Tea tree oil is sometimes used by owners as a ‘herbal’ or ‘natural’ treatment for fleas, but it is potentially hazardous, particularly when used undiluted. Poisoning has even occurred when pure tea tree oil has been added to water and sprayed on pets.
Source of tea tree oil
Tea tree oil, also known as melaleuca oil, is an essential oil obtained by steam distillation from the leaves and terminal branches of the Australian tea tree, Melaleuca alternifolia (Carson et al, 2006). Essential oils are produced by plants and give the plant their characteristic odour. Some people find the camphor-like odour of tea tree oil unpleasant. Essential oils are volatile oils that evaporate at room temperature (compared with fixed oils which do not). Tea tree oil is a colourless to pale yellow liquid containing a complex mixture of compounds including terpene hydrocarbons; more than 100 individual compounds have been identified in tea tree oil (Hammer et al, 2006).
Biological effects of tea tree oil
Tea tree oil is used in skin care products for its antibacterial and antifungal properties; it also has antiviral, antiprotozoal and anti-inflammatory activity (Carson et al, 2006). Use of the extracted oil rather than plant material itself began in the 1930s following the publication of papers demonstrating its antimicrobial activity (Carson et al, 2006).
There is no good quality evidence to show that tea tree oil is efficacious in any dermatological condition in humans (Ernst and Huntley, 2000; Cao et al, 2015).
Availability
Pure tea tree oil is readily available in ‘health food’ shops, typically in a 10 ml opaque bottle with a dropper top (Figure 1). The composition of tea tree oil sold is regulated by an international standard which sets maximum and minimum concentrations of 14 components (International Organization for Standardization (ISO), 2004). Tea tree oil is sold in an opaque bottle because when exposed to light, air and heat, tea tree oil undergoes oxidative degradation, yielding compounds that can increase the risk of adverse dermal effects.
In terms of human products, the European Cosmetic Toiletry and Perfumery Association (COLIPA) recommends ‘that tea tree oil should not be used in cosmetic products in a way that results in a concentration greater than 1% oil being applied to the body. When formulating tea tree oil in a cosmetic product, companies should consider that the sensitisation potential increases if certain constituents of the oil become oxidised. To reduce the formation of these oxidation products, manufacturers should consider the use of antioxidants and/or specific packaging to minimise exposure to light (COLIPA, 2002).
Tea tree oil is an active ingredient in numerous products such as facial washes, spot treatments, hand wash, hair care products and shower gels for humans. Products for pets include shampoos, skin creams, lotions and balms, insect repellents and ear cleaners. In the UK there are no licensed veterinary medicines containing tea tree oil.
Use of tea tree oil in companion animals
There is very limited information on the use of tea tree in the management of diseases in companion animals. Small studies have evaluated the effect of tea tree oil in various skin conditions in dogs.
In a study on the antifungal effect of tea tree oil on the yeast Malassezia pachydermatis isolated from dogs with cutaneous skin disease, all the strains tested showed high susceptibility to tea tree oil (Weseler et al, 2002).
In an open, multicentre study in 53 dogs with chronic dermatitis and pruritus a 10% tea tree oil cream was applied twice daily for 4 weeks. Assessment by investigating veterinarians showed a good or very good response to treatment for 82% of the dogs, moderate in 7.8% and unsatisfactory in 9.8%. At the end of the study a strong and significant reduction in signs and resolution of major signs were observed. Four dogs were withdrawn from the study for poor efficacy, non-compliance, concurrent disease or persistent licking of the application site. There were adverse events in nine dogs, three of which were withdrawn. Only two of the adverse events were assessed as possibly related to tea tree oil. One had local irritation and hyperaemia and the other had mild reversible hyperaemia, erythema and pruritus at the site of application (Fitzi et al, 2002).
A 10% cream was also evaluated in a randomised, double-blind controlled trial in 57 dogs with localised pruritic dermatitis. The tea tree oil cream (28 dogs) or the control cream (29 dogs) was applied twice daily. After 10 days, success rates of 71% for the tea tree oil cream and 41% for the control cream differed significantly. By day 10, the tea tree oil cream caused significantly more rapid relief than the control cream for pruritus (occurring in 84% of dogs) and alopecia. By day 20 there was clinically significant improvement in the severity of dermatitis in both treatment groups. Only one adverse event was reported in the tea tree oil group, but was not considered causally related to the study drug. Five other dogs withdrew from the study: two due to lack of efficacy (one in each group), one due to non-compliance (tea tree oil group), one due to inappropriate inclusion (control group) and one due to a mild concomitant disease leading to exclusion (tea tree oil group) (Reichling et al, 2004).
