ANIMAL BITES AND STINGS
FAITH T. FITZGERALD
Insects and Spiders
As travel, pet keeping, and out-of-doors activity increase, so does the risk of animal bites and stings. Injury can result from direct trauma, superinfection, allergy (by far the most morbid), and injected or applied toxins.
INSECTS AND SPIDERS
Although death is rare and due principally to Hymenoptera allergy, the morbidity of arthropod bites in the United States is considerable. Several species infest or attack humans. Nonvenomous but annoying and potentially dangerous arthropods are outlined in Table 58.1.
TABLE 58.1. NONVENOMOUS ARTHROPODS
Stings of wasps and bees, which are members of the order Hymenoptera, can be life- threatening to hypersensitive patients (see Chapter 173). Although the venom of Africanized honeybees is no more allergenic or toxic than that of European honeybees, the Africanized bees are more aggressive and multiple stings can lead to death. Local therapy in nonallergic individuals consists of removing the stingers and applying ice. With multiple stings, toxic systemic reactions include faintness, headache, fever, muscle spasms, and gastrointestinal upset. Fire ants, Solenopsis invicta and S. richteri, are nonwinged hymenopterans that are endemic in the southeastern United States, where the often multiple bites create painful, burning papules, vesicles, and pustules with potential for necrosis and scarring. Toxic systemic effects include nausea, vomiting, dizziness, and blurred vision. Allergic reactions may also occur.
Hard and soft ticks are noted mainly for their carriage of rickettsial, viral, bacterial, spirochetal, and protozoal diseases. Uninfected bites can cause local dermal reactions and, rarely, a neurotoxic ascending paralysis. Embedded mouth parts can stimulate a granuloma. Ticks should be removed by the application of petrolatum and careful use of blunt forceps. Surgical excision occasionally is required. Blister beetles do not bite or sting, but rough contact with their carapace, which contains the vesicant cantharidin, can give painful blisters, which are treated as chemical burns. Several species of caterpillars, including the saddleback, buck moth, and pus caterpillar of the southern United States, have hairs that penetrate skin and inject venom, causing intense local pain, urticaria, and, rarely, shock. Removal of hairs can be done with cellophane tape or tweezers; antihistamines, corticosteroids, analgesics, and intravenous calcium gluconate help pain and urticaria. The gypsy moth caterpillar in the northeastern United States causes an erythematous papular dermatitis.
Of centipedes, only the bite of the giant Scolopendra heros of the southwestern United States is life threatening, principally to children. It injects hemolytic and cardiotoxic venom, causing local necrosis, severe systemic symptoms, and even rhabdomyolytic renal failure. Treatment is supportive.
Many spiders bite, but there are only two major venomous species in the United States: the black widow, Latrodectus mactans, and the brown spiders, Loxosceles. The female black widow haunts privies, houses, garages, and woodpiles across the United States. It is about 6 mm in diameter and shiny black, with variable red ventral markings sometimes (not always) in an hourglass shape. It is aggressive. About 10% of black widow bites result in envenomation from a neurotoxin, causing an outpouring of various neurotransmitters that produce lactrodectism. A sharp local pain signals the bite. Within a half hour, there is local redness and swelling or a pallid area with a red–blue border. Systemic effects may follow and include muscular rigidity and fibrillations, abdominal pain of such severity as to be misdiagnosed as peritonitis, nausea, vomiting, hypersalivation, swings in blood pressure and pulse, ischemic electrocardiographic changes, cardiac dysrhythmias, hemolysis, rhabdomyolysis, renal failure, hyperglycemia, neutrophilia, coma, and, rarely, death. Muscle relaxants and narcotics treat pain and muscle spasm. An effective antivenin (Lyovac) is a horse serum product, likely to produce hypersensitivity reactions, and is used in severe cases (see Chapter 138).
Loxosceles species, the brown spiders (fiddlebacks), are small, nocturnal, shy creatures with a dark, violin-shaped mark on their backs. They live in homes and clothing, and bite when disturbed. They inject a venom containing necrotizing, hemolytic, and spreading factors. A sudden mild pain at the bite may produce only erythema, induration pain, and pruritus at the site or be followed within hours to days by a characteristic violaceous erythema and necrosis extending to the underlying muscle. Fever, chills, arthralgia, morbilliform rash, malaise, coagulopathy, jaundice, hematuria, renal failure, and, rarely, death may all complicate severe envenomation 24 to 48 hours after the bite. Supportive treatment with splinting and support of the affected part, analgesics, tetanus toxoid, antibiotics for superinfection, and occasional surgical debridement is required. The use of the sulfa drug dapsone, 50 mg orally twice daily for 2 days (increased to 100 mg orally twice daily thereafter if necrosis is progressing), is controversial as benefit is unclear and dapsone can cause agranulocytosis and methemoglobinemia. Intermittent cold packs applied to the site for the first 48 hours also may help.
