The Problem

Snakebite is a grossly neglected tropical health complication that, in addition to causing death, it can also lead to disability and disfigurement.

It is so serious that the World Health Organization, since late September 2017 has raised it to a Category A neglected tropical disease after appeal from Costa Rica and 17 other member countries at the tenth meeting of the WHO Strategic and Technical Advisory group for Neglected Tropical Diseases [1]. The true impact of envenoming, the severity and the morbidity it carries with it, are unknown due to underreporting in almost all affected areas [2].

However, it is conservatively estimated that up to five million people are bitten every year by snakes. Out of these 5 million people, about 2.4 million are estimated to be envenomed, resulting in 94,000-125,000 deaths annually [3].

Additionally, it is estimated that about 400,000 of the envenomed patients suffer either amputation of limbs, or display other severe health consequences, such as renal failure, necrosis, spontaneous bleeding, panhypopituitarism, diabetes, chronic neurological deficits, deformity and amputation of limbs.

The majority of snake bites occurs in South- and South-East Asia, Africa and South America. They are more common in rural areas, inhabited by people that depend on farming and other field working occupations for subsistence.

Agricultural and plantation workers, women and children are the groups mostly affected by snake bites. In these resource poor settings, people often have limited or no access to healthcare or antivenom, which increases the severity of the injuries and their outcomes [3].

Moreover, the socioeconomic impact on families and communities is adding to the burden of these injuries. In many occasions, the victims are the wage earners or care providers of the family, and child victims that suffer disabilities caused by snake bites are in need of greater support throughout the rest of their lives from their families. This has further important implications for the nutrition, growth and economy of the countries.

Antivenom remains the most effective antidote for snake envenoming, but is expensive and in short supply. As a consequence, it is unpractical or unavailable to rural and underdeveloped countries to carry due to challenged public health systems or poor infrastructure.

Furthermore, the proper antidote to apply is not straight forward, as the snake responsible for the envenoming is long gone and identification of such snake species is not the specialty of the medical personnel at the clinics.

Hence, if the clinic does carry antivenoms, in the majority of cases, these are polyvalent antivenoms. These broad spectrum antivenoms, which are often necessary due to unknown source of venom, must be administered at very high doses to cover the many potential venom components. This increases the vials needed, sometimes up to 10 vials amounting to 1000 USD during the course of the treatment in certain cases. Making this course of treatment a hard sell for a victim without financial means or adequate national health coverage.

In addition to that, administration of antivenom comes with a high risk of side effects. Acute reactions to the treatment cause problems of equal clinical importance as the envenomings themselves. Up to 40% of the victims can exhibit severe systemic anaphylaxis, including hypotension and cyanosis. Short term sickness of pyrogenic endotoxin nature and serum sickness in the long term are common type reactions [4].

Hence, antivenom should only be used in patients where the risk of envenoming is higher than the risk of the antidote. This assessment should be carried out by competent medical personnel and ideally at a specialized hospital. Realistically at the rural clinic, where the patient is treated first.

Since time is of the essence in proper care of snake envenomation, we have endeavoured to develop a rapid diagnostic tool for the identification of the type of snake responsible in order to provide optimal clinical care.

Current Detection Methods

Numerous venom detection methods have been developed, or modified from already existing molecular biology techniques to suit the needs of the problem throughout the years[3].

In order to determine which snake is responsible for the envenoming, clinical tests or laboratory tests can be conducted. For the clinical diagnosis, the symptoms of the patient are important. Those include swelling, blistering and necrosis at the sight of the bite. Depending on the snake species, symptoms such as haemorrhage, incoagulable blood, and hypovolaemic shock are more common in viper bites, while neurotoxic symptoms are caused mainly by elapid bites.

Laboratory diagnosis depends on tests that can be carried out in the laboratory and are mainly blood tests and immunologically-based assays [5].

An agar-stabilized precipitation test was first used to detect cobra venom in bite site tissue and later, gel immunodiffusion was used to detect venoms from common Nigerian snakes. The system was not sensitive enough to detect venom in serum and was of limited use.

Immunofluorescence has been used to detect specific venom in tissue samples, but not in body fluids. More recently, a single bead based immunofluorescence assay was developed with a detection sensitivity of 5-10 ng/ml with a 3 hour assay time. Immunoelectrophoresis was also used, but found to be of limited practical use in routine venom detection assays due to precipitating bands between venoms and antibodies of closely related species.

Radioimmunoassay was used to detect venom in serum, but, although highly sensitive, the method was impractical in patients as well as being very expensive, requiring sophisticated reading equipment for measuring isotope levels, and having problems related to the short half life of the isotopes.

Detection of specific venom using the Enzyme-linked Immunosorbent Assay (ELISA) or Enzyme Immunoassay (EIA) has also been described. The sandwich technique was used, linking soluble antigens to an insoluble solid phase (microtiter plate wells), while keeping the reactivity of the immunological components. It consists of binding specific venom antibody to the solid phase and then adding test material containing venom antigen. The detection of the complex is then carried out, by using an antibody conjugated to an enzyme. Substrate for that enzyme is then added, with the amount of reactivity being proportional to the amount of antigen present in the sample. This type of tests can be used for detection and quantification of venom antigen in other body fluids and wounds. With optimization of these techniques, the total assay can be reduced to less than 3 hours, which is still not rapid enough for the clinician to decide on whether or not to treat the patient with antivenom[5].

The detection methods covered in this section share disadvantages that prevent them from being used in field clinics for detection of the specific venom in the victim. Furthermore, these methods carry additional challenges including cost: the use of expensive and elaborate equipment, the need for trained personnel, and most importantly, the assaying time.

These factors prevent the clinician from being able to use assays for the identification of the specific venom, making them rely solely in the clinical diagnosis for deciding if, and which antivenom is necessary for the treatment. This costs valuable time that can prove critical for the survival of the patient.

Current Detection Methods

References

[1] Chippaux (2017). Snakebite envenomation turns again into a neglected tropical disease! Journal of Venomous Animals and Toxins including Tropical Diseases, 23:38.
[2] WHO (2017). Report of the Tenth Meeting of the WHO Strategic and Technical Advisory Group for Neglected Tropical Diseases
[3] WHO (2017). Animal Bites
[4] de Silva HA, Ryan NM, de Silva HJ (2016). Adverse reactions to snake antivenom, and their prevention and treatment. Br J Clin Pharmacol., 81(3):446-52 [5] Theakston RD, Laing GD (2014). Diagnosis of snakebite and the importance of immunological tests in venom research. Toxins (Basel), 6(5):1667-95