Re-emergent Threat of Equine
Herpesvirus-1 Neurologic Disease
Peter J. Timoney
Department of Veterinary Science
Gluck Equine Research Center
University of Kentucky, Lexington, KY 40546-0099
Outcomes of EHV-1 Infection in Horses
Abortion
Respiratory
Neonatal Death
Neurological
(EHM)
Ack. Dr. G. Allen (2008)
Equine Rhinopneumonitis
General features –
► Contagious
disease of equids endemic in vast
majority of domesticated equid populations.
► Term
encompasses range of syndromes caused by
either EHV-1 or EHV-4.
► Of
5 herpesviruses known to infect the horse, EHV1 & EHV-4 are the 2 of greatest veterinary medical
significance.
► Believed
EHV-1 / EHV-4 have co-evolved with
horses over millions of years.
► Neither
virus of public health significance.
3/14
EHV-1 and EHV-4 Infections
EHV-1 Infection not only of respiratory tract
epithelium and associated lymphatic
glands but also vascular endothelium
especially of nasal mucosa, lung, adrenal,
thyroid and in the case of some strains,
CNS and endometrium.
EHV-4 Infection restricted primarily to respiratory
tract epithelium and associated lymph
glands. Some strains can set up a
leukocyte-associated viremia.
3/14
Industry Concerns
► EHV-1
best known for its economic impact
as a cause of contagious abortion
worldwide.
► EHM
of concern not only economically but
also from a welfare viewpoint because of
the distressing nature of the disease.
► Lack
of a commercial vaccine of proven
capability to prevent EHM.
3/14
Equine Herpesvirus Myeloencephalopathy
1966 First definitive association between EHV-1 and
myeloencephalopathy following isolation of the
virus from brain and spinal cord of a horse with
severe neurologic disease. (Saxegaard, F.,
Nord. Vet. Med., 1966).
3/14
Equine Herpesvirus Myeloencephalopathy
General features –
► Syndrome
recorded with increasing frequency over
past 5-10 years, can be associated with high
morbidity & case fatality rate.
► Usually
a sequel to a primary respiratory infection,
febrile episode or abortion.
► Can
occur in horses of any age, breed or either
gender.
► Nature
of illness dependent on location & severity
of lesions in CNS.
► Disease
most frequently associated with infection
with neuropathogenic strains of EHV-1.
3/14
Equine Herpesvirus-1
Myeloencephalopathy
► Many
outbreaks of EHM associated with
venues / premises where significant
numbers of horses are congregated
together.
► Conditions
at shows, etc, conducive to
respiratory transmission of EHV-1 by direct
/ indirect means.
3/14
Increase in Incidence of Outbreaks of EHV-1
Neurologic Disease, 1970 - 2006
Time interval
No. of neurologic disease outbreaks
(US and UK) from which virus or
viral DNA were available
1970 – 75
1976 – 80
1981 – 85
1986 – 90
1991 – 95
1996 – 2000
2001 – 2006
1
3
4
6
5
6
33
Ack: Dr. G.P. Allen
Equine herpesvirus myeloencephalopathy
caused by the hypervirulent, mutant
(neuropathogenic) strain of the virus
designated by USDA a potentially
emergent disease of the horse.
(USDA: APHIS: VS: CEAH: Center for
Emerging Issues Information Sheet,
January 2007)
3/14
Association of Novel Genotype of EHV-1
with Neurologic Disease
► Majority
of severe and sometimes
extensive EHM events associated with
novel virus genotype.
► Novel
genotype characterised by single
point mutation on catalytic subunit of viral
DNA polymerase.
► Guanine
substituted for adenine at position
2254.
