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Venomous snake bites (snake envenomation) in horses, while not common, are a medical emergency. Without prompt veterinary attention, venomous snake bites can result in severe injury and, in some cases, death.
In North America, venomous snakes belong to either the Elapid (coral snakes) or Crotalid (pit vipers) family. Rattlesnakes are the most common source of equine snake envenomation on this continent.
Snake envenomation can cause a wide range of symptoms depending on the snake species, amount of venom injected, and location of the bite. Symptoms include severe swelling, cardiovascular irregularities, tissue death, and neurological deficits.
Treatment options vary depending on the severity of symptoms and species of snake involved. In all cases, the affected horse should be kept calm as agitation increases heart rate, speeding up the spread of venom throughout the body and amplifying its effects.
Preventing snake bites is complex as it is impossible to eliminate the risk of encounter. Environmental management strategies in high-risk areas include maintaining clean pastures and barns, as well as avoiding early evening rides in late summer.
Venomous Snake Bites
There are over 600 different species of venomous snakes around the world, each belonging to one of the following families: [1]
- Atractaspididae
- Colubridae
- Elapidae
- Viperidae
Snakes belonging to the Atractaspididae family are found in the Middle East and Africa. Colubridae snakes are found around the world and are mostly non-venomous, but five species are known to cause human fatalities. [2]
Snakes belonging to the Elapidae family, or elapids, are found throughout the American continent, with most populations concentrated around the Pacific coast and tropical or subtropical regions. [3]
Species of Elapidae venomous snake commonly found in these regions include: [3]
- Arizona coral snake (Micururoides euryxanthus)
- Texas coral snake (Micrurus tener)
- South Florida or Eastern coral snake (Micrurus fulvius)
North America is also home to over 200 species of pit vipers (Crotalids), a subfamily of the Viperidae family. The most common species include: [3]
- Rattlesnakes
- Cottonmouths
- Massasaugas
- Copperheads
In North America, cases of equine snake envenomation are related to venomous bites by snakes belonging to the Crotalid or Elapid families. [3]
Snake Venom
The composition of venom is highly differentiated between species and can contain anywhere from 20 to over 100 chemical components. Venom composition is also affected by other conditions and may even change from bite to bite within the same animal. [4]
Depending on the species, snake venom contains either neurotoxic peptides (Elapids) or a complex mixture of proteins and enzymes (Crotalids) that can all cause nervous system damage, necrosis (tissue death), heart and lung damage, and abnormal blood clotting. [5]
Toxic Effects on Horses
When horses are bitten by venomous snakes, toxic compounds enter the bloodstream through the bite wound. Depending on the location of the bite and how much venom is delivered, toxicity can affect multiple organ systems, causing a variety of life-threatening symptoms.
The term LD50 is a measurement of how lethal a substance is, used by toxicologists to assess a substance’s relative toxicity and establish safety guidelines for use. Substances with lower LD50 values are more toxic than those with higher LD50 values.
The LD50 of snake venom varies greatly depending on the species of snake, as shown in the following table. [6]
| Snake | LD50 | Venom Yield (mg) |
|---|---|---|
| Eastern diamondback | 1.68 | 590 |
| Timber rattlesnake | 2.69 | 140 |
| Copperhead | 10.92 | 60 |
| Western diamondback | 2.18 | 500 |
Toxicology of Snake Envenomation
Snake envenomation in horses occurs when a snake injects its venom into the horse’s body through a bite wound. In North America, envenomation in horses is typically caused by either coral snake or pit viper snake bites.
