Companion animal toxicology

Numerous potential toxicological hazards exist in and around the home – including plants, household chemicals, medicines, DIY and garden products, venomous animals and pesticides.

When presented with an animal with a potentially toxic exposure, appropriate and prompt treatment, where necessary – including stabilisation and decontamination – is essential.

History taking

Stabilisation of a critically ill animal is a priority, with a focus on the respiratory, neurological and cardiovascular systems. Thereafter, it is important to establish what the animal has been – or could have been – exposed to (Panel 1).

It is important to determine as many details of the exposure as possible – including the time since it occurred, and to estimate the quantity involved. This may include:

• questioning the owner
• examination of the packaging or material the owner has brought in
• counting any remaining tablets, in the case of medicines
• measuring the volume left in a product bottle
• looking through vomitus for evidence (Figure 1)

Risk assessment

Once what the animal has been exposed to has been established (where possible), it is necessary to determine whether the animal is at risk of developing toxic effects.

The substance may be a common cause of potential poisoning in animals, such as chocolate, and you may have experience in dealing with such cases; but where the substance is less familiar, part of the risk assessment may require seeking specialist advice from a veterinary poisons centre.

Poison centres can provide information on potential clinical signs – including time to onset and duration, specific treatments and prognosis.

This information will be useful when discussing the treatment plan with the owner, including the need for admission and estimated costs. It can also provide the reassurance in cases where toxic effects are likely to be minimal and treatment is not required.

It is advisable to telephone a poisons centre before treatment is commenced, to establish the correct plan (Panel 2). Advice is tailored to each case and can be helpful to prevent overtreatment (for example, inducing emesis for a non-toxic dose of a drug), prevent incorrect treatment (such as giving an emetic when it is contraindicated, or likely to be too late to be effective) and, most importantly, ensuring any treatment required is prompt, appropriate and timely.

Poisons centres will also be able to help with differential diagnoses in cases of unknown, but possible, poisoning.

Clinicians may also have to deal with the owner’s perceptions of the “poison” involved and whatever he or she has read on the internet, which, in some cases, is misleading. Seeking advice from an authoritative source may help ease an owner’s concerns in such cases.

Poisons centres also have a role in toxicovigilance and collect case data, even if advice is not required. Toxicovigilance is particularly important for new drugs or products, where information on overdose or toxic effects is likely to be limited, and for cases of poisoning in less common species, such as small furries and exotics.

Case data is essential for poison centres to highlight the potential risks with some drugs or products, establish toxic doses, and refine treatment guidelines and recommendations.

Decontamination

After the history and exposure details have been established, the risk assessment may indicate the need for decontamination.

Gastrointestinal decontamination is the most common method of decontamination used, and generally includes gastric evacuation (emesis or gastric lavage) and administration of an adsorbent.

In most cases, emptying the stomach is only worthwhile if ingestion was recent (within one to two hours), as the ingested material will still be in the stomach and the toxin will not have been absorbed. However, this is not always the case and some substances can remain in the stomach for longer (such as raisins and sultanas).

The physical form of the substance ingested (large tablets or a solution, for example) will also influence absorption rates and the time ingested material remains available in the stomach.

The chemical nature is important to consider, too – some substances are digested slowly, but others (such as alcohol) are absorbed directly from the stomach.

Emesis

Emesis is the most common method of emptying the stomach, but efficacy declines the longer the time between ingestion and emesis.

Emesis is not always suitable, and depends on the substance ingested and the clinical condition of the animal (Panel 1).

If the animal is very drowsy, having a seizure or unable to protect its airway, induction of emesis may lead to aspiration of the stomach contents.

A number of substances can be used for induction of emesis. Apomorphine is commonly used in dogs and vomiting typically occurs within a few minutes of administration.

Induction of emesis may be ineffective in some cases because the substance ingested has an antiemetic action (for example, cannabis).

Induction of emesis in cats can be challenging and apomorphine is generally not used because it is usually ineffective. Alpha-2 adrenergic agonists (such as xylazine, medetomidine and dexmedetomidine) can be used in cats (Thawley and Drobatz, 2015; Willey et al, 2016), as vomiting is an adverse – and, therefore, unreliable – effect of these drugs.

