. 2023 Dec 9;38(3):1872–1891. doi: 10.1111/jvim.16948

TABLE 2.

Summary of main findings from studies evaluating neurologic effects following EHV‐1 exposure.

Study identifier	Study design	Virus	Dose (route)	Outcome incidence	Main findings: viremia
Allen (2008)	RCT	Neuropathogenic: T953 (OH03)	10⁷ PFU (IN)	Neurologic signs: 9/24	Mean peak viral load (pcr relative quantification) asymptomatic horses = 52.9 ± 99.8; neurologic horses = 3740 ± 5600 (P = .02).
Allen (2008)	RCT	Abortogenic: T262	10⁷ PFU (IN)	Neurologic signs: 0/12	Mean peak viral load non‐neuropathogenic strain (Day 6) = 7.1 ± 23.7.
Allen and Breathnach (2006)	NRES	Abortogenic: T61, T75, T220, T480, T677	10⁷ PFU (IN)	Fever (>39°C): 10/10, nasal shedding: 10/10, duration of nasal shedding 4‐7 d, neurologic signs: 0/10.	Mean peak viral load: Day 9 = 29.5 ± 37.9. Viremia duration = 6 d.
Allen and Breathnach (2006)	NRES	Neuropathogenic: T313, T510, T672, T935, T946	10⁷ PFU (IN)	Fever (>39°C): 10/10, nasal shedding: 10/10, duration of nasal shedding 5‐12 d, neurologic signs: 2/10; 1/2 (T672); 1/2 (T935).	Mean peak viral load (pcr relative quantification): Day 7 = 165 ± 170. Viremia duration = 12 d. Duration of nasal shedding and viremia were greater in foals inoculated with neuropathogenic isolates.
Brosnahan (2010)	RCT	Ab4 siLuc [controls]	10⁷ PFU (IN)	Nasal discharge: 4/4, fever > 38.5°C: 4/4, neurologic signs: 3/4.	Viremia (qpcr) data not reported for neurologic and non‐neurological horses. Viral copies in CSF from neurologic horses had 0 to 1.6 × 10⁵ genome copies/mL (mean 5.9 × 10⁴ ± 7.5 × 10⁴. Viral copies in spinal cord from neurologic horses had 0.7 to 3.0 × 10⁶ genome copies/mL.
Brosnahan (2010)	RCT	Ab4 sigB3/siOri2	10⁷ PFU (IN)	Nasal discharge: 10/10, fever > 38.5°C: 10/10, neurologic signs: 2/10.
Eddington (1986)	NRES	Ab4	10^5.5 TCID₅₀ (IN) and 1.5 × 10^5.5 TCID₅₀ (SQ)	Fever ≥ 38.5°C: 8/8, neurologic signs: 3/8. CNS arteriolar and capillary thrombi unknown incidence. CNS hemorrhages from 2 ataxic mares were associated with extensive endothelial cell fluorescence and thrombus formation.	Viremia (equine kidney cell culture): 8/8, viremia from 3 to 10 dpi. Nasal shedding (8/8), duration from 1 to 10 d. Recovery of virus from CNS (1/8).
Eddington (1986)	NRES	2252	10^5.5 TCID50 (IN)	Fever ≥ 38.5°C: 2/5, neurologic signs: 0/5.	Viremia (equine kidney cell culture): 0/5. Nasal shedding (5/5) duration from 4 to 7 d.
Estell (2015)	Retrospective case series	Genotype D₇₅₂	Unknown	Fever: 4/7, neurologic signs: 7/7, case fatality: 2/7. Nonsurvivors with neurologic signs had focal CNS hemorrhages, multifocal vasculitis of small arterioles, nonsuppurative meningoencephalomyelitis, and multifocal CNS malacic lesions associated with vasculitis.	Viremia: 7/7. Peak viral load in blood was higher in nonsurvivors (2.05 × 10⁴ and 1.02 × 10⁵) vs survivors (143‐4340; median 3150 gB gene copies/million cells).
