Snake antivenoms: science, values and challenges
by Tan Choo Hock
Feared or loved, reviled or revered, the snake is a significant yet mystical subject throughout human civilisations. There are more than 3,000 snake species in the world. About 600 species of these are venomous and over 200 are considered medically important – their bites can cause envenomation, a condition characterised by the development of systemic or local toxicity due to the venom’s effect . Snakebite envenomation is prevalent in many tropical and subtropical countries, affecting mainly the poor rural populations. Worldwide, the annual mortality rate has been estimated to be around 100,000 deaths . Unfortunately, the persistent underreporting/underestimation of its true epidemiology has made it the most neglected disease condition, and ironic enough, this is further aggravated by its removal from the WHO-list of neglected tropical diseases in 2015.
Managing snakebite envenomation takes on multiple steps; antivenom remains the definitive and etiological therapy. Antivenom is perhaps one of the oldest “antibody-based treatments”, pioneered by Albert Calmette (recalling: BCG vaccine) in the late 19th century, which principle still applies today in the treatment of snakebites – while immunotherapy against infections (e.g. diphtheria) has basically been replaced by vaccination. Vaccination against snake venom is still controversial and has not been proven effective in the earlier trial. Antivenoms thus remain important, relevant, and life-saving to date. They are derived from animals (typically horses) that have been hyperimmunised with venom(s) of a single species (thus raising monovalent/monospecific antivenom) or several species (thus raising polyvalent/polyspecific antivenom). Current technologies process antivenoms in three main forms: whole immunoglobulin, the antigen-binding fragment (Fab), or the dimeric form F(ab’)2 . They act by binding to toxins in the venom, forming inactive immunocomplexes that can be eliminated through phagocytosis. Clinically, this neutralises the toxicity caused by venom, either by reversing or halting the progression of venom toxic effect.
Two all-time pertinent challenges surrounding the use of antivenom are: (1) the effectiveness or quality of antivenom; (2) the availability of antivenom . There is no universal antivenom or antidote, despite claims by some traditional medicine providers. The use of antivenom is species-specific to ensure effective treatment, and this is attributed to the vast variations in the composition of venoms from different or even a same species. Venom variations can manifest as differences in the subtypes of toxins, their relative abundances, peptide sequences and even the epitopes. The ramification of venom variation is far-reaching as it can lead to discrepancies in clinical presentation and syndrome progress, as well as suboptimal response to antivenom especially when the antivenom used was produced for a different species or from a distant country [3-5]. This explains the importance of species recognition in envenomation cases, as inappropriately administered antivenom would not only be futile in rescuing the victim but also increases the medical cost and exposing the patient to unnecessary risk of adverse effects of antivenom, for instance, hypersensitive reactions that can be fatal. Nevertheless, certain venoms share similar toxin profiles and hence may be cross-neutralised by a specific antivenom – for example, the monocled cobra antivenom manufactured in Thailand is effective in treating neurotoxic envenomation by the equatorial spitting cobra in Malaysia . For pit viper envenomations, venoms of the Malayan pit viper and the Asiatic lance-headed pit vipers of Trimeresurus complex , although exhibiting similar hemotoxic effects, have substantial antigenic differences in toxins that the respective specific antivenom does not cross-neutralise each other well, therefore warranting the use of different specific antivenoms .
On the other hand, antivenom manufacturing is highly costly and technically demanding. Several antivenom plants closed in recent years ostensibly for the high production cost and limited market demand by region. This further aggravates the already inadequate supply of antivenoms in many regions of the world. Note, although antivenoms are generally in high demand globally, unlike any other generic medicines, their formulations are limited by species and locality of concern, practically making each product some kind of an orphan drug – so, to produce or not to produce an expensive treatment to cater for a small market (limited by different snake species and geography) for consumers who are mostly poor – this is a serious but realistic question to answer. In Malaysia, the local antivenom production facility has long been closed down, and the country has been relying on imported antivenoms for the past few decades. This is a solution that many countries have adopted, however, the quality and effectiveness of these imported antivenoms, which were raised from different species (although may be closely related) from foreign lands, must be first rigorously tested at the laboratory level to provide insights into the suitability for use in local cases. In recent years, with the available emerging laboratory evidence and favourable clinical observations, Malaysia is moving towards the use of some antivenoms produced in Thailand for the common and closely related species shared by the two countries in close proximity [5-7]. Patients envenomed by some of the species in Malaysia can benefit from cross-neutralisation conferred by certain Thai antivenoms, due to the conserved antigenicity of principal toxins in the different lineages.
In this context, it should be noted that Professor Tan Nget Hong from the University of Malaya (UM) has led venom research in Malaysia for almost 40 years. Established recently, the Venom and Toxin Research Laboratory (VTRL, Faculty of Medicine, UM), which the author of this article is co-leading, has been continuously researching on snake venoms and antivenoms to unravel the complexity of snake venom toxins and the myth of venom-antivenom interactions. Findings that have been published include the transcriptomics and proteomics of several major or exotic species in the region [4, 8-11]. Coupled with functional, in vitro/in vivo characterisation of the venoms and their antivenom neutralisation profiles, the group is determined to provide detailed insights into the pathophysiology of envenomation and how treatment can be optimised – this aspect concerns the community most. Recently, the group has also identified some of the limiting factors in governing the immunogenicity of venom used in the production of antivenoms; research is now on-going to strategise an approach that can overcome the limitation of antivenom effectiveness [12-15]. It is hoped that the findings will be translated into a trans-border, pan-region collaborative research that will eventually seek the production of an affordable, “broad-spectrum” antivenom with high potency and multiple species coverage. In addition, the data generated by the group may be useful for future studies in drug discovery, biodiversity and wildlife conservation. Thus far, the group has unveiled the functional venom proteomes (with or without venom-gland transcriptomes) of several important lineages, including the monocled cobras (from Malaysia, Thailand and Vietnam), king cobra (Malaysia), hump-nosed pit viper and Russell’s viper (Sri Lanka), equatorial spitting cobra (Malaysia), beaked sea snake (Malaysia) and the exotic Malayan blue coral snake (Malaysia) [4; 8-11].
About the Author:
Dr. Tan Choo Hock (MBBS, PhD) is a Senior Medical Lecturer in Pharmacology from UM. His research interests span a wide range of topics in toxinology, including molecular and functional characterisation of venoms, antivenom pharmacology, and the “-omics” of venomous species. He enjoys passionately his research activities, from collecting samples in the wild to experiments conducted in the laboratory. He can be reached at [email protected]. Find out more about the author by visiting his profile at http://www.scientificmalaysian.com/members/tanchoohock/
This article first appeared in the Scientific Malaysian Magazine Issue 12. Check out other articles in Issue 12 by downloading the PDF version for free here: Scientific Malaysian Magazine Issue 12 (PDF version)
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