Snake bites from venomous snakes do not always contain venom, known as a “dry bite“. Production of venom has a high metabolic cost for the snake, so these species have evolved to use venom only when needed. If the bite is defensive in nature, snakes may reserve venom for attacking prey. [7][8]
Pit Viper Venom
The primary function of Crotalidae venom is to immobilize prey and initiate predigestion of tissues even before the snake begins to ingest its meal. Crotalidae snake venom contains a complex mixture of multiple enzymes, proteins, and toxins. [1]
Over 50 different components have been identified in pit viper venom, and the composition varies from bite to bite. [1]
Enzymes
Enzymes are proteins that speed up biochemical reactions. The enzymes in snake venom are involved in predigestion of prey and facilitate spread of venom throughout prey tissues. The main enzymes in pit viper venom are proteases and hyaluronidase: [1]
- Proteases: Enzymes that digest tissue proteins and peptides, leading to severe tissue damage. [1]
- Hyaluronidase: An enzyme that breaks down hyaluronic acid. This facilitates the spread of venom by breaking down connective tissue around the bite site. [1]
Proteins
Proteins in snake venom contribute to blood system dysfunction and abnormal blood clotting (coagulopathies). Snake envenomation can result in a reduction in platelet count (thrombocytopenia). [9]
Platelets are red blood cell fragments and are the primary mechanism of blood clotting. Reduced platelet count limits the body’s ability to clot normally, which can cause even minor wounds to bleed out. In severe cases this can result in life-threatening internal bleeding in affected horses.
Coral Snake Venom
Coral snake venom can cause a variety of physiological effects. Among the most severe are interruption of communication between the nerves and muscles, tissue damage, breakdown of red blood cells (hemolytic anemia), and vasodilation leading to low blood pressure. [10]
In cases of coral snake envenomation, the onset of symptoms can be delayed up to 12-18 hours after the snake bite, and can last up to two weeks. [10]
Coral snake venom contains a number of different compounds, and the toxicology of the venom is poorly understood. Most coral snake venom is known to contain neurotoxins and enzymes. [10]
Neurotoxins
Neurotoxins are compounds that disrupt nerve signaling between the muscles and central nervous system. These effects can result in muscle paralysis, which may be irreversible. [10]
Enzymes
Enzymes in coral snake venom can cause local tissue damage and death (necrosis). Phospholipase A is a specific enzyme found in venom that may damage red blood cell membranes, contributing to coagulopathies and hemolytic anemia.
What's your top priority with your horse's health?
What's your top priority with your horse's health?
Symptoms of Venomous Snake Bites
Symptoms of snake envenomation vary depending on the species of snake, location of the bite, and which organ system is most affected.
Coral Snake Envenomation
Coral snake bites are rare in horses as these snakes are generally non-aggressive and small in size. When envenomation occurs, symptoms may include: [5][10]
- Facial paralysis
- Difficulty breathing
- Excessive salivation
- Puncture wounds
Pit Viper Envenomation
The majority of reported cases of pit viper envenomation in horses affect the muzzle as the result of horses grazing in snake infested areas. [10] Bites from venomous pit vipers can result in respiratory symptoms, blood coagulation problems, musculoskeletal issues, cardiovascular effects and neurological symptoms.
Respiratory Symptoms
Muzzle bites from pit vipers can result in extreme facial swelling, which may lead to: [10]
- Obstruction of the airway: swelling obstructs the upper airway and can lead to difficulty breathing.
- Aspiration pneumonia: in rarer cases, swelling can lead to difficulty drinking, causing water to enter the horse’s lungs. This can further develop into pneumonia, which is life threatening.
Blood Coagulation Symptoms
Platelets are red blood cell fragments primarily responsible for blood clotting. Pit viper snake venom contains toxins that interfere with platelet count, which can lead to: [5]
- Thrombocytopenia (low platelet count)
- Prolonged clotting times
- Spontaneous bleeding around the eyes and nose
- Anemia (low hemoglobin; can result in inadequate oxygenation of tissue)
Musculoskeletal System Symptoms
Snake venom contains neurotoxins that disrupt nerve function in horses, specifically the neuromuscular junction (NMJ). The NMJ plays a central role in muscle function, connecting motor neurons to skeletal muscle fiber.
The neurotoxins contained in pit viper venom disrupt normal functioning of the NMJ and can cause neuromuscular paralysis. [15] Symptoms of neuromuscular paralysis include: [1][5][16]
- Lameness
- Swelling
- Generalized weakness
- Muscle tremors
- Recumbency (lying down)
- Tongue paralysis
- Abnormal pupils (different sizes, not responding to light)
Cardiovascular Symptoms
Cardiovascular symptoms are very common in cases of snake envenomation with up to 70% of horses presenting with cardiac arrhythmia.
