Foxglove (Digitalis Purpurea)
Foxglove (Digitalis purpurea) is a plant native to Europe with characteristic bell-shaped flowers, widely cultivated in gardens. All parts of the foxglove plant contain cardiotonic glycosides, also known as digitaloids or cardenolides, which are highly toxic to dogs and cats. There are about 100 different cardiotonic steroid glycosides in the plant, with digitoxin and digoxin being the most important active ingredients. These substances were formerly used as medications in human and veterinary medicine but are less common today due to the development of more effective and safer alternatives.
The toxicity of foxglove is classified as very high (+++). Even small amounts can lead to life-threatening poisoning in pets. For a medium-sized dog, just five dried leaves are considered potentially fatal. In addition to foxglove, other plants such as lily of the valley (Convallaria majalis) and oleander (Nerium oleander) also contain similar cardiotonic glycosides with comparable toxicity.
It is particularly dangerous that not only fresh or dried plant parts are poisonous, but also the water in vases where foxglove has stood can contain toxic concentrations of the active ingredients.
The most important facts at a glance
Poisoning by foxglove (Digitalis purpurea) is a medical emergency in dogs and cats. The cardiac glycosides contained in all parts of the plant influence heart function by inhibiting sodium-potassium ATPase, leading to increased heart contraction and slowed conduction. Overdosing results in life-threatening cardiac arrhythmias.
Symptoms initially include gastrointestinal complaints such as vomiting and diarrhea, followed by cardiac symptoms (arrhythmias, bradycardia) and neurological abnormalities. Cats are particularly at risk due to their limited glucuronidation capacity, as are dogs with the MDR-1 gene defect.
Diagnosis is based on anamnesis, clinical symptoms, and cardiological examinations. Therapy focuses on decontamination using emesis and activated charcoal, stabilization of the cardiovascular system, and symptomatic treatment. Unfortunately, a specific antidote (DigiFab) is not routinely available in veterinary medicine in Germany.
With early treatment, the prognosis is cautiously favorable, with follow-up care including regular cardiological check-ups. Preventive measures such as removing toxic plants from the pets’ environment are helpful to avoid poisoning cases.
Causes, development and progression
Foxglove poisoning in pets usually occurs through oral ingestion of plant parts. Despite the plant’s bitter taste, which deters many animals, poisoning cases still occur, especially in young, curious animals or those with Pica syndrome (pathological eating of non-nutritive substances).
The mechanism of action of cardiac glycosides is based on the inhibition of sodium-potassium ATPase in cardiac muscle cells. This enzyme is responsible for transporting sodium out of the cell and potassium into the cell. Inhibition leads to an increase in intracellular sodium concentration, which in turn leads to increased calcium influx. The increased calcium concentration causes an increased contractile force of the heart muscle (positive inotropic effect) and slowed conduction in the heart’s conduction system (negative dromotropic effect).
While these effects may be desirable in therapeutic applications for heart failure, in cases of poisoning, they lead to dangerous cardiac arrhythmias. Additionally, the glycosides affect the central nervous system and the gastrointestinal tract, which explains the neurological and gastrointestinal symptoms.
Cats are particularly at risk because their limited ability to glucuronidate (an important metabolic pathway for detoxification) makes them less able to break down toxins. Dogs with the MDR-1 gene defect, which is more common in Collies and related breeds, also show increased sensitivity to cardiac glycosides, as the defect impairs toxin excretion.
Mechanism of action
Cardiac glycosides inhibit specific enzymes (sodium-potassium ATPases) in heart muscle cells, leading to shifts in electrolyte concentrations (sodium, potassium, calcium) within the heart muscle cell.
The associated increase in calcium leads to an increase in the contractile force and speed of the heart muscle cell.
Furthermore, cardiac glycosides slow down nerve conduction in the heart’s nerve pathways, thereby reducing heart rate.
As the concentration of glycosides in the blood increases, the effects intensify.
Additionally, disturbances in the gastrointestinal tract and CNS occur.
Signs of intoxication, initially affecting only the gastrointestinal tract, are observed in dogs and cats at plasma levels of approximately 2.5 ng/ml.
For dogs, five dried foxglove leaves are considered lethal.
Cats are particularly sensitive to increasing plasma levels. The reason is their severely limited ability to break down certain substances via the glucuronidation metabolic pathway.
Dogs with the MDR-1 gene defect (Collies, etc.) are also particularly sensitive to increasing plasma concentrations.
