The lesser known equine herpesviruses

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The equine alphaherpesviruses: EHV-1, EHV-4, EHV-3

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

The equine gammaherpesviruses: EHV-2, EHV-5

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

• 52% were shedding at least one herpesvirus, as identified by quantitative polymerase chain reaction (qPCR) testing of a nasal swab.
• EHV-5 and EHV-2 were far more prevalent than EHV-1 and EHV-4.
• EHV-5 was detected in 40.7% of horses.
• 28.7% of samples were positive for EHV-2.
• Only 1.2% and 3% of horses were positive for EHV-1 and EHV-4, respectively.⁴
• The presence of clinical signs (e.g. fever, tachypnea, nasal discharge, lymphadenopathy) was not significantly correlated with qPCR status.⁴

EHV-2

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

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

• EMPF generally causes signs similar to equine asthma syndrome, including weight loss, tachypnea and tachycardia.
• Variable hyperthermia is often present, as well as wheezes and crackles on pulmonary auscultation.^10,11
• Lungs may appear normal on ultrasonography or may show evidence of bilateral pleural roughening. Radiography may show characteristic nodular lesions in addition to a diffuse interstitial pattern.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.

References

1. Marenzoni ML, Stefanetti V, Danzetta ML, et al. Gammaherpesvirus infections in equids: a review. Vet Med (Auckl) 2015;6:91-101.
2. Lunn DP, Davis-Poynet N, Flaminio MJBF, et al. Equine herpesvirus-1 consensus statement. J Vet Intern Med 2009;23(3):450-461.
3. Barrandeguy M, Thiry E. Equine coital exanthema and its potential economic implication for the equine industry. Vet J 2012;191:35-40.
4. Smith FL, Watson JL, Spier SJ, et al. Frequency of shedding of respiratory pathogens in horses recently imported to the United States. J Vet Intern Med 2018;32(4):1436-1441.
5. Rushton JO, Kolodziejek J, Tichy A, et al. Detection of equid herpesviruses 2 and 5 in a herd of 266 Lipizzaners in association with ocular findings. Vet Microbiol 2013;164(1-2):139-144.
6. Borchers K, Ebert M, Fetsch A, et al. Prevalence of equine herpesvirus type 2 (EHV-2) DNA in ocular swabs and its cell tropism for equine conjunctiva. Vet Microbiol 2006;118(3-4):260-266.
7. Kydd JH, Lunn DP, Osterrieder K. Report of the Fourth International Havemeyer Workshop on Equid Herpesviruses (EHV) EHV-1, EHV-2 and EHV-5. Equine Vet J 2019;51(5):565-568.
8. Plummer CE, Colitz CMH, Kuonen V. Ocular infections. In: Sellon D, Long M, eds. Equine infectious disease, 2nd ed. St. Louis, Missouri: Saunders Elsevier; 2014:109-188e3.
9. Back H, Ullman K, Treiberg Berndtsson L, et al. Viral load of equine herpesviruses 2 and 5 in nasal swabs of actively racing Standardbred trotters: temporal relationship of shedding to clinical findings and poor performance. Vet Microbiol 2015;179(3-4):142-148.
10. Marenzoni ML, Passamonti F, Lepri E, et al. Quantification of equid herpesvirus 5 DNA in clinical and necropsy specimens collected from a horse with equine multinodular pulmonary fibrosis. J Vet Diagn Invest 2011;23(4):802-806.
11. Wilkins PA, Lascola KM. Update on interstitial pneumonia. Vet Clin North Am Equine Pract 2015;31:137-157.
12. Davis E. Disorders of the respiratory system. In: Reed SM, Bayly WM, Sellon DC, eds. Equine internal medicine, 4th ed. St. Louis, MO: Saunders Elsevier; 2018:313-386.