BMJ  2004;329:1129 (13 November), doi:10.1136/bmj.329.7475.1129

Paper

Crotaline snake bite in the Ecuadorian Amazon: randomised double blind comparative trial of three South American polyspecific antivenoms

¹ Hospital Vozandes del Oriente, Shell, Pastaza, Ecuador, ² Alistair Reid Venom Research Unit, Liverpool School of Tropical Medicine, Liverpool L3 5QA, ³ Ministry of Health, Guayaquil, Ecuador, ⁴ Fundacion Herpetologica Gustavo Orcés, Quito, Ecuador, ⁵ Nuffield Department of Clinical Medicine, University of Oxford, John Radcliffe Hospital, Headington, Oxford OX3 9DU

Abstract

Objective To compare the efficacy and safety of three polyspecific antivenoms for bites from pit vipers.

Design Randomised double blind comparative trial of three antivenoms.

Setting Shell, Pastaza, southeastern Ecuador.

Participants 210 patients with incoagulable blood were recruited from 221 consecutive patients admitted with snake bite between January 1997 and December 2001.

Intervention One of three antivenoms manufactured in Brazil, Colombia, and Ecuador, chosen for their preclinical potency against Ecuadorian venoms.

Main outcome measures Permanent restoration of blood coagulability after 6 and 24 hours.

Results The snakes responsible for the bites were identified in 187 cases: 109 patients (58%) were bitten by Bothrops atrox, 68 (36%) by B bilineatus, and 10 (5%) by B taeniatus, B brazili, or Lachesis muta. Eighty seven patients (41%) received Colombian antivenom, 82 (39%) received Brazilian antivenom, but only 41 (20%) received Ecuadorian antivenom because the supply was exhausted. Two patients died, and 10 developed local necrosis. All antivenoms achieved the primary end point of permanently restoring blood coagulability by 6 or 24 hours after the start of treatment in > 40% of patients. Colombian antivenom, however, was the most effective after initial doses of 20 ml (two vials), < 70 ml, and any initial dose at both 6 and 24 hours. An initial dose of 20 ml of Colombian antivenom permanently restored blood coagulability in 64% (46/72) of patients after 6 hours (P = 0.054 compared with the other two antivenoms) and an initial dose of < 70 ml was effective at 6 hours (65%, P = 0.045) and 24 hours (99%, P = 0.06). Early anaphylactoid reactions were common (53%, 73%, and 19%, respectively, for Brazilian, Colombian, and Ecuadorian antivenoms, P < 0.0001) but only three reactions were severe and none was fatal.

Conclusions All three antivenoms can be recommended for the treatment of snakebites in this region, though the reactogenicity of Brazilian and Colombian antivenoms is a cause for concern.

Introduction

Each year in Ecuador 1200-1400 cases of snake bite are reported in 19 of the 21 provinces. In "El Oriente," east of the Andes, the principal venomous species are Bothrops atrox (common lancehead) and B bilineatus smaragdinus (two striped forest pit viper, also known as Bothriopsis bilineata smaragdina). B taeniatus (speckled forest pit viper, also known as Bothriopsis taeniata), the less common Bothrops and Bothrocophias species and Lachesis muta (bushmaster) cause some bites.^{1 2} People involved in farming and logging are all at risk of snake bite. B atrox is the leading cause of severe snake bites in the area. Envenoming by B bilineatus is usually less severe. Other species such as B brazili and L muta, although potentially as dangerous as B atrox, rarely bite people.²

In an earlier preclinical laboratory study, the Brazilian antivenom proved the most effective in neutralising the venoms of B atrox, B asper, and B xanthogrammus, followed by the Ecuadorian and two Colombian antivenoms.^{3 4} We selected the three most effective antivenoms for a double blind randomised comparative clinical trial in Ecuador.

