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Kate Hepworth-Warren is an equine internal medicine specialist who currently works as a clinical assistant professor of equine medicine at North Carolina State University in Raleigh. Outside of work, she enjoys traveling, reading, running and the beach.
I got a positive for EHV-2 and EHV-5 on a nasal swab. What does that mean?
The mention of equine herpesvirus (EHV) generally brings to mind respiratory disease, abortion and neurologic disease caused by the equine alphaherpesviruses EHV-1 and EHV-4,1 two big players in respiratory disease among young animals. In addition to causing abortion, EHV-1 is a major concern because of its link to frequently fatal neurologic disease. While vaccines do exist to protect against respiratory infection and abortion from both EHV-1 and EHV-4, no vaccine is considered protective against the neurologic disease associated with EHV.2
Of note, the horse also is the host of EHV-3, an alphaherpesvirus that causes equine coital exanthema but is not typically associated with systemic illness.3
What many practitioners may forget is that the equine gammaherpesviruses EHV-2 and EHV-5 are quite prevalent in the equine population as well and have been associated with multiple diseases. Despite their inclusion on some respiratory panels, the importance of EHV-2 and EHV-5 in horses with respiratory disease is uncertain.4
The following findings are from a recent study examining a group of 167 horses imported to the United States, mostly from Europe:4
EHV-2, deemed ubiquitous among horses with a seroprevalence close to 100% in some studies, is often identified in asymptomatic animals.1,5,6 It has been isolated from nasal and conjunctival swabs, white blood cells, trigeminal ganglia, the brain, lungs, spleen and kidneys.6 It is considered to play a role in respiratory disease in young animals and may be associated with ocular disease.7
Infection is thought to occur at a very young age, with seroprevalence reaching nearly 100% by 2 to 4 months of age, but then declining before the horse reaches 1 year of age.1 In foals, EHV-2 has been associated with pharyngitis with and without concurrent lymphadenopathy, rhinitis and pyrexia.1 In older horses, EHV-2 has been associated with a poor performance syndrome, along with conjunctivitis and keratitis.1,8
While EHV-2 has been identified in conjunctival swabs obtained from horses with ocular disease, positive swabs from horses without evidence of ocular disease have also been obtained. Interestingly, in one study, EHV-2 was actually detected in a larger proportion of horses without ocular disease (22/77), as compared with a group of horses with ocular disease (4/48).6
Horses with acute herpetic keratitis can present with superficial punctate lesions that may be quite painful, similar to what is seen in cats. Affected animals will be blepharoplastic with chemosis and conjunctival hyperemia in addition to serous ocular discharge.6
Visualization of lesions may require rose bengal (i.e. dichlorotetraiodo fluorescein) staining. Diagnosis can be difficult, but cytology often identifies lymphoplasmacytic inflammation without any bacterial or fungal agents.6
EHV-5 has been shown consistently to be even more prevalent than EHV-2 among the general horse population.4,9 A 2015 study looking at nasal swabs of 663 racing Standardbreds found that 74% were positive for EHV-5 and 27% were positive for both EHV-5 and EHV-2. Neither virus was associated with signs of poor performance or respiratory disease.9
The most widely known disease associated with EHV-5 is equine multinodular pulmonary fibrosis (EMPF), which is a fibrotic interstitial lung disease.7,10,11
Given the high prevalence of asymptomatic EHV-5 infection and the low prevalence of EMPF, it is important for practitioners to remember that a positive qPCR result for EHV-5 on a nasal swab does not confirm EMPF. The ideal samples for identification of EHV-5 in suspected EMPF cases are bronchoalveolar lavage fluid and a biopsy sample of the affected lung.11,12 Definitive diagnosis of EMPF is made only by histopathologic evaluation of a lung lesion.10-12
While some information on the prevalence of both EHV-2 and EHV-5 is available in the literature, along with documentation of each virus having some link to disease, the clinical significance of these viruses remains questionable. The studies described here have shown how both viruses are ubiquitous and can be identified in animals with and without disease, making it difficult to assess the significance of a positive PCR result from a nasal swab. Individual and co-infections with EHV-2 and EHV-5 may play roles in respiratory disease, ocular disease and poor performance, but at this time the significance of these viruses remains unknown.
Dr. Hepworth-Warren is an equine internal medicine specialist who works as a clinical assistant professor of equine medicine at North Carolina State University College of Veterinary Medicine in Raleigh. Outside of work, she enjoys travelling, reading, running and the beach.