Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

CHAPTER 25 OCULAR CLINICAL PHARMACOLOGY
able as a skin preparation usually used in the treatment
of skin burns. It is usually well tolerated. Studies have
shown that the silver is concentrated well in the
cornea.
Directions for use involve applying a generous amount
onto the affected cornea 4–6 times daily. Clinically the
author has found this preparation to be useful in treating
fungal keratitis.
ANTIVIRAL THERAPY
Ophthalmic viral infections are seen most commonly in
cats and are usually due to feline herpes virus. Ophthalmic
lesions caused by the virus include conjunctivitis,
keratitis (usually ulceration) or a combination of corneal
and conjunctival disease.
Antiviral therapy is of questionable value for the
treatment of viral conjunctivitis. It is mainly indicated
for active herpetic keratitis. Clinical signs of herpetic
keratitis are dendritic (linear tree-branching ulceration)
or geographic ulcers (a large superficial ulcer with loose
epithelial edges). Clinical response to antiviral treatment
regimens is variable.
All the antivirals used in veterinary practice are
human preparations. Recently, many antivirals previously
used to treat viral keratitis in humans have been
withdrawn from the market and replaced by aciclovir.
Unfortunately, aciclovir has much less efficacy against
feline herpes virus infections than the drugs (idoxuridine
and virabadine) it has replaced. In vitro testing has
revealed that the order of efficacy against feline herpes
virus is idoxuridine > virabadine > trifluridine. The least
effective drug is aciclovir.
Formulations and dose rates
Aciclovir
Apply a small amount of ointment to the affected eye six times daily.
It has been suggested that aciclovir therapy needs to be combined
with interferon to be effective. Clinically, the author has found a much
improved clinical response when aciclovir is used in combination with
interferon given by subcutaneous injection.
It has also been suggested that oral aciclovir may be of benefi t in
the treatment of feline herpetic keratitis. However, even at a dose of
50 mg/kg, the resultant concentration of aciclovir achieved was inadequate
to inhibit viral replication.
Trifluridine and idoxuridine
These drugs will usually need to be compounded by a manufacturing
pharmacist as they are no longer commercially made.
One drop every 1–2 h until a clinical response (reduction in the size
of the ulcer) is seen, then apply four times daily until the ulcer heals.
Trifl uridine and idoxuridine are only virostatic and therefore need to
be applied frequently to achieve a clinical response. Clinically, the
best results are seen when the drug is used hourly for the fi rst 24 h,
then 6–8 times daily until the corneal ulceration has resolved.
Both drugs can be irritating to the cornea, resulting in blepharospasm
and conjunctival hyperemia. If this occurs, drug treatment may
have to be discontinued.
ADDITIONAL THERAPIES FOR THE
TREATMENT OF VIRAL KERATITIS
Lysine
Lysine has been demonstrated to substantially reduce
replication and shedding of feline herpes virus when
given orally at a dose of 250 mg q.12 h. Clinically its
efficacy is unproven in controlled clinical trials but anecdotal
reports suggest that it may be of some use.
Interferon
Interferons are produced by leukocytes during an
immune response and induce an antiviral reaction in
cells. Experimentally, interferons greatly increase the
efficacy of aciclovir in in vitro testing against feline
herpes virus.
Formulations and dose rates for the
treatment of viral keratitis
TOPICAL THERAPIES
• Idoxuridine eye drops 0.1%: idoxuridine is the preferred drug for
viral keratitis. Apply 1 drop to the affected eye(s) hourly for the
fi rst 24 h, then 6–8 times daily. This will need to be compounded
by a pharmacist
Alternatively:
• Aciclovir (Zovirax, Glaxo SmithKline) ointment: apply to the
affected eye(s) 6–8 times daily
Alternatively:
• Betadine eye drops 1% or poviodine solution diluted in 9 parts
saline: one drop to the affected eye(s) 6–8 times daily. This
inexpensive treatment may resolve some cases of viral keratitis
ORAL THERAPIES
• Lysine (Lysine, Musashi): 250 mg (1/8th of a teaspoon) PO
q.12 h. Lysine may help reduce viral replication; its greatest
benefi t may be to reduce the risk of recurrence of viral
activation
• Doxycycline (Vibravet, Pfi zer) 5 mg/kg orally twice daily.
Antimicrobial treatment can be benefi cial as many cases of viral
keratitis are complicated by secondary chlamydial infection
IMMUNE STIMULATION
• Interferon injections subcutaneously 10,000 IU twice weekly.
Dilute Interferon (Interon A, Schering-Plough) in water for
injection. This treatment is thought to stimulate the antiviral
state of the immune system. In the author’s experience
concurrent interferon treatment enhances the effi cacy of
idoxuridine
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