Canine herpesvirus-1 is a varicellovirus of the subfamily Alphaherpesvirniae.1 The virus was first identified in the 1960s as an agent that causes systemic infections in neonatal dogs.2 The ocular disease associated with fetal and neonatal infection is a result of panuveitis caused by intraocular viral infection.3 More recently, CHV-1 was identified as an etiologic agent of ocular disease in mature dogs.4 In contrast to fetal and neonatal CHV-1 infections, the ocular lesions in mature dogs are restricted to the ocular surface and adnexa. Ocular lesions associated with CHV-1 infection include conjunctivitis, conjunctival ulcers, and corneal ulcers that may assume punctate, dendritic, or geographic patterns.4 Dendritic ulcerative keratitis is considered a classic and pathognomonic ocular lesion associated with alphaherpesvirus infection and is believed to result from viral replication in the basal layer of the corneal epithelium, linear spread of virus between cells, and concomitant attempts at corneal epithelial regeneration.4–7
Although CHV-1 possesses a host range restricted to domestic and wild canids, it is biologically and molecularly related to several alphaherpesviruses of ocular importance in humans and other animals, including HSV-1, HSV-2, varicella zoster virus, FHV-1, bovine herpesvirus-1, cervid herpesvirus-1, and cervid herpesvirus-2.1,8–10 Canine herpesvirus-1 has a worldwide distribution and is considered endemic in many regions.11 Studies12–14 from several countries have described the seroprevalence of CHV-1 in specific canine populations as ranging from 6% to 100%. Reported CHV-1 seroprevalences likely represent underestimations of the true infection rates in the canine populations evaluated because of rapidly decreasing serum antibody titers following active CHV-1 infection.15
Most primary CHV-1 infections in mature, immunocompetent dogs have a self-limited disease progression with establishment of lifelong latent infection in the sensory ganglia.14,16 Latent alphaherpesvirus infection may reactivate spontaneously or as a result of virus and host species–dependent stimuli, including immunosuppression associated with corticosteroid treatment or other events such as stress and ocular surgery or trauma.17 Viral reactivation is characterized by anterograde neuronal transport of the virus to the site of primary infection, viral replication, and viral shedding.18,19 Similar to other alphaherpesviruses associated with recurrent ocular disease, the probable site of reactivated CHV-1 in dogs with ocular infections is the trigeminal ganglion.11,16
Although ocular CHV-1 infection may be self-limited in some dogs, the goals of treatment are to shorten the disease course, reduce discomfort, limit viral shedding into the environment, and prevent severe complications.20 Clinical reports describing the use of topical ocular application of 1% trifluridine ophthalmic solution or 0.1% idoxuridine ophthalmic solution have indicated that these topical treatments are apparently successful in the treatment of dogs with CHV-1 infection.4,20 Trifluridine and idoxuridine are nucleoside analogues that are structurally related to thymidine and possess good activity against a range of herpesviruses.21,22 Given that these antivirals are virostatic, frequent administration is clinically important.21,23
Although its clinical use has been described, no controlled studies have evaluated the efficacy and safety of topical ocular treatment with trifluridine in dogs with ocular CHV-1 infection, to our knowledge. The purpose of the study reported here was to determine the effects of topical ocular application of 1% trifluridine ophthalmic solution in dogs with experimentally induced recurrent ocular CHV-1 infection during a randomized, masked, placebo-controlled study.
Materials and Methods
Animals and induction of latent CHV-1 infection
All protocols were approved by the Animal Care and Use Committee of Cornell University and were conducted in accordance with the Association for Research in Vision and Ophthalmology Statement for the Use of Animals in Ophthalmic and Vision Research. Ten 2.5-year-old, specific pathogen–free laboratory Beagles were used. Direct contact among dogs was prevented for the duration of the study. Strict bioisolation was maintained in the isolation facilities for all personnel in contact with dogs, including the use of hazardous material suits, gloves, and boot covers, throughout the study. Dogs were acclimated to housing facilities for a minimum of 12 weeks prior to the beginning of the study.
Twelve months prior to the beginning of the study, latent CHV-1 infection was experimentally induced in each dog by topical ocular inoculation with an ocular drop method as previously described.17,24 Briefly, CHV-1 seronegative dogs were topically inoculated in both eyes with 2 × 105 TCID50 of a field strain of CHV-1a isolated from corneal samples of a dog with dendritic ulcerative keratitis treated at the Cornell University College of Veterinary Medicine Hospital for Animals.24 Gentle manual massage of the closed eyelids was immediately performed for 60 seconds after ocular inoculation.