The main lethal effect of the venoms of these crotaline (rattlesnake-like) pit vipers is intracranial or gastrointestinal haemorrhage resulting from vascular endothelial damage, platelet dysfunction, and consumption coagulopathy. These antihaemostatic disorders and their reversal by specific antivenoms are reflected by whole blood coagulability which is easily assessed at the bedside using a simple but sensitive 20 minute whole blood clotting test.^{5 6} We used this test to measure the efficacy of the three antivenoms in restoring whole blood coagulability and observed early reactions to assess safety.

Methods

Participants—Between January 1997 and January 2002, all patients who presented with a history of snake bite at any time during the day or night to Hospital Vozandes del Oriente, Shell, Pastaza, Ecuador, were considered for the study, unless they had received antivenom within the previous 6 hours. Patients were included if the blood clotting test showed that their blood would not coagulate. All patients were admitted to the hospital and kept under observation for at least 48 hours. History, physical examination, and other relevant details were recorded on standard proformas. All members of the medical team were involved in the study, providing 24 hour cover.

Two striped forest pit viper (Bothrops bilineatus smaragdinus, also known as Bothriopsis bilineata smaragdina) from Napo Province, Ecuador, within the catchment area of this study. As many as 36% of our patients had been envenomed by this species Credit: D A WARRELL/FUNDACIÓN HERPETOLÓGICA GUSTAVO ORCËS, QUITO

 

Antivenom treatment and randomisation—We randomised patients in blocks of six to treatment with one of the three antivenoms (see bmj.com for details of the antivenoms). We calculated we would need to recruit 100 patients to each arm of the trial. After the 140th patient, the supply of Ecuadorian antivenom ran out, but strict randomisation was maintained to either Brazilian or Colombian antivenom. Medical staff treating and assessing the patient and the patient were blinded to the antivenom used. Most patients received an initial dose of 20 ml (two vials) of antivenom by slow intravenous injection over 10 minutes. A minority, considered on admission to be severely envenomed, were given higher initial doses of up to 70 ml (seven vials). Six hours after the start of antivenom treatment, we checked blood coagulability using the 20 minute clotting test. If the blood was still not coagulable, we administered a second dose of 20 ml of antivenom. Further doses were given at every six hours until blood coagulability was restored permanently.

General treatment—Early anaphylactoid reactions were treated with subcutaneous adrenaline (epinephrine) and intravenous diphenhydramine and hydro-cortisone. Pain was treated with oral paracetamol, intravenous pethidine, or tramadol. Patients received fresh blood if their packed cell volume fell below 20%, and routine tetanus prophylaxis was given. Local necrosis was treated by immediate surgical debridement, gentamicin, and chloramphenicol.

Snakes responsible for the bites—Dead snakes brought to the hospital were labelled and formally identified.

Laboratory investigations—Venous blood was sampled on admission, at 6, 12, 18, and 24 hours, and then daily until patients were discharged from hospital. We froze residual plasma/serum from the blood clotting test for detection of specific venom antigen by enzyme immunoassay. Enzyme immunoassays were developed for five different venoms (B atrox, B bilineatus smaragdinus, B taeniatus, B brazili, and L muta) (see bmj.com).

Statistical analysis—We compared the efficacy of antivenoms and reaction rates and determined the relation between the dose of antivenom required and the initial serum concentration of venom antigen.

Results

Randomisation and clinical features
We recruited 210 patients with incoagulable blood. Eighty seven patients (41.4%) received Colombian INS antivenom, 82 (39%) received Brazilian antivenom, but only 41 (19.5%) received Ecuadorian antivenom (table 1). The groups of patients receiving the three different antivenoms were similar in all respects on admission to hospital and before they received treatment.

View this table: [in this window] [in a new window]   Table 1 Comparison of groups of patients bitten by snakes in Ecuador, at randomisation before treatment with antivenom*

 

Snakes responsible for bites
Twenty nine patients (14%) brought in the snake responsible for the bite. With the help of herpetologists and the results of enzyme immunoassay we identified the snake responsible for envenoming in 187 (89%) cases. The distribution of bites by the different species was similar among the three groups (table 1).

