When a snake bites your dog PDF Print E-mail
News - Rubrieke
Monday, 25 January 2016 20:31
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Dr Liesel van der Merwe is a small animal medicine specialist. Send her your questions: This e-mail address is being protected from spambots. You need JavaScript enabled to view it .


Dr Liesel van der Merwe

This past few months I have seen several dogs which have been bitten by snakes, so I thought it was a good time for a summary.

There are basically three kinds of snake bite scenarios. Generally adders, such as puff adders, cause cytotoxicity which means severe tissue damage. Cobras, rinkhals and black mambas are neurotoxic and cause muscle paralysis, and the boomslang and green mamba cause a problem with blood clotting.

Obviously avoidance is the best route, but if you live in a more rural setting there isn’t much chance of that.

Dry bite
Just because your dog has been bitten, doesn’t mean that it has been envenomated. Snakes can control how much poison they inject. Some patients are bitten by non-venomous snakes and 10–50% of those bitten by venomous snakes are not envenomed (so called ‘dry bites’).

However, once symptoms of envenomation start appearing things can progress very rapidly. Even if you are not sure if your dog has actually been bitten, rather get it to the vet as soon as you can so that observation and waiting for evidence of a bite can occur in a controlled environment where help is close at hand.

Anti-venom
Anti-venom should be given as soon as possible once signs of systemic or severe local envenoming are evident. Although the polyvalent anti-venom is more effective when given early (within six hours after the bite), it may be administered up to 24–48 hours or later in serious envenomations. It is never too late to give anti-venom. Administration of polyvalent anti-venom will not reverse, but may limit further tissue damage.

Anti-venom is available and many veterinary practices will have a few bottles stored in the fridge. Polyvalent anti-venom (SAIMR Polyvalent Snakebite Antiserum SAVP) is supplied in 10 ml ampoules.

Used
Venoms of the following snakes are used in the preparation of the anti-venom: puff adder, gaboon adder, rinkhals, green mamba, Jameson’s mamba, black mamba, Cape cobra, forest cobra, snouted cobra and Mozambique spitting cobra.

Polyvalent anti-venom is ineffective in treatment of bites caused by the berg adder, other dwarf adders, night adders, the burrowing asp and back-fanged snakes (boomslang and vine snake).

Anti-venom neutralises a fixed amount of venom. Since snakes inject the same amount of venom into any sized animal, the same dose/volume of anti-venom must be administered to small and large dogs. It is, however, extremely expensive per vial and in humans between five and 10 vials are commonly given to neutralise the venom.

Often this is just not possible, volume and cost wise, in a veterinary practice, so if we see clinical signs we will give one to three vials and then reassess based on the size of the patient, type of snake and degree of symptoms.


Puff adder

Puff adder
With puff adder bites there is generally bruising and swelling within a short space of time. This swelling is not just inflammation but also accumulation of blood from leaking and damaged blood vessels and dead damaged red blood cells. This can lead to shock due to loss of blood volume.

Dogs are generally bitten about the face and neck. This in some ways is good, as there is place for accumulation of fluids and swelling and there is no damage to the muscles from compartment syndrome, which occurs when the bite is on the leg.

However, it is also easy to underestimate the degree of swelling in this area and it may also compress the windpipe and compromise breathing. Often dogs are bitten inside the mouth and swelling here can also affect breathing. In adder bites it may not always be essential to use anti-venom and conservative therapy may suffice, especially in larger dogs. In smaller dogs it does seem to improve survival.

Breathing
With neurotoxic snake bites death is caused by paralysis of the breathing muscle. This may occur rapidly to up to a few hours after the bite. Observation needs to be careful and prolonged. Generally the patient is placed into a kennel/cage but taken out every five to ten minutes initially to see if there is any indication of weakness, which would indicate that it has received a dose of venom and not just a dry bite. In these patients the risk of respiratory paralysis means that anti-venom is essential.

The recommended dose is often not possible and our aim is to keep the animal breathing even though it may be a bit weak or unable to stand. The effect of the toxin will generally wear off after 24–48 hours.

Signs
Neurotoxic signs improve slowly after several hours (two to six hours), often unconvincingly. It must be emphasised that the administration of polyvalent anti-venom in the acute phase of neurotoxic snake envenoming will not always prevent progression of neurotoxic effects, most notably respiratory paralysis.

Respiratory support is the only life-saving treatment, but the administration of adequate doses of anti-venom will decrease the duration of muscle paralysis. If the animal has breathing difficulties, we have to place an endotracheal tube as we would for anaesthesia and place the animal on a ventilator or manually breathe for it using a normal anaesthetic machine or ambubag.

Ventilator
It is advisable to still administer anti-venom to prevent further blockage of the nerve endings. The animal will generally need to remain on the ventilator for anything from six to 24 hours depending on the amount of venom injected.

The venom of some snakes has both neurotoxic and cytotoxic effects. These may cause intense pain and tissue damage with minimal swelling as well as some neurological signs.

So, there is no certainty with snake bites. It could be a dry bite even though the snake was massive and the dog tiny and everyone goes home happy. Or, the full dose of toxin could have been administered with serious physical consequences to your pet and financial consequences to yourself.

Have a plan if you are at risk where you live. Find out which vets keep anti-venom. Find out where emergency clinics are located.

 

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