Sedation is common, but can be reversed with atipamezole.

Sodium carbonate (washing soda) crystals are an effective emetic in dogs and cats; however, administration can be a problem as washing soda is now more commonly available as a powder rather than crystals, and severe gastrointestinal and respiratory side effects may occur from its use (Watson and Indrawirawan, 2019).

Some vets choose to add water to the powder to make a thick paste and place a small amount on the back of the tongue, which can help to minimise the complications.

Three per cent hydrogen peroxide is sometimes used an as emetic in dogs, but it can cause local tissue damage – with haematemesis and gastritis in dogs (Niedzwecki et al, 2017) and cats (Obr et al, 2017). Consequently, it is not a recommended treatment and owners should be warned not to try to self-treat their dogs at home using this method.

A number of substances have been used as emetics in the past, but they are obsolete and potentially dangerous (Panel 3).

Gastric lavage

Gastric lavage is another method of gastric decontamination. It will only retrieve material from the stomach and is generally used in potentially severe cases of poisoning, particularly where rapid onset of signs is present and emesis is contraindicated (for example, metaldehyde poisoning).

Adsorbents

The most commonly used adsorbent is activated charcoal. It binds many toxins and further reduces gastrointestinal absorption.

Activated charcoal is a finely powdered material that has been treated to give it a huge surface area, which is capable of binding a variety of drugs and chemicals. The charcoal is not systemically absorbed or metabolised, but passes through the gut.

Activated charcoal is given as a single dose or in repeated doses, depending on the toxin involved, and is given after emesis or gastric lavage.

Timing of administration is important, as efficacy declines the longer the period between ingestion and administration. A single dose is most useful when the substance ingested is still in the stomach – and in most cases of poisoning this is all that is required.

Repeat dose administration of activated charcoal is particularly important when the agent is enterohepatically recirculated (such as theobromine in chocolate), because it can interrupt enterohepatic recycling and/or promote drug exsorption from the systemic circulation into the gut lumen (Pond, 1986; McLuckie et al, 1990).

Note that activated charcoal does not bind everything and is not recommended/worthwhile for a number of substances (Panel 4).

Activated charcoal administration is not without risks. If aspirated, it can cause respiratory complications (Caudill et al, 2019). It also stains faeces black and slows gut transit time; therefore, co-administration of a laxative can be considered.

Activated charcoal should be used with care in small furries for this reason.

The use or timing of activated charcoal administration should be considered when oral treatments are to be used, as the charcoal will also absorb these and reduce their efficacy. A period of at least two hours should be allowed between administration of charcoal and oral medication.

Dermal decontamination

In most cases, warm water and mild detergents – for example, shampoo and washing-up liquid – are suitable for dermal contaminants (Figure 2).

In small patients, it is important to monitor the body temperature closely to ensure the animal does not develop hypothermia.

Contamination with oily, greasy substances – or other substances that are not water soluble – may require stronger degreasing detergents, such as a heavy-duty hand cleanser. It is important to note these are more effective if applied to dry fur, as it is water soluble. After application to the affected area, the cleanser should then be thoroughly washed off, as it contains a petroleum solvent.

Other options for removal of sticky agents – such as glue traps or fly paper – are vegetable oil, margarine or butter (Bough, 2003; Wild, 2010). After decontamination, these substances can then be washed off, although ingestion of a small quantity is not hazardous.

Solvents – such as alcohol or white spirit – are not recommended for dermal decontamination because they can spread the contaminant further and may irritate the skin.

If heavy contamination is present, or large clumps of material adhering to the fur, it may be more practical to clip decontaminated hair, particularly in long-haired animals.

Corrosive substances – such as acids, and particularly alkalis – can cause serious tissue damage. Acids and alkalis on the skin should never be neutralised – the neutralisation reaction gives off heat and this can cause further tissue damage.

Dermal exposure to strong alkalis – such as sodium hydroxide and potassium hydroxide – requires very thorough decontamination. These chemicals can cause deep penetrating burns that are often painless initially, but progress over a few hours.

Washing the contaminated area will only remove surface alkali; therefore, repeated irrigation is required.