Gardiner (2012)	NRES	OHO3	5 × 10⁷ PFU (IN)	Fever ≥ 1°C rise above baseline: 7/7, neurologic signs: 1/7	Viremia (pcr): 7/7. Viremic between Days 5 and 8 post‐inoculation, with a duration of 1‐4 d. Mare with neurologic signs: peak viremia (6 dpi): 10^3.4; no abortion (5‐7 dpi): 10^2.9‐10^3.6 EHV‐1 gB copies/10⁶ beta β copies
Gardiner (2012)	NRES	Ab4	5 × 10⁷ PFU (IN)	Fever ≥ 1°C rise above baseline: 7/7, neurologic signs: 0/7	Viremia (pcr): 7/7. Viremic between Days 5 and 8 with a duration of 1‐3 d. Peak viremia (5‐9 dpi): 10^2.3‐10^3.2 EHV‐1 gB copies/10⁶ β Actin copies
Goehring (2010a)	RCT	OH03	5 × 10⁷ PFU (IN)	Fever ≥ 38.6°C: 24/24, neurologic signs: 0/24	Viremia (pcr) control group: Peak mean (8 dpi) = 10^2.9 EHV‐1 gB copies/10⁶ beta Actin copies. Viremia (pcr) vaccinated groups: Peak mean (6 dpi) = 10^1.6 EHV‐1 gB copies/10⁶ β Actin copies.
Goehring (2010b)	NRES	Neuropathogenic EHV‐1 strain	5 × 10^7.6 PFU (NP)	Fever ≥ 38.5°C: 4/4, mild neurologic signs: 4/4, neuropathology 1/4.	Viremia (pcr): 1/4, Peak viremia (12 dpi) = 1.5 × 10⁵ DNA copies/2.5 × 10⁶ PBMC
Goehring (2013)	NRES	Ab4	5 × 10⁷ PFU (NP)	Fever: 8/8, neurologic signs: 1/8	Viremia (pcr): 8/8. Viremia duration = 2‐6 d with a median = 3.5 d; 0‐7 d duration with a median of 2.5 d in group 2, and for 0‐8 d with a median of 4 d in group 3.
		Ab4	5 × 10⁷ PFU (NP)	Fever: 8/10, neurologic signs: 0/10	Viremia (pcr): 8/10. Viremia duration = 0‐7 d median = 2.5 d.
		OH03	1 × 10⁷ PFU (NP)	Fever: 8/10, neurologic signs: 6/8	Viremia (pcr): 9/10. Viremia duration = 0‐8 d median = 4 d.
Goodman (2006)	RCT	OHO3 Controls	5 × 10⁶ PFU (NP)	Fever ≥ 38.5°C: 5/5, nasal shedding: 5/5; duration of nasal shedding: 3.8 d, neurologic signs: 3/5.	Viremia (qpcr): 5/5. Peak mean viremia (9 dpi) 1.5 × 10⁵ genome copies per 10⁹ 18S rRNA gene copies.
		OHO3 MLV vaccine		Fever ≥ 38.5°C: 5/5, nasal shedding 1/5, duration of nasal shedding: 0.2 d, neurologic signs: 0/5.	Viremia (qpcr): 5/5. Peak mean viremia (9 dpi) 3.3 × 10⁵ genome copies per 10⁹ 18S rRNA gene copies.
		OHO3 Inactivated vaccine		Fever ≥ 38.5°C: 5/5, nasal shedding controls: 5/5, duration of nasal shedding: 3.2 d, neurologic signs: controls: 3/5.	Viremia (qpcr): 5/5. Peak mean viremia (9 dpi) 8.1 × 10⁴ genome copies per 10⁹ 18S rRNA gene copies.
Gryspeerdt (2010)	NRES	03P37 (neuropathic)	10^6.5 TCID₅₀ (50:50: IN/PO)	Fever ≥ 38.5°C: 5/5, nasal shedding 5/6, neurologic signs: 0/5.	Viremia (plaque assay): 4/5. Quantity of viremia was variable between animals (n = 2‐3) after 3 dpi. At 3 dpi mean number of plaques/10⁷ PBMC = 3.3. Maximum = 36 plaques/10⁷ PBMC (4 dpi, n = 1).
Gryspeerdt (2010)	NRES	97P70 (nonneuropathic)	10^6.5 TCID₅₀ (50:50: IN/PO)	Fever ≥ 38.5°C: 5/5, nasal shedding 6/6, neurologic signs: 0/5.	Viremia (plaque assay): 5/5. Quantity of viremia was variable between animals (n = 2‐3) after 3 dpi. At 3 dpi mean number of plaques/10⁷ PBMC = 1.0. Maximum = 42 plaques/10⁷ PBMC (4 dpi, n = 1).
Heldens (2001)	NRES	Ab4 Controls	2 × 10^6.0 TCID₅₀ (IN)	Fever (>38.8°C): 4/5; nasal shedding: 5/5, neurologic signs (ataxia): 1/5	Viremia (horse kidney cells): controls: 5/5; viral titers (PBMC) not provided, mean duration of viremia: 4.3 d.
