Snakebite Injuries Treated in United States Emergency Departments,

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1 Wilderness and Environmental Medicine, 18, (2007) ORIGINAL RESEARCH Snakebite Injuries Treated in United States Emergency Departments, Mary Elizabeth O Neil, MPH; Karin A. Mack, PhD; Julie Gilchrist, MD; Edward J. Wozniak, DVM, PhD From RTI International, Social and Statistical Sciences, Atlanta, GA (Ms O Neil); the Centers for Disease Control and Prevention, National Center for Injury Prevention and Control, Division of Unintentional Injury Prevention, Atlanta, GA (Dr Mack); the Centers for Disease Control and Prevention, National Center for Injury Prevention & Control, Atlanta, GA (Dr Gilchrist); and Texas A&M University Health Science Center, Institute for Biosciences and Technology, Houston, TX (Dr Wozniak). Objective. Venomous and nonvenomous snakes are found throughout most of the United States. While the literature on treatment is robust, there is not a current national epidemiologic profile of snakebite injuries in the United States. National estimates of such injuries treated in emergency departments (EDs) are presented along with characteristics of the affected population. Methods. Data on snakebite injuries were abstracted from the National Electronic Injury Surveillance System All Injury Program ( ). Variables included age, gender, body part affected, cause, disposition, and treatment date. When available, location, intentionality of the interaction, and snake species were coded based on narrative comments. Estimates were weighted and analyzed with SPSS Complex Samples. Results. An estimated 9873 snakebites were treated in US EDs each year between 2001 and Males were more frequently seen in the ED for snakebites than were females (males: 72.0% [95% confidence interval (CI), ]; females: 28.0% [95% CI, ]). Approximately 32% of patients were known to be bitten by venomous species. Overall, more than one quarter of patients were hospitalized (27.9% [95% CI, ]), although 58.9% of patients with known venomous bites were hospitalized (95% CI, ). Conclusions. While they are rare events, snakebites cause nearly visits to EDs for treatment every year. Epidemiologic data regarding snakebites provide practicing physicians with an understanding of the population affected and can help guide public health practitioners in their prevention efforts. Key words: snake, snakebite, venomous snake, envenomation, wounds and injuries, surveillance Introduction Snakebites from venomous snakes can be painful and may lead to disfigurement, permanent loss of normal limb function, and death. Even dry, nonenvenomating snakebites and those from nonvenomous snakes can result in puncture wounds that require medical evaluation. 1 The literature provides regional analyses of the affected population and an extensive discussion of treatment guidelines; however, there is little epidemiologic information at the national level. The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention. Corresponding author: Mary Elizabeth O Neil, MPH, Research Analyst, RTI International, Social and Statistical Sciences, 2951 Flowers Rd South, Suite 119, Atlanta, GA ( MONeil@rti.org). There are over 100 species of snakes in the United States, about 20 of which are categorized as venomous. 2 Two families of venomous snakes are indigenous to this country: the Viperidae, which includes rattlesnakes, cottonmouths (or water moccasins), and copperheads; and the Elapidae (coral snakes). Bites from captive nonindigenous venomous snakes, such as cobras, may also present at medical facilities given exotic pet practices in the United States. 3 5 The literature about snakebites lacks current national descriptions of such injuries treated in medical facilities. One frequently cited source derived estimates from a survey of selected hospitals and a random sample of physicians conducted in the late 1950s. Based upon survey responses, an estimated 6680 venomous snakebites were treated in inpatient and outpatient facilities annu-

2 282 O Neil et al ally. 6 Twenty years later, an additional 1000 bites were suggested to be unreported, resulting in an estimate of 7000 to 8000 total venomous snakebites per year in the United States. 7 This range of values is cited in recent works The American Association of Poison Control Center s Toxic Exposure Surveillance System reported 7000 telephone calls nationally for snakebites in 2004; 43% of these calls regarded venomous snakebites, 27% were for nonvenomous snakebites, and the remainder were unspecified. 11 According to a recent report of all noncanine bites and stings treated in US emergency departments (EDs), there were just over 3000 venomous snakebite injuries annually between 2001 and 2004; however, the report did not describe the affected population. 12 Regional and case reports of snakebites have used data from poison control centers and local or tertiary care facilities. 