Why Perform an Essential Fatty Acid and Antihistamine Trial?

In many cases pets with skin disease require life-long medical management to control itch. Unfortunately the most effective anti-itch therapy corticosteroids (prednisolone, dexamethasone, Prednil®, Macrolone®) have a significant potential for untoward effects particularly at high doses and/or after prolonged therapy. In cases where we cannot reduce the doses of corticosteroids to amounts we are comfortable with we recommend performing an essential fatty acid and antihistamine trial. Although these drugs have been recorded as being effective as sole therapies for itchy skin disease their success rate is quite low but they can often reduce the dose rate of corticosteroids required to keep pets comfortable therefore reducing the potential for deleterious effects of the corticosteroids.

When to do the trial?

Antihistamines and essential fatty acids will rarely have any effect if there is infection, parasite infestation, food hypersensitivity or severe inflammation present. Therefore your vet will usually treat any infections or parasite infestations before starting the trial. When assessing response to the antihistamine trial you are asked to monitor the severity of the itch as each treatment is introduced. It is therefore essential that other therapy be tapered to the lowest (alternate day if possible) dose that keeps the patient comfortable but not completely free of itch. Otherwise it would be impossible to notice any improvement.

Which antihistamine and essential fatty acids should I use?

Unfortunately there are limited studies comparing the efficacy of antihistamines and many of these assess antihistamines that are no longer available in Australia. In general first generation (older) antihistamines are cheaper and have a shorter duration of action and therefore are given three times daily. Second generation antihistamines tend to be longer acting and more expensive. Some antihistamines are contraindicated if there is concurrent heart or liver disease and treatment with some antibiotics or anti fungal drugs may also affect the antihistamine choice. I usually try antihistamines from different classes and may try up to 5 different types. Occasionally two antihistamines may be combined

A mixed Omega 3 & 6 fatty acid supplement should be used. I usually prefer to use a reputable manufacturer’s product with some form of air protection as the essential fatty acids (EFAs) can become rancid with storage and lose efficacy.

Side effects and contraindications of antihistamines:

Antihistamines are generally very safe for long-term use but some points should be noted.

First generation antihistamines tend to result in sedation, hypersalivation, diarrhoea, vomiting and occasionally CNS excitement. The sedation often reduces after 2-3 days. Care should be taken with animals having concurrent glaucoma, urinary retention or cardiac disease. Fexofenadine (Telfast®) is associated with ventricular arrhythmias in people so should be avoided in animals with significant cardiac disease.

Performing the trial

In most cases the essential fatty acid therapy can be started before the animal is completely stable but the antihistamines must be evaluated on animals that are slightly pruritic but are stable on medication otherwise a response to treatment is hard to assess. If not already on EFAs I usually recommend their use for 1 month before starting the antihistamines. Each antihistamine should be used for 1 week followed by 3 days without before giving the subsequent one. If there is significant sedation I usually persist as it often reduces within a few days. I ask the owners to record the itch and note any side effects of therapy. If the side effects are significant you should contact your veterinary surgeon for advice. If the pruritus is significantly reduced while being treated with any antihistamines an increase should be noted in the 3-day period between drugs. If this does not occur there may have been an unrelated change in the underlying pruritus. After the trial any antihistamine that appears effective should be evaluated for 1 month while any concurrent glucocorticoid therapy is tapered to assess its efficacy. This may subsequently become the maintenance dose.

Reproduced with permission of Dr Andrew Carter, BVSc(hons), DVD(RCVS)