SNAKE AND ARTHROPOD BITES: A COMPREHENSIVE REVIEW
The objectives of this lecture are to give a general ability to identify venomous
snakes and arthropods and give a general understanding of the clinical presentation
of an envenomed patient.
Envenomations have been documented by man for centuries and fossils of snakes and arthropods have been found and dated as far back as four million years ago. So, you can imagine that since ancient man and cultures had to deal with envenomations, many superstitions became prevalent. Particularly, the snake has been credited with supernatural powers, including medicinal and sexual powers, stealth and guile, and great evil. Some nations continue to worship the serpent as a sign of power and fertility and some modern religions use the snake in many rituals, today.
Venomous snakes, in general, have distinct characteristics that are important in the diagnosis of snake bite. There is one major exception I will discuss later. The importance of learning these is to aid in the diagnosis of venomous bites and allow good treatment and disposition of the victim.
Rattlesnakes, or crotalids, belong to the Crotalidae family and are most commonly thought of when you think of venomous snakes. However, the Crotalidae, or pit viper, also includes the cottonmouths and water moccasins, or Agkistrodon, species. They all have a distinctly triangular head caused by the protrusion of the venom glands on the sides of the mandibles.
They also have characteristically elliptical pupils, and fangs that are hinged and retract when not in use. The fangs are hollow and are attached to the venom glands by ducts and muscles that regulate the amount of venom deposited with each bite.
The tail of most venomous snakes has a double row of subcaudal plates under the tail by the anus. The crotalids have the characteristic rattle which is formed each time the snake molts, however, the Agkistrodon, don't. The rattle doesn't represent the age of the snake, as commonly thought, because snakes molt several times a year.
The "other" mentioned on the slide is the pit that pit vipers get their name. This is an infrared sensory organ that senses the size, distance and location of the prey the snake is hunting or defending itself from. It's located between the nostril and the eye on top of the head.
This leads me to the exception I mentioned earlier in identifying venomous snakes. The coral snake, or the elapid, has a rounded head, round pupils, fixed teeth without fangs, and only has a single row of subcaudal plates. These characteristics are similar to many non-venomous snakes and the coral snake is often confused with the king snake. The way to tell the difference is a common mnemonic: "Red on yellow kills a fellow, red on black venom lack". This is describing the pattern of colorful rings that circle the bodies of each of these snakes.
The venom snakes produce has three generally agreed purposes. These are defense, immobilization and killing of the prey, and to aid in digestion of prey before the snake ingests the prey.
At least 26 different enzymes have been isolated from different venom but each particular snake usually only has 5 to 6 of these. There are also lethal proteins that cause capillary leaking and loss of intravascular volume. This capillary leaking is responsible for many of the clinical effects of venom.
Besides the type of venom deposited, there are several other factors that affect the venom's toxicity. The age and health of the snake and victim are important in that extremes of age adversely affect each. Also, the number of bites the snake delivers and the particular species of the snake greatly affect the venom's toxicity.
In general, venom does not penetrate the CNS, however, the venom the Mojave rattlesnake and the coral snake have mainly central affects and I'll talk about this a little later.
The clinical presentation can vary greatly from patient to patient and the initial symptoms can be misleading . Up to 25% of snake bites will be dry bites, or a bite where no venom was deposited.
Patients with bites from crotalids will have the most significant initial local symptoms. These can range from mild to severe and pain and ecchymoses, blistering, bleeding and tissue necrosis and swelling. Fang marks will be present in 100% of bites from crotalids, so you have to look carefully.
Systemic symptoms from crotalid bites usually develop slowly but depending on the area of the bite, they can occur from minutes to hours. The most severe bites are intravenous with almost immediate onset of systemic symptoms. Most of the patients will die before presentation to treatment but. luckily, these are very rare. Bites to the face, head, or neck are followed by bites to the upper extremities in suddenness of onset.
Systemic effects include hematuria, hemorrhage, tachycardia, sweating, and hypotension with more listed in your hand-out. Lab abnormalities occur and the most helpful in determining the severity of the bite are hemoglobin and hematocrit, platelets, and fibrinogen. The H&H is usually initially elevated but as hemorrhage progresses they drop. The platelets will be decreased because the venom proteins bind them and make them non-functional. This is important later in treatment.
The major cause of death in crotalid bites is cardiovascular collapse except from Mojave bites. Mojave's venom contains a potent neurotoxin that causes stridor, muscle weakness, and respiratory arrest. The important point with Mojave bites is symptoms can be delayed hours and the local symptoms are usually mild. There are case reports of patients being discharged with the diagnosis of minimal envenomation and return later due to respiratory arrest.
Elapid bites present similarly to Mojave bites in that there are minimal local effects. There are also 15% of victims that will have no fang marks and the patients can be misdiagnosed initially. They will then present up to 15 hours later with respiratory paralysis; this can last up to 5 weeks but is completely reversible. So the major cause of death in elapid bites is respiratory arrest.