Cardiac arrhythmias (abnormal variations in heart rate) can cause a wide range of effects ranging from reduced performance to sudden death. [11][12][13]
The severity of cardiovascular symptoms depends on the subspecies of snake. In severe cases, horses may suffer from tissue death of the heart muscle (myocardial damage) which can lead to congestive heart failure. [10][11]
Skin (Integumentary) Symptoms
Regardless of the location of the bite wound, rattlesnake venom contains enzymes that cause tissue damage. Common skin symptoms include: [5]
- Skin discoloration
- Swelling
- Pain
In severe cases, horses may experience tissue necrosis (death) at the bite site. [10]
Nervous System Symptoms
In rare cases, neurotoxins contained in the venom combined with reduced oxygenation of the brain can result in neurological symptoms such as: [10]
- Ataxia (incoordination)
- Bladder dysfunction
- Localized paralysis (facial)
Risk Factors
Horses of all ages, sexes and breeds can receive venomous snake bites. Animals living in endemic areas are at a higher risk of envenomation, especially between spring and early autumn, when snakes are most active. In North America the majority of snake bites in horses are reported between the months of March and October. [10]
As with all forms of poisoning, toxicity is directly related to the size of the dose relative to body weight. Given this, miniature horses and foals are more likely to have severe symptoms compared to larger or more mature horses.
Diagnosis
If an encounter with a snake is suspected but not confirmed, diagnosis of envenomation is based on clinical signs, geographic location, and time of year.
In addition to a physical examination and medical history, specific tests are aimed at determining blood clotting time and platelet count. These include: [10]
- Complete blood count
- Urinalysis
- Biochemistry
- Clotting time tests: ACT, PT, and PTT
Diagnostic imaging may be recommended to visualize internal damage.
Differential Diagnosis
A number of other conditions cause symptoms that may be confused with snake envenomation, including:
- Allergic reactions
- Infections
- Heart failure
- Black widow spider envenomation
- Some types of injuries
Treatment
In cases of snake bites, it’s important to keep the horse calm. Agitation increases heart rate, which can facilitate rapid spread of venom through the body.
The key procedures for treating rattlesnake envenomation are protecting the airways and administration of antivenom. Extreme swelling of the muzzle associated with rattlesnake envenomation can lead to life-threatening airway obstruction. [10]
Administration of antivenom stops the progression of swelling and reverses the decrease in platelet count. In most cases, one dose of antivenom is sufficient, but if clotting times and platelet count do not improve within four hours of administration, an additional dose may be required. [10]
In some cases, affected horses may require broad-spectrum antibiotics to treat infection at the bite site. Antibiotics are best prescribed only when there is clinical evidence of an infection, as overuse and misuse can lead to antibiotic resistance. [10]
Additionally, if the horse is not up to date with tetanus vaccination, a booster of the vaccine is required. [14] Tetanus is a life-threatening bacterial infection caused by the bacterium Clostridium tetani that is commonly found in soil and is introduced into the body through puncture wounds. [14]
Supportive treatment after the initial toxic effects resolve includes: [10]
- IV fluids
- Stall rest
- NSAIDs to treat inflammation
- Wound care
Specific treatment depends on the species of snake, severity, and location of the bite wound. [10] It is important that the toxin is completely cleared from the body before administering NSAIDs, as NSAIDs may exacerbate the bleeding disorder induced by the venom. NSAIDs are not suggested for horses with abnormal blood test results following a snake bite and also not for horses bitten by a rattlesnake, due to the possible occurrence of late-onset toxicity. [17]
Prognosis
With prompt medical attention, pit viper and coral snake envenomation both have excellent prognoses. The majority of horses make a complete recovery, and there is a low incidence of long-term complications. [10]
Swelling caused by pit viper envenomation generally subsides within 72 hours of treatment. [10]
Recovery from coral snake envenomation is generally longer than that of pit viper envenomation and can take up to 14 days. [10]
Long-term neurological and cardiovascular complications are rare but can occur. [10] All recovering horses require supportive care including assistance with eating and drinking. [10]
Prevention
Prevention of venomous snake bites in horses is complex as it is impossible to completely eliminate the risk of an encounter. Environmental management strategies in areas endemic to venomous snakes include:
- Maintaining clean pastures and barns
- Keeping up to date with vaccinations
- Avoid riding during peak snake activity (late afternoon in North American fall season)
There is a vaccine against rattlesnake bites available, but any incidence of snake envenomation requires prompt medical attention, regardless of vaccination status.