Study design
A randomized, masked, placebo-controlled, 30-day trial was performed. Baseline study data were collected 1 day prior to initiation of the trial (day 0). Latent CHV-1 infection was experimentally reactivated in the dogs by administering an immunocompromising dosage of prednisolone (3.0 mg/kg, PO, q 24 h) for 7 days, starting on day 1. Dogs were randomly assigned to 1 of 2 groups (5 dogs/group): a treatment group that received 1 drop of 1% trifluridine ophthalmic solutionb in each eye/treatment or a placebo group that received 1 drop of an artificial tear ophthalmic solutionc in each eye/treatment. Trifluridine solution or placebo was administered 6 times daily for 2 days starting on day 4 (administrations at 8:00 am, 10:00 am, 12:00 pm, 2:00 pm, 4:00 pm, and 8:00 pm). Beginning on day 6, trifluridine solution or placebo was applied to each eye 4 times daily for the following 12 days (administrations at 8:00 am, 12:00 pm, 4:00 pm, and 8:00 pm).
Study solutions
Study solutions were used in a manner that masked the administrator's and investigators’ awareness of whether dogs were receiving trifluridine solution or placebo. The trifluridine and placebo ophthalmic solutions were placed in identical external container packaging and stored at 4°C until use.
Clinical examination and sample collection
Complete ophthalmic examinations were performed on both eyes of each dog 1 day prior to the initiation of the study (baseline examinations) and at 2-day intervals throughout the 30-day study. These examinations included slit-lamp biomicroscopyd before and after corneal application of lissamine green stain.e A modified ocular surface disease clinical scoring system24 was used to semiquantify examination findings, with a minimum daily clinical score of 0 and a maximum daily score of 18. Clinical signs of blepharospasm, ocular discharge, conjunctival hyperemia, and chemosis were scored by means of the following classifications: 0 = none, 1 = mild, 2 = moderate, and 3 = severe. Conjunctival and corneal epithelial ulceration were scored by means of the following classifications: 0 = none, 1 = punctate ulcerations, 2 = 1 or more linear or dendritic ulcerations, and 3 = geographic ulcerations. The same investigator (ECL) performed all clinical examinations and clinical ocular disease score calculations. A single cumulative ocular surface disease score was calculated for each dog on each examination day. A mean score for each group at each time point was calculated. Overall total clinical scores (sum of the clinical scores for all dogs in a given group for the duration of the study) were also calculated.
Following clinical ocular disease scoring but prior to lissamine green application, conjunctival swab samples were collected for CHV-1 PCR assay by gently brushing a sterile polyester-tipped swabf across the conjunctival fornices of both eyes of each dog. Ocular swab samples were collected 1 day prior to the initiation of the study and at 3-day intervals throughout the duration of the study. These samples were stored in sterile tubes at −80°C until analysis. All virologic samples were collected by the same investigator (CBS).
Schirmer I tear testingg was performed on both eyes of each dog 1 day prior to the initiation of the study (baseline testing) and on study days 15 and 30. Blood samples were collected from each dog by peripheral venipuncture 1 day prior to the initiation of the study (baseline samples) and on study days 15 and 30 for a CBC, serum biochemical analysis, and assessment of CHV-1 serum neutralization antibody titers. Samples for CBCs and serum biochemical analyses were processed immediately, and serum samples for antibody titer assessments were stored at −80°C until analysis.
CHV-1 serum neutralization antibody titer assessment
Serum samples were analyzed for CHV-1 serum neutralization antibody titers concurrently in a single session. The test sample and high-positive control, low-positive control, and negative control sera were heat-inactivated in a 56°C water bath for 30 minutes. Test medium composed of 50 µL of minimum essential medium-E with heat-inactivated 10% fetal bovine serum and 2% penicillin-streptomycin solution was added to plate wells along with 50 μL of test sample, and 2-fold serial dilutions were performed. The working dilution of virus (100 TCID50/50 μL) and back titrations of 10−1, 10−2, and 10−3 dilutions were prepared and added to test wells. Plates were incubated for 1.5 hours at 37°C. Canine kidney DE-2 cellsh were added to the wells and incubated for 3 days in a 37°C humidified 5% CO2 incubator. An inverted microscope ocular was used to examine wells for cytopathic effect, and antibody titers were calculated.
