Researchers in Belgium trying to find a relationship between eye conditions and equine herpesviruses found no statistical proof of a link, but the rate of infection in horses with eye-related findings was higher than previously described in other research.
The study, centred on the University of Veterinary Medicine in Vienna, involved four checks on stabled Lippizzaners – an initial assessment and three further checks at six-monthly intervals.
The authors, James Rushton, Jolanta Kolodziejek, Alexander Tichy, Norbert Nowotny and Barbara Nell, conducted the research in a bid to determine the potential influence of herpesviruses – specifically equine herpevirus 2 and equine herpesvirus 5 – on the development of equine ocular diseases.
In the end, the researchers found no statistical proof of a link between EHV-2 or EHV-5-infection and the presence of ocular conditions.
“However the rate of infection of horses with ocular findings was higher than previously described in the literature,” the researchers wrote in their findings, published in the open-access journal, PLoS ONE.
They also noted an increase in corneal issues as the horses aged, possibly due to an immune-mediated process due to EHV-2.
The study involved 266 Lipizzaners across three Austrian states. Each horse underwent a complete ophthalmologic examination four times. Blood samples, as well as nasal and conjunctival swabs, were obtained at the same time and used for the detection of EHV-2 and EHV-5.
In all, 266 horses were examined in the initial assessment, with 261, 249 and 230 horses revisited in the subsequent assessments.
Ophthalmic findings consistent with herpesvirus infections included conjunctival and corneal issues.
Statistical analysis revealed that the probability of positive herpesvirus results decreased with progressing age. However, the presence of corneal findings increased over time.
At the time of each examination, 45.1%, 41.8%, 43.0%, and 57.0% of horses with conjunctival or corneal findings, respectively, were positive for EHV-2 and/or EHV-5.
However, 31.6%, 17.6%, 20.1%, and 13.0% of clinically sound horses were positive for these herpesviruses at each examination period, too.
Based on the results of the study, there was a significant correlation between young horses and EHV-2 and/or EHV-5 infection. Corneal issues increased over time and with progressing age, but whether the identified findings were caused by herpesviruses could not be unequivocally determined, they said.
Three types of boarding systems were used to accommodate the horses – open boxes, closed boxes and freestalls.
“A plethora of non-inflammatory (non-vision threatening), and findings of suspected inflammatory origin (potentially vision-threatening) were identified in the study population,” they said.
The researchers focused in the paper on findings consistent with suspected inflammatory origin, to determine any influence of herpesvirus infection on the presence of respective findings.
Ophthalmic findings of inflammatory origin consisted of conjunctivitis, inflammatory corneal diseases and lesions. There were no cases of acute or chronic uveitis in any horse at any examination period.
Regarding conjunctivitis, 118 (44.4%), 145 (55.6%), 151 (60.6%), and 170 (73.9%) cases were identified in each consecutive examination period.
A total of 31 (11.7%) of the horses were positive for EHV-5 in the first study period, 51 (19.5%) in the second, 39 (15.7%) in the third, and 65 (28.3%) in the fourth.
EHV-2 was positive in 62 (23.3%), 30 (11.5%), 69 (27.7%), and 34 (14.8%) cases.
Double infection of both EHV-2 & -5 was detected in 105 (39.5%), 68 (26.1%), 42 (16.9%), and 58 (25.2%) horses.
Conjunctivitis, they noted, was a symptom associated with several causes, both infectious and non-infectious.
“It is a subjective finding and largely dependent on the examiner’s experience, unless clinically overt ocular discharge and blepharospasm are present. In this study moderate to severe redness and chemosis of the conjunctiva were termed conjunctivitis.”
The researchers said an additional noteworthy finding was the rate of newly acquired corneal findings between examination periods, the researchers said.
Previously, the influence of EHV-2 on host immune response has been described, which may predispose horses to secondary infections with other pathogens.
Further research on the host immune response to herpeviruses, rather than the influence of outside factors should be continued, they said.
Rushton JO, Kolodziejek J, Tichy A, Nowotny N, Nell B (2013) Clinical Course of Ophthalmic Findings and Potential Influence Factors of Herpesvirus
Infections: 18 Month Follow-Up of a Closed Herd of Lipizzaners. PLoS ONE
8(11): e79888. doi:10.1371/journal.pone.0079888
The full study can be read here.