Snake Envenomation

Chapter 95 Snake Envenomation



Michael E. Peterson, DVM, MS



KEY POINTS



• Severity of the envenomation depends on the species of snake, nature and location of the bite, and the size of the victim.

• Snake envenomation can be unpredictable and can progress, necessitating continuous monitoring and reassessment.

• Antivenin therapy is the only proven treatment that will improve the outcome of moderate to severe pit viper envenomations.

• Continued progression of clinical signs or failure of clinical signs to improve may be an indication for repeated antivenin therapy.

• Coral snake envenomation causes flaccid paralysis with little or no local tissue reaction.


PIT VIPER ENVENOMATION


Pit vipers are the largest group of venomous snakes in the United States and are involved in an estimated 150,000 bites annually of dogs and cats. In North America members of the family Crotalidae belong to three genera: the rattlesnakes (Crotalus and Sistrurus spp) and the copperheads and cottonmouth water moccasins (Agkistrodon spp).


Agkistrodon species, the copperheads and water moccasins, are found throughout the eastern and central United States. Copperheads are responsible for most venomous snakebites to humans in North America because of their proclivity for living next to human habitation. Water moccasins can be pugnacious and have a greater tendency to deliver venom when they bite. Rattlesnakes (Crotalus and Sistrurus spp) are found throughout the continental United States and account for most deaths in both human and animal victims. Clinicians should become familiar with their regional indigenous poisonous snake species.


The venom is not considered more toxic during the summer months; however, snakes show increased aggression and venom yield with environmental warming and longer photoperiod (as in the spring and summer). The maximum venom yields occur during the hottest months of summer.


Pit vipers control the amount of venom they inject during a bite. The amount injected depends on the snake’s perception of the situation. Initial defensive strikes often are not envenomating. Offensive bites meter a given amount of venom into the victim, and agonal bites deliver the entire venom load and are therefore the most dangerous.


The severity of any pit viper bite is related to the volume and toxicity of the venom injected as well as to the location of the bite, which may influence the rate of venom uptake. As a generalization, the toxicity of pit viper venoms ranges in descending order from the rattlesnakes to the water moccasins and then to the copperheads. The toxicity of rattlesnake venom varies widely. Of the rattlesnakes, 9 species and 12 subspecies have populations with venoms containing proteins that are immunologically similar to the potent neurotoxic Mojave toxin. It is possible for pit viper venom to be strictly neurotoxic with virtually no local signs.


Pit viper venoms are a complex combination of enzymatic and nonenzymatic proteins. The primary purpose of the venom is not to kill but rather to immobilize the prey and predigest its tissues. The venom is 90% water and has a minimum of 10 enzymes and 3 to 12 nonenzymatic proteins and peptides in any individual snake.


The envenomation syndrome reflects the complexity of the venom. The body has to respond to the effects of multiple venom fractions, metabolize each, and cope with the resultant myriad of metabolites. In addition to the individual pharmacologic properties of these proteins and their metabolites, it has been demonstrated that some components act synergistically in producing specific effects or reactions. The net effect of this interaction of venom with the victim’s response is a metabolic stew of toxic peptides and digestive enzymes. Additionally, the traditional categorization of pit vipers as having only hematoxic venoms should be reevaluated because some subpopulations of rattlesnakes possess only neurotoxic venom.


The onset of clinical signs after a snakebite may be delayed for several hours. Forty percent of all severe envenomations in humans are graded as mild to without envenomation sometime during the syndrome. In humans it is estimated that 20% of all pit viper bites are without envenomation (i.e., dry), with an additional 25% classified as mild.


Every pit viper envenomation is different. The victim affects the severity of an envenomation by such factors as species of victim, body mass, location of bite, post-bite excitability, and use of premedications (e.g., nonsteroidal antiinflammatory drugs in older dogs that may make the victim more susceptible to clotting defects). The snake affects the severity of the envenomation by species and size of snake, age of snake, motivation of snake, and degree of venom regeneration since last use.


Cats are more resistant, on the basis of milligram of venom per kilogram of body mass, to pit viper venom than dogs. However, cats generally arrive for veterinary care in a more advanced clinical condition. This is probably due to the cat’s smaller body size and the proclivity of cats to play with the snake, thereby antagonizing it and inducing an offensive strike, often to the torso. Additionally, cats commonly run off and hide after being bitten before they return home, thus delaying the time from bite to veterinary care. Because dogs generally receive more defensive strikes, have a larger body mass, and more frequently seek immediate human companionship after injury, they are more likely to receive medical attention promptly.



Clinical Signs


It is possible that a life-threatening envenomation may occur with no local clinical signs other than the puncture wounds themselves. Local tissue signs of pit viper envenomation include puncture wounds, one to six from a single bite, which may be bleeding. Rapid onset of pain may ensue with progressive edema. Ecchymosis and petechiation may manifest. Tissue necrosis may occur, particularly in envenomations to areas without a significant subcutaneous tissue mass. The presence of fang marks does not indicate that envenomation has occurred, only that a bite has taken place. It must be reiterated that the severity of local signs does not necessarily reflect the severity of the systemic envenomation.