Supplements
For dogs and cats, even the ingestion of small amounts of fresh plant material poses a potentially life-threatening danger.
- Main Toxic Substances
The most important cardiac glycosides in foxglove are:
- Digoxin
- Digitoxin
- Gitoxin
- Digitalin
These substances are lipophilic, well absorbed through the gastrointestinal tract, and act directly on cardiac muscle tissue as well as indirectly via the autonomic nervous system.
- Mechanism of Action at the Cellular Level
Cardiac glycosides competitively inhibit the sodium-potassium ATPase (Na⁺/K⁺ pump) in the cell membrane of cardiomyocytes (heart muscle cells).
A) Primary Effect: Inhibition of Na⁺/K⁺-ATPase
- This pump is essential for maintaining the electrolytic homeostasis of the heart muscle cell.
- Inhibition leads to an increase in intracellular sodium concentration.
- This leads to an inhibition of the Na⁺/Ca²⁺ exchanger, causing calcium to accumulate in the cell.
- The increased intracellular calcium leads to increased contractility of the heart muscle (positive inotropic effect).
B) Secondary Effects
- However, the increased calcium concentration also leads to electrical instability → cardiac arrhythmias.
- Additionally, vagal tone is increased (parasympathetic activity), which can lead to a slowing of the heart rate (bradycardia).
- Toxic Effects of Overdose
The therapeutic window of foxglove glycosides is very narrow; an overdose quickly leads to intoxication. In dogs and cats, the range between effective and toxic doses is particularly narrow, which is why any ingestion of the plant is considered potentially dangerous.
Main Effects:
- Cardiotoxic: Arrhythmias (e.g., ventricular extrasystoles, atrial block, AV block), bradycardia or tachycardia
- Central nervous: Lethargy, disorientation, in severe cases seizures
- Gastrointestinal: Vomiting, diarrhea, hypersalivation – often the first clinical signs
- Specific Toxicity in Dogs and Cats
- Dog: More prone to ingesting larger quantities due to curious behavior (e.g., playing with plant parts). First signs are usually gastrointestinal (vomiting, nausea), followed by cardiac arrhythmias and neurological abnormalities.
- Cat: Less frequently affected due to their more selective eating habits, but highly sensitive to cardiac glycosides. Even small amounts can cause bradycardia, hypothermia, and massive weakness.
- Summary of Toxic Mechanisms of Action
| Level of Action | Effect of Foxglove Glycosides |
| Cellular (Heart) | Inhibition of Na⁺/K⁺-ATPase → Calcium accumulation → increased contraction |
| Electrophysiology | Conduction disturbances, arrhythmias |
| Autonomic Nervous System | Increased vagal tone → Bradycardia, AV block |
| Gastrointestinal Tract | Direct irritation → Vomiting, Diarrhea |
| Central Nervous System | Disorientation, weakness, possibly seizures |
- Conclusion
Foxglove is one of the most dangerous poisonous plants for dogs and cats. The cardiac glycosides it contains disrupt ion homeostasis and directly affect heart function by inhibiting Na⁺/K⁺-ATPase. Even tiny amounts of the plant can trigger severe, sometimes fatal, cardiac arrhythmias. Typical symptoms include vomiting, lethargy, slowed pulse, and eventually syncopal events or sudden death.
Symptoms of intoxication
The symptoms of foxglove poisoning typically develop within 2–6 hours after ingestion of the plant and can be divided into several phases.
In the initial phase, gastrointestinal symptoms dominate:
- Increased salivation (hypersalivation)
- Repeated vomiting, often severe
- Diarrhea, which can be bloody in severe cases
- Inappetence and refusal to eat
- Abdominal pain, recognizable by a tense abdominal wall
As poisoning progresses, cardiovascular symptoms come to the forefront:
- Irregular heartbeat (arrhythmias)
- Bradycardia (slowed heartbeat) or tachycardia (accelerated heartbeat)
- Heart block of various degrees
- Weak pulse
- Pale or bluish mucous membranes
- Collapse and weakness
Neurological symptoms may also occur:
- Gait instability (ataxia)
- Tremors or muscle twitching
- Disorientation
- Seizures
- In severe cases, impaired consciousness leading to coma
In advanced poisoning, life-threatening complications can occur:
- Ventricular fibrillation
- Cardiac Arrest
- pulmonary edema
- Multi-organ failure
- Death
The severity of symptoms correlates with the amount of plant material ingested and the time until treatment. In cats, symptoms may appear more quickly and severely than in dogs due to their particular sensitivity.
