horses can be easy to miss early. A horse may simply seem dull, picky with feed, lose condition, act strangely, or develop mild colic-like signs before anything looks dramatic.

The danger is that some plant and feed toxins damage the liver quietly over time. By the time jaundice, neurological signs, or severe photosensitivity appear, the disease may already be advanced.

The goal is not to make every owner panic about every weed in the paddock. The goal is to know which exposures matter, what early signs to take seriously, and when to call a vet before liver damage becomes irreversible.

Quick Answer

Plant and feed toxins can cause liver disease in horses, especially when horses eat pyrrolizidine alkaloid plants such as ragwort, groundsel, Paterson’s curse, heliotrope, or Crotalaria species, contaminated hay, mouldy grain, cocklebur seedlings or seeds, or water affected by toxic blue-green algae. Some toxins cause slow cumulative liver damage, while others can cause sudden severe illness. A horse with poor appetite, weight loss, jaundice, odd behaviour, photosensitivity, diarrhoea, colic signs, weakness, or neurological signs needs veterinary assessment, especially if toxic plant, feed, or water exposure is possible. (MSD Veterinary Manual)

Why Liver Disease In Horses Can Be Hard To Spot

The liver has a large reserve capacity. That means early damage may not look obvious from the outside.

A horse can have abnormal liver values before showing clear signs of liver failure. When liver dysfunction becomes clinically obvious, signs may include weight loss, jaundice, abnormal behaviour, photosensitivity, diarrhoea, constipation, dullness, or neurological changes linked with hepatic encephalopathy. (Merck Veterinary Manual)

The practical problem is this:

By the time a horse looks obviously liver-sick, the damage may already be significant.

That is why exposure history matters. If a horse has been grazing risky weeds, eating questionable hay, drinking from stagnant water, or consuming mouldy or suspect feed, do not wait for dramatic signs before involving your vet.

What Does The Liver Do?

The liver is one of the body’s major processing organs. It helps handle toxins, metabolism, bile production, protein production, energy balance, and waste processing.

When the liver is damaged, toxins that would normally be processed can build up. This can affect the brain and behaviour, causing hepatic encephalopathy. Signs can include depression, lethargy, head pressing, circling, aimless walking, poor coordination, difficulty swallowing, persistent yawning, sleepiness, aggression, seizures, stupor, or coma. (MSD Veterinary Manual)

Liver disease can also cause photosensitivity. This happens when a light-reactive compound called phylloerythrin builds up and reacts with ultraviolet light, damaging skin, especially white or lightly pigmented areas. (MSD Veterinary Manual)

Early Signs Of Liver Disease In Horses

Liver disease does not always start with yellow gums.

Early signs may include:

1. Reduced appetite

2. Weight loss

3. Dullness or lethargy

4. Poor performance

5. Behaviour changes

6. Recurrent mild colic signs

7. Loose manure or constipation

8. Persistent yawning

9. Head pressing or aimless walking

10. Photosensitivity, especially sunburn-like damage on white markings

11. Jaundice, seen as yellow gums or yellowing around the eyes

12. Neurological signs such as circling, blindness, staggering, or seizures

A useful checkpoint:

A horse that is not eating normally, losing condition, acting strangely, or showing photosensitivity should not be treated as simply “off colour.” Liver disease needs to be considered.

The Main Plant And Feed Toxins That Damage The Liver

There are many possible hepatotoxins, but these are some of the most important for horse owners.

Pyrrolizidine Alkaloid Plants

Pyrrolizidine alkaloids are one of the most important plant toxin groups linked with chronic liver damage in horses.

These toxins are found in several plant groups, including:

1. Senecio species, including ragwort and groundsel

2. Crotalaria species, including rattlebox or rattlepod

3. Heliotropium species, including heliotrope

4. Amsinckia species, including fiddleneck

5. Echium species, including Paterson’s curse

6. Cynoglossum species, including hound’s tongue

MSD Veterinary Manual describes pyrrolizidine alkaloidosis as a chronic toxicosis that can result in hepatic failure, with common plant genera including Senecio, Crotalaria, Heliotropium, Amsinckia, Echium, Cynoglossum, and Trichodesma. (MSD Veterinary Manual)

The dangerous part is that these toxins are cumulative. A horse may not become sick after one mouthful, but repeated exposure over weeks, months, or seasons can gradually damage the liver.

Why Hay Can Be More Dangerous Than Fresh Weeds

Many toxic plants are not very palatable when fresh. Horses may avoid them when good pasture is available.

