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ASG Specialties:

  • Hip Dysplasia
  • Patella Luxation
  • Medial Patella Luxation
  • Elbow Dysplasia
  • Fragmented Coronoid Process
  • OCD Osteochondritis Dissecans Fractures
  • Arthritis
  • Ruptured Cruciate Ligament
  • Ruptured Cranial Cruciate Ligament
  • Ruptured Anterior Cruciate Ligament


Rattlesnake Envenomation

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Rattlesnake envenomation is a common and potentially life-threatening condition in Southern California in the spring through autumn months. There are at least 50 different enzyme activities that have been described in pit viper snake venom. These have direct or indirect toxic effects on the victim’s blood cells, heart, blood vessels, muscles, soft tissues, respiratory, and nervous systems.

By Peggy McMahon, DVM
DVM, Emergency + Critical Care

Clinical Signs

Patients can have a wide range of clinical signs because of the combined varying degrees of systemic and local effects caused by the venom. These include:

  • Progressive hemorrhagic lymphedema near bite
    • 80% of dog bites on head
    • 60% of cat bites on limbs
  • Pain
  • Ecchymosis
  • Hypotension
  • Tachypnea
  • Tachycardia
  • Neurotoxicity
    • Muscle fasciculation thought to be due to interaction of venom components with Ca2+ or Ca2+ binding sites on nerve membrane.
    • Generalized weakness, CN deficits, respiratory paralysis (associated with Mojave rattlesnake) are believed to be caused by action of Mojave toxin inhibiting acetylcholine release at the presynaptic terminal of neuromuscular junction, causing inhibition of neuromuscular transmission and eventually complete neuromuscular blockade.
  • Pigmenturia
    • This is common and is due to either solely or a combination of hematuria, myoglobinuria, or hemoglobinuria.

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Diagnostics

Rattlesnake venom can affect all body systems. Therefore, baseline diagnostics including serial complete blood count, serum chemistry profile, blood smear, urinalysis, coagulation profile (PT and PTT) are recommended.

Common Diagnostic Findings

  • Hematologic abnormalities in 81% of cases
  • Echino-spherocytosis in approximately 70-90% of dogs
    • The mechanism involves incorporation of lysolecithin in the outer erythrocyte membrane, leading to expansions and morphologic changes consistent with echinocytosis and spherocytosis.
  • Coagulopathies documented in 56% of dogs
    • The most common mechanism of coagulopathy is defibrination. Many venoms contain fibrinolysins and fibrinogenases that destroy fibrinogen and fibrin and lead to friable clot formation. The depletion of fibrinogen and fibrin limits the animal’s ability to form clots and results in a hemorrhagic defibrination coagulopathy, or venom-induced consumptive coagulation. There are also many other pro- and anticoagulant properties of venom.
  • Thrombocytopenia in 35-88% of dogs
  • Hypokalemia in approximately 80% of dogs
    • It is theorized that this is caused by release of endogenous epinephrine that stimulates beta receptors on the cell surface, thereby stimulating the Na-K-ATPase pumps, leading to intracellular shift of potassium.

Treatment

First aid: First aid treatment should not be a cause for delayed transport to an emergency facility. Recommendations for people include immobilizing the patient to slow the spread of venom, applying a light-constricting tourniquet that will only compress the lymphatics, and performing local incision and suction. These techniques are likely less efficacious in veterinary patients, as the majority affected patients are dogs and the majority of their bites occur on the head. Incision and suction are only useful if performed within 5 minutes of the bite and is seldom effective due to the depth of bite and rapid spread of venom.

Mainstays of treatment include volume resuscitation and maintenance with crystalloids, pain management (opioids), and antivenin.

Antivenin: Antivenin works by binding and neutralizing venom toxins, facilitating their redistribution from tissues and their elimination from the body. Blood products (plasma, fresh whole blood) should only be used to correct coagulopathies and severe anemia, respectively, when the patient is refractory to adequate treatment with antivenin. The administration of plasma to human victims has been reported as ineffective because of the addition of the extra substrate. Unless all of the venom is neutralized, it is believed there may be a procoagulant effect, which may accelerate fibrinolyisis and further hemorrhage. Early use of antivenin, within 4 hours of the snake bite, is recommend for maximum efficacy. However, there may be some beneficial effects as late as 24 hours following the bite.

Blocking Drugs: There is some theoretical use for H1 and H2 blocking drugs in regards to treating the venom properties as opposed to restricting its use as a medication for antivenin hypersensitivities. Antihistamines are a necessary therapy for acute hypersensitivity associated with administration of antivenin. There is limited veterinary data regarding the utility of these drugs for treatment of venom properties without evidence of hypersensitivity.

Glucocorticoids: Glucocorticoid use is also controversial, and most of the current medical literature does not recommend glucocorticoid drugs for the management of snakebites in humans and animals. While these agents can have significant anti-inflammatory effects, they have not been shown to be beneficial for treatment of primary venom effects. Glucocorticoids can also interfere with normal wound healing. An outcome benefit has not been demonstrated in human or veterinary patients.

Non steroidal anti-inflammatory drugs: Nonsteroidal anti-inflammatory drugs are not recommended for rattlesnake envenomations due to their ability to impair platelet aggregation, which can worsen the bleeding associated with the venom-induced coagulopathy.

