Page 1088 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Nervous system 1063

VetBooks.ir Management Aetiology/pathophysiology
WNV is classified as a flavivirus within the fam-
There is no specific treatment available and cur-
rent therapy is based on supportive care. Anti-
viruses that are usually spread by mosquitoes and
inflammatory drugs (flunixin meglumine [1 mg/kg ily Flaviviridae, which encompasses a wide range of
i/v or p/o q12–24 h] or dimethylsulphoxide (DMSO) ticks. Other related viruses of veterinary interest
[1 g/kg administered as a 10% solution i/v or via include bovine viral diarrhoea, classical swine fever,
nasogastric tube q12–24 h]) are commonly used to St. Louis encephalitis virus and tick-borne encepha-
reduce CNS inflammation. In addition, good nurs- litis virus. There are several WNV lineages, but
ing care, monitoring of hydration status and pro- only lineages 1 and 2 have been reported to affect
tection from self-induced trauma are important. horses. There has been a surge in the pathogenic-
In many regions these diseases must be reported to ity and geographical distribution of WNV lineage 2,
government authorities. with several epidemics of neuroinvasive equine cases
Prevention is based on vaccination and limiting in Europe in the past few years, having previously
exposure to vectors. Monovalent, bivalent and triva- only been reported as a cause of endemic flu-like dis-
lent killed vaccines are available. Recommendations ease in Africa.
vary depending on the region, with vaccination 2–4 WNV encephalomyelitis is a mosquito-borne
times annually recommended in temperate areas disease that affects a broad range of animals includ-
where vectors may survive all year round. In areas ing birds, cats, dogs, horses and humans. The virus
bordering Central America or for horses travelling to cycles between bird reservoir hosts and mosquitoes.
endemic areas, twice-yearly vaccination against VEE Competent bird reservoir hosts sustain an infectious
is recommended. Foals from vaccinated dams are gen- viraemia for 1–4 days following exposure and then
erally protected for 6–7 months. Vaccination of foals develop life-long immunity. Horses become infected
against EEE should be started at 4 months of age in via the bites of infected mosquitoes, particularly
endemic areas and repeated at 6 and 12 months of age. those of the Culex genus. Humans, horses and most
Limiting exposure to vectors involves efforts to other mammals are considered incidental dead-end
eliminate mosquitoes or their habitats, use of repel- hosts and, because they do not develop a substan-
lents and stabling at dawn and dusk when mosqui- tial enough viraemia to infect biting vectors, WNV
toes are most active. is not transmissible from horses to humans or vice
versa. Many species can develop an immune response
Prognosis without demonstrating signs of disease. Equine cases
Prognosis varies greatly among the different dis- are most commonly seen in late summer to early
eases. In general, the prognosis for EEE is grave, autumn, but may be observed year round in areas
with mortality rates of 75–100% being reported. with prolonged vector seasons. No significant age or
Mortality rates for VEE and WEE of 40–80% and breed predisposition has been identified. More males
25–50%, respectively, have been reported. Residual than females were affected in one report.
neurological deficits may be present in survivors.
Clinical presentation
WEST NILE VIRUS ENCEPHALOMYELITIS In the horse, the majority of infections are sub-
clinical; only a small percentage of infected horses
Definition/overview develop clinical signs. Initially, an affected horse
WNV is a geographically important cause of enceph- shows vague signs of illness including pyrexia,
alitis in horses. It was first recognised in Africa and anorexia and depression within 9–11 days of infec-
the Middle East and has made periodic incursions tion. The onset of neurological signs consistent
into wider geographical areas with increasing fre- with an encephalomyelitis is fairly rapid. Changes
quency over the past 20 years. WNV has emerged in mentation include a change in normal habits,
as an important cause of encephalitis in horses and attitude and reaction to the environment and may
other species in North America and Europe. progress into aggression, somnolence and coma.