Heldens (2001)	NRES	Ab4 Vaccinates	2 × 10^6.0 TCID₅₀ (IN)	Fever (>38.8°C): 2/4; nasal shedding: 5/5, neurologic signs (ataxia): 0/5	Viremia (horse kidney cells): 5/5, viral titers (PBMC) not provided, mean duration of viremia: 4.0 d.
Holz (2017)	NRES	Ab4	5 × 10⁷ PFU (IN)	Fever (>38.6°C) 8/8; neurologic signs: Ab4: 3/8.	Viremia (pcr): 8/8. Peak mean viremia (pcr): 6 Dpi: 699 ± 751 Gb copy number/10⁵ β‐Actin copies. Ab4‐infected horses had higher viremia vs Ab4 N752‐infected horses (P < .001) and approached significance when compared to Ab4 gD4‐infected horses (P = .07). Duration of viremia = 3 d.
		Ab4 N752 (ORF mutant)		Fever (>38.6°C) 9/9; neurologic signs: 0/9	Viremia (pcr): 9/9. Peak mean viremia (pcr): 7 dpi: 68 ± 78 Gb copy number/10⁵ β‐Actin copies
		Ab4 gD4		Fever (>38.6°C): 8/8. neurologic signs: 0/8	Viremia (pcr): 8/8. Peak mean viremia (pcr): 50 ± 142 Gb copy number/10⁵ β‐Actin copies
Hussey (2013)	NRES	Ab4	5 × 10⁷ PFU (IN)	Fever: 10/10, neurologic signs: 0/10	Viremia (plaque assay): 10/10. Peak median viremia (experiment 3) Ab4 (6 dpi): 2.0 × 10³ log gB copies/10⁶ β‐Actin copies. Viremic for 1‐7 d between 3 and 9 dpi.
		Ab4Δ75‐LacZ	5 × 10⁷ PFU (NP aerosol)	Fever: 12/12, neurologic signs: 1/12	Viremia (pcr): 12/12. Viremic for 1‐7 d between 3 and 9 dpi.
		Ab4 or Ab4GFP	5 × 10⁷ PFU (NP aerosol)	Fever: 12/12, neurologic signs: 0/12	Viremia (pcr): 11/12. Peak median viremia (7 dpi): 4.9 × 10³ log gB copies/10⁶ β‐Actin copies. Viremic for 1‐7 d between 3 and 9 dpi.
Maxwell (2017)	RCT	T953 (OH03) Controls	10⁷ PFU (IN)	Fever: 6/6, ataxia 4/6, neuropathology: negative.	Viremia (pcr): 6/6. Peak viremia at 9 dpi: mean = 23 000 copies/10⁶ cells.
		T953 (OH03) Prophylactic valacyclovir (1 wk)		Fever: 4/6, ataxia: 1/3, neuropathology: negative.	Viremia (pcr): 3/3, nasal shedding 3/3. Peak viremia at 8 dpi: mean = 960 copies/10⁶ cells.
		T953 (OH03) Prophylactic valacyclovir (2 wk)		Fever: 4/6, ataxia: 0/3, neuropathology: negative.	Viremia (pcr): 3/3, nasal shedding 3/3. Peak viremia at 9 dpi: mean = 59 copies/10⁶ cells.
Mumford (1991)	NRES	AB4 Controls	10⁷ TCID₅₀ (IN)	Fever: 9/9, neurologic signs (quadriplegia): 1/9	Viremia (rabbit kidney cells): 9/9
Mumford (1991)	NRES	AB4 Vaccinated	10⁷ TCID₅₀ (IN)	Fever: 10/10, neurologic signs (quadriplegia): 0/10	Viremia (rabbit kidney cells): 10/10
Mumford (1994)	NRES	V592	30 or 50 × 10^7.5 TCID₅₀ (IN aerosol)	Fever: 8/10, neurologic signs (quadriplegia): 0/10	Viremia (rabbit kidney cells): 10/10, mean duration of viremia: 2.5‐3.5 d.
		Ab4	20 × 10^6.7 TCID₅₀ (IN aerosol)	Fever: 8/8, neurologic signs (quadriplegia): 0/8	Viremia (rabbit kidney cells): 8/18, mean duration of viremia: 3.1 d.
			30 × 10^7.5 TCID₅₀ (IN aerosol)	Fever: 7/7, neurologic signs (quadriplegia): 2/7	Viremia (rabbit kidney cells): 7/7, mean duration of viremia: 8.2 d.