CHV-1 real-time PCR analysis
A CHV-1 real-time PCR assay was performed with CHV-specific primers and probes as described previously.25 Samples were maintained at −80°C until the batch was processed. The DNA was extracted by means of a 96-well magnetic bead-based processi with a commercial DNA extraction kit.j
Statistical analysis
Significance was set at α (P value) < 0.05 for all analyses. The overall total clinical scores (sum of the clinical scores for all dogs in a given group for the duration of the study) for dogs that received trifluridine and those for dogs that received placebo were compared by use of a 2-sample Student t test. The analysis was performed with statistical software.k A least squares linear regression analysis, with concentration on days 5 to 21, was performed to assess the mean total clinical scores over time of the trifluridine and placebo groups.
Survival analysis was used to assess the significance of differences in CHV-1 shedding duration for the treatment and placebo groups. The shedding duration for the 2 groups was plotted with the Kaplan-Meier method and tested for significance with a log-rank test. The analysis was performed with statistical software.l A 2-sample Student t test was used to compare the CHV-1 serum neutralization antibody titers and Schirmer tear test values on days 0, 15, and 30 between the treatment group and placebo group.
Results
Prestudy and baseline examinations
Following initial ocular CHV-1 inoculation to induce latent infection, all study dogs developed bilateral ocular disease and serologic responses typical of primary ocular CHV-1 infection.24 The ocular disease was characterized by bilateral conjunctival hyperemia, chemosis, and mucopurulent ocular discharge that resolved over a 2- to 3-week period in each dog. All dogs seroconverted during the primary infection and had detectable CHV-1 serum neutralization antibody titers by 15 days after viral inoculation. Following resolution of the primary ocular infection, dogs remained free of clinical signs of ocular disease until induction of recurrence in the present study.
One day prior to the initiation of the trial, no abnormalities were detected in any of the study dogs during physical and ophthalmic examinations. Schirmer I tear test results were > 15 mm/min in both eyes of each dog. Lissamine green stain was not retained by the cornea or conjunctiva of either eye in any dog. Results of CBCs and serum biochemical analyses were unremarkable for each dog.
Clinical ophthalmic examinations
Clinical signs consistent with recurrent ocular CHV-1 infection were detected in all dogs in the treatment and placebo groups by day 4. Ocular disease was characterized by intermittent blepharospasm, ocular discharge, conjunctival hyperemia, chemosis, and conjunctival epithelial ulceration. No corneal epithelial ulcerations were detected by lissamine green application in any dog during the study.
The dogs that received trifluridine developed mild conjunctivitis, and their ocular disease clinical scores reached maximum values on day 6 (mean ± SD clinical score, 2.6 ± 1.2); thereafter, scores rapidly decreased, returning to baseline values by day 20. The dogs that received placebo developed moderate conjunctivitis, and their ocular disease clinical scores reached maximum values on day 8 (mean clinical score, 4.0 ± 0.89); thereafter, scores slowly returned to baseline values by day 28. Between days 4 and 24, the placebo group had consistently higher mean total clinical scores than the treatment group (Figure 1).
Mean ± SD total clinical scores for eyes of adult dogs with experimentally induced recurrent ocular CHV-1 infection treated with 1% trifluridine ophthalmic solution (gray squares; n = 5) or artificial tears solution (black diamonds; 5). Recurrent ocular CHV-1 infection was experimentally induced by oral administration of prednisolone for 7 days beginning on day 1. Dogs were administered either the treatment solution or placebo solution topically in both eyes, 6 times daily for 2 days and then 4 times daily for 12 days beginning on study day 4. Clinical ocular disease scores were calculated on the basis of complete ophthalmic examination findings 1 day before initiation of the study (day 0) and every 2 days for 30 days. A modified ocular surface disease clinical scoring system24 was used to semiquantify examination findings, with a minimum daily clinical score of zero and a maximum daily score of 18. Clinical signs of blepharospasm, ocular discharge, conjunctival hyperemia, and chemosis were scored by means of the following classifications: 0 = none, 1 = mild, 2 = moderate, and 3 = severe. Conjunctival and corneal epithelial ulceration were scored by means of the following classifications: 0 = none, 1 = punctate ulcerations, 2 = 1 or more linear or dendritic ulcerations, and 3 = geographic ulcerations. A cumulative clinical score was calculated for each dog on each examination day. Mean total clinical scores for each examination day were then determined for dogs receiving the treatment solution and dogs receiving the placebo solution.
