Quantitative molecular viral loads in 7 horses with naturally occurring equine herpesvirus-1 infection.
Authors: Estell K E, Dawson D R, Magdesian K G, Swain E, Laing S T, Siso S, Mapes S, Pusterla N
Journal: Equine veterinary journal
Summary
# Quantitative Viral Load and Survival in Equine Herpesvirus-1 Myeloencephalopathy This retrospective analysis of seven horses with naturally occurring EHM establishes the first direct correlation between measurable viral load and clinical outcome in this disease, addressing a significant gap in equine herpesvirus literature. Researchers at UC Davis tracked neurological status, administered standardised treatment (anti-inflammatories, valacyclovir, and supportive care including slings), and performed daily quantitative PCR on nasal secretions and whole blood samples from May to September 2011. Strikingly, non-surviving horses (2/7) carried viral loads approximately 1000-fold higher in nasal secretions (1.9–2.2 × 10⁹ copies) and tenfold higher in blood (1.02–2.05 × 10⁴ gB gene copies per million eukaryotic cells) compared with survivors (nasal peak median 5.11 × 10⁴; blood median 3146 copies). Notably, initial neurological grade—ranging from 3/5 to 5/5—did not predict survival, but progression to encephalopathy in the two fatal cases did correlate with markedly elevated viral burden. For practitioners, these findings suggest that quantitative viral load assessment may carry prognostic value beyond clinical presentation alone, potentially guiding treatment intensity and owner counselling in suspected EHM cases, though larger prospective studies are needed to establish viral load thresholds and their clinical utility.
Read the full abstract on PubMed
Practical Takeaways
- •High viral loads (especially in nasal secretions) detected by PCR appear to be a poor prognostic indicator in EHM cases and may warrant aggressive intervention or case counseling about prognosis
- •Initial neurological grade alone should not be used to predict outcome; viral load quantification may provide additional prognostic information
- •All affected horses in this series were infected with the D752 genotype of EHV-1; practitioners should consider viral genotyping if available for prognostic or epidemiological purposes
Key Findings
- •Nonsurvivors had viral loads 1000-fold higher in nasal secretions and 10-fold higher in blood than survivors
- •Peak viral load in nasal secretions ranged from 6.9×10³ to 2.81×10⁵ in survivors and 1.9-2.2×10⁹ in nonsurvivors
- •Case fatality rate was 2/7 (28.6%), with both deaths occurring in horses that progressed to encephalopathy
- •Initial neurological grade did not predict survival, but high viral load and encephalopathy were negatively associated with survival