Snakebite

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Snakebite
ICD-10 ICD10 F84.0-F84.1
ICD-9 989.5
MedlinePlus 000031

A snakebite is just that, a bite inflicted by a snake. In the wild, when a snake bites a human being (or almost any domestic animal) it's a defensive reaction: the snake strikes out of an instinctive fear for its life - with or without real cause. When a person is the one bit by a snake, that "fear for one's life" is often transferred, perhaps just as instinctively, and generally just as regardless of whether or not any real threat exists. Although some snakes are venomous, and their bite can trigger death or serious injury, most snake's bite causes only a very minor injury. However, so many people fear all snakes, including the most harmless sorts, that any snake bite can be a nightmarish occurrence. This outsized fear of snake bite can produce such alarm, that many doctors and other experts have claimed, on average, the emergency treatment administered by well-meaning but panicked people has caused as much devastation as the snake! This article explores the reality of snakebite.

A part of that reality seems surreal, and that is the fact that in the developed world, snake bite is usually the result of people purposefully keeping and handling dangerous snakes.

In a practical sense, if faced with a snake bite, there are only a few things to know. Some of the most important of these require extremely specialized knowledge, and that's one reason the rapid transport of the person or animal that has suffered a bite to experts is so important. Only an expert can determine what kind of snake did the biting, and whether or not the bite was 'dry'. If the bite resulted in the injection of venom, only an expert can obtain and administer the correct anti-venom that will effectively treat the bite. Bringing a picture, description, or the actual snake to that expert is helpful, as long as it can be accomplished without risking additional snakebite. Other important aspects to snake bite will be obvious to anyone faced with the problem: where in the world - and where on the person, did the bite occur? As with most health issues, prevention is better than cure and interestingly enough, most bites can be easily avoided with the right knowledge. Finally, snakebite is a much bigger problem in the tropical regions of the developing world than in places with rapid transportation systems and sophisticated medical care. In areas of Africa, Asia and Oceania, snakebite is a problem that is often made worse by the lack of health science networks.

Snakes and human injury

Why does a snake bite?

A 'bite' occurs either as a defensive manoeuvre on the part of the snake, or as the first step in catching and holding a prey animal. All snakes are strict carnivores, eating only animals, not plants, as food. Most snakes have a preferred diet of particular kinds of prey animals, rarely seeking domestic animals, and never seeking humans, to eat. There is one simple reason for this: size. When snakes do eat, they swallow their prey whole. Since a snake can unhook its jaws and make its mouth large, these reptiles can swallow larger prey than their small closed mouths might suggest. No matter the size of the mouth, that prey has to fit in the body of the snake and so very few snakes can eat any but small animals. Even if starving, a snake will not attempt to eat anything that appears too big to swallow, and so only one of the rare giant snakes of the world would be large enough to be a maneater. That means, almost always, a snake that bites a person or any of the larger domestic animals is not hunting - but striking defensively.

Now, if a person keeps a snake and feeds it, it is quite possible for the snake to bite a hand or other human body part, especially if that part has the smell of the snake's food on it, mistaking it for the prey.

Several hundred of the thousands of different kinds of snakes are called "poisonous", because they are venomous animals. The giant snakes large enough to actually swallow a human are all constrictors, and none of these are venomous. It is the venomous snakes that are capable of causing serious human injury, and since none of them are large enough to try to eat a person, none of them will ever bite a person except to try to protect themselves. With rare exception, this happens when the snake is pursued or physically contacted. Sometimes people are bitten when purposefully handling venomous snakes, and many of the case reports from the medical literature in the developed world include examples of people bitten by captive venomous snakes. Often, in the field, the person contacting the venomous snake had no idea it was there, but put a hand or foot into the snake's hiding place. That's why certain activities, like gardening without gloves or walking along stream banks without shoes, are hazardous in areas where venomous snakes frequent. Even then, the bite is not always disastrous. Despite the potential danger, bites from snakes that do not include any injection of venom (whether the bite comes from a venomous or non-venomous snake) rarely cause any serious harm to people or their pets or livestock. When venomous snakes strike, they often bite without injecting venom.

