• Open Access

Nasal Shedding of Equine Herpesvirus-1 from Horses in an Outbreak of Equine Herpes Myeloencephalopathy in Western Canada


  • This work was supported by the Western College of Veterinary Medicine at the University of Saskatchewan, and by the Program for Economically Important Infectious Animal Diseases at Colorado State University through a grant from the USDA:CSREES.

Corresponding author: Dr Morley, Department of Clinical Sciences, Colorado State University, 1678 Campus Delivery, Fort Collins, CO, 80523; e-mail: paul.morley@colostate.edu.



There is little information on the duration of nasal shedding of EHV-1 from horses with naturally occurring equine herpesvirus myeloencephalopathy (EHM).


To evaluate the duration of nasal shedding of EHV-1 in horses affected by EHM.


One hundred and four horses naturally exposed to EHV-1, 20 of which had clinical signs of EHM.


All horses on affected premises were monitored. Those horses developing EHM were sampled in a longitudinal outbreak investigation. Nasal swabs were collected daily from 16 of 20 horses affected by EHM. A qPCR was performed on 98 of 246 nasal swab samples to determine nasal shedding duration. Historical and clinical information was analyzed to evaluate potential risk factors for developing EHM and duration of shedding during this outbreak.


The last day shedding was detected in any horse was Disease Day 9. EHV-1 was detected in two-thirds of horses tested on Disease Days 0–3. The amount of EHV-1 DNA found in nasal swabs varied markedly and was not associated with disease severity or age. The odds of developing EHM were greater for febrile horses (OR = 20.3; 95% CI 3.4–390.3; P = .01) as well as for horses attending the riding clinic (OR = 4.1; 95% CI 0.84–21.65; P = .08).

Conclusions and Clinical Importance

Biosecurity measures should be implemented for a minimum of 14 days beyond the onset of clinical signs of EHM. Animal managers cannot rely on the severity of clinical signs to predict the duration of EHV-1 shedding.