Page 236 - WSAVA2018
P. 236

 25-28 September, 2018 | Singapore
usually nonulcerative, substantial conjunctival thickening and hyperemia can occur secondary to inflammatory
cell infiltration. Corneal involvement is relatively
frequent in recrudescent disease compared to primary infection and may involve the corneal epithelium or stroma. With epithelial involvement, dendritic and later geographic corneal ulceration may be seen just as in primary infections. Corneal stromal disease is typically immunopathological (i.e., immune-mediated, but not necessarily autoimmune) in origin and includes stromal neovascularization, edema, stromal cell infiltration, and ultimately fibrosis usually under an intact epithelium. Consensus has not been reached regarding the antigens responsible for the subepithelial immunological response within cornea and/or conjunctiva. Some believe the process is driven by viral antigens, while others are suspicious that altered self-antigens are the focus of the immunological response.
FHV-1-Associated Disease Syndromes
The following diseases have been associated with detection of FHV-1 in affected tissues; however the causative role of the virus in each syndrome has been variably proven.
Symblepharon. There is little question that symblepharon can be a sequela to severe primary FHV-1 infection. It is commonly seen in young animals, and presumably occurs as a result of widespread ulceration with exposure of the conjunctival substantia propria
and sometimes also the corneal stroma. FHV-1 is almost certainly the predominant cause of symblepharon formation in cats and other infectious agents are unlikely to cause symblepharon formation.
Corneal sequestration. Experimentally, FHV-1 inoculation (in cats receiving corticosteroids) can result in corneal sequestration. However, the prevalence of detectable FHV-1 in samples collected from cats with sequestra
has varied widely in the clinical setting and the link between FHV-1 and sequestra has not been shown
to be causative. It seems likely that sequestration is a non-specific response to stromal exposure or damage and that FHV-1 is just one possible cause of this disease. This is borne out in a study by Nasisse et al who reported identification of FHV-1 DNA in 86 of 156 (55%) of sequestra analyzed (compared with only 6% of clinically normal corneas). A lower prevalence of FHV-1 DNA was found in corneas of Persian and Himalayan cats with sequestration, suggesting that other non-viral causes of sequestration are more likely to be operative in these breeds.
Eosinophilic keratitis. Prior clinical studies have suggested a link between FHV-1 infection and eosinophilic keratitis. PCR testing of corneal scrapings from cats with cytology-confirmed eosinophilic keratitis has revealed 76% (45/59) of cases to be FHV-1 positive.
However, PCR performed on tears collected onto a
STT was negative in 10 cats with cytologically proven eosinophilic keratitis. As with corneal sequestra, the role of the virus in the initiation or exacerbation of this disease has not been determined; however anecdotally some patients with this syndrome improve with antiviral therapy alone.
Dermatitis. Periodically, FHV-1 has been identified as
a cause of dermatological lesions, particularly those surrounding the eyes and involving nasal skin of domestic and wild felidae. This is not surprising when one considers the marked epithelial tropism of this
virus and the reliability with which HSV-1 causes dermal lesions. We have recently examined the diagnostic
utility of FHV-1 PCR for this disease. FHV-1 DNA was detected in all 9 biopsy specimens from 5 cats with herpetic dermatitis but in 1 of 17 biopsy specimens from the 14 cats with nonherpetic dermatitis, and was not detected in any of the 21 biopsy specimens from the 8 cats without dermatitis. This is in sharp contrast to the use of this technique in ocular tissues where the extent of viral shedding in normal animals dramatically reduces the sensitivity of a positive test in affected animals. When results of histologic examination were used as the gold standard in this study of cats with dermatitis, sensitivity and specificity of the PCR assay were 100% and 95%, respectively. We concluded that FHV-1 DNA can be detected in the skin of cats with herpetic dermatitis,
that the virus may play a causative role in the disease, and that this PCR assay may be useful in confirming a diagnosis of herpetic dermatitis.
Diagnosing Cats with Keratoconjunctivitis
One of my least favorite questions is “What is the best laboratory test for cats with corneal or conjunctival disease?”. In reality there is not one. Explaining this position requires an understanding of an essential
fact about feline herpesvirus (FHV-1) - clinically normal cats (and lots of them) can shed FHV-1 at their ocular surface. Because PCR is more sensitive than IFA or VI, this assay exacerbates this problem. In fact, in some humane shelter-based populations, about half of all normal cats are shedding FHV-1 DNA as determined by PCR. Therefore, in some circumstances, the number of false positive test results we can expect is extraordinarily high and we may be better to flip a coin than to run
that PCR assay! Given the predictably high rate of false positive (particularly with serology and PCR) and negative test results (particularly with VI and IFA), I now no longer conduct laboratory tests for FHV-1 or Chlamydia felis (previously Chlamydia psittaci and, before that, Chlamydophila felis) in individual cats with keratoconjunctivitis. Rather, I resort to good old fashioned clinical acumen. My diagnostic “tests” now are (i) the history and clinical exam findings followed by (ii) response to therapy. This requires acceptance

   234   235   236   237   238