Coral Snake Envenomation
Coral snakes are the only elapid snakes native to North America. There are two genera of coral snake in the United States. Genus Micrurus includes the eastern coral snake (Micrurus fulvius), which is found in the southeastern United States from North Carolina, throughout Florida, and as far as the Mississippi River. The Texas coral snake (Micrurus fulvius tener) is found in Texas, Louisiana, and Arkansas.
Genus Micruroides includes the Arizona coral snake and the sonoran coral snake. Micruroides venom is far less toxic than Micrurus venom, and antivenin is rarely required for envenomations by snakes in this genus.
The venom of elapid snakes is primarily neurotoxic and might cause little or no local tissue reaction.
The venom of coral snakes is neurotoxic and results in parathesias and muscle fasciculations at the site of the bite that can progress over hours to weakness, dizziness, nausea, vomiting, paresthesias, hypersalivation, and respiratory paralysis. Bulbar symptoms such as diplopia, dysphagia, and slurred speech can precede respiratory failure and might be delayed for more than 10 hours from the time of the initial bite. Death is due to respiratory failure, but cardiac failure has also been reported.
Coral snakes are small and shy and bites are unusual; however, a large coral snake can administer up to 20 mg of venom in a single bite. This is four to five times the lethal dose for an adult. Fortunately, up to 50% of bites do not result in envenomation.
Elapid snakes deliver venom by latching tightly onto their prey and then making chewing motions with their jaws. If the snake is attached for as little as 10 to 30 seconds, significant envenomation can occur. Victims often report that the snake is difficult to remove and describe removal to be like peeling layers of Velcro apart.
The onset of neurologic symptoms can be delayed for several hours after the bite, and since there is little or no local reaction, it is easy to mistake true envenomation for a "dry bite."
Neurologic symptoms can be local (numbness and tingling of the affected extremity) or systemic. Once systemic symptoms are present, they can be very difficult to reverse even with administration of antivenin. Many patients have residual neurologic symptoms for months or permanently.
There is currently no FAB fragment antivenin for coral snake venom. The available antivenins are manufactured with horse serum. Furthermore, antivenin can be difficult to find. There is only one manufacturer in the United States (Wyeth), and supplies are limited. In addition to hospital pharmacies, local zoos might keep coral snake antivenin. An additional source is the national antivenin bank, which is a joint project of the American Zoo and Aquarium Association and the American Association of Poison Control Centers.
When antivenin is unavailable locally, the local poison center (800-222-1222) can, given time, find a source through the antivenin bank.
Critical Actions - Possible Diagnosis (1)
The coral snake’s neurotoxic venom can affect airway protective reflexes and respiratory drive. The presence of these symptoms is an indication for management of the airway.
Any neurologic symptom, even local symptoms, is an indication for antivenin administration. Additionally, because the onset of serious symptoms can be delayed for many hours and because it can be very difficult to determine which victims were envenomated, some authorities recommend administration of antivenin to anyone with a reliable history of a coral snake bite.
One vial of antivenin is sufficient to neutralize 2 mg of venom. The authors of the largest case series of coral snake bites to date recommend that a minimum of 4 to 6 vials of antivenin be given. Ten to 15 vials might be required in cases of severe envenomation.
Antivenin is administered as follows:
A test dose of horse serum should be given prior to the administration of the full dose. Prior to the administration of the test dose, emergency airway equipment and medications should be brought to the bedside, and an attending physician should be present. The patient should receive 0.02 to 0.1 cc of 1:00 horse serum intradermally. Some practitioners prefer to pretreat the patient with antihistamines. A skin wheal (usually present within 5 to 30 minutes in sensitive patients) indicates sensitivity to horse serum.
Provided that the skin test does not indicate sensitivity to horse serum, antivenin treatment should begin with a minimum of 4 to 6 vials. Each vial of antivenin should be reconstituted with 10 mL of sterile water or normal saline and gently agitated. The total dose of antivenin should then be diluted in an age-appropriate volume of normal saline (ideally 250–500 mL) and administered over 1 to 2 hours. Some authorities prefer to administer a portion of the total dose over 30 minutes and then decrease the rate of administration to the maximum safe rate for age and clinical condition. Because a serious reaction to horse serum can occur at any time, emergency medications for allergic reactions to the antivenin include epinephrine and vasopressor agents. Up to one fourth of patients can be expected to exhibit signs of hypersensitivity to horse serum during treatment, and about half will develop serum sickness during the treatment or soon afterward.
If a reaction occurs during treatment, the infusion should be stopped and the patient treated with antihistamines, steroids, and, if necessary epinephrine. The infusion can then be restarted at a slower rate of administration or the antivenin can be further diluted. The decision to administer antivenin to to a person who has demonstrated sensitivity to horse serum is a difficult one. In cases of severe envenomation, antivenin can be lifesaving, but the practitioner must be prepared to treat anaphylaxis. Such individuals are best stabilized and rapidly transported to a facility capable of treating severe complications.
Core Knowledge Points - Possible Diagnosis (2)
Non-native elapid snakes are kept as pets. Many of these snakes are highly venomous and aggressive. Although they occasionally escape or are released into the wild, these snakes usually bite while they are being handled by their keepers or other individuals. In such cases, it is critical that the snake be correctly identified so that a source of antivenin can be located. If possible, the snake should be examined by a professional herpetologist. If this is not possible, high-quality digital photos of the snake may be sent to a herpetologist via the Internet. Alternatively, the keeper might be able to correctly identify the snake and might even have a supply of appropriate antivenin. Treatment for these types of bites should be conducted in concert with a poison center or a knowledgeable expert.
In this case, the description of the snake and the bite pattern do not suggest a large, non-native elapid bite.
The patient was admitted to the ICU and treated with 8 vials of antivenin.
Over the next few days, his neurologic symptoms improved, but he developed erythema multiforme and a low-grade fever. The team caring for him appropriately determined that he had serum sickness related to horse-serum–based antivenin and administered steroids.
He was extubated on the third day after the bite and discharged from the hospital 2 days later. He had some residual weakness for 3 weeks but suffered no permanent sequelae.