Systemic clinical manifestations encompass a wide variety of problems, including pain, weakness, dizziness, nausea, severe hypotension, thrombocytopenia, fasciculations, regional lymphadenopathy, alterations in respiratory rate, increased clotting times, decreased hemoglobin concentration, abnormal electrocardiogram findings, increased salivation, echinocytosis of red blood cells, cyanosis, proteinuria, bleeding (e.g., melena, hematuria, hematemesis), obtundation, and convulsions. Not all of these clinical manifestations are seen in each patient, and they are listed in descending order of frequency as seen in human victims.


Severe hypotension results from pooling of blood within the “shock organ” of the species bitten (i.e., the hepatosplanchnic [dogs] or pulmonary [cats] vascular bed) in addition to fluid loss from the vascular compartment secondary to severe peripheral swelling. This swelling can be significant.


The victim’s clotting anomalies largely depend on the species of snake involved. Coagulopathies range from direct blockage or inactivation of various factors in the patient’s clotting cascade to the possible destruction of megakaryocytes in the circulating blood and bone marrow. Approximately 60% of envenomated patients develop a coagulopathy, by far the most common being hypofibrinogenemia with prolonged clotting times. Venom-induced thrombocytopenia occurs in approximately 30% of envenomations, with an unmanaged nadir usually occurring between 72 and 96 hours. Syndromes resembling disseminated intravascular coagulation are possible with pit viper envenomations.


Myokymia, a type of fasciculation of various muscle groups, frequently is reported in humans after bites received by timber rattlesnakes (Crotalus horridus horridus) and western diamondback rattlesnakes (Crotalus atrox).1



Monitoring


Monitoring of the severity and progression of the clinical envenomation syndrome may be difficult. A tool that has proven useful is the envenomation severity scoring system (Box 95-1). Use of this system more accurately quantifies the severity of the patient’s condition over time and allows a more objective assessment than is usual without it.2 It is recommended that a severity score be acquired upon entry and 6 hours, 12 hours, and 24 hours after admission.



Box 95-1 Snake Bite Severity Score

















































































































Score Severity
Pulmonary System
0 Signs within normal limits
1 Minimal: Slight dyspnea
2 Moderate: Respiratory compromise, tachypnea, use of accessory muscles
3 Severe: Cyanosis, air hunger, extreme tachypnea, respiratory insufficiency, or respiratory arrest from any cause
Cardiovascular System
0 Signs within normal limits
1 Minimal: Tachycardia, general weakness, benign arrhythmia, hypertension
2 Moderate: Tachycardia, hypotension (but tarsal pulse still palpable)
3 Severe: Extreme tachycardia, hypotension (nonpalpable tarsal pulse or systolic blood pressure <80 mm Hg), malignant arrhythmia, or cardiac arrest
Local Wound
0 Signs within normal limits
1 Minimal: Pain, swelling, ecchymosis, erythema limited to bite site
2 Moderate: Pain, swelling, ecchymosis, erythema involves less than half of extremity and may be spreading slowly
3 Severe: Pain, swelling, ecchymosis, erythema involves most or all of one extremity and is spreading rapidly
4 Very severe: Pain, swelling, ecchymosis, erythema extends beyond affected extremity, or significant tissue slough
Gastrointestinal System
0 Signs within normal limits
1 Minimal: Abdominal pain, tenesmus
2 Moderate: Vomiting, diarrhea
3 Severe: Repetitive vomiting, diarrhea, or hematemesis
Hematologic System
0 Signs within normal limits
1 Minimal: Coagulation parameters slightly abnormal, PT <20 sec, PTT <50 sec, platelets 100,000 to 150,000/mm3
2 Moderate: Coagulation parameters abnormal, PT 20 to 50 sec, PTT 50 to 75 sec, platelets 50,000 to 100,000/mm3
3 Severe: Coagulation parameters abnormal, PT 50 to 100 sec, PTT 75 to 100 sec, platelets 20,000 to 50,000/mm3
4 Very severe: Coagulation parameters markedly abnormal with bleeding present or the threat of spontaneous bleeding, including PT unmeasurable, PTT unmeasurable, platelets <20,000/mm3
Central Nervous System
0 Signs within normal limits
1 Minimal: Apprehension
2 Moderate: Chills, weakness, faintness, ataxia
3 Severe: Lethargy, seizures, coma
Score
0 to 20 Total score: 20 possible points
Interpretation:
<2 minimal severity
2 to 3 moderate severity
4 to 8 more severe
>8 very severe

PT, Prothrombin time; PTT, partial thromboplastin time.

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  4. Allergic Airway Disease in Dogs and Cats and Feline Bronchopulmonary Disease

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Snake Envenomation

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