Citation: American Journal of Veterinary Research 77, 10; 10.2460/ajvr.77.10.1140
Mean ± SD total clinical scores for eyes of adult dogs with experimentally induced recurrent ocular CHV-1 infection treated with 1% trifluridine ophthalmic solution (gray squares; n = 5) or artificial tears solution (black diamonds; 5). Recurrent ocular CHV-1 infection was experimentally induced by oral administration of prednisolone for 7 days beginning on day 1. Dogs were administered either the treatment solution or placebo solution topically in both eyes, 6 times daily for 2 days and then 4 times daily for 12 days beginning on study day 4. Clinical ocular disease scores were calculated on the basis of complete ophthalmic examination findings 1 day before initiation of the study (day 0) and every 2 days for 30 days. A modified ocular surface disease clinical scoring system24 was used to semiquantify examination findings, with a minimum daily clinical score of zero and a maximum daily score of 18. Clinical signs of blepharospasm, ocular discharge, conjunctival hyperemia, and chemosis were scored by means of the following classifications: 0 = none, 1 = mild, 2 = moderate, and 3 = severe. Conjunctival and corneal epithelial ulceration were scored by means of the following classifications: 0 = none, 1 = punctate ulcerations, 2 = 1 or more linear or dendritic ulcerations, and 3 = geographic ulcerations. A cumulative clinical score was calculated for each dog on each examination day. Mean total clinical scores for each examination day were then determined for dogs receiving the treatment solution and dogs receiving the placebo solution.
Citation: American Journal of Veterinary Research 77, 10; 10.2460/ajvr.77.10.1140
Mean ± SD total clinical scores for eyes of adult dogs with experimentally induced recurrent ocular CHV-1 infection treated with 1% trifluridine ophthalmic solution (gray squares; n = 5) or artificial tears solution (black diamonds; 5). Recurrent ocular CHV-1 infection was experimentally induced by oral administration of prednisolone for 7 days beginning on day 1. Dogs were administered either the treatment solution or placebo solution topically in both eyes, 6 times daily for 2 days and then 4 times daily for 12 days beginning on study day 4. Clinical ocular disease scores were calculated on the basis of complete ophthalmic examination findings 1 day before initiation of the study (day 0) and every 2 days for 30 days. A modified ocular surface disease clinical scoring system24 was used to semiquantify examination findings, with a minimum daily clinical score of zero and a maximum daily score of 18. Clinical signs of blepharospasm, ocular discharge, conjunctival hyperemia, and chemosis were scored by means of the following classifications: 0 = none, 1 = mild, 2 = moderate, and 3 = severe. Conjunctival and corneal epithelial ulceration were scored by means of the following classifications: 0 = none, 1 = punctate ulcerations, 2 = 1 or more linear or dendritic ulcerations, and 3 = geographic ulcerations. A cumulative clinical score was calculated for each dog on each examination day. Mean total clinical scores for each examination day were then determined for dogs receiving the treatment solution and dogs receiving the placebo solution.
Citation: American Journal of Veterinary Research 77, 10; 10.2460/ajvr.77.10.1140
The mean total clinical scores for the treatment group were significantly (P ≤ 0.001) lower over time, compared with findings for the placebo group. The mean daily difference in the mean total clinical score values between dogs that received trifluridine and dogs that received placebo was 1.12 over the course of the study. The overall total clinical scores for the treatment group were also significantly (P = 0.001) lower than those for the placebo group (Figure 1).
Schirmer I tear test results were > 15 mm/min in both eyes of each dog on days 15 and 30. There were no significant differences in Schirmer I tear test results between the dogs that received trifluridine or placebo. No signs of immediate ocular irritation or increased discomfort associated with study agent administration were detected in any dogs.
Ocular viral shedding
Ocular viral shedding (detected by CHV-1 PCR assay) in the study dogs occurred on study days 6 through 12. In the treatment group, CHV-1 PCR assays yielded positive results for 4 ocular samples collected on 2 days (3 samples on day 6 and 1 sample on day 9). Three dogs had positive assay results on 1 or more days. The median CHV-1 shedding duration for dogs that received trifluridine was 1 day (range, 0 to 4 days). In the placebo group, CHV-1 PCR assays yielded positive results for 11 ocular samples collected on 3 days (5 samples on day 6, 3 samples on day 9, and 3 samples on day 12). All 5 individual dogs in this group had positive assay results on 1 or more days. The median CHV-1 shedding duration for dogs that received placebo was 7 days (range, 1 to 7 days). The treatment group had a significantly (P = 0.04) shorter median shedding duration, compared with that of the placebo group (Figure 2).