Venomous snakes

Venomous snakes manufacture a very toxic saliva that can be introduced into an animal that is bitten. Snake venoms contain some of the most powerfully toxic biological agents known, but since every bite from a venomous snake does not always include the venom,some people have been bitten by such snakes without harm. When a bite from a venomous snake does not include the injection of venom, that's called a dry bite. When a bite from a venomous snake does include the venom, effects can range from minor inflammation around the area of the bite, to death. The effect of a bite by any venomous snake depends on the amount of venom actually injected into tissues, the exact kind of snake, and the size and kind of animal bitten. Why go on about the fact that so much depends on the kind of snake, and on whether or not any venom was actually injected? In snake bite, the treatment can be more dangerous than the bite. Proper treatment of snakebite has everything to do with being sure that a snake, rather than some other creature, inflicted the bite, and on knowing the kind of snake it was. When there is certainty or near certainty that the creature was a venomous snake, and there are signs that venom was injected, the next steps have everything to do with whether the victim is within range of being transported to a sophisticated hospital, or not.

There are some general rules in avoiding snakebite and for giving first aid when it occurs. These will be discussed in the next sections. In some situations, however, snakebite is a danger because people are in the same immediate environment as venomous snakes but are focused on tasks that demand total attention and leave them vulnerable to inadvertent contact with snakes. In this way, soldiers in desert wars of North Africa and the Middle East are subject to bites by Horned vipers, and timbermen logging in the pine forests of the southeastern United States are subject to accidental contact with the Eastern Diamondback Rattlesnake.

Frequency and statistics

File:Snakebite morbidity map.png
Map showing global distribution of snakebite morbidity.

Some diseases and injuries must, by law, be reported to health authorities. Snake bite is not a medical condition that falls into that category. Since reportage of snakebite is not legally mandatory in any country, there are no firm statistics on how often they occur. Not only do many snakebites go unreported, but the places where most medically significant snakebites are expected to happen are the very places that reportage to health authorities is most unlikely. That's because the greatest numbers of venomous snakes that live in areas where people also live are in rural areas of the tropics, and much of that part of the world is in medically under-served developing nations. Consequently, no accurate study has ever been conducted to determine the frequency of snakebites on the international level. Despite this, many encyclopedia and news articles quote statistics on snakebite as if each bite by a venomous snake has been logged into a record book. In reality, none of these statistics are anything more than estimates, and the more "exotic" the location, the rougher and more inaccurate is that guess.

In areas of the rural tropics where attempts have been made to ascertain the precise number of venomous snake bites, the frequency has been many times that of places such as the United States. "The overall incidence for Central Province ( Papau New Guinea) is 215·5 per 100 000 population, but Kairuku subprovince has an incidence of 526 per 100 000, which is amongst the highest in the world". (reference for quote: D. G. Lalloo, A. J. Trevett, A. Saweri, S. Naraqi, R. D. G. Theakston and D. A. Warrell:The epidemiology of snake bite in Central Province and National Capital District, Papua New Guinea • Transactions of the Royal Society of Tropical Medicine and Hygiene, 89, 1995, Pages 178-182)

A late 1950s study estimated that 45,000 snakebites occur each year in the United States (Parrish 1966). Despite this large number, only 7,000-8,000 of those snakebites were actually caused by venomous snakes, and, back then, those several thousand bites of venomous snakes resulted in an annual average of 10 deaths. [1] [2] The majority of bites in the United States occur in the southwestern part of the country, in part because rattlesnake populations in the eastern states are much lower (Russell 1983). Changes in the United States over the last half century have made venomous snake bite less likely to happen at all, and much less likely to cause death or serious injury.

Most snakebite related deaths in the United States are attributed to eastern and western diamondback rattlesnake bites. Children and the elderly are most likely to die (Gold & Wingert 1994). The state of North Carolina has the highest frequency of reported snakebites, averaging approximately 19 bites per 100,000 persons. The national average is roughly 4 bites per 100,000 persons (Russell 1980).

Most snakebites are caused by non-venomous snakes. Of the roughly 3,000 known species of snake found worldwide, only 15 percent are considered dangerous to humans (Russell 1990). Snakes are found on every continent except Antarctica. The most diverse and widely distributed snake family, the Colubrids, has only a few members which are harmful to humans. Of the 120 known indigenous snake species in North America, only 20 are venomous to human beings, all belonging to the families Viperidae and Elapidae. [3] However, every state except Maine, Alaska, and Hawaii is home to at least one of 20 venomous snake species. [4]

Since the act of delivering venom is completely voluntary, all venomous snakes are capable of biting without injecting venom into their victim. Such snakes will often deliver such a "dry bite" (about 50% of the time [5]) rather than waste their venom on a creature too large for them to eat. Some dry bites may also be the result of imprecise timing on the snake's part, as venom may be prematurely released before the fangs have penetrated the victim’s flesh.