Prehospital management has changed dramatically in the last ten years and now the only treatment recommended is reassurance, moving from the snake's territory and transport to a hospital as soon as possible. Tourniquets and other therapies have all been found to increase morbidity and mortality.
There is one device that is acceptable in the field if it doesn't delay transport. This is the Sawyer Extractor and if it's used properly, can reduce the venom pool deposited by up to 35%.
Hospital management starts with the ABC's of resuscitation. Most crotalid victims will be volume depleted and will need fluid. Remember to get history of tetanus status and a good H&P focusing on medical problems, history of any prior bites or a sensitivity to horse serum.
Patients with moderate and severe bites definitely require antivenin. The antivenin
product is shipped in a box with a test dose of horse serum and full instructions
on their use. But, in general, you give a skin test of horse serum and, whether
the patient reacts or not, this is followed by antivenin administration.
The skin test just prepares the clinician for any pending allergic reaction
including anaphylaxis and allows the patient to be pretreated.
Again, remember that up to 25% of crotalid bites are dry bites and the victim will have no symptoms other than fang marks or very local pain. With crotalid bites, its recommended to observe the patient for 4 to 8 hours. If no or very minimal symptoms occur and there are no lab abnormalities the patient can be discharged home. Patients with minimal envenomations will require close follow-up for signs of infection and bite complications.
All patients with either Mojave or elapid bites require admission for at least 24 hours due to the potential late onset of symptoms.
This moves us on to arthropod envenomations. The arthropods include a huge range of species but this talk will focus on bites from the brown recluse, scorpion and black widow.
The brown recluse is half-dollar sized brown spider with a dark, violin-shape on its back (otherwise known as it's cephalothorax). This spider is unaggressive and only bites when disturbed. It hides in woodpiles and clothing as well as bedsheets.
The presentation varies but the victim will usually have local pain within
8 hours. This progresses to blistering and hemorrhage followed by the formation
of a deep ulcer. The ulcer heals over 2 to 5 weeks and leaving a large scar.
Treatment is similar for all arthropod bites and consists of ice, elevation of the affected area and NSAID's. This is followed by debridement of the necrotic tissue only after the ulcer forms.
Other modalities, including early debridement, have all been shown to be ineffective. Only dapsone, is advocated by some poison control centers.
Disposition of the patient is fairly straight forward. If they have systemic signs such as renal failure from DIC, they need admission. If no severe local signs and no systemic symptoms occur, the patient can be discharged with close follow-up.
The scorpion is located mainly in the southwestern United States. It's pale yellow to brown and had the characteristic "stinger" at the end of it's tail. They can reach up to 7.5 centimeters and hide in wood and rockpiles. They commonly climb in camper's boots and clothes. They are most active at night.
The sting is characterized by being more painful than the local edema and erythema would suggest. The systemic effects are caused by a neurotoxin found in some scorpion's venom and can affect the sympathetic, parasympathetic, and neuromuscular systems. So, patients can present with a variety of symptoms including opisthotonos, seizures, pulmonary edema or respiratory arrest. However, mortalities in humans rarely occur in patients under 6 years old.
As usual, ice, elevation and NSAID's is sufficient mild envenomations. Remember to ask about tetanus status and prophylax as needed. If the patient has systemic signs, they may need cardiovascular or respiratory support. Benzodiazepines are the recommended treatment for seizures and hyperactivity and calcium gluconate is effective for muscle spasms. There is no antivenin therapy in the United States for scorpion bites.
Although hospitalization is rare, any patient presenting with systemic signs needs at least 4 hours of observation. Again, any cardiovascular or respiratory compromise requires admission.
The black widow is a small, glossy black spider that has a characteristic red hour-glass on the underside of its abdomen. They are found in wood and rockpiles also, but have an uncanny predisposition to the bottom of the seat in out-houses. This has lead to multiple victims being bitten in the genital area or buttocks.
Patients can have a variety of initial symptoms from no pain to moderate swelling with severe pain. However, within 60 minutes the neurotoxin in the black widows venom can cause severe neuromuscular symptoms. This lactrotoxin cause muscle cramping, most commonly in the abdomen, which is usually severe and can give the appearance of an acute, board-like abdomen.
Other symptoms include priapism, restlessness, respiratory muscle weakness and bronchorrhea. The highest risk group for respiratory arrest are children that weigh less than 15 kg. Also, pregnant women can experience uterine contractions strong enough to induce premature delivery.
Treatment is the usual ice and elevation with NSAID's and systemic symptoms
are treated symptomatically. Severe muscle cramping can be treated with calcium
gluconate or muscle relaxants if that fails. Any patient with respiratory
compromise, seizures, uncontrolled hypertension or pregnancy will require
the administration a antivenin.