			10³ to 10⁷ TCID₅₀ (IN)	Fever: 35/41, neurologic signs (quadriplegia): 3/41	Viremia (rabbit kidney cells): 15/15
Perkins (2013)	RCT	Ab4 siRNA	10⁷ PFU (IN)	Fever: 7/7, nasal shedding: 7/7. Neurologic signs with histologic evidence of lymphocytic perivascular cuffs in CNS: 3/7	Viremia (qpcr): 7/7, maximum genome copies/10⁶ cells: 18 700 (606‐55 600)
Perkins (2013)	RCT	Ab4 siLuc control	10⁷ PFU (IN)	Fever: 6/6, nasal shedding: 6/6. Neurologic signs with histologic evidence of lymphocytic perivascular cuffs in CNS: 2/6	Viremia (qpcr): 6/6, maximum genome copies/10⁶ cells: 7735 (1510‐45 100)
Perkins (2019)	RCT	Null/Ab4	10⁷ PFU (IN)	Fever: 5/5, nasal shedding: 5/5, neurologic signs: 0/5	Viremia (qpcr): 5/5; peak viremia at 5 dpi: 32.1 (31.2‐32.6)
		Ab4/Ab4		Fever: 0/5, neurologic signs: 0/5	Viremia (qpcr): 0/5, peak viremia at 5 dpi: not detected
		Ab4ΔORF1/71/Ab4		Fever: 0/5, neurologic signs: 0/5	Viremia (qpcr): 0/5, peak viremia at 5 dpi: not detected
Pusterla (2008)	Retrospective case series	Febrile	Unknown	Fever: 12/12, nasal shedding: 12/12, neurologic signs: 0/12	Viremia (qpcr): 12/12, mean viral load blood: 1.7 × 10⁴ gene copies/10⁶ cells
		Neurologic		Fever unknown, nasal shedding: 15/15, neurologic signs: 15/15	Viremia (qpcr): 9/15, mean viral load blood: 1.6 × 10² gene copies/10⁶ cells
		Subclinical		Fever 0/41, nasal shedding: 40/41, neurologic signs: 0/41	Viremia (qpcr): 5/41, mean viral load blood: 4.5 × 10¹ gene copies/10⁶ cells
Pusterla (2012)	Retrospective case series	Asymptomatic (n = 27)	Unknown	Neurologic signs: 0/27	Viremia (qpcr): 27/27, mean viral load blood: 157 ± 30 gene copies/10⁶ cells.
Pusterla (2012)	Retrospective case series	Neurologic (n = 6)	Unknown	Neurologic signs: 6/6	Viremia (qpcr): 6/6, mean viral load blood: 2820 ± 740 gene copies/10⁶ cells.
Pusterla (2021)	Retrospective case series	Natural infection	Unknown	Fever: 26/31, nasal shedding: 26/31, neurologic signs: 4/31.	Viremia (qpcr): 13/41, neurologic horses 4/4. Median viral load blood (Day 0 of outbreak) = 6269 [range = 572 to 2.1 × 10⁵] gene copies/10⁶ cells (n = 13).
Schnabel (2018)	RCT	Ab4	10⁷ PFU (IN)	Fever: 8/8, nasal shedding: 8/8, mild ataxia: 1/8	Viremia (qpcr): 8/8, mean viral load blood (6 dpi): 32.3 ± 1.1 Ct values for the gB gene.
Schnabel (2018)	RCT	Ab4Δ ORF2	10⁷ PFU (IN)	Fever: 8/8, nasal shedding: 8/8, mild ataxia: 1/8	Viremia (qpcr): 8/8, mean viral load blood (6 dpi): 33.4 ± 1.1 Ct values for the gB gen. On d8 pi, higher amounts of viral DNA were detected in PBMC of the Ab4ΔORF2 group vs Ab4 group (P < .01)
Schnabel (2019)	RCT	Null/Ab4	10⁷ PFU (IN)	Fever: 8/8, nasal shedding: 8/8, mild ataxia: 1/8	Viremia (qpcr): 8/8; peak viremia at 4‐8 dpi: mean (n = 8): 32.3 ± 1.6 Ct values for the gB gene.
		Ab4/Ab4		Fever: 0/8, nasal shedding: 3/8, neurologic signs: 0/8	Viremia (qpcr): 3/8, peak viremia at 6‐8 dpi: mean (n = 3): 34.3 ± 3.6 Ct values for the gB gene.
		Ab4ΔORF2/Ab4		Fever: 1/8, nasal shedding: 1/8, neurologic signs: 0/8	Viremia (qpcr): 0/5, peak viremia at 8 dpi (n = 1): 37.0 Ct values for the gB gene.
Soboll (2010)	NRES	Army 183 A3/B2 vacc	5 × 10⁷ PFU (IN)	Mean maximum clinical score (1 dpi, n = 10): 0.9, nasal shedding: 10/10, neurologic signs: 0/10.	Viremia (qpcr): 1/10, Mean peak viremia at 8 dpi (n = 10): 0.4 Log gB copy numbers/10⁶ copies of β‐Actin.