Global evaluation of snakebites [6]
Landmasses Population (x106) Total number of bites No. of envenomations No. of fatalities
Europe 730 25,000 8,000 30
Middle East 160 20,000 15,000 100
USA and Canada 270 45,000 6,500 15
Central and South America 400 300,000 150,000 5,000
Africa 760 1,000,000 500,000 20,000
Asia 3,500 4,000,000 2,000,000 100,000
Oceania 20* 10,000 3,000 200
Total 5,840 5,400,000 2,682,500 125,345

*Population at risk

Prevention

Snakes are most likely to bite when they feel threatened, are startled, provoked, and/or have no means of escape when cornered. If you or your party encounters a snake you should always assume it is dangerous and leave the immediate vicinity. Unless you are an expert, there is no practical way to safely identify any snake species as appearances vary dramatically.

Snakes are likely to approach residential areas when attracted by prey, such as rodents. If it is a feasible option, practice pest control and snakes should not be an issue. If you live in an area with many snakes you may want to educate yourself on the species in your area. Likewise, if you plan to spend extended periods of time in areas of the world such as Africa, Australia, India, and southern Asia, it may be wise to research some of the more dangerous snakes inhabiting the region. If anything, you may at least be more wary of their presence and, as a result, less likely to be bitten.

Sturdy over-the-ankle boots, loose clothing and responsible behavior offer excellent protection from snakebites when in the wilderness. Give snakes plenty of warning that you are approaching by putting slight emphasis on your footsteps. The rationale behind this is that the snake will feel the vibrations and flee from the area. However, this generally only applies to North America as some larger and more aggressive snakes in other parts of the world, such as king cobras and black mambas, will actually protect their territory. In this case, if you run into a snake, stop moving and wait for several minutes. If the snake has not yet fled, slowly back away from the area.

If you are camping and decide to gather firewood at night, use a flashlight and, for your sake, do not go outside barefoot. Approximately 85% of the natural snakebites occur below the victims' knees. [7] Snakes may be unusually active during especially warm nights with ambient temperatures exceeding 70˚F., and a person not wearing footwear will have no protection from a potential bite.

It is advisable not to reach blindly into hollow logs, flip over large rocks, and enter old cabins or other potential snake hiding-places. If you are a rock climber, do not grab ledges or crevices without first looking (this does not mean poking your finger or a stick in the crevice) as snakes are coldblooded creatures and oftentimes sunbathe atop rock ledges.

If you own a pet snake that you know is capable of causing injury, or handle snakes as a hobby, always act with caution - approximately 65% of snakebites occur to the victims’ hands or fingers. Use your common sense and do not drink alcohol, or you may start acting foolishly once you are intoxicated. In fact, in the United States more than 40% of snakebite victims intentionally put themselves in harms way by attempting to capture wild snakes or by carelessly handling their dangerous pets. Further yet, 40% of that number had a blood alcohol level of 0.1 percent or more (Kurecki, et al 1987).

Avoid snakes that appear to be dead, as some species will actually rollover on their backs and stick out their tongue to fool potential threats. Even if a snake's head is detached from its body you should not attempt to pick it up as the reflex action of a snake's jaw muscles may cause it to "bite" you. This may be just as dangerous as a bite from a live snake (Gold & Barish 1992).

What to do if bitten by a snake

What to do for a person or pet bitten by a snake

The most important thing to do is to keep the individual as calm as possible and get medical help.

What NOT to do

Do not do anything that will delay getting the victim to a hospital. Some snake experts have advised that the most effective snake bite kit consists of car keys and a working phone!(reference: Roger Conant and Joseph T. Collins: A Field Guide to Reptiles and Amphibians. Eastern and Central North America. Peterson Field Guides. Houghton Mifflin Boston / New York , 1998 page 33


Do not do anything that will make things worse. Many of the traditional remedies offered the snakebite victim often do just that- make things worse.