Venom antigen detection
We assessed the serum concentrations of venom antigen from admission to discharge in 148 patients out of the 180 who received an initial dose of 20 ml of antivenom (samples from 32 patients were lost during transport). Concentrations on admission correlated significantly with the total volume of antivenom required to restore blood coagulability permanently (P < 0.0001) (fig 1). The time to venom clearance was also longer in patients who required higher doses of antivenom (fig 2).

View larger version (19K): [in this window] [in a new window]   Fig 1 Relation between median serum concentrations of venom antigen on admission in patients requiring treatment with 20 ml (two vials), 40 ml (four vials), 60 ml (six vials), and >60 ml of antivenom

 

View larger version (29K): [in this window] [in a new window]   Fig 2 Clearance of venom antigen from 148 patients treated with one dose (venom antigen completely cleared by all three antivenoms within 6 hours of start of antivenom); two doses (venom antigen completely cleared from circulation within 12 hours after start of antivenom in patients whose coagulopathy did not respond to initial 20 ml dose); and three doses (venom antigen finally completely cleared from circulation within 18 hours after start of antivenom in patients whose coagulopathy did not respond to first two 20 ml doses)

 

Efficacy of antivenom treatment
Two patients died. A 4 year old boy treated with Brazilian antivenom and a 38 year old woman treated with Colombian antivenom died 11 hours and 3 days, respectively, after being bitten by B atrox. Both developed acute pulmonary oedema. Ten patients (5% in each treatment group) developed local necrosis. All other patients were well on discharge from hospital.

Table 2 gives details of the comparative efficacy of the antivenoms. One hundred and eighty patients were treated with an initial dose of 20 ml (two vials). More patients who received this dose of Colombian antivenom had their blood coagulability restored at 6 hours compared with those who received Brazilian and Ecuadorian antivenoms. Colombian antivenom also proved superior when the initial dose was < 70 ml and after any initial dose of antivenom (table 2). Eighty one patients (45%) required more than one dose of antivenom, including 11 patients whose blood became incoagulable again after a normal result from the whole blood clotting test at 6 hours. There was no difference in median total doses of the three antivenoms.

View this table: [in this window] [in a new window]   Table 2 Permanent restoration of blood coagulability after various initial doses of antivenom at 6 and 24 hours after start of treatment

 

Therapeutic concentrations of antivenom
Of the 103 patients in whom we measured serum concentrations of therapeutic antivenom, 63 had received an initial dose of 20 ml with or without subsequent doses. Antivenom was detectable in the serum when serum venom antigenaemia had become undetectable and for at least 48 hours, even in those who had received only a single dose of antivenom.

Antivenom reactions
Early reactions to antivenom were common, including rash, vomiting, abdominal pain, fevers and chills, pruritis, and, more seriously, dyspnoea and hypotension. In those receiving an initial antivenom dose of 20 ml, reaction rates were 19% (7/37) in those receiving Ecuadorian antivenom, 73% (56/71) for Colombian, and 53% (37/70) for Brazilian antivenom. These rates were significantly different from each other (P < 0.0001). Two patients who developed hypotension had been treated with Colombian antivenom, one with Brazilian antivenom, and one with Ecuadorian antivenom.

Discussion

The primary end points of our trial were the resolution of coagulopathy at 6 and 24 hours after the initial dose of antivenom. We chose the 6 hour time interval because of the frequently reported observation that, in envenomed patients, blood coagulability is usually restored within 3-6 hours of the administration of a dose of antivenom sufficient to neutralise circulating procoagulant toxins (see bmj.com). If an initial two vials (20 ml) of antivenom failed to correct the coagulopathy within 6 hours, a further two vials were given. Judged by this criterion, Colombian antivenom proved the most effective.

