APPROPRIATE ANTIVENOM DOSES FOR SIX TYPES OF ENVENOMATIONS CAUSED BY SNAKES IN TAIWAN

Six of the 15 species of venomous snakes found in Taiwan are responsible for most of the clinically significant envenomations in the country. These species are: Trimeresurus mucrosquamatus, Trimeresurus stejnegeri, Naja atra, Bungarus multicinctus, Deinagkistrodon acutus and Daboia russelii siamensis, which together can be subdivided into three groups based on their venom effects. Primary treatment consists of rapid administration of appropriate antivenoms. The present study aimed to identify a proper dose of antivenom for each snake group as well as to describe hemorrhagic, neurotoxic, and mixed effects of their venoms. A retrospective chart review identified 72 snakebite cases referred to an emergency department. Data on epidemiology, examination findings, snake identification, treatment, antivenom dose and complications were collected. After excluding 14 patients, data from 58 victims were analyzed. Most studied cases were male (86%). Significantly higher doses of antivenom were administered against neurotoxic envenomations (mean dose: three vials) compared with the other two (p < 0.05). Moreover, patients affected by neurotoxic bites were more likely to develop blurred vision and other complications (p < 0.05). Multivariate logistic regression analysis indicated that neurotoxic envenomation was a risk factor for complications (OR: 8.84, 95% CI: 1.06-73.73). Neurotoxic envenomations and complication occurrence were positively correlated with antivenom dosage. In conclusion, patients affected by neurotoxic envenomations received higher doses of antivenom than others whereas incidence of complications was associated with higher antivenom doses.


INTRODUCTION
Taiwan, located in the South Pacific, has a subtropical environment with more than 40 snake species; 15 of these are venomous snakes.Six of these 15 are responsible for most of the clinically significant snakebites (1).Two of these six species belong to the Elapidae family that includes cobra, coral and sea snakes, while the other four belong to the Viperidae family, vipers and adders.
These six species can be divided into three groups based on local or systemic clinical effects produced by their venoms, namely hemorrhagic, neurotoxic or mixed symptoms.The hemorrhagic venom causes disorders of the clotting cascade such as prolonged bleeding, primary fibrinolysis and disseminated intravascular coagulopathy (2,3).Snakes that present hemorrhagic venom are Trimeresurus stejnegeri (Taiwan bamboo viper), Trimeresurus mucrosquamatus (turtle-designed snake) and
The mixed envenomation manifests as a combination of neurotoxic and hemorrhagic effects previously described, as well as rhabdomyolysis and acute renal failure (5).

PATIENTS AND METHODS
Our medical center has a standard protocol for treatment of snakebite patients.A retrospective review was conducted based on records of the emergency department (ED) ranging from January 1993 to December 2002.Seventy-two patients who received medical treatment for snakebites were identified.Of them, 14 were unable to identify the snake species and were excluded.The following information of the remaining 58 patients was observed: • basic epidemiological characteristics including age, gender, the time of the bite, bitten area of the body, season when the bite occurred and clinical manifestations; • examination of snakebite evidence (fang marks); • snake identification based on matching photographs, description, envenomation symptoms or the dead animal; • associated therapy including laboratory tests, antivenom administration, supportive care, debridement, respiratory support; This study was approved by the Institutional Review Board, Kaohsiung Medical University, protocol KMUH-IRB-960302.

Definitions
Each complication was defined according to associated venom effects.

Antivenom
Antivenoms employed in the treatment were produced by the Center for Disease Control, Department of Health, Taipei, Taiwan.Antivenoms were prepared by immunizing horses after snake envenomations.Each vial of antivenom contains over 1,000 Tanaka units (mean, 1200 TU).Each unit can neutralize one minimum lethal dose (MLD) of 12 to 14 g of mouse body weight (6).A skin test dose of 0.1 mL of a 1:100 solution was intradermally injected in the forearm of the patient.Epinephrine was available prior to test dose injection.Patients were given diphenhydramine and corticosteroids for evidence of allergic reaction.Before the administration, the antivenom was diluted in 300 to 500 mL of normal saline and infused over 30 to 60 minutes.

Statistical Analysis
The Kruskal-Wallis test was used for baseline comparisons of continuous variables, i.e., age and antivenom dosage.Chi-square and Fisher's exact tests were employed for analyses of categorical variables between risk factors and venomous types.
Logistic and linear regression analyses were employed to establish a model for predicting the probability of complications and dosage of antivenom, respectively.SAS 9.0® (SAS Institute Inc., USA) was utilized to perform statistical analysis; p < 0.05 was significant.