Plot of the probability of CHV-1 shedding for eyes of dogs with experimentally induced recurrent ocular CHV-1 infection that were treated with 1% trifluridine ophthalmic solution (gray line; n = 5) or artificial tears solution (black line; 5) for 14 days (beginning on study day 4). Ocular viral shedding was evaluated every 3 days for 30 days. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 77, 10; 10.2460/ajvr.77.10.1140
Plot of the probability of CHV-1 shedding for eyes of dogs with experimentally induced recurrent ocular CHV-1 infection that were treated with 1% trifluridine ophthalmic solution (gray line; n = 5) or artificial tears solution (black line; 5) for 14 days (beginning on study day 4). Ocular viral shedding was evaluated every 3 days for 30 days. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 77, 10; 10.2460/ajvr.77.10.1140
Plot of the probability of CHV-1 shedding for eyes of dogs with experimentally induced recurrent ocular CHV-1 infection that were treated with 1% trifluridine ophthalmic solution (gray line; n = 5) or artificial tears solution (black line; 5) for 14 days (beginning on study day 4). Ocular viral shedding was evaluated every 3 days for 30 days. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 77, 10; 10.2460/ajvr.77.10.1140
CHV-1 serum neutralization antibody titers
On day 0, CHV-1 serum neutralization antibody titers were detected in all dogs. The CHV-1 serum neutralization antibody titer increased, albeit not significantly, in both the treatment and placebo groups over the course of the study period (Figure 3). There was no significant difference in CHV-1 serum neutralization antibody titers between the dogs that received trifluridine and those that received placebo on days 0, 15, or 30.
Mean ± SD CHV-1 serum neutralization antibody titers in dogs with experimentally induced recurrent ocular CHV-1 infection that were treated with 1% trifluridine ophthalmic solution (gray bars; n = 5) or artificial tears solution (black bars; 5) for 14 days (beginning on study day 4). Serum samples were collected for analysis before initiation of the study (day 0) and on days 15 and 30. Antibody titers are reported as the reciprocal of the highest dilution of serum that neutralized the infectivity of the virus. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 77, 10; 10.2460/ajvr.77.10.1140
Mean ± SD CHV-1 serum neutralization antibody titers in dogs with experimentally induced recurrent ocular CHV-1 infection that were treated with 1% trifluridine ophthalmic solution (gray bars; n = 5) or artificial tears solution (black bars; 5) for 14 days (beginning on study day 4). Serum samples were collected for analysis before initiation of the study (day 0) and on days 15 and 30. Antibody titers are reported as the reciprocal of the highest dilution of serum that neutralized the infectivity of the virus. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 77, 10; 10.2460/ajvr.77.10.1140
Mean ± SD CHV-1 serum neutralization antibody titers in dogs with experimentally induced recurrent ocular CHV-1 infection that were treated with 1% trifluridine ophthalmic solution (gray bars; n = 5) or artificial tears solution (black bars; 5) for 14 days (beginning on study day 4). Serum samples were collected for analysis before initiation of the study (day 0) and on days 15 and 30. Antibody titers are reported as the reciprocal of the highest dilution of serum that neutralized the infectivity of the virus. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 77, 10; 10.2460/ajvr.77.10.1140
General diagnostic assays and examinations
Results of CBCs and serum biochemical analyses on days 15 and 30 were unremarkable for each dog. No overt systemic clinical abnormalities were observed in any dog at any time during the study.
Discussion
In the present study, frequent topical ocular application of 1% trifluridine ophthalmic solution was well tolerated in all dogs with experimentally induced recurrent ocular CHV-1 infection. The trifluridine treatment regimen used in the study effectively reduced mean total and overall total clinical ocular disease scores in the study dogs, compared with findings in dogs that received placebo. Dogs in the treatment group developed conjunctivitis that was milder and of shorter duration than that which developed in dogs in the placebo group. In addition, the number of dogs with detectable ocular CHV-1 shedding and the duration of viral shedding were reduced in the trifluridine-treated dogs. These results suggested that 1% trifluridine ophthalmic solution can be effectively used for the clinical management of ocular CHV-1 infection in dogs.