These studies demonstrated a positive effect of 10% tea tree oil cream in dogs with chronic dermatitis and pruritus, but it is worth noting that there appear to be no studies evaluating tea tree oil treatment in cats or any studies on the use of the oil in the control of fleas.
Clinical signs
The various constituents of essential oils are highly lipophilic (fat soluble) and therefore are relatively rapidly absorbed from the gastrointestinal tract and across the skin. The oils are local irritants of mucous membranes, but the mechanism of systemic toxicity remains unknown. Most exposures are dermal or oral and dermal; oral exposure alone is less common (Khan et al, 2014).
Signs of essential oil toxicity in companion animals can occur within 2–8 hours (Villar et al, 1994) and typically less than 24 hours (Genovese et al, 2012), but can initially be non-specific and easily overlooked. Recovery often occurs within 24 hours, but can be longer.
Hypersalivation (particularly in cats), depression, lethargy, ataxia, weakness, tremor, muscle fasciculation, paresis and abdominal discomfort may occur in pets following oral or dermal exposure to essential oils (Thornton, 1990; Villar et al, 1994; Bischoff and Guale, 1998; Kaluzienski, 2000; Khan et al, 2014). Vomiting and hypothermia occur occasionally (Bischoff and Guale, 1998). In severe cases there may be collapse, coma and convulsions (Thornton, 1990). The breath, vomitus, urine and faeces may smell strongly of the oil. Hepatic and renal effects may occur following exposure to some essential oils (Bischoff and Guale, 1998). In addition, essential oils are volatile and can be aspirated into the lungs resulting in aspiration pneumonia (Wismer and Means, 2012).
Essential oils are also irritant to the skin and may cause erythema, dermatitis, pruritus, rash, alopecia or burns (Norris, 1990; Khan et al, 2014).
Adverse effects have been reported in cats and dogs with other ‘natural’ products containing essential oils used for the control of fleas, even when the product was used as directed (Genovese et al, 2012).
Adverse effects reported in cats
In a review of 106 feline cases of pure (100%) tea tree oil exposure the most common signs were hypersalivation (44%), ataxia (23%), lethargy or listlessness (20%), coma or recumbency (16%), tremor or muscle fasciculations (9%), hypothermia (8%) and dermatitis, pruritus or rash (2%). The outcome was known in only six cases and all six cats recovered (Khan et al, 2014).
In a case series three Angora cats became unwell after being shaved and treated with 100% tea tree oil; 60 ml was used in total on the three cats. Within 5 hours cat 1 was hypothermic and uncoordinated. On admission on the day of application cat 2 was comatose with dehydration and hypothermia. Cat 3 was alert, nervous, mildly ataxic and trembling. All cats had elevated liver enzymes and were noted to have a strong odour of tea tree oil and were washed with a detergent. Cat 3 recovered within 24 hours of admission and cat 1 recovered after 48 hours. Cat 2 improved over days 2 and 3 but remained ataxic, dehydrated and obtunded. On day 3 it began to regulate its own body temperature, but was found dead that evening. Pooled urine from all the cats was positive for terpinen-4-ol, a component of tea tree oil (Bischoff and Guale, 1998).
Death occurred in an 8-month-old chinchilla cat 14 hours after she had been sprayed with a water and tea tree oil mixture as a flea repellent. Signs included collapse, laboured respiration, significant abdominal pain, dermatitis, depression and a strong odour of the oil (Norris, 1990).
Adverse effects reported in dogs
In a review of 337 canine cases of pure tea tree oil exposure the most common signs were depression, lethargy, listlessness or being subdued (54%), paresis or weakness (45%), ataxia (43%), tremor or muscle fasciculation (10%), vomiting (6%), coma, collapse or recumbency (5%), dermatitis, pruritus or rash (4%), and elevated liver enzymes (2%). Where the outcome was known 24 dogs (94%) recovered at home (with bathing and observation), two recovered with veterinary care and two died (Khan et al, 2014). In one fatal case a 7.5-year-old English Sheepdog was accidentally given 0.3–0.4 ml of tea tree oil intravenously. He immediately collapsed and died in asystole. In the other reported fatal case a 15-year-old miniature poodle was treated dermally with approximately 28.5 ml of pure tea tree oil applied over 3 days. After the third dose he became ataxic and was taken to a veterinary practice. He appeared to recover with supportive care and was discharged, but died at home 60 hours after the last application (Khan et al, 2014).