5,9,13 25 To provide a descriptive profile and national estimates of snakebite injuries treated in US EDs, an in-depth review in the National Electronic Injury Surveillance System All Injury Program (NEISS-AIP) was conducted. The demographics of the patients and, when possible, the snake species, the location of the interaction, and whether the interaction with the snake was intentional were reported. Methods NEISS-AIP is derived from a national, stratified, probability sample drawn from all US hospitals with 6 beds that provide 24-hour emergency service. NEISS hospitals were sampled with 5 strata, 4 of which were defined by hospital size (ie, small, medium, large, very large), based on the annual number of ED visits, plus 1 stratum for children s hospitals. This surveillance system is a collaborative effort of the Centers for Disease Control and Prevention, National Center for Injury Prevention and Control (Injury Center) and the US Consumer Product Safety Commission. This system provides continuous surveillance of ED-treated nonfatal injuries and allows researchers to follow national trends, evaluate interventions, and identify emerging injury problems. The system has been widely used in reports of intentional and unintentional injuries Trained data abstractors collect information on all types and external causes of nonfatal injuries treated in the NEISS-AIP EDs. Individuals are included in the surveillance system if they present for a first-time visit for a condition that receives an injury diagnosis after medical evaluation in the ED. Patients who are transferred to a NEISS-AIP hospital are not eligible to be included in the surveillance system and are not double counted because they do not qualify as a first-time visit. Repeat visits for injuries treated in the same or another ED are excluded from the surveillance system. Individuals whose reason for visit is pain or possible injury but who are not diagnosed as having an injury are also excluded. More detailed descriptions of the NEISS-AIP system have been reported in previously published reports. 32,33 NEISS-AIP data were analyzed for a 4-year period from 2001 to For this study, cases were defined as persons treated at an NEISS-AIP hospital for bite injuries in which the source of the bite was specified as a snake. Persons who were dead on arrival or who died in the ED were excluded, because mortality data are not captured completely by NEISS-AIP. Variables abstracted from the ED record included age, gender, body part affected, cause, disposition, treatment date, and a brief narrative description of the injury incident. Transferred cases were combined with cases of those hospitalized since both dispositions indicate a need for a higher level of care. The narrative descriptions were coded for the source of the bite. The source was abstracted from the narrative using text string queries, and then each comment line was read to verify that the correct one was coded. This verification was replicated twice. Intentionality of the interaction and location of the snake-biting incident were also abstracted from the comment line narrative description when possible. For example, Patient at home was bitten by a cottonmouth snake; patient caught the snake and was going to keep it when it bit him was coded as species venomous; intentionality intentional interaction, and location home. A sample weight is calculated for each injured person treated at a NEISS-AIP hospital based on the inverse probability of selection of that hospital. In addition, sample weights are adjusted for nonresponse and were poststratified to adjust for changes in the annual number of ED visits over time. 33 Rates were calculated using 2001 through 2004 bridged race population estimates from the US Census Bureau. 34 Injury estimates were identified as unstable if the national estimate was 1200, if the number of sample cases used was 20, or if the coefficient of variation (CV) was 30%, where CV (standard error/national estimate) Analyses were conducted with SPSS Complex Samples (2005 SPSS Complex Samples, SPSS Inc, Chicago, IL). Results Between 2001 and 2004, an estimated 9873 snakebite injuries were treated in US EDs annually; the crude rate of snakebites was 3.4 per persons (the Table). Venomous snakes accounted for 3188 bites, and 6684

3 Snakebite Injuries Treated in US EDs bites were from nonvenomous or unspecified snake types. The crude rate was highest for children aged 10 to 14 years, at 5.5 per population. Males were more likely to present in the ED with a snakebite than females (males: 72.0% [95% confidence interval (CI), ]; females: 28.0 [95% CI, ]). The extremities of the body, including the arms, hands, legs, and feet, were more likely to be treated for snakebites than were other body regions (arm/hand: 50.3% [95% CI, ]; leg/foot: 39.2% [95% CI, ]). Overall, two thirds of the patients were treated and released from the ED (68.6% [95% CI, ]). Most of the bites occurred between spring and early fall. The majority of the records did not indicate the species of snake; these were classified as unspecified snakebites (54.3% [95% CI, ]). Less than one fifth of the records specifically noted that the species was nonvenomous (13.4% [95% CI, ]). The nonvenomous snake species implicated included anacondas, black snakes (black racers or black rat snakes), boa constrictors, corn snakes, garter (garden) snakes, pythons, unspecified rat snake species, and unidentified nonvenomous snakes. None of these species was mentioned in sufficient numbers to report estimates. Approximately 30% of the snakes were identified as venomous species. Of