Trifluridine is incorporated into viral and, to a reduced extent, host cellular DNA and inhibits DNA synthesis at low concentrations.26 Results of several in vitro studies26–28 have indicated that trifluridine is a potent inhibitor of the cytopathic effects of HSV-1. De Clercq et al27 determined that a mean concentration of 0.7 μg of trifluridine/mL inhibited the cytopathic effects of clinical and laboratory strains of both HSV-1 and HSV-2 by 50%. In addition, trifluridine is substantially more potent in vitro than idoxuridine, vidarabine, bromovinyldeoxyuridine, and acyclovir against strains of FHV-1 in cultured feline kidney cells.29
Trifluridine can penetrate intact corneal epithelium, although corneal ulceration and uveitis increase its intraocular penetration.30,31 In contrast, idoxuridine does not penetrate the cornea unless the epithelial barrier has been compromised.30,31 In rabbits with experimental HSV-1 keratitis, treatment with 1% trifluridine ophthalmic solution 6 times daily for 2 days dramatically reduced viral titers within the corneal epithelium relative to findings for the placebo or prednisolone treatment.32 Topical application of trifluridine 1% ophthalmic solution was effective at resolving experimental stromal herpetic keratitis induced by injection of HSV-1 into the corneal stroma of rabbits.33 In rabbits with iontophoretically induced recurrent HSV-1 infection, frequent topical ocular treatment with 1% trifluridine ophthalmic solution significantly decreased the recovery of HSV-1 from the tear film, but did not affect the incidence of corneal epithelial lesions.34 Romanowski et al35 performed a study in rabbits with experimentally induced HSV-1 keratitis to determine the effects of ophthalmic preparations of 1% trifluridine (9 applications daily for 3 days, then 4 applications daily for 4 days), 1% cidofovir (2 applications daily for 7 days), 0.5% cidofovir (2 applications daily for 7 days), and 3% acyclovir (5 applications daily for 7 days) on HSV-1 ocular disease. Results of that study35 indicated that treatment with any of these antiviral medications led to significantly lower HSV-1 ocular viral titers, decreased total number of HSV-1–positive eyes during the treatment period, lower clinical ophthalmic scores, fewer eyes with keratitis, and a shorter duration of keratitis, compared with findings in the control group treated with control vehicle (2 applications daily for 7 days).
Trifluridine has been studied in masked comparative trials involving several other topical ophthalmic antiviral preparations in humans with herpetic keratitis.36,37 In 2 systematic, quantitative assessments of the pertinent published literature, trifluridine was determined to have a greater clinical efficacy than topical application of either idoxuridine or vidarabine for the treatment of HSV-1 epithelial keratitis in humans.37,38 In a study36 evaluating the relative efficacy of trifluridine and idoxuridine treatments, 22 of 23 (96%) trifluridine-treated herpetic corneal ulcers were healed within 14 days, whereas only 10 of 16 (63%) idoxuridine-treated corneal ulcers were healed in that time period. Several additional masked, controlled studies39–44 have confirmed the effectiveness of trifluridine for resolving dendritic herpetic keratitis in humans and have indicated that the efficacy of trifluridine is similar, or superior, to that of acyclovir, adenine arabinoside, idoxuridine, or bromovinyldeoxyuridine. In another study45 specifically evaluating HSV-1 geographic corneal ulcers, ocular application of 1% trifluridine ophthalmic solution was found to be a more effective treatment than ocular application of adenine arabinoside.