Epidemiological and demographic characteristics of patients bitten by venomous
snakes are presented in Table 1.The majority of the victims were male (86%), which was statistically significant (p < 0.05).More than half of the patients bitten by Viperidae and Elapidae serpents were attacked near their homes, in suburban areas.
Fingers and palms were more frequently affected by Viperidae (hemorrhagic venom) and Elapidae snakes (neurotoxic venom), while toes and the lower leg were more commonly bitten by Viperidae (mixed venom).However, the locations of bitten areas were no statistically significant difference among the three venomous groups (Table 1).
Local pain and swelling accounted for more than 50% of patients' complaints on the arrival at the ED in the three groups.Blurred vision was predominantly found among persons bitten by Elapidae snakes (neurotoxic venom) (p < 0.05), otherwise, there was no significant difference in symptoms among the three groups (Table 1).
Further analysis using a univariate logistic regression model indicated important correlations among venom complications and venomous types, respiratory distress and duration of treatment (

DISCUSSION
Roughly, one quarter of all the snake bite patients could not confirm the species of snake that had attacked them.Hung (1) found that of the 282 cases he studied, 50 patients were unable to identify the snake species that bite them.This may have clinical significance, as it could delay the antivenom administration.Based on this retrospective review, patients bitten by snakes of the family Elapidae (neurotoxic venom) required higher antivenom doses and had greater chances for complications, even after antivenom therapy.We found similar epidemiological patterns in previous studies including male predominance (86%), bites on distal extremities of the body (98%) and a large incidence of attacks occurring near the individual residence (57%) (3,4,(7)(8)(9).Correct antivenom administration remains the mainstay of therapy, with a suggested elapsed time from the moment of the bite to the antivenom administration of six to eight hours (10).Most patients (72%) in our study received antivenom therapy within six hours from the time they were bitten.
The exact antivenom dose required remains a matter of discussion.Yeung et al. (11) in a study of 35 brown snake envenomations in Australia found that initial antivenom doses were very small, leading to a continued circulation of venom.Paul et al. (2) conducted a study with 100 patients bitten by snakes from three venom groups (hemorrhagic, neurotoxic and mixed).They further divided the patients into two groups; the first was treated with high doses of antivenom and the other group with low doses.There was no statistical difference in clinical outcomes between the two groups (2).
Antivenom can present adverse effects ranging from local skin reactions to severe anaphylaxis and death.Additionally, delayed reactions that include serum sickness may occur.Therefore, the goals of antivenom treatment comprise administration of an appropriate dose of antivenom to neutralize the toxin, avoidance of complications related to the envenomation and prevention of additional complications (12,13).
In our study, the median dose of antivenom administered for the family Viperidae (hemorrhagic venom) was one vial and the complication rate was 7.14% (2/28), suggesting that the dose of antivenom may be suitable for most patients.Two of these victims had coagulopathy with compartment syndrome and required fasciotomies.Three victims were bitten by D. acutus and received antivenom for T. stejnegeri and T. mucrosquamatus since the antivenom against these two species presents weak cross-reactivity with D. acutus.
Complications consisted of tissue necrosis, compartment syndrome, coagulopathy, respiratory failure and acute renal failure.Coagulopathy and compartment syndrome were consequences of hemorrhagic venoms.Respiratory failure and tissue necrosis or fasciitis requiring debridement or fasciotomy were complications of neurotoxic venoms.Coagulopathy and acute renal failure resulted from mixed venoms.Coagulopathy was defined as a prolonged prothrombin time (PT), activated partial thromboplastin time (aPTT) or thrombocytopenia.Compartment syndrome comprised excessive pain with passive stretching, decreased sensation of pinprick or light touch, decreased discrimination of two points or necessity of fasciotomy.Measurement of intracompartmental pressure in affected limbs is required in an ideal situation; however, physical findings described above may substitute it in emergency circumstances.Acute renal failure was defined as a 50% decline in creatinine clearance or a 50% increase in serum creatinine from baseline.Acute respiratory failure was defined as arterial oxygen tension < 60 mmHg, arterial oxygen saturation Chieh-Fan C et al.Appropriate antivenom doses for six types of envenomations caused by snakes in Taiwan.J Venom Anim Toxins incl Trop Dis.2009;15(3):482 < 90%, or arterial carbon dioxide tension > 45 to 55 mmHg.

Table 1 .
Epidemiological characteristics of 58 patients bitten by venomous snakes who received antivenom treatment in the medical center of Taiwan, from January 1993 to December 2002 Chi-square test for gender, location, biting position, complaint and complications.Number of subjects for data analysis of variables does not correspond to the total number of subjects because of more than one complaint from each subject.* p < 0.05.
* Data are presented as mean for age and dosage of antivenom and number (percentage) for other variables.a Viperidae family with hemorrhagic venom: T. stejnegeri, T. mucrosquamatus, D. acutus; Elapidae family with neurotoxic venom: N. atra, B. multicinctusmulticinctus; and Viperidae family with mixed venom: V. russellii.b Kruskal-Wallis for age and dosage of antivenom.c d

Table 2 .
Logistic regression analysis of complications derived from 58 cases of venomous snake bites, the medical center of Taiwan, January 1993 to December 2002

Univariate logistic regression Multivariate logistic regression a
a Adjusted for sex and age.b Dashes represent no 95% CI for reference groups.*p < 0.05.

Table 3 .
Linear regression analysis of the association between antivenom dose and effectors in 58 patients who received treatment in the medical center of Taiwan, from January 1993 to December 2002 a Adjusted for sex and age.b Dashes represent no p value for the reference groups.* p < 0.05.