these, rattlesnakes were the most often reported (22.4% [95% CI, ]), followed by copperheads (6.3% [95% CI, ]). Other venomous snakes (3.7% [95% CI, ]) included cottonmouths (water moccasins), coral snakes, and unspecified venomous snake species (values not reported in the Table). The location of the interaction indicated that a large proportion of the bites occurred in or around the home; 35.3% (95% CI, ) noted home as the location and another 11.2% (95% CI, ) indicated that the person was outside the house but in his/her yard. Numerous interactions occurred during outdoor recreational activities, such as hiking and camping (9.2% [95% CI, ]). Another 3.8% (95% CI, ) specifically mentioned that the snakebite occurred while the patient was fishing or golfing. Overall, few bites occurred at work (6.4% [95% CI, ]) and in other locations (5.0% [95% CI, ]), which included walking down the street or at school. However, when the bites were stratified on venomous/nonvenomous criteria, approximately 30% of the venomous snakebites occurred at work (the Table). Nearly one third of the narratives did not specify a location (29.1% [95% CI, ]). Most patients seen in the ED for venomous snakebites were hospitalized (58.9% [95% CI, ]), whereas none reportedly bitten by a nonvenomous snake were 283 hospitalized (see Table). When considering venomous snakebites only, the rate of ED visits shows the same gender distinctions (eg, for venomous snakebites only, 79.6% of injuries were sustained by males [95% CI, ] and 20.4% by females [95% CI, ]). Yet the percentage of males and females hospitalized was not significantly different (eg, for venomous snakebites only, 55.1% of males [95% CI, ] and 74.0% of females [95% CI, ] were hospitalized) (values not reported in the Table). Adults aged 19 years or older were significantly more likely to present for venomous or unspecified snakebites than were youth aged 18 years or younger. Most records (65.9%) did not provide enough detail to determine the nature of the interaction and could only be coded as unknown. Among the records that could be coded, 3.8% were the result of handling a pet or captive snake (95% CI, ; an estimated 375 bites annually), 6.8% involved an intentional interaction (95% CI, ), and roughly one quarter of the records were coded as unintentional interactions with the snake (23.4% [95% CI, ]). Of the patients in the unintentional interaction cases, 16.5% mentioned that the bite occurred at home (95% CI, ); 32.4% occurred outside the home in the yard (while the patient was engaging in activities such as gardening or mowing the lawn [95% CI, ]), 18.7% while the patient was outdoors (hiking or camping [95% CI, ]), and 10.7% while the patient was golfing or fishing. The unintentional interactions were nearly evenly distributed between the arm/hand or leg/foot (arm/hand, 41.6% [95% CI, ]; leg/foot, 50.3% [95% CI, ]). A little over half (53.5% [95% CI, ]) of the bites from intentional interaction occurred in or around the home. The bites from intentional interaction were largely to the arm/hand (91.6% [95% CI, ]). Discussion This report characterizes the nature of venomous and nonvenomous snakebite injuries treated in EDs in the United States. Results show that during 4 years, over persons were treated in EDs for snakebites. Annually, there are an estimated 3188 venomous and 6684 nonvenomous or nonspecified snakebites treated in EDs. These estimates do not include snakebites treated in outpatient facilities, those that were self-treated, or the approximately 5 deaths that occur annually from such bites. 35 The demographics of the affected population in this study are similar to those described in regional reports of snakebites treated in medical facilities. Males com-

4 Table. Demographic characteristics of emergency department (ED) patients treated for snakebites, United States, (n 9873) All Types 95% CI* Overall Crude Rate (per ) Total Venomous 95% CI Nonvenomous 95% CI Unspecified Age, y ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) Gender Male 72.0 ( ) ( ) 69.4 ( ) 68.2 ( ) Female 28.0 ( ) ( ) 30.6 ( ) 31.8 ( ) Primary body part affected Head, neck, or trunk Arm/hand 50.3 ( ) 47.0 ( ) 79.9 ( ) 45.0 ( ) Leg/foot 39.2 ( ) 33.8 ( ) ( ) More than 1 area or unknown 7.7 ( ) 18.2 ( ) ( ) Disposition Treated/released 68.6 ( ) 34.8 ( ) 99.8 ( ) 80.9 ( ) Hospitalized 27.9 ( ) 58.9 ( ) ( ) Observed, left without being seen, or unknown 3.6 ( ) Month of ED visit January March 5.7 ( ) ( ) April June 39.3 ( ) 39.3 ( ) 38.7 ( ) 39.5 ( ) July September 42.0 ( ) 41.6 ( ) 41.7 ( ) 42.3 ( ) October December 13.0 ( ) 14.5 ( ) Location Home 35.3 ( ) 29.5 ( ) 49.1 ( ) 35.3 ( ) Home, outside 11.2 ( ) 11.0 ( ) ( ) 95% CI continued 284 O Neil et al