For humans with ocular HSV-1 infection, the manufacturers of 1% trifluridine ophthalmic solution recommend topical ocular application every 2 hours while awake, followed by less frequent application (every 4 hours) for an additional week when clinical improvement is observed.22,46 In general, ocular treatment with trifluridine is well tolerated by humans; however, punctate epithelial keratopathy, corneal epithelial dysplasia, follicular conjunctivitis, lacrimal punctal occlusion, conjunctival cicatrization, and anterior segment ischemia are reported potential complications of trifluridine application.41,47–49 Most of the adverse reactions reported for ocular treatment with trifluridine in humans are associated with long-term administration, and it is generally recommended to avoid the prolonged use of trifluridine when possible.38 No overt signs of local ocular toxicosis or ocular irritation associated with 14 days of application were observed in the dogs of the present study; however, assessment of the effects of longer durations of treatment in dogs is warranted. Trifluridine has been described as well tolerated by dogs following topical application at a frequency of 6 to 8 times daily for the first 48 hours and then 4 times daily until resolution of clinical signs.4
The dogs with experimentally induced, recrudescent CHV-1 ocular disease that were used in the present study were otherwise systemically healthy. As a result, the present research methods may not mimic all clinical scenarios for herpetic diseases in dogs. Clinical reports suggest that dogs with ocular CHV-1 infections are frequently receiving immunosuppressive medications or affected by immunosuppressive conditions.4,20,50 An additional limitation of the present study was that the 2 groups of dogs evaluated were small. Episodes of ocular CHV-1 shedding may also have been missed in some dogs owing to the frequency of ocular sample collection in this study. When a quantitative PCR assay was used to assess swab samples for HSV in a study of 25 HSV-2–seropositive and 18 HSV-1–seropositive healthy adults, it was reported that 24% of anogenital HSV-2 reactivations and 21% of oral HSV-1 reactivations lasted ≤ 6 hours, and 49% of anogenital HSV-2 reactions and 39% of oral HSV-1 reactions lasted ≤ 12 hours.51 Ocular swab samples collected at more frequent intervals might have detected shedding of CHV-1 in additional dogs in the present study.
Given that 1% trifluridine ophthalmic solution may require a frequent application schedule and good client compliance to be effective in dogs, a nonirritating, effective, long-acting antiviral preparation would be advantageous for treatment of ocular CHV-1 infections in dogs. Cidofovir is an acyclic nucleoside monophosphate analogue of cytosine that is active in vitro against CHV-1.52 The twice-daily topical administration of 0.5% cidofovir ophthalmic solution was described in a case report50 to be clinically effective against ocular CHV-1 infection in a dog, and an experimental study53 revealed its effectiveness in cats with ocular FHV-1 infection. Cidofovir treatment involves a convenient dosing schedule, but results of a recent study52 evaluating the effects of the twice-daily topical ocular application of 0.5% cidofovir in dogs with experimental ocular CHV-1 infection indicated that, although that regimen effectively reduced the duration of ocular viral shedding, cidofovir treatment was associated with marked local ocular toxic effects that may preclude its routine clinical use in dogs. These toxic effects included exacerbation of herpetic ocular disease, increased corneoconjunctival leukocyte infiltration, ulcerative blepharitis, and conjunctival pigmentation.52
In a study22 comparing trifluridine ophthalmic solution, cidofovir ophthalmic solution, and penciclovir ophthalmic ointment for the treatment of HSV-1 keratitis in New Zealand White rabbits, topical ocular trifluridine treatment was highly effective even when only applied once a day. There was no significant difference in keratitis severity grades between eyes administered trifluridine 2, 4, or 7 times a day and eyes administered cidofovir 2 times a day.22 Even though rabbits are not the natural host for HSV-1, those data indicate that trifluridine may be effective for ocular herpesvirus infections when administered less frequently than is typically recommended. Additional research is warranted to test the efficacy of less frequent topical ocular application of 1% trifluridine ophthalmic solution in dogs with CHV-1 infection.
Acknowledgments
Supported in part by the Merial Veterinary Scholars Program.
ABBREVIATIONS
| CHV-1 | Canine herpesvirus-1 |
| FHV-1 | Feline herpesvirus-1 |
| HSV-1 | Herpes simplex virus-1 |
| HSV-2 | Herpes simplex virus-2 |
Footnotes
Cornell University College of Veterinary Medicine Hospital for Animals, Ithaca, NY.
Viroptic ophthalmic solution, Pfizer Inc, New York, NY.
Polyvinyl alcohol 1.4% artificial tears solution, Rugby Laboratories Inc, Duluth, Ga.
Kowa SL-15, Kowa Co, Tokyo, Japan.
Lissamine green ophthalmic strips, Rose Stone Enterprises, Alta Loma, Calif.
Sterile polyester-tipped applicators, Puritan Medical Products Co, Guilford, Me.
Schirmer tear test standardized sterile strips, Intervet Inc, Summit, NJ.
Developed by the New York State Animal Health Diagnostic Center, Ithaca, NY.
Mag Max 96, Life Technologies Corp, Grand Island, NY.
AM 1840, Life Technologies Corp, Grand Island, NY.
Statistix, version 9.0, Analytical Software, Tallahassee, Fla.
SPSS, version 20, IBM Corp, White Plains, NY.
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