The two most dangerous species of scorpions in the southwestern United States are Centruroides sculpturatus and C. gertschi. Their venom, delivered by sting from a tail sac, is potent but seldom lethal. Local reactions to many scorpion stings can be severe, but the most venomous southwestern scorpions usually produce little local response. Neurotoxin injection leads within hours to perinasal and oral itching; nystagmus, trismus, numbness, dizziness, agitation, confusion, convulsions; circulatory, renal, and respiratory failure; hemolysis; and death. Therapy is with local ice packs and support. In severe reactions, in very young children, which can be lethal, specific antivenin may be used.
Of snake bites in the United States, fewer than 25% are poisonous, and death is rare. Most venomous bites in the United States are caused by rattlesnakes, copperheads, and cottonmouths (all Crotalidae, or pit vipers) or by the coral snake (Elapidae). A few snake bites are by imported exotic species. Confronted by a snake bite victim, the physician must determine the species of snake, which is best done if the reptile is brought in (dead, in a closed container) with the patient. A snake with catlike pupils, a triangular head, and pits between the nostrils and eyes is a pit viper. The coral snake has black bands abutting yellow (“black on yellow, kill a fellow”), and the harmless look-alike snake has red bands adjacent to black (“red on black, venom lack”). Even if the snake is poisonous, not all bites contain venom, and of those that do, the fate of the patient may depend on the size of the snake, the size of the bite, the interval between the bite and the onset of local or systemic signs and symptoms, the field therapy that was given, and the age, size, and general condition of the victim.
Field therapy for snake bite is controversial, but it probably is best to do no more than immobilize the body part, calm the victim, and transport him or her to the hospital as soon as possible. Tourniquets, pressure bands, incision and suction, snake venom extractors, electric shock therapy, and the application of heat or cold to the bite are of no proven benefit and might be harmful. Ingestion of alcohol should absolutely be avoided. Definitive therapy in the hospital depends on the type and severity of envenomation. A severity score for crotalid snake bites has been developed and is useful in predicting the course of events and need for antivenin.
Local effects of pit viper bites include pain, erythema, ecchymosis, local bleeding, and necrosis. Systemic venom produces perioral tingling, a metallic taste, muscle fasciculation, hypotension, disseminated intravascular coagulation, respiratory distress, and renal failure. Large-bore intravenous access should be established and initial blood samples drawn for a complete blood cell count, coagulation studies, renal function tests, and blood for type and cross match. Physical examination should be repeated frequently and laboratory studies repeated after 4 hours in the asymptomatic patient. Electrocardiograms should be obtained on the severely ill and all patients older than 40 years of age. In patients with major systemic symptoms or laboratory derangements, antivenin therapy is given. Because antivenin is horse serum, the patient first should be skin-tested for immediate hypersensitivity. Ten to twenty vials of antivenin, depending on the patient’s degree of envenomation, is given intravenously in 500 mg of 5% dextrose and water or normal saline over 1 to 2 hours. Then, based on signs and symptoms, more is given as needed. Local wound care may include debridement, broad-spectrum antibiotics, and tetanus prophylaxis. Should fear of compartment syndrome arise, compartment monitoring and expert surgical consultation are mandated, with fasciotomy only if the pressure is increasing measurably. Extensive debridement is no longer recommended. Serum sickness occurs about 10 days after the receipt of 5 or more vials of antivenin in almost all patients.
Coral snake venom is neurotoxic, and the onset of any neurologic symptoms in a patient with multiple punctures or scratches in the bite (the snake injects venom by chewing) requires specific antivenin (after skin testing), with 3 to 5 vials given intravenously. This may be repeated if symptoms progress. The patient should be observed for at least 48 hours, even in the absence of symptoms, because the onset of toxin effects may be delayed.
Nonvenomous snakes can transmit anaerobic and enteric bacteria, and their bite wounds require careful cleaning and cultures. Antibiotics may be needed if the bites are infected, although rates of infection are low and prophylaxis with antibiotics is not necessary. Tetanus immunization or booster should be considered.
