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 hemoylitic anemia.

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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.

While identifying the species of snake can be useful to direct administration of a specific antivenom, trying to catch a venomous snake is hazardous and is not advised. If it is possible to get a photo of the snake, it may aid in diagnosis, but it is more important to seek urgent veterinary care. Do not bring a live poisonous snake to a veterinary clinic.

 

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.

Regardless of the type of snake bite, supportive treatment includes: [10]

Specific treatment depends on the species of snake, severity, and location of the bite wound. [10]

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 prevent infection at the bite site. [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 though puncture wounds. [14]

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 medication attention, regardless of vaccination status.

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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References

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  2. Modahl, C. M. et al. Venoms of Colubrids. In Venom Genomics and Proteomics, P. Gopalakrishnakone and J. J. Calvete, Eds., Dordrecht: Springer Netherlands, 2016, pp. 51–79. doi: 10.1007/978-94-007-6416-3_9.
  3. Schmidt, M.C. Snakes of North America. University of Pittsburgh (accessed Mar. 22, 2024).
  4. Oliveira, A. L. et al. The Chemistry of Snake Venom and Its Medicinal Potential. Nat Rev Chem. 2022. doi: 10.1038/s41570-022-00393-7.
  5. Hovda, L. R., Benson, D., & Poppenga, R. H. Blackwell’s Five-Minute Veterinary Consult Clinical Companion: Equine Toxicology. Wiley Blackwell, 2022.
  6. Plumlee, K. H., Ed. Clinical veterinary toxicology. St. Louis, Mo: Mosby, 2004.
  7. Gilliam, L. L. Snake Envenomation. Veterinary Clinics of North America: Equine Practice. 2024. doi: 10.1016/j.cveq.2023.08.003.
  8. Pucca, M. B. et al. Current Knowledge on Snake Dry Bites. Toxins. 2020. doi: 10.3390/toxins12110668.
  9. Plumb, D. C. Plumb’s veterinary drug handbook, 6th ed. Stockholm, Wis.; Ames, Iowa: PharmaVet; Distributed by Blackwell Pub, 2008.
  10. Lavoie, J.-P., Ed. Blackwell’s five-minute veterinary consult. Equine, Third edition. Hoboken, NJ: Wiley-Blackwell, 2019.
  11. Cardiac Arrhythmias in Horses. Royal Veterinary College (accessed Mar. 22, 2024).
  12. Slade, J. et al. Wilderness Cardiology: A Case of Envenomation-Associated Cardiotoxicity Following a Rattlesnake Bite. Cardiol Ther. 2021. doi: 10.1007/s40119-021-00215-9.
  13. Gilliam, L. L. et al. Cardiotoxicity, Inflammation, and Immune Response after Rattlesnake Envenomation in the Horse. J Vet Intern Med. 2012. doi: 10.1111/j.1939-1676.2012.01022.x. View Summary
  14. Tetanus in Horses – Horse Owners, MSD Veterinary Manual (accessed Mar. 22, 2024).
  15. Rodríguez Cruz, P. M. et al. The Neuromuscular Junction in Health and Disease: Molecular Mechanisms Governing Synaptic Formation and Homeostasis. Front. Mol. Neurosci. 2020. doi: 10.3389/fnmol.2020.610964.
  16. Bamford, N. J. et al. Elapid Snake Envenomation in Horses: 52 Cases (2006–2016). Equine Veterinary Journal. 2018. doi: 10.1111/evj.12735. View Summary