Frequently Asked Questions
Here are some frequently asked questions about venomous snake bites in horses:
Pit vipers are the venomous snakes most often involved in equine snake bites in North America, especially rattlesnakes. Other pit vipers include copperheads, cottonmouths, and massasaugas. Coral snakes can also bite horses, but these cases are less common. Venom effects vary by species and may include swelling, tissue damage, blood clotting problems, breathing difficulty, or neurological signs.
Equine snake bite risk is highest from spring through early autumn, especially between March and October in North America. Snakes are more active during warmer months, which increases the chance of encounters in pastures, barns, trails, and grazing areas. Horses living in regions where venomous snakes are common face the greatest risk during this seasonal activity window.
A horse bitten by a snake needs urgent veterinary care, even if symptoms seem mild at first. Keep the horse calm, restrict movement, and call your veterinarian immediately. Do not cut the wound, apply a tourniquet, or try to catch the snake. A photo may help with identification, but only if it can be taken safely.
Venomous snake bite signs in horses may include puncture wounds, rapid swelling, pain, skin discoloration, weakness, tremors, lameness, abnormal pupils, bleeding around the eyes or nose, and difficulty breathing. Muzzle bites are especially dangerous because swelling can block the airway. Coral snake bites may cause delayed signs, including facial paralysis, salivation, and respiratory distress.
Rattlesnake bites are dangerous for horses because pit viper venom can damage tissue, interfere with blood clotting, and cause severe swelling. Muzzle bites may obstruct the airway, while systemic effects can involve the heart, muscles, and nervous system. Some horses develop abnormal heart rhythms, weakness, bleeding problems, or tissue death around the bite site.
A horse can survive a venomous snake bite with prompt veterinary treatment, and the prognosis is often excellent when care is started quickly. Pit viper swelling commonly improves within 72 hours after treatment. Coral snake recovery may take longer, sometimes up to 14 days. Delayed care increases the risk of breathing problems, tissue damage, and life-threatening complications.
Treatment for venomous snake bites in horses focuses on keeping the airway open, slowing venom effects, and supporting recovery. Antivenom may be used to stop worsening swelling and improve platelet counts. Additional care may include IV fluids, stall rest, wound care, antibiotics if infection is present, and a tetanus booster if vaccination is not current.
Antivenom may be needed after a venomous snake bite when swelling, clotting problems, or systemic signs are present. For rattlesnake envenomation, antivenom can stop swelling from progressing and help reverse venom-related platelet changes. Some horses need only one dose, while others may need more if blood work does not improve within four hours.
A dry snake bite can happen when a venomous snake bites but does not inject venom. Even so, horse owners should treat any suspected venomous bite as an emergency. It can be difficult to tell early on whether venom was delivered, and symptoms may worsen quickly. Veterinary assessment is needed to monitor swelling, blood clotting, and breathing.
Snake bite risk can be reduced by keeping barns, pastures, and fence lines clean and less inviting to snakes. Remove tall grass, debris, and clutter where snakes may hide, and store feed securely to avoid attracting rodents. In high-risk areas, avoid riding during peak snake activity, especially late afternoon and evening in warm seasons.
Summary
In North American horses, snake envenomation is caused by venomous bites from snakes belonging to one of two families: pit vipers and coral snakes.
- Snake envenomation is cause for immediate medical attention as it can lead to severe injury and death
- Vaccines for some types of venom are available, but affected horses require veterinary care regardless of vaccine status
- With prompt care, the prognosis is excellent, and horses can easily return to successful athletic careers
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