We could not detect any difference in the ability of the three antivenoms to eliminate or reduce local effects of envenoming. Only 10 patients (5%) developed local necrosis, the main cause of persistent morbidity in those who survive severe envenoming. The pathophysiology of local envenoming may involve direct and indirect inflammatory mechanisms that are independent of neutralisation of venom toxins mediated through antivenom.^7-10

The high incidence of symptoms of early anaphylactoid reactions (100/178, 56%) was reminiscent of a study in Brazil in which reaction rates as high as 84% were recorded.¹¹ In other studies, early reaction rates ranged from 3% to 54%.^{12 13 14} The higher reactogenicity of Colombian antivenom (73%) may reflect its higher protein content (61 mg/ml) compared with the two other antivenoms (Ecuadorian 47 mg/ml, Brazilian 44 mg/ml)³ and the fact that it is a whole IgG ammonium sulphate precipitated preparation, whereas the other two antivenoms are pepsin digested F(a)₂ fragments.³ Severe reactions were rare.

B atrox and B bilineatus smaragdinus are responsible for most snake bites in the Pastaza region. B atrox is notorious as the leading cause of severe snake bites wherever it occurs in South America.^2-15 In contrast, B bilineatus, a distinctive bright green arboreal snake, is less well known as a snake of real medical importance.¹⁶ In this study 36% of our patients were bitten by this species and 6% by B taeniatus, B brazili, or L muta.

Conclusions
All three of the tested antivenoms proved clinically adequate in eastern Ecuador. Colombian antivenom, however, was the most effective judged by speed and efficiency in permanently correcting venom induced coagulopathy. The Ecuadorian antivenom was significantly less likely than either of the other two antivenoms to cause early anaphylactoid reactions. We recommended that the Ecuadorian Ministry of Health should increase production of this antivenom or, failing that, import either Colombian or Brazilian antivenoms to improve the treatment of snake bite envenoming in the Ecuadorian Amazon region.

What is already known on this topic Most studies on effectiveness of antivenom in snake bite have been carried out by estimating neutralising potency in experimental animals Few well designed clinical trials have been carried out on the treatment of human snake bite in the rural tropics What this study adds A simple, accurate, and robust whole blood clotting test can be used to assess the ability of three antivenoms (Brazilian, Colombian, and Ecuadorian) to eliminate venom induced coagulopathy permanently Enzyme immunoassay enabled the accurate identification of the snake species responsible for envenoming Although the Colombian antivenom proved the most effective, all three antivenoms proved suitable for treating snakebite victims in this region The Brazilian and Colombian antivenoms caused a high incidence of anaphylactoid reactions. This should prompt their manufacturers to improve production procedures

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This is an abridged version; the full version is on bmj.com

We thank the staff (medical, administrative, nursing, and technical) of Hospital Vozandes del Oriente, Shell, Pastaza; I Hastings (statistical advice) and A Richards (technical help), Liverpool School of Tropical Medicine; and the Ecuadorian Ministry of Health. We thank Philip Cooper, Hospital Vozandes, Quito, who arranged the transport of specimens from Ecuador to Liverpool.

Contributors: See bmj.com

Funding: European Union (contract No ERBIC18-CT96-0032). The Instituto Butantan (San Paulo, Brazil), the Instituto Nacional de Salud (Bogota, Colombia), and the Instituto Nacional de Higiene y Medicina Tropical (Guayaquil, Ecuador) donated the antivenoms.

Competing interest: None declared.

Ethical approval: Ecuadorian Ministry of Health and Hospital Vozandes del Oriente, Shell, Pastaza, Ecuador.