Partial paralysis of the hind limbs, ataxia and depressed behaviour was reported in two dogs treated with only 7–8 drops of 100% tea tree oil as a flea repellent applied along their spines. Signs resolved quickly with decontamination and they both recovered (Kaluzienski, 2000).
A cross breed was euthanased approximately 4 days after 30 ml of tea tree oil was rubbed into a raw eczematous skin surface. Initial signs included intense thirst and respiratory distress. She progressed to vomiting, ataxia, drowsiness, convulsions, putrid diarrhoea, constricted pupils, dehydration, increased respiratory rate, abdominal pain and muscle fasciculation and was euthanased after failing to respond to supportive care (Thornton, 1990).
Management
Treatment of essential oil toxicity is supportive. Emesis is contraindicated if the oil has been ingested, as essential oils are volatile and there is a risk of aspiration pneumonia if vomiting occurs (Villar et al, 1994; Wismer and Means, 2012). Activated charcoal can be given (Alpert, 2016), but its efficacy in essential oil toxicosis is unknown and the amount ingested is often small. Activated charcoal is contraindicated in animals with central nervous system depression or vomiting, because of the risk of aspiration.
If the tea tree oil has been applied dermally, decontamination is essential. If the owner applied the oil they should be asked about the amount of oil given, the concentration of the oil and where it was applied. The animal should be washed promptly with water and detergent, as essential oils are not readily soluble in water. Tea tree oil has a strong odour, so this can be used as a guide to determine the effectiveness of dermal decontamination, and collaring may be useful to prevent further grooming. The animal should be examined carefully for any signs of dermal irritation, which should be treated with antihistamines or corticosteroids, as required (Khan et al, 2014).
Animals should be assessed for evidence of aspiration, e.g. coughing, noisy breathing (which could occur following ingestion or possibly from grooming), and in those with heavy contamination and significant signs, liver enzymes should be monitored.
Further management is supportive. Methocarbamol can be given for tremor or muscle fasciculation, and diazepam is the drug of choice for convulsions. Liver protectants could be considered in severe cases (Khan et al, 2014).
Poison centres collect information on adverse reactions to and inappropriate use of essential oils. A poison information centre will also be able to advise on appropriate management in a specific case.
Prognosis
Prognosis is good in animals with mild signs of tea tree oil exposure that resolve rapidly following decontamination, but in animals with heavy contamination and neurological or prolonged signs failing to respond to supportive care, the prognosis is more guarded.
Prevention
If an owner asks about the use of tea tree oil for the management of fleas or a skin condition, they should be advised that pure (100%) or concentrated tea tree oil should never be given orally or applied directly to the skin or fur, even when mixed in water (the oil is not miscible with water); even a few drops of 100% tea tree oil applied dermally has caused clinical signs in companion animals. Cats may be more at risk of adverse effects because their grooming behaviour may lead to a higher exposure after dermal application (Khan et al, 2014), and because metabolism of the terpene in tea tree oil involves glucuronidation (Kohlert et al, 2000) and cats are poor glucuronide conjugators. In addition, animals with liver disease may be more at risk of toxicity. Alternative treatments for the management of fleas should be discussed with the owner.
Products containing tea tree oil and formulated for use on the skin or fur are not expected to cause essential oil toxicity, since the concentrations used are small.
Conclusion
Some owners may find natural or herbal products attractive as treatments for fleas in pets, and pure (100%) tea tree oil is readily available. Natural does not mean safe, however, and dermal application of tea tree oil can cause poisoning which typically manifests as neurological (ataxia, tremor, lethargy, weakness, coma) and dermatological signs (dermatitis, pruritus or rash). Cats and dogs had died after dermal application of tea tree oil, and even a few drops can cause clinical signs. If a pure or concentrated essential oil has been applied, the skin should be washed promptly with detergent and water. Management is supportive.