Venomous snakes poisonous to pets: Part 2

(Timber rattlesnake picture courtesy of J.D. Willson, PhD, University of Arkansas Department of Biological Sciences)

When it comes to venomous snakes, there are two families represented by several different genera and species throughout most of North America. These include the Elapidae family (coral snakes), and the Viperidae family (rattlesnakes, copperheads, and water moccasins). All have a mixture of different pharmacologically active components in their venoms, and varying clinical signs may result following a venomous snakebite. Owners, team members, and veterinarians should have basic knowledge of what venomous species are native to the geographic region they live in.

Viperidae, subfamily Crotalinae (pit vipers)

In North America, the Crotalinae subfamily includes the genera Agkistrodon (cottonmouths, also known as water moccasins, and copperheads) as well as Crotalus and Sistrurus (rattlesnakes). These snakes are found throughout the United States, but are most represented in the southeastern and southwestern regions of North America. Venom toxicity varies between species. Rattlesnakes are generally considered to have more potently toxic venom, followed by cottonmouths (water moccasins), then copperheads. However, copperhead bites frequently result in considerable morbidity due to extensive swelling and local venom effects. Copperheads account for the majority of venomous snakebites and they are often found in areas of human habitation.

How do I identify a pit viper snake?

Members of the Crotalinae subfamily have a variety of different colors and patterns, and are identifiable by a triangle-shaped head and narrow neck, and most have a stout body. They have retractable fangs, and a heat-sensing pit (loreal pit) between the nostril and the eyes. The pupil is vertically elliptical (like a football standing on end). Their subcaudal scales are not divided, and as such appear as a single scale below the cloaca/vent.

What is the threat to pets?

Any curious animals that encounter these snakes are at risk. Bites often occur to the face and front legs and although the puncture marks may be evident, they can also be easily missed due to the animal's hair. Pit viper venoms primarily consist of numerous proteins (both enzymatic and nonenzymatic), small peptides, and non-protein substances. Together, they cause issues with blood clotting (which can lead to significant coagulopathies) and tissue integrity. Altered blood and serum distribution, leaking of blood from vessels leading to pooling in other tissue and organ systems (which results in a drop in blood pressure leading to shock), and depending on the species of snake, blocking CNS and neuromuscular functions can also be observed.

Several species of rattlesnakes (Mojave rattlesnake-Crotalus scutulatus, Timber Rattlesnake-C horridus, and Southern Pacific rattlesnake-C helleri) have subpopulations with venom containing potent neurotoxins. It's possible for a snake to have both hemoatologically toxic and neurotoxic types of venoms. If the venom is primarily neurotoxic, the development of coagulopathy may not occur.

A venomous snakebite by species within the Crotalinae subfamily does not necessarily mean an envenomation has occurred. In humans, 20 percent to 25 of these bites are dry bites with no venom injected. In cases where envonmation has occurred, typically early localized swelling is seen (neurotoxic venom may cause no local signs other than bite wounds), followed by edema, excessive bleeding from the wound, bleeding of the gums and mucous membranes, and airway obstruction may occur with bites to the tongue, lips, or nose. If coagulopathy develops, further serious complications can result.

(Cottonmouth picture courtesy of J.D. Willson, University of Arkansas Department of Biological Sciences)

What will I see if a pet is bitten?

Typically clinical signs and symptoms develop within six hours. Bites to the face and extremities generally have slower uptake than bites to the torso or tongue, which typically have rapid absorption. Subcutaneous bites have the slowest onset of systemic symptoms, with intramuscular being slightly faster, and intravenous envenomation being the most rapid in effects. The tissue distribution pharmacokinetic properties of venom components are variable, and as such redistribution of venom from tissues back into blood may result in recurrent toxicity, which can occur days after the bite.

Local signs may include the following:

• Angioedema (a vascular reaction involving the deep dermis or subcutaneous or submucosal tissues, representing localized edema caused by dilatation and increased permeability of the capillaries, and characterized by the development of giant areas of subcutaneous fluid blisters/blebs)

• Fang punctures (Note: not always two punctures, may be multiple scratches/punctures, and may not always be visible)

• Edema and swelling

• Bleeding from the bite site

• Erythema and ecchymosis

• Lymphangitis (inflammation of a lymphatic vessel or vessels)

Systemic signs may include the following:

• Hypotension

• Dyspnea

• Weakness

• Bleeding (epistaxis, gingival, hematuria, melena, retinal hemorrhage)

• Excessive salivation

• Myokymia/fasciculations

• Vomiting-hematemesis

• Diarrhea

• Oliguria

• Respiratory depression

• Cardiac rhythm abnormalities (rare, but in cases of severe envenomation may occur)

How do I treat it?

Again, if the snake is available (or a photo of snake, cell phone photo), accurate identification is useful to determine if the snake species is venomous or nonvenomous. However, snakes have bitten pet owners, and it's best not to take actions that will increase the time for transport of the pet to veterinary medical care facility. It's best to avoid having owners performing any first aid at home. Immediate transportation to clinic is most effective. Note: Have owners remove collars or other restrictive devices, prior to transport. Try to keep animal calm with comforting reassurance and minimize movement.

At the veterinary hospital:

• Examine for fang marks and areas of broken skin. Clip and clean the area of the bite, and examine for localized tissue swelling, erythema or ecchymosis.

• Baseline CBC, serum chemistry, UA, and coagulation profile should be obtained and repeated as necessary (particularly coagulation panel, PCV, and TP). Platelets and fibrinogen are important monitoring parameters. Hematocrit and hemoglobin should also be followed.

• Monitor ECG and blood pressure.

• Minimize exercise/movement.

• Observe closely for airway obstruction and be prepared to intubate.

• Anti-venom: Because of the complex makeup of snake venom with numerous different toxins, there is potential for venom-drug interactions. Anti-venom drug interactions are rare, as are endogenous antibody interactions with the anti-venom.

• Corticosteroids have no documented value during initial treatment and may in fact worsen the condition.

• Colloids (fresh frozen plasma, whole blood) are useful in cases of severe envenomation, but if available, ideally anti-venom should be given prior to colloid replacement. Fluid replacement (crystalloid) is useful for treating hypotension due to large redistribution of blood fluids from the vasculature. Monitor fluids to avoid fluid overload.

• DMSO (dimethyl sulfoxide) enhances uptake and spreads venom.

• Heparin should not be used as venom-induced coagulopathies by pit viper venom act by a different pharmacological mechanism. It has no clinical value and again, may worsen the condition.

What is the prognosis?

Animals may suffer recurrent symptoms following apparent recovery. These can be both local and systemic, so animals should be monitored closely for up to one to three weeks following snakebite and associated treatments. For this reason owners should be informed to not let their animals be overly active during this time as bruising and other forms of mild trauma may result in active bleeding complications.