The risk increases when:

1. Pasture is sparse

2. Drought reduces normal forage

3. Horses are hungry

4. Weeds are baled into hay

5. The plant dries and becomes harder for the horse to identify

6. Owners cannot easily see what is mixed through the forage

Pyrrolizidine alkaloid plants can remain toxic after drying, which means contaminated hay can still be dangerous. MSD Veterinary Manual notes that toxins from these plants survive the drying process and remain active in baled hay. (MSD Veterinary Manual)

In practice, contaminated hay is one of the biggest traps because the owner may never see the horse actively eating a toxic plant in the paddock.

Paterson’s Curse And Australian Horse Risk

For Australian horse owners, Paterson’s curse is an important example.

NSW WeedWise states that Paterson’s curse contains pyrrolizidine alkaloids that can cause liver damage when livestock graze the weed over extended periods. It also notes that horses and pigs are highly susceptible, and affected horses may show gradual loss of condition, listlessness, poor appetite, head pressing, blindness, and aimless walking. (WeedWise)

This is a good example of why regional plant knowledge matters. A toxic weed list from one country may not match your paddock. Horse owners should learn the common toxic plants in their local area and get help from local veterinarians, agronomists, land services, or weed officers when unsure.

Cocklebur

Cocklebur can cause severe poisoning, particularly from seedlings and seeds.

The high-risk stage is the young seedling, especially the two-leaf stage. Cocklebur seeds can also contaminate hay or grain. The University of Kentucky notes that cocklebur seedlings contain large quantities of carboxyatractyloside, and horses have been poisoned by seed contamination in hay or grain. (Equine Programs)

This matters because cocklebur risk can rise when forage is limited and horses are more likely to eat plants they would normally avoid.

A practical checkpoint:

If seedlings appear after rain, drought break, or pasture disturbance, inspect paddocks before horses are allowed to graze heavily.

Aflatoxin And Mouldy Feed

Aflatoxins are toxins produced by Aspergillus moulds. They can contaminate feed ingredients such as corn, peanuts, nuts, cottonseed, rice, and other cereals when conditions allow mould growth. Aflatoxins can cause liver damage, reduced protein production, poor growth, depression, haemorrhage, jaundice, and death. (MSD Veterinary Manual)

A major mistake is assuming feed is safe because mould is not obvious. The University of Kentucky notes that aflatoxin can be present in toxic concentrations with or without visible mould. (Equine Programs)

Feed risk increases with:

1. Poor storage

2. Damp feed rooms

3. Heat and humidity

4. Rodent contamination

5. Old or damaged grain

6. Insect-damaged crops

7. Feed bags left open

8. Unknown or unreliable feed sources

If feed smells musty, looks clumped, is discoloured, has visible mould, or horses suddenly refuse it, do not keep feeding it.

Fumonisins And Fusarium Moulds

Fumonisins are mycotoxins produced mainly by Fusarium moulds and are especially associated with mouldy corn. In horses, fumonisin exposure is classically associated with equine leukoencephalomalacia, a severe neurological disease, but hepatic involvement can also occur. MSD notes that signs in equids can include apathy, drowsiness, blindness, circling, staggering, recumbency, and icterus when the liver is involved. (MSD Veterinary Manual)

This is one reason mouldy feed should never be treated casually in horses. It is not just a digestive upset risk. It can be a brain and liver risk.

Blue-Green Algae

Blue-green algae are actually cyanobacteria. Under the right conditions, they can form harmful algal blooms in ponds, dams, lakes, streams, and other water sources.

Some cyanobacteria produce hepatotoxins that can cause acute liver injury. MSD Veterinary Manual notes that harmful freshwater algal blooms may look blue, green, brown, like pea soup, like spilled paint, or like streaks in the water, but may also be invisible. It also states that some cyanotoxins can remain in water for weeks after cyanobacteria are killed, and prevention is critical because prognosis is guarded at best. (MSD Veterinary Manual)

Treat suspect water seriously.

If there is a bloom, scum, strange colour, dead wildlife near water, or multiple animals affected, remove horses from that water source immediately and provide clean water.

How Worried Should You Be?