According to physicians, "using any sharp instrument to incise fang marks in the field does more harm than good by exacerbating local bleeding (especially in the face of coagulopathy), introducing bacteria into the wound under nonsterile conditions, and further devascularizing the wound when perfusion may already be impaired". (reference for quote Auerbach: Wilderness Medicine, 4th ed., Copyright © 2001 Mosby, Inc.). In plainer English, this means that the notion of cutting the skin around the bite to make an opening to "draw out" the venom is a poor idea that should never be done. Components of snake venom can sabotage the body's normal clotting systems, and so the bleeding that occurs from making a cut in the area of the bite can be tremendous. Since this cutting is not being done in a sterile manner, this sort of incision can also lead to a serious infection. Since the cutting is not being done by a trained surgeon, blood vessels can be injured that cause the area around the bite to lose the vital blood supply needed for healing.

Treatment

It is not an easy task determining whether or not a bite by any species of snake is life-threatening. A bite by a copperhead on the ankle is usually a moderate injury to a healthy adult, but a bite to a mall child’s abdomen or face by the same snake may well be fatal. The outcome of all snakebites depends on a multitude of factors; the size, physical condition, and temperature of the snake, the age and physical condition of the victim, the area and tissue bitten (e.g., foot, torso, vein or muscle, etc.), the amount of venom injected, and finally the time it takes for the patient to be treated and the quality of treatment.

Snake identification: Geographic considerations

Identification of the snake is important in planning treatment, but not always possible. Ideally the dead snake would be brought in with the patient, but in areas where snake bite is more common, local knowledge may be sufficient to recognise the snake. In countries where polyvalent anti-snake venoms are available identification of snake is not of as much significance.

A scoring systems can be used to try and determine biting snake based on clinical features,[1] but these scoring systems are extremely specific to a particular geographical area.

Even where there are laws against the keeping venomous snakes in captivity, enforcement is not strict enough to prevent this entirely. Additionally, though rarely, snakes can be introduced into distant locations through importation of goods. Therefore, a bite by a venomous snake that is not native to a particular geographic region is possible. However, statistically, the number and type of snake bites in the general population occurs in a geographic distribution that reflects the native habitat of these snakes, and, sometimes, occupations and recreational practices by residents and travellers that are at higher risk for snake bite. Most envenomations from snakes occur in tropical countries. In areas where antivenom is available, along with technologically sophisticated medical care, mortality from venomous snake bite is very low. The World Health Organization has indicated that treatment for venomous snake bite is a health issue for the developing world.

North America

Most snakebites in North America that require medical intervention are from pit vipers. "Eastern and western diamondback rattlesnakes (Crotalus adamanteus and C. atrox, respectively) are responsible for most snakebite deaths in the United States. However,the mortality rate is <1% for victims receiving antivenom". [13] Elapid envenomations do occur in the USA and Mexico from coral snakes. North American coral snakes include the eastern coral snake (Micrurus fulvius), the Texas coral snake (M. tener), and the Arizona (Sonoran) coral snake (Micruroides euryxanthus).

Central America

South America

Despite the paucity of native venomous snakes in Europe, there are reports of venomous snakebite. In 1970–77, 17 people in the UK were victims of 32 bites by foreign venomous snakes. In North Africa and the Middle East, the desert horned vipers (genus Cerastes) is a distinctive snake of the desert sands, implicated in cases of snake bite reported in Dharan, Saudi Arabia. This snake is not uncommonly kept as a pet, and some cases reported by physicians have been due to the snake biting its captor during handling and occurred in places like Switzerland. [14]

Africa

There are many venomous snakes that are native to Africa, particularly in tropical regions. As elsewhere, a common name that sound very exact may actually refer to more than one kind of snake, for example the Kenyan carpet viper is not a single species. When a common name is used for both a venomous and non-venomous species, misplaced fear easily occurs. Although it's not the snake that kills the most people annually, the black mamba is by far the most feared snake on the continent of Africa. The reason this species is so feared is because they tend to be extremely aggressive and carry virulently toxic venom. Untreated bites from a black mamba have a mortality rate of 100 percent. The snake that is responsible for the majority of human fatalities is the puff adder. Puff adders are very common throughout much of sub-Saharan Africa and they are often seen in human settlements.