5 Snakebite Injuries Treated in US EDs 285 Table. Continued Unspecified Nonvenomous Venomous Overall Crude Rate (per ) All Types 95% CI 95% CI 95% CI 95% CI* Outdoors (hiking, camping, other) 9.2 ( ) Golfing or fishing 3.8 ( ) Work 6.4 ( ) 30.1 ( ) ( ) Other (street, car, school) 5.0 ( ) 12.7 ( ) ( ) Unknown 29.1 ( ) ( ) *CI indicates confidence interval. Estimate may be unstable. posed 72% of the patient population, which is similar to results in other published work. 4,19,20 This gender distribution has also been found for most other injury mechanisms, including drowning, burns, and fire-related injuries. 36 The majority of persons treated for a snakebite injury in the ED were treated and released; as would be expected, however, among those treated for venomous snakebites, most were hospitalized. Similar to the results of other work, bites from intentional interaction were found to be primarily to the arms or hands. 8 Bites from unintentional interaction were equally likely to occur to the arm/hard or leg/foot regions of the body. It was also confirmed that snakebite injuries occurred most often between the spring and fall, corresponding with seasons of increased reptile activity and increased human outdoor activities. 19,20,24 Our study relied on self-reported identification of the snake species involved in the incident. Over half of snake species were not identified in the comment field (54.3 [95% CI, ]), as has been the case in other retrospective studies. 37 A study conducted in California found that the general public could accurately distinguish between venomous and nonvenomous snakes 81% of the time. 38 It should be noted, however, that all of the native venomous species in California are rattlesnakes and that the study participants were often unable to accurately identify the snakes to the species or subspecies level of taxonomic distinction (ie, California common kingsnake [Lampropeltis getulus californiae], Mohave rattlesnake [Crotalus scutulatus scutulatus], or Pacific rattlesnake [Crotalus viridis helleri, now C. oreganusu helleri]). These findings indicate that the self-reported categories of venomous vs nonvenomous snakes can be reliable for some, but perhaps not all, geographic regions. Of the identified venomous snakebites, 69% were reported to be inflicted by rattlesnakes; 20% by copperheads; and 11% by other unidentified venomous snakes. Regionally, the distribution of snake species will vary. One study in Arizona reported that among the identified snakebites treated in 5 hospitals, 98% of bites were inflicted by rattlesnakes. 20 Another study in central Missouri found that all the venomous snakebites treated were inflicted by copperheads, which are the most widely distributed and common venomous snake species in that region. 23 We could only determine the intentionality of the snake interaction in 34% of the cases. About a quarter of the interactions were coded as unintentional interactions (23.4%) and 6.8% as intentional (or provoked) interactions. A retrospective follow-up of patients treated in a level-1 trauma center found that 67% of snakebites were the result of intentional interactions and that 40%

6 286 O Neil et al of patients had consumed alcohol prior to the bite. 8 We were unable to code intoxication with a high degree of accuracy, although some narrative descriptions clearly indicated that intoxication was a factor at the time of the incident. Most unintentional exposures in our population occurred at home during activities such as gardening and yard work; similarly, other work found that 43% of patients were bitten by rattlesnakes in or around the house 20 and that 74% of copperhead bites occurred around the home. 16 The findings of this report are subject to limitations. First, only nonfatal injuries treated in hospital EDs were included. Injuries treated in health care facilities outside of an ED (eg, a physician s office or an urgent care center) or those for which no professional medical care was received were not included. Therefore, our estimates likely underestimate the total burden of snakebite injuries in the United States. Second, only national estimates can be made with NEISS-AIP. Regional differences in snake habitats will correlate with the distribution of snakebites. Morgan et al 39 reviewed 20 years of National Vital Statistics System data and found that 3 Southern states (Texas, Florida, and Georgia) accounted for 44% of venomous snake deaths. Other studies provide regional estimates of venomous snakebites. 5,9,13 16 Third, results were not presented by race or ethnicity because a high percentage of records did not specify race/ethnicity. Finally, we could not code the snake species for the majority of cases, and in the cases in which species was coded it should be cautioned that a layperson s ability to identify snake species is not always reliable. Snakebites can result in significant morbidity to patients and, in rare cases in the United States, death. 1,40 43 Recovery time from venomous snakebites can be substantial. 16 Future descriptive studies investigating snakebite occurrences, behaviors of individuals with snakebite exposures, and location in the community in which the snakebites occurred would help to further elucidate the circumstances surrounding bites and aid in developing prevention measures. For example, the majority of interactions in this study occurred at home during usual activities such as gardening and yard work. An investigation exploring the extent to which individuals living in areas with endemic species of venomous snakes are familiar with 1) identification and habits of the indigenous species, 2) the medical significance of snake bites, and 3) how to avoid contact/bites while performing outdoor activities would add to injury prevention knowledge. 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