Risk Level	What It Looks Like	What It May Mean	What To Do
Low risk	Horse appears normal, but toxic weeds are found in paddock or hay source is questionable	Exposure risk without obvious illness	Remove the risk, inspect hay and feed, monitor closely, consider veterinary advice
Moderate risk	Reduced appetite, dullness, weight loss, poor performance, mild manure changes, mild photosensitivity	Possible early liver disease or another systemic illness	Arrange a vet check and blood testing
High risk	Jaundice, worsening photosensitivity, head pressing, circling, aimless walking, persistent yawning, colic signs, diarrhoea, marked weight loss	Possible significant liver dysfunction or hepatic encephalopathy	Veterinary assessment is needed urgently
Critical	Seizures, collapse, severe neurological signs, severe weakness, not eating, severe diarrhoea, suspected blue-green algae exposure, multiple animals sick, severe jaundice	Possible acute liver failure, severe toxicosis, or life-threatening systemic disease	Emergency veterinary care immediately

The most important rule:

Do not wait for jaundice before taking suspected liver toxin exposure seriously.

What Else Can Look Like Liver Disease?

Not every dull horse with yellow gums has toxic liver disease.

Important differentials include:

1. Fasting hyperbilirubinaemia

2. Infectious or inflammatory liver disease

3. Cholangiohepatitis

4. Theiler disease

5. Hepatic lipidosis

6. Cholelithiasis

7. Chronic progressive hepatitis

8. Endotoxaemia from another illness

9. Severe gastrointestinal disease

10. Haemolysis

11. Neurological disease unrelated to the liver

12. Severe systemic infection

13. Parasite-related disease

14. Toxin exposure not primarily affecting the liver

Merck Veterinary Manual lists multiple causes of hepatic disease in horses, including pyrrolizidine alkaloid toxicosis, hepatic lipidosis, cholangiohepatitis, chronic progressive hepatitis, Theiler disease, Tyzzer disease in younger foals, cholelithiasis, aflatoxicosis, fumonisin toxicosis, Panicum toxicosis, alsike clover toxicosis, and other less common causes. (Merck Veterinary Manual)

This is why bloodwork, exposure history, and veterinary examination matter. You cannot reliably diagnose the cause from appearance alone.

Why Yellow Gums Can Be Misleading

Jaundice is important, but it is not always straightforward in horses.

Horses can develop elevated bilirubin from reduced feed intake or fasting, and that does not always mean primary liver failure. Merck notes that fasting hyperbilirubinaemia is a more common cause of icterus in horses and can be differentiated from true liver disease by documenting normal liver enzyme activity. (Merck Veterinary Manual)

So the question is not just: “Are the gums yellow?”

The better questions are:

1. Is the horse eating?

2. Are liver enzymes abnormal?

3. Are bile acids elevated?

4. Are there neurological signs?

5. Is there photosensitivity?

6. Was there exposure to toxic plants, mouldy feed, or contaminated water?

That is proper veterinary reasoning.

How Do Vets Diagnose Liver Disease In Horses?

A vet may recommend:

1. Physical examination

2. Full history, including pasture, hay, grain, water, supplements, and recent feed changes

3. Blood tests for liver enzymes

4. Bilirubin testing

5. Bile acids to assess liver function

6. Complete blood count and biochemistry

7. Coagulation testing if biopsy or severe liver disease is suspected

8. Ultrasound of the liver and bile system

9. Feed, hay, water, or plant testing where relevant

10. Liver biopsy in selected cases

MSD Veterinary Manual notes that routine biochemical tests can detect liver disease before liver failure occurs, but additional tests may be needed to assess function. It also states that liver biopsy is the definitive way to diagnose tissue changes, fibrosis, and regenerative capacity, while ultrasound can help assess liver appearance and related structures. (MSD Veterinary Manual)

When Is This An Emergency?

Treat this as urgent if your horse shows:

1. Jaundice with dullness or poor appetite

2. Head pressing

3. Circling or aimless walking

4. Blindness or severe disorientation

5. Seizures

6. Collapse or severe weakness

7. Severe photosensitivity with swelling, peeling, or painful skin

8. Persistent colic signs

9. Severe diarrhoea

10. Refusal to eat

11. Suspected blue-green algae exposure

12. Known ingestion of toxic plants or mouldy feed

13. Multiple horses affected on the same property

14. Dark red, pale, purple, or tacky gums

15. Rapid deterioration over hours

A horse with neurological signs from suspected liver disease can be unpredictable and dangerous to handle. MSD notes that horses with hepatic encephalopathy may show aggressive or unpredictable behaviour and may require sedation. (MSD Veterinary Manual)

If you suspect acute toxin exposure, do not move slowly through a checklist. Call your vet.

