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)