Diagnosis
The diagnosis of foxglove poisoning is based on several pillars and requires a careful anamnesis as well as clinical and laboratory diagnostic examinations.
The anamnesis is crucial and should include the following aspects:
- Possible access to foxglove or other cardiac glycoside-containing plants
- Observed ingestion of plant parts
- Time of suspected exposure
- Onset and course of symptoms
The clinical examination focuses on:
- Cardiovascular status (heart rate, heart rhythm, pulse quality, mucous membrane color)
- Neurological status
- Hydration status after vomiting and diarrhea
Laboratory diagnostic measures include:
- Blood count and serum chemistry to assess organ function
- Electrolytes (especially potassium, as hyperkalemia can occur in severe poisoning)
- Acid-base status
- ECG for detecting cardiac arrhythmias (typical changes include ST segment depression, prolonged PR intervals, AV blocks)
- In specialized laboratories: Detection of digitoxin or digoxin in blood or urine
Differential diagnosis includes other poisonings with cardiotoxic effects, primary heart diseases, and metabolic disorders. Particularly important is the differentiation from poisonings with other cardiac glycoside-containing plants such as oleander or lily of the valley, which can cause similar symptoms.
A definitive diagnosis can be made by direct detection of cardiac glycosides in the blood, but these tests are not universally available, and results are often not available quickly enough to influence acute treatment decisions.
Therapeutic principles
The antidote DigiFab available in human medicine
is not available for veterinary medicine in Germany.
Atropine counteracts some of the toxic effects of cardiac glycosides on the heart.
Lidocaine and phenytoin can also be used to treat severe cardiac arrhythmias.
Decontamination is achieved by emptying the stomach and administering activated charcoal. Since cardiac glycosides are excreted in bile and reabsorbed in the intestine (enterohepatic circulation), activated charcoal should be administered repeatedly.
Symptomatic therapy particularly addresses the correction of fluid, electrolyte, and acid-base balance disturbances, as these can enhance the toxic effect of glycosides on the heart.
Treating foxglove poisoning requires a rapid and comprehensive approach. As it is a medical emergency, therapy should be initiated immediately.
Decontamination is the first priority:
- In cases of recent ingestion (within 1–2 hours), vomiting can be induced under veterinary supervision, provided the animal is conscious and shows no neurological symptoms.
- Gastric lavage under anesthesia for larger ingested amounts or if vomiting is contraindicated.
- Administration of activated charcoal (initial 1–4 g/kg body weight) to reduce toxin absorption. Since cardiac glycosides undergo enterohepatic circulation, activated charcoal should be repeated every 4–6 hours for 24–48 hours.
Symptomatic and supportive therapy includes:
- Intravenous fluid therapy to stabilize circulation and promote renal excretion of toxins.
- Correction of electrolyte and acid-base disturbances, especially hyperkalemia.
- For cardiac arrhythmias: Antiarrhythmics such as lidocaine or phenytoin.
- Atropine can be used for severe bradycardia, as it counteracts some of the toxic effects of cardiac glycosides on the heart.
- Oxygen therapy for dyspnea or cyanosis.
- Anticonvulsant medications for neurological symptoms.
In human medicine, DigiFab (digoxin-specific Fab fragments) is available as a specific antidote, but it is not routinely available in veterinary medicine in Germany. In severe cases, the use of this antidote can be life-saving and should be considered.
Intensive care monitoring is essential and should be continued for at least 48–72 hours, as toxins are slowly excreted and delayed complications can occur.
Prognosis & follow-up care
As always, the prognosis depends on the amount of toxin (plant parts) ingested and rapid and comprehensive decontamination. Since cardiac glycosides taste bitter, there is a high probability that only a small amount of plant material will be ingested.
The prognosis is generally good. Symptoms usually subside within 2–3 days.
The prognosis for foxglove poisoning depends on several factors: the amount of toxin ingested, the time until treatment, the animal’s general health, and the presence of risk factors such as the MDR-1 gene defect in dogs or impaired kidney function.
With early and adequate treatment, the prognosis is cautiously favorable. Most animals recover completely within 2–3 days if decontamination is performed promptly and no severe cardiac arrhythmias have occurred. The fact that cardiac glycosides taste bitter, and many animals therefore ingest only small amounts, further improves the prognosis.