What Should You Do Right Now?

Step 1: Remove the suspected source

Take the horse off the suspect pasture, hay, grain, or water source.

If multiple horses have access, remove all horses until the risk is assessed.

Step 2: Call your vet

Tell your vet:

1. What signs you are seeing

2. When they started

3. What the horse has been eating

4. Whether new hay or feed was introduced

5. Whether weeds are present

6. Whether any water source looks abnormal

7. Whether other horses are affected

8. Whether there are neurological signs or jaundice

Step 3: Save samples

Keep samples of:

1. Hay

2. Grain

3. Suspect plants

4. Water

5. Photos of the paddock

6. Photos of the water source

7. Feed bag labels and batch information

Do not throw everything away before your vet has a chance to advise. Testing may matter.

Step 4: Move photosensitive horses out of sunlight

If the horse has sunburn-like lesions, swelling, crusting, or peeling on white skin, reduce UV exposure while your vet investigates. Shade, stabling, fly protection, and wound care may be needed.

Step 5: Do not medicate randomly

Do not give human medications, leftover drugs, herbal detox products, or extra supplements without veterinary advice.

A damaged liver is not the time for guesswork.

What Not To Do

Common mistakes include:

1. Waiting until the horse is yellow before calling a vet

2. Assuming mouldy feed is safe because only part of the bag looks affected

3. Feeding hay that contains unidentified weeds

4. Leaving horses on sparse pasture where they are forced to eat plants they normally avoid

5. Ignoring behaviour changes such as yawning, dullness, aimless walking, or head pressing

6. Assuming photosensitivity is just sunburn

7. Treating recurrent colic signs without investigating liver values

8. Throwing away all evidence before feed, water, or plant testing can be considered

9. Adding “liver support” supplements instead of removing the toxin source

10. Forgetting to check water sources during hot weather

The liver can sometimes recover if the insult is removed early enough. But chronic fibrosis, severe toxin exposure, and advanced liver failure may not be reversible.

Myth Versus Reality

Myth	Reality
“My horse will avoid poisonous plants.”	Horses may eat toxic plants when pasture is sparse, when plants are dried in hay, or when normal forage is limited.
“If hay looks mostly clean, it is safe.”	Toxic weeds can be hidden in hay, and some toxins remain active after drying.
“No visible mould means no mycotoxin risk.”	Some feed toxins can be present even when mould is not obvious.
“Yellow gums always mean liver failure.”	Horses can develop yellowing from reduced feed intake, so blood tests are needed to interpret it properly.
“Photosensitivity is just sunburn.”	Photosensitivity can be a sign of liver dysfunction and should be investigated.
“Liver detox supplements will fix it.”	Removing the toxin source and getting a diagnosis matters more than adding supplements.

How To Prevent Plant And Feed Toxin Liver Disease

Inspect pastures regularly

Walk paddocks every few weeks, especially:

1. After rain

2. During drought

3. In spring when seedlings emerge

4. Before flowering and seed set

5. When moving horses to a new paddock

6. Before cutting hay

Learn the toxic weeds common in your region. Take photos and ask your vet, agronomist, local weed authority, or extension service if you are unsure.

Maintain good pasture cover

Hungry horses eat risky things.

Good pasture management helps reduce toxin exposure by reducing weed dominance and preventing horses from being forced onto unpalatable plants.

Do not bale toxic weeds into hay

This is one of the biggest prevention points.

Many owners look at the paddock. Fewer inspect the hay properly. Toxic plants hidden in hay can be more dangerous because the horse may eat them without recognising or avoiding them.

Buy hay and feed from reputable suppliers

Ask where hay was grown, what weeds are common in the area, and whether the hay was inspected before baling.

For grain and concentrates, buy from reliable suppliers and avoid old, damp, clumped, musty, discoloured, or pest-contaminated feed.

Store feed properly

Feed should be stored:

1. Cool

2. Dry

3. Off the ground

4. Protected from rodents

5. Protected from insects

6. Sealed after opening

7. Rotated so old feed is not left sitting

Warm, damp storage conditions and pests can increase mould and mycotoxin risk. (Equine Programs)

Check water sources

In warm weather, inspect ponds, dams, and troughs.

Be suspicious of water that looks:

1. Green

2. Blue-green

3. Brown or paint-like

4. Scummy

5. Streaky

6. Thick like pea soup

7. Associated with dead fish, birds, or wildlife

If a bloom is suspected, remove horses and provide clean water immediately.