References

1. Touzet JM. Mordeduras de ofidios venenosos en la comunidad de los indigenas Siona-Secoya de San Pablo de Kantesyia y datos sobre la fauna de reptiles y anfibios locales. Publicaciones Museo Ecuatoriano de Ciencias Naturales 1986;5: 163-90.
2. Warrell DA. Snakebites in Central and South America: epidemiology, clinical features, and clinical management. In: Campbell JA, Lamar WW, eds. The venomous reptiles of the western hemisphere. Vol 2. New York: Cornell University Press, 2004: 709-62.
3. Theakston RDG, Laing GD, Fielding CM, Freire Lascano A, Touzet J-M, Vallejo F, et al. Treatment of snake bites by Bothrops species and Lachesis muta in Ecuador: laboratory screening of candidate antivenoms. Trans R Soc Trop Med Hyg 1995;89: 550-4.[Medline]
4. Campbell JA, Lamar WW. The venomous reptiles of the western hemisphere. New York: Cornell University Press, 2004.
5. Warrell DA, Davidson NMcD, Greenwood BM, Ormerod LD, Pope HM, Watkins BJ, et al. Poisoning by bites of the saw-scaled or carpet viper (Echis carinatus) in Nigeria. Q J Med 1977;46: 33-42.[Medline]
6. Sano-Martins IS, Fan HW, Castro SCB, Tomy SC, França FOS, Jorge MT, et al. Reliability of the simple 20 minute whole blood clotting test (WBCT20) as an indicator of low plasma fibrinogen concentration in patients envenomed by Bothrops snakes. Toxicon 1994;32: 1045-50.[Medline]
7. Chaves F, Barboza M, Gutierrez JM. Pharmacological study of edema induced by venom of the snake Bothrops asper (terciopelo) in mice. Toxicon 1995;33: 31-9.[Medline]
8. Farsky SHP, Walber J, Coista-Cruz M, Cury Y, Teixera CF. Leukocyte response induced by Bothrops jararaca crude venom: in vivo and in vitro studies. Toxicon 1997;35: 185-93.[Medline]
9. Trebien HA, Calixto JB. Pharmacological evaluation of rat paw edema induced by Bothrops jararaca venom. Agents Actions 1989;26: 292-300.[Medline]
10. Laing GD, Clissa PB, Theakston RDG, Moura-da-Silva A, Taylor MJ. Inflammatory pathogenesis of snake venom metalloproteinase-induced skin necrosis. Eur J Immunol 2003;33: 3458-63.[Medline]
11. Cardoso JLC, Fan HW, França FOS, Jorge MT, Leite RP, Nishioka SA, et al. Randomized comparative trial of three antivenoms in the treatment of envenoming by lance-headed vipers (Bothrops jararaca) in São Paulo, Brazil. Q J Med 1993;86: 315-25.[Web of Science][Medline]
12. Fan HW, Marcopito LF, Cardoso JLC, França FOS, Malaque C, Ferrari RA, et al. Sequential randomised and double blind trial of promethazine prophylaxis against early anaphylactic reactions to antivenom Bothrops snake bites. BMJ 1999;318: 1451-3.[Abstract/Free Full Text]
13. Malasit P, Warrell DA, Chanthavanich AP, Virivan C, Mongkolsapaya J, Singhthong B, et al. Prediction, prevention and mechanism of early (anaphylactic) antivenom reactions in victims of snake bites. BMJ 1986;292: 17-20.
14. Otero R, Gutierrez JM, Nunez V, Robles A, Estrada R, Segura E, et al. A randomised double-blind clinical trial of two antivenoms in patients bitten by Bothrops atrox in Colombia. Trans R Soc Trop Med Hyg 1996;90; 696-700.[Medline]
15. Pardal PP, Souza SM, de Cassia da Costa Monteiro MR, Fan HW, Cardoso JLC, França FOS, et al. Clinical trial of two antivenoms for the treatment of snake bites in the eastern Amazon region of Brazil. Trans R Soc Trop Med Hyg 2004;98: 28-42.[Medline]
16. Silva-Haad JJ. Las serpentes del género Bothrops en la Amazonia colombiana. Comando Unificado del Sur, Amazonia 1982;82: 45-50.

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