Epizootic/epidemic cycle: equids are amplifying host (high, prolonged viraemias) and infect a wide
range of mosquitoes which are not restricted to equid hosts
o non-vector spread of disease via direct contact or aerosols has been proposed; horse-
to-human and human-to-human transmission has not been recorded
o role of non-equid vertebrates in transmission is not clear but likely minor
Swiftness of spread depends on: VEE virus subtype, density of competent vectors and population
of susceptible hosts; large, geographically disperse epizootics of VEE have depended on ability of
virus to produce high viraemias in equids.
Sources of virus
Equids (horses, donkeys and zebras) are a primary source of epizootic virus strains during an
outbreak
o inter-epidemic maintenance of epizootic strains is not known though three hypotheses
prevail:
incomplete inactivation of subtype I-AB VEE vaccines (suggested by
sequencing studies)
epizootic/epidemic subtype I-AB and I-C VEE strains arise from genetic
mutation of enzootic subtype I-D strains (supported by genetic studies)
recent studies of epizootic I-C VEE strains in Venezuela would indicate
persistent, low-level sylvatic maintenance in and yet unknown cycle (cryptic
transmission cycle)
Haematophagous vectors become infected with high titres of virus from infected horses
Enzootic/endemic strains remain in tropical ecosystems in cycle between rodents and
mosquitoes; and for some subtypes, birds also.
Occurrence
Enzootic/endemic VEE viruses are known to be circulating continually in lowland tropical and sub-tropical
forests and swamps of South and Central America, Mexico, and areas of the United States. Enzootic VEE
viruses do not usually produce clinical encephalomyelitis in the equine species; however, in 1993 and
1996 in Mexico, the 1-E enzootic subtype caused limited epizootics in horses. Everglades virus is a
subtype II VEE virus that infects rodents and dogs in Florida.
Historically, epizootic VEE has been limited to northern and western South America (Venezuela, Colombia,
Ecuador, and Peru) and the Caribbean Island of Trinidad (1944). A panzootic of VEE I-AB began
spreading through Central America in 1969 reaching the United States (Texas) in 1971. Epizootics of VEE
caused by I-AB or I-C virus have not occurred in North America and Mexico since 1972. Recent equine
and human isolations of epizootic VEE virus were subtype 1-C strains from Venezuela in 1993, 1995, and
1996 and Colombia in 1995. More recent VEE detections in equids have been reported to the OIE from
Belize (1996, 1998, 2003, 2004, 2005, 2007), Colombia (1996, 1997, 1998, 1999, 2001, 2002, 2003,
2005–2007), Costa Rica (2001, 2002), Honduras (1997, 2000, 2001, 2002, 2003, 2007), Guatemala
(1998, 2005-2008), Guyana (2006), Panama (1999, 2005), and Venezuela (2000, 2003, 2004, 2005)
[cases may be from prior year of report]. More recently, sporadic outbreaks have occurred in Mexico,
Central America, and northern and western parts of South America. Human enzootic subtypes reach more
broadly into northern Central America and South America.
For more recent, detailed information on the occurrence of this disease worldwide, see the OIE
World Animal Health Information Database (WAHID) interface
[http://www.oie.int/wahis/public.php?page=home] or refer to the latest issues of the World Animal
Health and the OIE Bulletin.
DIAGNOSIS
Incubation period is usually recognised as 1– 5 days with high fever appearing within a day and neurologic
signs manifesting themselves at approximately 5 days.
Clinical diagnosis
Although a presumptive diagnosis of “equine encephalomyelitis” can be made when susceptible animals in
tropical or subtropical areas display clinical signs of encephalomyelitis where haematophagous insects are