3/14
Nucleotide Substitution in Neuropathogenic Strains
of EHV-1
EHV-1 DNA
Replicase
gene
Abortion Strains:
-- GTC AAC TAC --
Paralytic Strains:
-- GTC GAC TAC --
Ack: Dr. G.P. Allen
(neutral)
Asparagine
Aspartic acid
(acidic)
Outbreaks of EHV-1 Neurologic Disease in
USA, 2000 - 2006
--- Genotype of Virus Isolates ---
Time Span
No. of EHV-1
CNS Outbreaks
2000 – 2006
Wild-Type Outbreaks
26
Wild-Type
Mutant
2
24
Mutant Outbreaks
• Low neurologic morbidity
• High neurologic morbidity
• Low to zero neurologic mortality • High neurologic mortality
Ack: Dr. G.P. Allen
Clinical Outcome in Relation to Virus
Genotype Involved
► In
terms of both neurologic-attack and
case-fatality rates, clinical outcome can
vary depending on genotype of EHV-1.
► Outbreaks
caused by G2254 tend to be
more extensive and clinically more severe.
► In
comparison, A2254 strains associated
with lower neurologic-attack and casefatality rates.
3/14
Equine Herpesvirus Myeloencephalopathy
Characteristics of Vasculitis
► Perivascular
cuffing with mononuclear cells and
neutrophils.
► Extension
of inflammatory cells from intima into
media and adventitia of vessel wall.
► Endothelial
► Necrosis
proliferation and necrosis.
of media.
► Occasionally,
thrombin in vessel lumen.
3/14
EHV-1 Paralysis Results from Endothelial
Cell Infection
Spinal Cord Blood Vessel of Paralyzed Horse
EHV-1 infected
endothelial cells
Fibrin thrombus
Inflammatory
lymphocytes
Ack: Dr. G.P. Allen
Neuropathogenic Strains of EHV-1
Summary of properties –
► Most
frequently but not invariably associated
with a single point mutation in the catalytic
subunit of the gene (ORF30) encoding the viral
DNA polymerase gene.
► "Turbo-charged"
versions of wild type virus.
► Total
body burden of mutant strains of EHV-1
much greater than wild type virus.
► No
evidence of neurotropism.
3/14
Viremia in Foals after Inoculation with G2254
Mutant or Wild Type Strains of EHV-1
n = 10 foals/group
Magnitude of Viremia
400
Mutant EHV-1
300
200
Wild Type EHV-1
100
0
5
10
15
Days Post-Inoculation with EHV-1
20
Ack: Dr. G.P. Allen
Replicative Capacities of A2254 and G2254
Genotypes of EHV-1
► A2254
and G2254 genotypes differ significantly in
their respective replicative capacities.
► Cell-associated
viremia and duration of
respiratory shedding greater in cases of G2254
infection.
► Infection
with G2254 strains results in vasculitis
in the CNS that is more severe and more
widespread.
3/14
Consequences of Mutation on Pathogenicity
of Genetic Variants of EHV-1
►
Enhanced replicative capacity
►
Elevated level of viremia
►
More widespread vasculitis
►
Greater severity of vasculitis
►
Greater mortality from neurologic disease
Ack: Dr. G.P. Allen
Clinical Outcome following Infection with
Neuropathogenic Strains of EHV-1
► Infection
with G2254 strains may not necessarily
result in development of neurologic disease.
► Individual
animal outcomes can be influenced
by age, innate immunity, acquired immunity,
challenge dose, hormonal status and
environmental factors.
3/14
Evidence that A2254 Nucleotide Substitution not
the Only Determinant of Neuropathogenicity
Report that 24% of the isolates from horses with
neurological disease possessed the A2254 and not
the G2254 genotype (Perkins et al., 2009).
Identification of viruses with nonsynonymous
nucleotide substitutions in ORF30 besides A2254 to
G2254 from horses without signs of neurologic
disease (Smith et al., 2010).
(continued)
Ack: Dr. U. Balasuriya (2011)
Evidence that A2254 Nucleotide Substitution not
the Only Determinant of Neuropathogenicity
(continued)
Sequence analysis of EHV-1 field strains has
identified other mutations outside of the small region
of ORF30 sequenced by Nugent et al. (2006).
Mutations found in same gene or genes encoding
proteins of viral elongation complex or viral envelope
proteins.
Ack: Dr. U. Balasuriya (2011)
Factors Involved in the Epidemiology of
Equine Herpesvirus Myeloencephalopathy
► Virus
strain.