Asia

Pit vipers exclusive to China include the large and spectacular Mt. Mang Viper (Trimeresurus mangshanensis) of the Hunan province. Many of the widespread elapid snakes, such as the King Cobra (Ophiophagus hannah), in southern China , are also native to Southeast Asia, including India and the Philippines. In Southern Asia, cobras are large snakes with potent venom that adapt to living in areas of human habitation. Although it is estimated that up to 45% of their bites are dry, in Burma and India, an annual mortality incidence of between 3 and 10 per 100,000 has been reported (but as most snake bites occur in areas without consistent medical reporting, estimates are very imprecise). Every species of snake native to Australia is venomous. These include tiger snakes (Notechis), brown snakes (Pseudonaja).

First Aid

Snakebite first aid recommendations vary, in part because different snakes have different types of venom. Some have little local effect, but life-threatening systemic effects, in which case containing the venom in the region of the bite (e.g., by pressure immobilization) is highly desirable. Other venoms instigate localized tissue damage around the bitten area, and immobilization may increase the severity of the damage in this area, but also reduce the total area affected; whether this trade-off is desirable remains a point of controversy.

Because snakes vary from one country to another, first aid methods also vary; treatment methods suited for rattlesnake bite in the United States might well be fatal if applied to a tiger snake bite in Australia. As always, this article is not a legitimate substitute for professional medical advice. Readers are strongly advised to obtain guidelines from a reputable first aid organization in their own region, and to beware of homegrown or anecdotal remedies.

However, most first aid guidelines agree on the following:

  1. Protect the patient (and others, including yourself) from further bites. While identifying the species is desirable, do not risk further bites or delay proper medical treatment by attempting to capture or kill the snake. If the snake has not already fled, carefully remove the patient from the immediate area.
  2. Keep the patient calm and call for help to arrange for transport to the nearest hospital emergency room, where antivenom for snakes common to the area will often be available.
  3. Make sure to keep the bitten limb in a functional position and below the victim's heart level so as to minimize blood returning to the heart and other organs of the body.
  4. Do not give the patient anything to eat or drink. This is especially important with consumable alcohol, a known vasodilator which will speedup the absorption of venom. Do not administer stimulants or pain medications to the victim, unless specifically directed to do so by a physician.
  5. Remove any items or clothing which may constrict the bitten limb if it swells (rings, bracelets, watches, footwear, etc.)
  6. Keep the patient as still as possible.
  7. Do not incise the bitten site.

Many organizations, including the American Medical Association and American Red Cross, recommend washing the bite with soap and water. However, do not attempt to clean the area with any type of chemical.

Pressure immobilization

Pressure immobilization may not be appropriate for cytotoxic bites such as those of most vipers[2][3] [4], but is highly effective against neurotoxic venoms such as those of most elapids[5][6][7]. Developed by Struan Sutherland in 1978[8], the object of pressure immobilization is to contain venom within a bitten limb and prevent it from moving through the lymphatic system to the vital organs in the body core. This therapy has two components: pressure to prevent lymphatic drainage, and immobilization of the bitten limb to prevent the pumping action of the skeletal muscles. Pressure is preferably applied with an elastic bandage, but any cloth will do in an emergency. Bandaging begins two to four inches above the bite (i.e. between the bite and the heart), winding around in overlapping turns and moving up towards the heart, then back down over the bite and past it towards the hand or foot. Then the limb must be held immobile: not used, and if possible held with a splint or sling. The bandage should be about as tight as when strapping a sprained ankle. It must not cut off blood flow, or even be uncomfortable; if it is uncomfortable, the patient will unconsciously flex the limb, defeating the immobilization portion of the therapy. The location of the bite should be clearly marked on the outside of the bandages. Some peripheral edema is an expected consequence of this process.

Apply pressure immobilization as quickly as possible; if you wait until symptoms become noticeable you will have missed the best time for treatment. Once a pressure bandage has been applied, it should not be removed until the patient has reached a medical professional. The combination of pressure and immobilization can contain venom so effectively that no symptoms are visible for more than twenty-four hours, giving the illusion of a dry bite. But this is only a delay; removing the bandage releases that venom into the patient's system with rapid and possibly fatal consequences.

For more information on this technique visit http://www.snakebite-firstaid.com/. Besides an easy-to-use First Aid description, much info on snakes and their venom.