		Army 183 A3‐non‐B2 vacc		Mean maximum clinical score (10 dpi, n = 5): 2.4, nasal shedding: 5/5, neurologic signs: 0/5.	Viremia (qpcr): 5/5, Mean peak viremia at 9 dpi (n = 5): 2.8 Log gB copy numbers/10⁶ copies of β‐Actin.
		Army 183 Non‐A3 vacc		Mean maximum clinical score (6 dpi, n = 5): 2.4, nasal shedding: 5/5, neurologic signs: 0/5.	Viremia (qpcr): 3/6, Mean peak viremia at 8 dpi (n = 6): 1.2 Log gB copy numbers/10⁶ copies of β‐Actin.
		Army 183 Control		Mean maximum clinical score (4 dpi, n = 6): 1.5, nasal shedding: 6/6, neurologic signs: 0/6.	Viremia (qpcr): 5/5, Mean peak viremia at 8 dpi (n = 5): 2.3 Log gB copy numbers/10⁶ copies of β‐Actin.
Sutton (2020)	NRES	FR‐56628 (C2254)	5 × 10⁷ TCID₅₀ (IN)	Fever: 4/4 nasal shedding: 4/4, lethargy and tail hypotonia: 4/4	Viremia (qpcr): 4/4, mean peak viremia at 9 dpi (n = 4): 5.6 ± 0.3 log₁₀ copy numbers/mL.
Van de Walle (2009)	NRES	Ab4 (rNY03_N752)	1.5 × 10⁷ PFU (IN)	Fever: 3/3, neurologic signs: 0/3.	Mean viremia score (RK 13 cells) at 8 dpi: 1.6. Viral genome copies in cerebrospinal fluid (pcr) = 0 for all horses.
Van de Walle (2009)	NRES	Ab4 rNY03_D752 (ORF mutant)	1.5 × 10⁷ PFU (IN)	Fever: 6/6, moderate to severe ataxia: 2/6.	Mean viremia score (RK 13 cells) at 5 dpi: 1.8. Viral genome copies in cerebrospinal fluid (pcr) for horses without neurologic signs = 0 (n = 2), 118, and 178. Viral genome copies in cerebrospinal fluid (pcr) for horses with neurologic signs = 0 and 178. 630 genome copies/mL.
Wilson (2019)	NRES	Ab4	5 × 10⁷ PFU (IN)	Fever: 8/8, neurologic signs (9 dpi): 3/8 including mild hindlimb weakness (n = 1) or reduced tail tone, hindlimb ataxia, recumbency resulting in euthanasia (n = 2).	Viremia (pcr): 8/8, Peak mean viremia at 6 dpi: 705 ± 760 Gb copy number/10⁶ copies of β‐Actin.
Wimer (2018)	RCT	Ab4	10⁷ PFU (IN)	Peak mean rectal temperature at 2.5 dpi = 39.5 ± 0.3°C. Nasal shedding: 5/5. Neurologic signs: 0/5.	Viremia (pcr): 5/5, peak mean viremia at 5 dpi: 32.3 ± 0.4 copy number/5 × 10⁶ PBMC (Ct).
Wimer (2018)	RCT	Ab4ΔORF1/71	10⁷ PFU (IN)	Peak mean rectal temperature at 3.5 dpi = 38.8 ± 0.1°C. Nasal shedding: 1/5. Neurologic signs: 0/5.	Viremia (pcr):5/5, peak mean viremia at 6 dpi: 33.4 ± 0.5 copy number/5 × 10⁶ PBMC (Ct).
Zarski (2021)	NRES	Ab4	5 × 10⁷ PFU (IN)	Fever: 7/7, nasal shedding: 7/7, severe neurologic signs requiring euthanasia: 1/7.	Viremia (pcr): 7/7. Peak viremia at 7 dpi seen in horse developing neurologic signs: 120 EHV‐1 copy number per 500 ng template DNA. Duration of viremia in this horse = 1 d. Peak viremia at 5‐7 dpi seen in four horses without neurologic signs: 299‐951 EHV‐1 copy number per 500 ng template DNA. Duration of viremia in these horses = 1‐3 d. Remaining horses had lower peak values.

Abbreviations: Ct, cycle threshold; d, day; IN, intranasal; NRES, non‐randomized experimental study; pi, post‐infection; PBMC, peripheral blood mononuclear cell; pcr, polymerase chain reaction; qpcr, quantitative real time PCR; RCT, randomized controlled trial; RK, rabbit kidney.