Gila monsters (Heloderma species) are the only venomous lizards in the United States and are found in the Southwest. Bites are rare, but the lizard is bulldog-like in the tenacity of its grip. In 70% of bites, a serotonin-containing toxin is injected, causing local edema, tenderness, lymphangitis, and lymphadenitis. Occasionally, the victim experiences weakness, faintness, dizziness, sweating, nausea, vomiting, shock, or coagulopathy. Treatment is supportive.
Information and consultation on venomous bites may be obtained by calling the Antivenom Index in Tucson, Arizona (602-626-6016).
The number of sports divers and snorkelers is increasing, and injuries caused by reef and beach creatures are correspondingly more common. Marine injuries and envenomation caused by simple contact are outlined in Table 58.2.
TABLE 58.2. MARINE CONTACT ENVENOMATIONS
Many species of fish and rays carry toxic barbs on their tails (e.g., stingray) or fins (e.g., catfish, lionfish, scorpionfish, weaverfish, stonefish, toadfish, ratfish, rabbitfish, leatherback, some sharks). Injury is by deposition of the barb and venom, or the venom alone, leading to local injury or destruction of neural or cardiac tissue, which can be lethal. The local site should be cleaned and soaked in warm [115°F to 120°F (46°C to 48°C)] water for 30 to 60 minutes to inactivate protein toxins. Support and surgical removal of the barb might be required. Antivenins are available for some species. Superinfection with Vibrio or Aeromonas species can occur.
A bite from the horny beak of the blue-ringed octopus of the Pacific, from Australia to Japan, releases a mix of tetrodotoxin, histamine, serotonin, and other poisons into the wound, causing numbness, paralysis, hypotension, and respiratory failure. Another dangerous species, found in the Caribbean, is Octopus joubini. Because venomous octopi are small, no octopi less than 6 in. in length should be handled without protective gloves. There is no antivenin.
Numerous species of venomous sea snakes live in Indo-Pacific waters. Bites, although rarely lethal, can cause death by rhabdomyolysis and hyperkalemia. Antivenin is available for those associated with extreme toxicity.
The most common mammal bites are those of dogs, cats, humans, and rats. When dogs bite, infection rates reportedly range from 3% to 18%. Cultures may show polymicrobial aerobic and anaerobic organisms, including Pasteurella canis, P. multocida, Staphylococcus, Streptococcus, Corynebacterium, and Capnocytophaga canimorsus (which in sepsis mimics a Shwartzman reaction), as well as many other. Tetanus, rabies, and mycobacterial disease rarely are transmitted by dog bite in the United States. Cat bites become infected 28% to 80% of the time. Pasteurella multocida is the major pathogen; it may cause local injury, such as cellulitis, osteomyelitis, septic arthritis, or tenosynovitis, or it may lead to systemic disease with bacteremia, especially in patients with liver disease. Other pathogens include Erysipelothrix, Streptococcus, Staphylococcus, Moraxella, and many others. Cat scratches also can carry P. multocida, and both bites and scratches can lead to cat-scratch fever (see Chapter 292). Human bites typically occur on the knuckles during fistfights, and the potential for both direct injury and infection is high. Organisms implicated in human bite infections include Streptococcus, Staphylococcus, Corynebacterium, Bacteroides, and Eikenella corrodens. Anaerobes are common. Syphilis, hepatitis B, and, rarely, rabies have been reported from human bites, but no cases of the acquired immunodeficiency syndrome have been reported. Rat bites can transmit rat-bite fever and hemorrhagic fever caused by Hantaan virus.
In evaluating bites, initial steps are careful examination for bone or tendon injury, performance of aerobic and anaerobic cultures, meticulous wound irrigation with 1% povidone-iodine (Betadine), and categorization as low or high risk for the development of infection (Table 58.3). In low-risk wounds, antibiotics are unnecessary and suturing may be appropriate. High-risk wounds should be treated with antibiotics but not sutured. Empirical antibiotic therapy for bites should be directed against pasteurellae, streptococci, staphylococci, and anaerobes. A b-lactam antibiotic with b-lactamase inhibitor, and a second-generation cephalosporin with anaerobic activity, or penicillin plus first-generation cephalosporin, or clindamycin and fluoroquinolone, are recommended. In suspected rabies exposure, therapy includes rabies vaccine (1 mL in the deltoid on days 0, 3, 7, 14, and 28) combined with 20 IU rabies immunoglobulin per kilogram of body weight. As much as possible of the immunoglobulin should be infiltrated at the wound site and the residual given intramuscularly at a site distant from the wound. Bite victims should be considered for tetanus and hepatitis B prophylaxis, as well as rabies prophylaxis (see Chapter 318).
TABLE 58.3. BITE WOUND RISK FOR DEVELOPING INFECTION
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