transmitted by mosquitoes. Outbreaks of disease occur in horses and humans in the USA, in
all states west of the Appalachian mountains, and also in western Canada. (B47)
- "Although WEEV is distributed across the United States, northward into western
Canada and southward into South America, it causes epidemics only in far-western North America, in the
prairie states and provinces, and in Argentina and Uruguay".
Incubation One to three weeks. (B47)
Seasonal: June to November except in the most southerly USA.
Sporadic disease in the summer, with epizootics in August and September
then decreasing to cease by November in most areas (all mosquitoes killed
by frosts). (B47)
Susceptibility: Young horses may be more susceptible than older
- Fever, sometimes with depression. (B47)
- The horse may not develop further illness but recover. (B47)
- If the nervous system is involved (encephalitis), deranged
- Initially restlessness or mild excitement (e.g. walking in
circles, crashing though fences, aimless walking and ignoring
obstacles, refusal of food and water.
- Later sleepiness, head lowered; arousal is possible but left
alone the horse returns to drowsiness, the head resting on a
manger or fence. Unnatural posture may be noted - e.g. crossing of
the front legs, or sitting on the hindquarters.
- Paralysis of parts of the body, e.g. lip pendulous, tongue
protruding, problems in walking due to reduced control of the hind
- General paralysis, recumbency and inability to rise.
- Death usually within one or two days of the onset of nervous
- Some individuals recover, others show residual paralysis for
weeks or permanently.
- Some individuals partially recover but become
"dummies", having lost the ability to react to normal
- Some horses develop pneumonia and die from this.
- Duration of illness may be as short as a few days from the onset of
clinical signs, more usually two to four days, a week or more with
- Mortality usually 20-30%, sometimes up to 50%.
- No characteristic lesions. (B47)
- CNS: Grey matter affected more than white matter. Neuronal
degeneration, perivascular cuffing (monocytes and polymorphonuclear
cells), grey matter infiltration with polymorphonuclear leucocytes,
glial cell proliferation. (B47)
- By mosquitoes, also ticks e.g. Dermacentor andersoni and
possibly other bloodsucking insects. (B47)
- In the west, the main mosquito vector is Culex tarsalis
which usually feeds on birds. Aedes
melanimon (feeds on mammals) is also a vector of this disease. (B209.4.w4)
albifasciatus is thought to be a WEEV vector in northern Argentina.
- Direct transmission (intranasal) may also occur. (B47)
- Virus isolation. (B47)
- Histopathological lesions indicate a viral encephalitis, but do not
distinguish which virus is responsible. (B47)
- In horses, vaccination. (B47)
- Clinical illness and deaths can occur in humans. (B47)
- In humans, WEE may cause illness varying from mild fever and
headache, to aseptic meningitis and encephalitis. Signs include
fever, headache, chills, nausea, vomiting, occasionally respiratory
symptoms, progressing with CNS involvement to lethargy, drowsiness,
nuchal rigidity, photophobia and vertigo, and, in severe cases, stupor
or coma. In infants under one year old and particularly under two
months of age, irritability with focal or generalised convulsions,
tremors and upper motor neuron deficits (seen in 90% of babies
under one year of age, decreasingly in older individuals. (J128.7.w1)