Neglected tropical diseases

Snakebite

Envenoming resulting from snake bites is a particularly important public health problem in rural areas of tropical and subtropical countries situated in Africa, Asia, Oceania and Latin America. A recent study estimates that at least 421,000 envenomings and 20,000 deaths occur worldwide from snakebite each year, but warns that these figures may be as high as 1,841,000 envenomings and 94,000 deaths. The highest burden of snakebites is in South Asia, Southeast Asia, and sub-Saharan Africa.

Snake bite is primarily a problem of the poorer rural populations in these regions and affects mainly those involved in subsistence farming activities. Poor access to health services in these settings and, in some instances, a scarcity of antivenom, often leads to poor outcomes and considerable morbidity and mortality. Many victims fail to reach hospital in time or seek medical care after a considerable delay because they first seek treatment from traditional healers. Some even die before reaching hospital. Hospital statistics on snakebites therefore underestimate the true burden. In addition to mortality, some snakebite victims survive with permanent physical sequelae due to local tissue necrosis and, sometimes psychological sequelae. Because most victims are young, the economic impact of snakebite can be considerable.

Play now audio summary–The burden of snakebites
00:04:55 [mp3 2,8Mb]

The major groups of snakes causing envenoming are the elapids (cobras, kraits, mambas etc.) and vipers, and in some regions, sea snakes. Snake venom is a complex mixture of many different compounds. The composition and effects of venom varies considerably between species to species, but can broadly be divided into categories which include i) cytotoxins causing local swelling and tissue damage, ii) haemorrhagins which disturb the integrity of blood vessels, iii) compounds which lead to incoagulable blood, iv) neurotoxins causing in neurotoxicity and iv) myotoxins which cause muscle breakdown,

The clinical features of the bites of venomous snakes reflect the effects of these venom components. These include, local tissue damage ranging from swelling of the bitten limb to skin and muscle necrosis, abnormal blood clotting and bleeding, hypotension and shock, neurotoxicity sometimes leading to paralysis of respiratory muscles requiring assisted ventilation, and renal toxicity. Although the most obvious explanation for a confirmed snake-bite with no clinical manifestations is a bite by a non-venomous species, bites by venomous species do not always cause symptoms, and only 50-70% of bites by a venomous species will actually cause envenoming.

Reassurance and immobilization of the affected limb with prompt transfer to a medical facility are the cornerstones of the immediate care of snakebite. Pressure immobilization is used for some elapid species. The mainstay of treatment of snakebite is the infusion of an appropriate antivenom. Snake venoms are diverse, and therefore the efficacy of antivenoms is geographically and biologically restricted, meaning that a large number of antivenoms have been developed for use in different settings. Antivenoms are manufactured by immunizing horse or sheep with venom from a particular species and then processing the serum from the animal. Both monospecific and polyspecific antivenoms are available; monospecific antivenoms are produced using the venom of a single species, whereas polyspecific antivenoms are produced against the venoms of several species that are prevalent in a geographic region. In the rural tropics, victims are often bitten in an agricultural field or jungle, and in many instances the biting species is not identified. In such situations, treatment with polyspecific rather than monospecific antivenoms may be more appropriate. The need for many different antivenoms means that production is not always economically viable and in some parts of the world, particularly Africa, there is a crisis regarding production, distribution and accessibility of antivenoms. In addition, the need to restrict the cost of these products when manufactured on a small scale means that relatively limited processing leads to high adverse reaction rates with some products, including life threatening anaphylaxis. Another problem is that currently available antivenoms are not effective against the local necrotic effects of snake venom which can lead to long-term disability and disfigurement. In addition to antivenom therapy, management of snakebite also includes supportive therapy, such as ventilation for respiratory paralysis, which is often lifesaving for victims bitten by species which cause severe neurotoxicity.

Envenoming following snakebite, is largely a neglected threat to public health. It affects mainly the poor in deprived rural areas where health facilities and are limited and anti-venoms may be hard to obtain. Training of health staff in the management of envenoming is often neglected, despite good evidence that it improves outcome. Concerted action is needed to ensure supplies of effective antivenoms and to develop systems that deliver good quality health care to snake bite victims so that we can deal effectively with this problem, which causes severe disability, brings misery to families and which kills thousands of people.

Links:

International Society of Toxinology

Venom Week' 2009
Third International Scientific Symposium, 1 to 4 June 2009, Albuquerque, NM.

Animal Sera
Antivenom therapy is key to the medical management of snakebite and other venomous bites and stings. Unfortunately there are a number a problems for developing countries in accessing and using antivenoms.
A photogallery of clinical features of snakebites is under construction.

The global burden of snakebite: A literature analysis and modelling based on regional estimates of envenoming and deathspublished 03 November 2008.

Effects of Bothrops asper Snake Venom on Lymphatic Vessels: Insights into a Hidden Aspect of Envenomation
PLoS Medicine paper, by Javier Mora, Rodrigo Mora, Bruno Lomonte, José María Gutiérrez, published 15 October 2008.

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