However, in severe poisoning with pronounced cardiac arrhythmias or neurological symptoms, the prognosis is cautious to unfavorable. Ventricular fibrillation and higher-grade AV blocks, in particular, can lead to death despite intensive therapy.
Aftercare following acute poisoning includes:
- Regular cardiological check-ups using ECG and echocardiography, as cardiac glycosides can also cause delayed or chronic damage to the heart muscle.
- Monitoring kidney function, as toxin excretion occurs via the kidneys and can strain them.
- Dietary adjustments during convalescence, possibly easily digestible food for persistent gastrointestinal symptoms.
- Strict rest in the first few days after poisoning to spare the cardiovascular system.
In the long term, pet owners should take preventive measures to prevent re-poisoning. This mainly includes removing foxglove and other poisonous plants from the garden or securely fencing off such areas.
Research outlook
Research into foxglove poisoning in pets is continuously evolving. Current studies focus on several areas:
In toxicology, new detection methods for cardiac glycosides are being developed to enable faster and more precise diagnoses. Point-of-care tests could simplify and accelerate the detection of digitoxin and digoxin in veterinary practice in the future.
In the field of therapy, research is being conducted on improved antidote strategies. The development of veterinary-specific Fab fragments against cardiac glycosides could revolutionize treatment options. In addition, new formulations of activated charcoal are being investigated, which should enable more efficient binding of toxins in the gastrointestinal tract.
Genetic studies deal with the differing sensitivity of various animal breeds to cardiac glycosides. In addition to the known MDR-1 gene defect, further genetic factors that could influence the metabolism and toxicity of cardiac glycosides are being investigated.
In preventive veterinary medicine, digital tools such as apps for identifying toxic plants are being developed to help pet owners recognize potential dangers in the garden or during walks. In addition, educational measures for pet owners are being evaluated to raise awareness of plant poisonings.
Research into the long-term consequences of survived foxglove poisoning is another important field. This concerns the question of whether and to what extent subclinical heart damage can occur after a survived poisoning and how this can be detected and treated early.
Frequently asked questions (FAQs)
- How do I recognize foxglove poisoning in my pet?
Look for sudden vomiting, drooling, diarrhea, weakness, irregular heartbeat, and unsteady gait. If you suspect poisoning, seek veterinary attention immediately. - Can I treat poisoning at home?
No, foxglove poisoning is a medical emergency that requires immediate veterinary treatment. Attempts at self-treatment can be dangerous. - Is foxglove poisoning always fatal?
No, with early treatment, many animals have a good chance of survival. However, if left untreated, the poisoning can lead to death from cardiac arrest. - Is foxglove still poisonous when dried?
Yes, the toxins remain active in dried plant parts and can even be more concentrated than in fresh plants. - Which animals are particularly at risk from foxglove poisoning?
Cats and dogs with the MDR-1 gene defect (like Collies) are particularly sensitive. Young, curious animals have a higher risk of eating plant parts. - How long does recovery take after foxglove poisoning?
With successful treatment, symptoms usually subside within 2–3 days. However, full recovery can take 1–2 weeks. - Can I make my garden pet-friendly?
Yes, avoid poisonous plants like foxglove or plant them in fenced areas that your pets cannot access. - Are there breed-specific differences in sensitivity to foxglove?
Yes, dogs with the MDR-1 gene defect (especially Collies, Australian Shepherds, and related breeds) are more sensitive to cardiac glycosides. - Can recovered foxglove poisoning cause long-term damage?
Yes, in some cases, permanent heart damage may remain, which is why regular cardiological check-ups after poisoning are important.
What other plants contain similar toxins to foxglove?
Lily of the valley, oleander, hellebore, and sea onion also contain cardiotonic glycosides and can cause similar poisoning symptoms.
Literature
- Kovalkovičová N, Šutiaková I, Pistl J, et al. Some food toxic for pets. Interdisciplinary Toxicology. 2019;12(1):45-51.
- Plumlee KH. Clinical Veterinary Toxicology. 2nd ed. St. Louis: Mosby; 2020.
- Gupta RC. Veterinary Toxicology: Basic and Clinical Principles. 3rd ed. Academic Press; 2018.
- Poppenga RH, Gwaltney-Brant SM. Small Animal Toxicology Essentials. 2nd ed. Wiley-Blackwell; 2021.
- Löwe G, Löwe O. Poisoning in Dogs and Cats – A Veterinary Guide. 2nd edition. Kreuztal: Kynos-Verlag. 2021; 208 p.