After the initial decontamination, which should continue until the pH of the skin is neutral (litmus paper or a urine dipstick can be used), it is important to wait 15 minutes. This allows residual alkali to diffuse up from the deeper regions of the dermis. At this point, recheck the pH of the area. If the skin is strongly alkaline again, irrigation should be repeated.

Decontamination should be repeated until the run-off fluid remains neutral. In some cases, this can take several hours of repeated decontamination.

Some mildly alkaline substances, such as detergents, can cause burns if left on the skin for a prolonged period.

Common drugs used in the management of poisoning

As well as apomorphine and activated charcoal, a number of other drugs are used in the management of poisoning (Table 1). These include drugs to support the gastrointestinal system, including gastroprotectants and antiemetics. Analgesics, sedatives, anticonvulsants and anaesthetics are also commonly used.

Table 1. Some drugs and products used in the management of poisoning
Product	Usage
Acetylcysteine	Hepatoprotectant, particularly for paracetamol.
Activated charcoal	Adsorbent.
Antivenom	Bites and stings from venomous animals.
Apomorphine	Emetic.
Atropine	Carbamates, organophosphate insecticides, muscarinic fungi
Bisphosphonates	Vitamin D.
Blood products	Substances causing anaemia, methaemoglobinaemia or bleeding.
Cyproheptadine	Serotonin antagonist in the management of serotonin syndrome from drugs such as 5-hydroxytryptophan.
Deferoxamine	Iron chelating agent.
Ethanol	Ethylene glycol.
Flumazenil	Benzodiazepine reversal agent.
Lipid emulsion	Various cardiotoxic and lipophilic compounds.
Methocarbamol	Muscle relaxant for substances causing tremors.
Methylene blue (methylthioninium chloride)	Methaemoglobinaemia.
Misoprostol	NSAIDs
Naloxone	Opioid/opiate reversal agent.
S-adenosylmethionine	Hepatoprotectant.
Vitamin K1	Anticoagulant rodenticides.

Methocarbamol is a skeletal muscle relaxant and can be useful in the management of poisoning with substances that cause tremors, such as permethrin, metaldehyde and tremorgenic mycotoxins.

Hepatoprotectants – such as acetylcysteine and S-adenosylmethionine – are used for paracetamol and xylitol toxicity.

Atropine is used in the management of hypersalivation, and as an antidote for carbamate poisoning and some fungi (for example, those containing muscarine).

Vitamin K is the specific antidote to anticoagulant rodenticides, which are very commonly ingested by dogs.

Snake bite should be managed with the appropriate antivenom.

Fluid therapy is important to main cellular perfusion, correct dehydration and electrolyte imbalance, protect the kidneys after ingestion of nephrotoxic substances (such as grapes, raisins and lilies), and to correct hypotension. It is not indicated or useful in every case, however, so vets should be sure it is necessary before administration.

Blood products may also be required for animals with toxin-induced anaemia or with blood loss from anticoagulant rodenticide toxicity.

Parenteral lipids for lipid infusion should also be available. This is used in the management of toxicity from cardiotoxic drugs or lipophilic compounds, such as permethrin, ivermectin and numerous other compounds.

Lipid emulsion is relatively, easy to give and can be used in the management of poisoning with a variety of drugs and chemicals. It is inexpensive and often life-saving.

If rarely used drugs are not stocked, it is important to know how and where to obtain them in an emergency. Poison centres usually have this information, which they can provide.

Admit or refer?

After stabilisation and initial treatment have been carried out – and with information on the risks and potential prognosis in a particular case – the decision about admission or referral should be made.

If the clinical signs of poisoning are likely to be minor and/or short-lived, it is likely the animal can be managed at the practice.

Referral may be necessary if severe signs are expected or already present, if specialist monitoring or therapy with drugs that are not readily available is required, or if treatment is likely to be prolonged.

Conclusion

Initial management of a poisoned patient will involve stabilisation – if necessary – and obtaining a history, followed by a risk assessment to determine the right approach to decontamination and further treatment.

A poisons information centre can provide specific advice to aid the clinician and ensure every animal exposed to toxins is treated appropriately.

• Use of some drugs mentioned in this article would be under the cascade.