Page 1089 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

1064 CHAPTER 10

VetBooks.ir Hyperaesthesia is also reported. Signs associated and spinal cord. The spinal cord lesions become pro-
gressively worse caudally. Histopathological changes
with spinal cord pathology include ataxia (which
is sometimes asymmetrical and may involve mul-
unlike other causes of viral encephalitis, WNV has
tiple limbs), flaccid paralysis (due to LMN disease), include monocellular perivascular cuffing and,
paresis and recumbency. Muscle fasciculations (par- a predilection for the basal ganglia, thalamus, pons
ticularly of the muzzle) are relatively common in and medulla. WNV cases have limited viral load in
affected horses, in comparison with other causes their neurological tissue, and therefore detection of
of encephalitis. Some authors also report a stiff antibody or WNV antigen should focus on the mid-
gait that may initially be mistaken for lameness. brain and hindbrain areas to maximise sensitivity of
Reported CN abnormalities include weakness of these tests.
the tongue (CN XII), muzzle deviation (CN VII),
head tilt/vestibular ataxia (CN VIII) and difficulty Management
swallowing (CN IX). There are no antiviral therapies licensed for use in
neuroinvasive flavivirus infections and therefore,
Differential diagnosis as with alphavirus encephalitides, therapy is based
Other viral causes of encephalitis should be consid- on supportive care. Anti-inflammatory treatment
ered, depending on the geographical region and time such as flunixin meglumine (0.5–1 mg/kg i/v q12 h)
of year. Other relevant differential diagnoses include may be helpful. Corticosteroid therapy is contro-
hepatic encephalopathy, tremorigenic toxicity (rye- versial, without an evidence-base, and may increase
grass staggers), hypocalcaemia, head trauma, poly- peripheral and CSF viral load as has been demon-
neuritis equi, equine herpes myeloencephalopathy, strated in other species. Anti-oedema therapy with
verminous meningomyeloencephalitis and botulism. mannitol (0.5–1.0 g/kg as a 20% solution i/v via a
blood filter q12–24 h for 2–3 days) may be benefi-
Diagnosis cial in cases with rapidly progressing neurological
WNV encephalomyelitis should be suspected in signs. Commercial hyperimmune serum or plasma
any horse showing signs of neurological disease products are available. Neutralising antibody is the
in an area (or in close geographical proximity to predominant antibody response and appears impor-
an area) in which WNV activity has been docu- tant in the blocking of intracellular infection in vitro.
mented. Definitive diagnosis of a positive case of At the time of writing, the clinical efficacy of these
WNV encephalitis is challenging because of the products had not yet been proven. The hypothesis
cross-reactivity of antigens and the use of vac- is that the administration of neutralising antibody
cines. An ante-mortem diagnosis usually depends against WNV early in the course of the disease will
on the demonstration of: specific IgM antibod- reduce the severity and shorten the duration of clini-
ies in serum and/or CSF; a four-fold increase in cal signs in the horse. The use of interferon alpha
competitive (c)ELISA (IgG) titres (ideally 10 days (3 million units diluted in 250 ml saline s/c or i/v
apart); or PRNT (plaque reduction neutralisa- q12 h [recumbent horses] or q24 h [standing horses]
tion test)-positive serum samples taken during the for 5–7 days) is based on anecdotal reports in the
acute phase and convalescent phase. The cELISA human and veterinary literature.
and PRNT tests are affected by recent vaccination, Prevention is based on limiting exposure to mos-
and although it was initially reported to differenti- quito vectors and on vaccination. Vaccines licensed
ate vaccinated from recently infected horses, there for prevention of WNV viraemia include whole
is now debate in the literature about the effect of inactivated virion vaccines, one recombinant inacti-
vaccination on IgM titres as detected via the IgM vated vaccine and a canarypox-vectored live recom-
capture (MAC) ELISA. binant vaccine. Protection against WNV lineage 2
Clinicopathologically, neuroinvasive flaviviruses is recommended because there has been a surge in
such as WNV cause a polioencephalomyelitis, neuroinvasive WNV disease from this lineage in
mainly involving the grey matter of the hindbrain recent years.