See also

References

  1. Pathmeswaran A, Kasturiratne A, Fonseka M, et al. (2006). "Identifying the biting species in snakebite by clinical features: an epidemiological tool for community surveys". Trans R Soc Trop Med Hyg 100: 874–8 issue=9.
  2. Tony Celenza, MBBS, FACEM, FFAEM. "Simulated Field Experience in the Use of the Sam Splint for Pressure Immobilization of Snakebite". Wilderness and Environmental Medicine 13 (2): 184–185.
  3. Bush SP; Green SM; Laack TA; Hayes WK; Cardwell MD; Tanen DA (December 2004). "Pressure-Immobilization Delays Mortality and Increases Intra-compartmental Pressure after Artificial Intramuscular Rattlesnake Envenomation in a Porcine Model". Annals of Emergency Medicine 44 (6): 599–604. Retrieved on 2006-06-25.
  4. Sutherland SK; Coulter AR (March 1981). "Early management of bites by the eastern diamondback rattlesnake (Crotalus adamanteus): studies in monkeys (Macaca fascicularis)". The American Journal of Triopical Medicine and Hygiene 30 (2): 497–500. Retrieved on 2005-06-25.
  5. Ken D. Winkel, PhD (Australian Venom Research Unit, University of Melbourne, Victoria, Australia). "Struan Sutherland's “Rationalisation Of First-Aid Measures For Elapid Snakebite”—A Commentary". Wilderness and Environmental Medicine 16 (3): 160–163. Retrieved on 2006-06-25.
  6. Sutherland SK (December 1992). "Deaths from snake bite in Australia 1981-1991". The Medical journal of Australia 157 (11–12): 740–746. Retrieved on 2006-06-25.
  7. Sutherland SK; Leonard RL (December 1995). "Snakebite deaths in Australia 1992-1994 and a management update.". The Medical journal of Australia 163 (11–12): 616–618. Retrieved on 2006-06-25.
  8. S.K. Sutherland; A.R. Coulter; R.D. Harris (1979). "Rationalisation of first-aid measures for elapid snakebite". Lancet 313 (8109): 183–186.
  • Gold, Barry S., Willis A. Wingert, et al. "Snake venom poisoning in the United States: A review of therapeutic practice", Southern Medical Journal, June 1994, 87(6):579-89.
  • Gold, Barry S., Barish RA. “Venomous snakebites: current concepts in diagnosis, treatment, treatment, and management.” Emerg Med, Clin North Am 1992;10:249-67.
  • Kitchens CS, Van Mierop LHS. “Envenomation by the eastern coral snake (Micrurus fulvius fulvius): a study of 39 victims.” JAMA 1987;258:1615-8.
  • Kurecki, Brownlee, et al. The Journal of Family Practice, 1987, 25(4):386-392
  • Palm Beach Herpetological Society. Venomous Snake Bite". Retrieved on 2006-06-26.
  • Parrish HM. “Incidence of treated snakebites in the United States.” Public Health Rep 1966;81:269-76.
  • cf Postgrad Med, 1987, Oct;82(5):32; Postgrad Med, 1987, Aug;82(2):42; Ann Emerg Med, 1988, Mar;17(3):254-256; Toxicon, 1987;25(12):1347-1349; Ann Emerg Med, 1991, Jun;20(6):659-661.
  • Riggs et al. Rattlesnake evenomation with massive oropharyngeal edema following incision and suction (Abstract) AACT/AAPCC/ABMT/CAPCC Annual Scientific Meeting, 1987.
  • Russell, Findlay E. Ann Rev Med, 1980, 31:247-59.
  • Russell, Findlay E. “Snake venom poisoning.” Great Neck, N.Y.: Scholium, 1983:163.
  • Russell, Findlay E. “When a snake strikes.” Emerg Med, 1990;22(12):20-5, 33-4, 37-40, 43.
  • "Suction for Venomous Snakebite: A Study of 'Mock Venom' Extraction in a Human Model", February 2004, Annals of Emergency Medicine, p. 181.
  • Sullivan JB, Wingert WA, Norris Jr RL. North American Venomous Reptile Bites. Wilderness Medicine: Management of Wilderness and Environmental Emergencies, 1995; 3: 680-709.
  • U.S. Food and Drug Administration (November 2002) "For Goodness Snakes! Treating and Preventing Venomous Bites". Retrieved December 30, 2005.
  • World Health Organization. "Animal sera". Retrieved December 30, 2005.

Further reading