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of a couple of critical facts: first I have to be willing to be wrong when making an educated guess regarding the etiological diagnosis and, second, I have to use the absolute best therapeutic trial and demand excellent owner compliance in executing that trial.
Diagnosing Keratoconjunctivitis Using Clinical Signs as Your Guide
Using clinical signs of surface ocular disease as a “diagnostic assay” requires a philosophical approach that I liken to adding pebbles to one of two sides of
an old-fashioned scale or balance. I start with the paradigm that feline keratoconjunctivitis is infectious
till proven otherwise and that by far and away the most commonly implicated infectious organisms are FHV-1 and Chlamydia felis. I then consider the clinical signs outlined in the table. Using each feature as a discerning feature I aim to place one of my “diagnostic pebbles” on the herpetic or chlamydial sides of the balance, thereby making a clinical judgment at the end of the examination as to which of these 2 organisms is more likely to be the cause of the disease seen.
although some have been tested for safety in this species. Agents with a reasonable safety profile in humans are not always or predictably non-toxic when administered to cats and all require safety and effica- cy testing in vivo.
· Many antiviral agents require host metabolism before achieving their active form. These agents are not reliably or predictably metabolized by cats and phar- macokinetic studies in cats are required.
· Antiviral agents tend to be more toxic than do anti- bacterial agents since viruses are obligate intracel- lular organisms and co-opt or have close analogues of the host’s cellular “machinery”. This limits many antiviral agents to topical (ophthalmic) rather than systemic use.
· All antiviral agents currently used for cats infected with FHV-1 are virostatic. Therefore, they typically require frequent administration to be effective.
The following antiviral agents have been studied to varying degrees for their efficacy against FHV-1, their pharmacokinetics in cats, and/or their safety and efficacy in treating cats infected with FHV-1.
Trifluridine (TFU or trifluorothymidine) is too toxic to be administered systemically but topically administered trifluridine is considered one of the most effective drugs for treating HSV-1 keratitis. This is in part due
to its superior corneal epithelial penetration. It is also one of the more potent antiviral drugs for FHV-1. It is commercially available in the USA as a 1% ophthalmic solution that should be applied to the affected eye 5-6 times daily. Unfortunately, it is expensive and is often not well tolerated by cats, presumably due to a stinging reaction reported in humans.
Idoxuridine (IDU) is a nonspecific inhibitor of DNA synthesis, affecting any process requiring thymidine. Therefore, host cells are similarly affected, systemic therapy is not possible, and corneal toxicity can occur.
It has been used as an ophthalmic 0.1% solution or 0.5% ointment. This drug is reasonably well tolerated by most cats and seems efficacious in many. It is no longer commercially available in the USA but can be obtained from a compounding pharmacist. It should be applied to the affected eye 5-6 times daily.
Vidarabine (VDB) interferes with DNA polymerase and, like idoxuridine, is non-selective in its effect and so is associated with notable host toxicity if administered systemically. Because it affects a viral replication step different from that targeted by idoxuridine, vidarabine may be effective in patients whose disease seems resistant to idoxuridine. As a 3% ophthalmic ointment, vidarabine often appears to be better tolerated than many of the antiviral solutions. Where it is not available commercially, it can be obtained from a compounding pharmacist. Like idoxuridine, it should be applied to the affected eye 5-6 times daily.
Note that both agents cause some of the signs and that it is a weighted assessment. This introduces a notable element of subjectivity into the assessment.
I unashamedly tell clients this and explain that I still believe that this is better than wasting their money
on a laboratory test. I also use this time to introduce the concept that the clients themselves will form the critical next step in the diagnostic process – “response to therapy”. We will discuss this more fully in the next session.
Antiviral Therapy
If we are to use response to therapy as a “diagnostic test”, then we must choose the optimum therapeutic approach possible for each cat. Although a large variety of antiviral agents exists for oral or topical treatment of cats infected with feline herpesvirus type 1 (FHV-1), some general comments regarding these agents are possible: · No antiviral agent has been developed for FHV-1;
although many have been tested for efficacy against this virus. Agents highly effective against closely-re- lated human herpesviruses are not necessarily or predictably effective against FHV-1 and all should be tested in vitro before they are administered to cats.
· No antiviral agent has been developed for cats;
Your Singapore, the Tropical Garden City
  Clinical Signs
  FHV-1
   C. felis
  Conjunctival hyperemia
  +++
   +
  Chemosis
   +
    +++
  Conjunctival ulceration
 +/-
  -
  Keratitis
   +/-
    -
  Dendrites
 Pathognomonic
  -
  Respiratory signs/malaise
   ++
    +/-
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