Antimicrobials: Antimicrobials are generally not necessary when only hemorrhagic lymphedema is present. However tissue necrosis may justify bacterial culture and sensitivity, antimicrobial treatment, and surgical debridement, if indicated.

Types of rattlesnake antivenin most commonly used in the United States:

ANTIVENIN TYPES…
TYPE COMPOSITION COVERAGE OTHER
ACP: Antivenin (Antivenin (Crotalidae) Polyvalent) Made of complete IgG and albumin from immunized horses (more allergenic) because uses both the Fc and the Fab portions of the immunoglobulin South American, Eastern and Western rattlesnakes, Fer-de-lance No neurotoxin efficacy. Dose typically 1-5 vials, depending on severity
Crofab Purified, ovine derived, cleaved Fab Ig (contains less than 3% of immunoglobulin Fc fragments). Smaller and more rapidly eliminated, so may need to be reduced. Effective against Mojave toxin Eastern, Western, and Mojave rattlesnakes, cottonmouths 5 times as potent as ACP. Dose typically 1-3 vials
VenomVet Polyvalent antivenin of equine origin. F(ab)2 antivenin that has 2 antigen binding sites per molecule North American rattlesnakes, copperheads, and water moccasins Dose typically 1-2 vials

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Prevention

  • Snake encounters with pets are most likely to occur between late April and October.
  • Vaccination – The benefits of prophylactic vaccination include more time to get to a veterinary hospital, reduction in the amount of pain and swelling experienced, faster recovery times, and a decrease in the mortality rate. It is not meant as a sole means of protection. Depending on the region and length of rattlesnake season and exposure, the typical vaccine protocol includes a first dose 4-6 weeks before rattlesnake season, a booster one month later, then booster every year. The vaccine was designed to provide protection against the Western Diamondback Rattlesnake and does not provide protection against the Mojave rattlesnake, Eastern Diamondback rattlesnake, cottonmouths or coral snakes.
  • Walk or hike in areas with an obvious path.
  • Use a walking stick or other device to rustle the shrubs along the side of route to alert snakes of your presence.
  • Keep dogs on a leash when outside of the home or away from the yard.
  • Do not let dogs wander into tall grass or thick shrubs where rattlesnakes may be hiding.
  • Be careful along streams and riverbeds, as snakes are often found in the tall grass near water sources.
  • Stay a minimum of 10 feet or more away from the snake and walk around the area or take another route.
  • Always have a nearby 24-hour veterinary hospital phone number and address available.

What to Expect

Rattlesnake bites can results in wide variety of signs and these will vary from patient to patient. Mortality rates in dogs are variable and have been reported from 1% to 30%. There are many factors to consider when determining prognosis in dogs and cats. Smaller animals tend to receive a larger dose of venom based upon body mass and typically carry a poorer prognosis. Time to treatment and location of the bite on the body will also affect prognosis. In general, animals treated with antivenin as soon as possible following the bite, carry a fairly good prognosis.

 


REFERENCES: [1] Armenian R and Schaer M. Overview and controversies in the medical management of pit viper envenomation in the dog. J Vet Emerg Crit Care 2011; 21(5): 461-470. [2] Gold B, Barish R, and Dart R. North American snake envenomation: diagnosis, treatment and management. Emery Clin N Am 2004; 22: 423-443. [3] Holster C, Miller M, Warmth M, et al. Crotalid snake envenomation. Brit Care Clin 1997; 13(4): 889-921. [4] Hoose J and Carr A. Retrospective analysis of clinical findings and outcome of cats with suspected rattlesnake envenomation in Southern California: 18 cases (2007-2010). J Vet Emerg Crit Care 2013; 23(3): 314-320. [5] Julius T, Kaelble M, Leech E, et al. Retrospective evaluation of neurotoxic rattlesnake envenomation in dogs and cats: 34 cases (2005-2010). J Vet Emerg Crit Care 2012; 22(4): 460-469. [6] McCown J, Cooke K, Hanel R, et al. Effect of antivenin dose on outcome from crotalid envenomation: 218 dogs (1988-2006). J Vet Emerg Crit Care 2009; 19(6): 603-610. [7] Najman L and Sephardi R. Rattlesnake envenomation. Compound Contin Educ Vet 2007; 29(3): 166-176. [8] Pashmakova M, Bishop M, Black D, et al. Multi center evaluation of the administration of crotalid antivenom in cats: 115 cases (2000-2011). J Am Vet Med Assoc 2013; 243: 520-525. [9] Peterson M, Matz M, Semibold K, et al. A randomized multi center trial of Crotalidae polyvalent immune Fab antivenom for the treatment of rattlesnake envenomation in dogs. J Vet Emerg Crit Care 2011; 21(4): 335-344. [10] Woods C and Young D. Clinical safety evaluation of F(ab’)2 antivenin (Crotalus durissus – Bothrops asper) administration in dogs. J Vet Emerg Crit Care 2011: 21(5): 565-569.

Animal Specialty Group

DVM, Diplomate American College of Veterinary Emergency + Critical Care

After receiving her Doctor of Veterinary Medicine degree with honors from the University of Illinois at Champaign-Urbana in 2011, Dr. McMahon migrated to Los Angeles where she completed both a Small Animal rotating internship as well as a Small Animal Emergency and Critical Care residency at Animal Specialty and Emergency Center. Shortly after joining Animal Specialty Group’s Emergency Team in 2016, she became board certified in veterinary emergency and critical care.


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