Despite its prevalence and seriousness, snakebite envenoming remains a neglected tropical disease (NTD).
Dr Amy Marriott and Dr Stuart Ainsworth from the Centre for Snakebite Research and Interventions at the Liverpool School of Tropical Medicine, UK, are addressing vital issues in regulations and testing.
Read more in Research Features
Read the original research: doi.org/10.1371/journal.pntd.0008579
Image Source: Adobe Stock Images / Pabitra
Hello and welcome to Research Pod! Thank you for listening and joining us today.
In this episode, we look at the new innovative research of Dr Amy Marriot and Dr Stuart Ainsworth from the Centre for Snakebite Research and Interventions at the Liverpool School of Tropical Medicine, UK on snakebite envenoming, which is classsed as a neglected tropical disease, or NTD for short. Ainsworth and Marriott are developing research techniques to improve the efficacy of antivenoms, and while doing so, are developing alternative models to substantially improve mouse welfare in antivenom pre-clinical experiments.
Humans and snakes have a long symbolic and cultural history. However, snakebite envenoming, which has plagued humanity since antiquity, still kills between 85,000 to 130,000 people and maims a further 400,000 each year. NTDs affect a significant proportion of the world’s population – but are underserved in terms of resources, research, and treatment needed to halt their devastating impact on extremely disadvantaged and socioeconomically vulnerable communities, especially prevalent in India, sub-Saharan Africa, Latin America, and southeast Asia. This common but devastating disease causes death, disability, deprivation, and destitution for hundreds of thousands of people each year.
Snakebite envenoming occurs when someone is bitten by a venomous snake. Snakebites are always accidental in nature and are most common amongst agricultural workers, who may disturb snakes while working in farmland, or in individuals walking at night – when snakes are harder to see and to avoid. Once injected, snake venom can cause a myriad of potentially lethal effects on the human body. In the aftermath of a bite, symptoms range from uncontrollable bleeding, tissue damage, paralysis, and ultimately death. The effects of envenoming can be localised – primarily affecting the bite site – or systemic, where the venom rapidly spreads throughout the body. Or, if you’re unlucky, both. Snakebite envenoming is therefore a medical emergency, and the time taken to receive medical treatment is critical in terms of positive outcomes. Once a victim is bitten, toxic effects can occur in as little as 30 minutes.
Research on treating snakebite envenoming has been miniscule compared to other NTDs. The only treatment currently available for snakebite envenoming is antivenom, which consists of antibodies purified from horses immunised with non-toxic quantities of venom. When given to a snakebite victim, the antibodies in the antivenom will bind and neutralise the potentially fatal effects of the venom’s toxins. As the World Health Organization, known as WHO, states: ‘Good-quality antivenoms can literally make the difference between life and death’ for snakebite victims.
Snake venoms and their toxins vary from species to species, meaning that different antivenoms are required for each variety of snake. This has made developing a safe and universally effective antivenom extremely difficult. The resources needed to develop, produce, store, and administer species-specific antivenoms mean that the costs of treatments are high for the populations most affected. Many snakebite victims are simply unable to afford the resulting huge medical bills that plunge them further into poverty. Unfortunately, even if victims can afford antivenom, the antivenom they receive may not be as effective as they hoped or even claimed by the manufacturer.
Like most other drugs, methods for testing an antivenom’s effectiveness and safety start with animal experiments using mice. However, unlike most other drugs, which will then go through phases of human clinical trials, antivenoms are unusual in that they completely rely on these animal experiments to predict their potential efficacy in humans.
Unfortunately, the mouse model is not 100% reliable in its ability to predict an antivenoms performance in humans. Marriott and Ainsworth explain that ‘the current model is very crude, relying on mixing venom and antivenom together and injecting it into a mouse. In very basic terms, if the animal survives, the therapy is considered effective.’ This approach, they say, ‘does not reflect real-world envenoming, as venom and therapy would never be premixed or injected directly’ and runs the risk of overestimating the effectiveness of an antivenom. The premixing of venom and antivenom also makes detailed assessments of the antivenom’s pharmacokinetic properties, something that is routinely assessed for other drugs, nearly impossible.
Naturally, questions remain as to the and reliability of antivenoms assessed in this way. Ainsworth and Marriott argue that while many antivenoms are effective, the limitations with the current model can result in ‘Ineffective antivenoms being licensed and reaching patients – ultimately resulting in unnecessary deaths.’
Importantly, Marriott and Ainsworth highlight the high cost of these experiments on mouse welfare: ‘The current model is outdated in terms of the large number of mice used to determine if an antivenom may be effective or not, and the substantial pain and distress the mice experience during the experiment.’ To combat this, the researchers want to bring the science of antivenom preclinical testing – both in terms of being able to accurately predict an antivenom’s effectiveness in humans and in terms of animal welfare – into the 21st century.
There is a general agreement on the need for a coordinated effort between government and research institutions and agencies, antivenom manufacturers, and international regulating bodies to tackle the issues. The current model has been used for many decades with little to no modification. ‘There has been attempts to improve the assay in terms of mouse welfare, for instance introducing painkillers which clearly make a difference, but still the model relies on a crude live/dead readout.’ These types of severe models are becoming substantially less common globally as researchers search for better or animal-free models. Fortunately, Marriott and Ainsworth are spearheading work to improve testing procedures for antivenoms – that will benefit both the patient and the animals involved in saving their lives.
Marriott and Ainsworth say that animal-free testing of antivenom is still some way off. ‘The current reality of antivenoms not having to go through human clinical trials means that regulators are likely to insist on animal models for assessing envenoming therapies in the foreseeable future.’ However, they say ‘This does not mean we should not strive to make the required animal testing as stress-free and painless for the mice as possible.’ The NC3Rs is a UK government funding agency whose remit is to fund research which will work towards the replacement, refinement, and reduction of animal use in research, and Marriot and Ainsworth recently obtained NC3Rs funding to develop a new model mouse of envenoming. Ainsworth explains, ‘We hope to use our knowledge of venom toxins and their modes of action to rigorously assess antivenoms, preventing ineffective therapies reaching snakebite victims, while using substantially fewer mice and subjecting them to the lowest possible level of suffering.’
The key to their new models is to use the smallest possible dose of venom which can induce a measurable change in biomarkers, such as blood fibrinogen levels or prothrombin time, which may also be used to clinically assess human snakebite victims and their response to antivenom. The use of assays and non-invasive monitoring equipment means a much more detailed and rigorous readout can be obtained versus the current binary and not very informative ‘alive or dead’ readout.
Importantly, while still in the early days of development, the welfare of the animals in the new models seem much improved compared to the current model. Marriot explains: ‘The mice receiving such small quantities of venom do not show any classical signs of pain or distress and outwardly behave no differently from control non-envenomed mice.’
All of this paves the way towards exciting alternative antivenom therapies, such as small molecule inhibitors and monoclonal antibodies, which are showing great promise However, Ainsworth notes ‘Unlike traditional snakebite therapies, these newer therapies will be required to undergo normal regulatory assessment, meaning rigorous and expensive human clinical trials’.
The current mouse model does not lend itself to supporting these clinical trials, as its readout does not have the resolution to distinguish between promising therapies and ones which might fail in the clinic – again, a process which is routine for most drugs under pharmaceutical development. Marriott and Ainsworth’s new models will hopefully allow better early identification of promising and not-so-promising candidates and thereby speed up the translation of exciting new therapeutics to clinical study in humans.
That’s all for this episode – thanks for listening. Links to Marriot and Ainsworth’s original research will be will be linked in the show notes below and, as always, stay subscribed to Research Pod for more of the latest science.
See you again soon.