► Modes
of transmission.
► Immune
status of individual animals / groups of
horses.
► Existence
► Various
of the carrier state.
management practices.
3/14
Ack: Dr. G.P. Allen
EHV-1 and EHV-4 Infections
Latency –
► Latency
of EHV-1, EHV-4 widespread (40-60%) in
adult equids.
► Individual
animals may be carriers of one or both
viruses.
► Sites
of latency of EHV-1 / EHV-4: lymphoreticular
tissues associated with the respiratory tract,
circulating CD3+ lymphocytes, and the trigeminal
ganglia (EHV-1).
(continued)
3/14
EHV-1 and EHV-4 Infections
Latency (cont.) –
► Carrier
state probably life-long.
► No
infectious virus present unless latent virus has
been reactivated.
► Latent
virus can be reactivated by environmental /
pharmacological stimuli.
3/14
Expansion in the Reservoir Size of the Latent G2254
Neuropathogenic EHV-1 Strains in Kentucky TB Mares
Mutant Strain of EHV-1
(% of Total Isolates)
20%
n = 450 abortion isolates of EHV-1
15%
10%
5%
1960’s
1970’s
1980’s
1990’s
2000’s
Decade
Smith, K. 2007. Master’s Thesis. University of Kentucky
Ack: Dr. G.P. Allen
Prevalence of Latent, G2254 Neuropathogenic
Strains of EHV-1 in TB Mare Population of Kentucky
Sub-maxillary lymph nodes collected from 132 necropsied TB
mares.
Tested for latent EHV-1 DNA by PCR.
46% of tested mares harbored latent wild-type EHV-1 DNA.
8% of tested mares harbored G2254, neuropathogenic
strains of EHV-1 (=18% of total latent reservoir of EHV-1).
8%
M
46%
WT
132 TB broodmares
EHV-1 DNA in
SMLN tissue
of TB mares
Ack: Dr. G.P. Allen
Development of Equine Herpesvirus
Myeloencephalopathy
Risk factors –
► Existing
levels of cytotoxic T-lymphocyte precursor
(CTLP) cells specific for EHV-1 critically important.
► Significantly
greater risk in elderly horses (≥ 20 y.o.).
► Significantly
greater risk in horses exposed to
ORF30G2,254genotype of EHV-1.
► No
significant correlation with pre-exposure levels of
serum neutralising antibodies to EHV-1).
(G.P. Allen, AJVR, 69:1595-1600, 2008)
3/14
Viremic Load (Log10)
Relationship Between EHV-1 Cellular Immunity
and Viremic Load
Cellular Immunity
(Pre-Infection CTLp Frequency per million PBMC)
Ack: Dr. G.P. Allen
Dr. Roger Doll, 1960’s
Equine Herpesvirus Myeloencephalopathy
Key points –
►
One of 5 clinical syndromes caused by EHV-1
and infrequently, certain strains of EHV-4.
►
An emergent disease of increasing veterinary
medical and economic significance since
2000.
►
Usually a sequel to a primary herpesvirus
respiratory infection, febrile illness or abortion.
►
Can occur in horses of any age, breed or
either gender.
(continued)
2/12
Equine Herpesvirus Myeloencephalopathy
Key points (cont.) –
►
Nature of illness depends on location and
severity of lesions in CNS.
►
Majority of outbreaks caused by hypervirulent,
neuropathogenic (mutant) strains of EHV-1.
►
Neuropathogenic EHV-1 strains give rise to
much greater body burdens of virus than wild
type virus.
►
Neuropathogenic EHV-1 strains cause higher
morbidity and case-fatality rates.
(continued)
2/12
Equine Herpesvirus Myeloencephalopathy
Key points (cont.) –
►
Evidence of increasing prevalence of latent
infection with neuropathogenic strains of EHV-1.
►
Risk factors associated with development of
EHM:
 Age (≥ 20 years old).
 Infection with neuropathogenic strain of EHV-1.
 Level of CTLP cells specific for EHV-1.
►
Very doubtful current vaccines effective in
preventing EHM.
2/12