Do not rely on taste avoidance

Some toxic plants are unpalatable when fresh, but that does not protect horses reliably.

Risk increases when plants are dried, when horses are hungry, when pasture is poor, or when weeds contaminate hay or grain.

Consider blood screening after known exposure

If a horse has had ongoing exposure to risky plants or feed, ask your vet whether blood testing is appropriate even if the horse looks normal.

Early liver enzyme changes may be detected before obvious liver failure.

How Vets Think About These Cases

The most important question is not, “Which toxin is it?” at the very start.

The first question is:

Is this horse clinically stable, and is there evidence of liver dysfunction?

Then we work backwards:

1. What has the horse been eating?

2. Has the hay changed?

3. Are weeds present in the pasture?

4. Is pasture sparse?

5. Is there mouldy grain or old feed?

6. Is the water source clean?

7. Are other horses affected?

8. Are signs acute or chronic?

9. Are there neurological signs?

10. Are blood tests consistent with liver disease?

The mistake I see owners make is trying to identify every plant before calling the vet. Plant identification helps, but a sick horse needs assessment first.

Can The Liver Recover?

Sometimes, yes.

The liver has regenerative ability, and some horses improve if the toxic source is removed early and supportive care is started.

But recovery depends on:

1. The toxin involved

2. How much was ingested

3. How long exposure continued

4. Whether fibrosis has developed

5. Whether neurological signs are present

6. Whether the horse is still eating

7. Whether the damage is acute or chronic

8. How quickly the source is removed

Chronic pyrrolizidine alkaloid damage can be especially serious because fibrosis may be advanced by the time signs appear.

The practical takeaway:

Early detection gives the horse a better chance. Waiting for severe signs removes options.

Helpful Related Reading

This article fits naturally with:

1. Early signs of colic in horses

2. When a horse needs colic surgery

3. Right dorsal colitis from NSAID use in horses

4. How to prevent enteroliths in horses

5. What to feed a horse with poor appetite

6. How to monitor manure, weight, and behaviour changes

7. Photosensitivity in horses

8. Safe pasture management for horses

These topics connect because many horse emergencies start as vague changes before they become obvious crises.

FAQs

What plants cause liver disease in horses?

Important liver-toxic plants include pyrrolizidine alkaloid plants such as ragwort, groundsel, Paterson’s curse, heliotrope, Crotalaria species, fiddleneck, and hound’s tongue. Other risks include cocklebur seedlings and seeds, some grasses such as Panicum species, alsike clover, mouldy feed, and contaminated water sources. (MSD Veterinary Manual)

Can horses recover from plant toxin liver disease?

Some horses can recover if exposure is stopped early and liver damage is not advanced. Chronic liver fibrosis or severe liver failure carries a much more guarded prognosis.

What are the first signs of liver disease in horses?

Early signs may include reduced appetite, weight loss, dullness, poor performance, mild colic signs, behaviour changes, photosensitivity, or manure changes. Jaundice and neurological signs usually make the situation more concerning.

Is mouldy hay dangerous for horses?

Yes. Mouldy hay or feed can contain mycotoxins that may affect the liver, gut, nervous system, or overall health. Do not feed hay or grain that is mouldy, musty, clumped, discoloured, or contaminated with weeds.

When should I call a vet?

Call a vet if your horse is not eating, losing weight, jaundiced, photosensitive, dull, behaving strangely, showing colic signs, has diarrhoea, or may have eaten toxic plants, mouldy feed, or contaminated water. Call urgently if there are neurological signs, collapse, severe weakness, suspected blue-green algae exposure, or multiple horses affected.

Final Thoughts

Plant and feed toxin liver disease is one of those problems where prevention matters more than rescue.

Once advanced liver damage is obvious, treatment becomes harder and the prognosis can be poor. The best protection is good pasture management, clean hay, safe feed storage, reliable water, early recognition, and fast veterinary involvement when something does not look right.

The horse that worries me is not only the one with yellow gums. It is the horse that has been quietly losing weight, picking at feed, acting odd, getting sunburn-like skin lesions, or grazing poor pasture full of weeds.

That is where owners can make a real difference. See the pattern early, remove the risk, and get the liver checked before the damage becomes irreversible.

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If your horse has possible toxin exposure, abnormal behaviour, photosensitivity, jaundice, or unexplained weight loss, ASK A VET™ can help you understand what signs to monitor and when veterinary care is needed.