Eastern, Western and Venezuelan equine encephalitis
viruses (EEEV, WEEV and VEEV)
The equine encephalitis viruses are
"Group A"
arboviruses (Alphaviruses in the Togavirus family) that cause
encephalitis and/or febrile disease in both horses and humans.
Mosquitoes are obligatory biological vectors for transmission
of these viruses.
EEEV and WEEV
The reservoir hosts for these equine encephalitis viruses are
primarily wild birds (also rodents AND HORSES for VEEV). In the
reservoir hosts, the virus replicates to high enough titer to
be infectious for mosquitoes and, therefore, to perpetuate the
virus in nature. Horses and humans, in contrast, are
"dead-end"
hosts for EEEV and WEEV.
- For EEEV, Culiseta spp. mosquitoes are the
vectors involved in the enzootic reservoir cycle among birds,
whereas Aedes spp. mosquitoes transmit the virus
to horses, humans and other clinically-affected hosts.
- For WEEV, Culex spp. are the primary mosquito
vectors for both transmission among wild birds enzootically and
transmission to humans.
Geographically, both viruses exist throughout Central America
and northern portions of South America. In the U.S., EEEV and
WEEV exist in the eastern and western 2/3 of the country, respectively.
EEEV and WEEV clinical disease:
Clinical disease occurs in the summer during the mosquito season.
Following the bite of an infected mosquito, the virus replicates
locally at the site of the bite in the skin, then enters the blood
stream and spreads to the CNS.
In horses:
- Infections with EEEV or WEEV begin with fever, inappetence
and lethargy, progressing to various degrees of excitability
and then drowsiness, ultimately ending in paresis, seizures and
coma in fatal cases. (5-10 day course)
- Mortality is much higher with EEEV infections (up to 90%)
than with WEEV infections (<20-40 li="li">
In humans:
- Fever, headache, depression and nausea progress to altered
mentation, paralysis and coma.
- The mortality rate can be as high as 65% with EEEV infections.
- Infections with EEEV are most severe in children, and they
often have residual sequelae such as paralysis, mental retardation
and persistent seizure disorders.
- Virtually all people infected with EEEV will develop clinical
disease. WEEV infection is much less threatening, with a clinical
disease-to-infection ratio of ~1:1000.
Cases of equine encephalitis virus infections in humans in
a given locality generally appear about 2 weeks after cases in
horses. Thus, the first appearance of disease in horses should
set off intensive efforts to control mosquitoes and to conduct
virus surveillance by local health departments. Veterinarians
can, therefore, play a very important role in protecting the community
by prompt recognition and reporting of suspected equine encephalitis
cases in horses.
Ratites (emus, ostriches) and swine:
- These animals can also be infected with EEEV/WEEV and develop
clinical disease. However, the infections in these species may
present as hepatitis and/or GI tract disease, rather than encephalitis.
VEEV
For
VEEV, the enzootic and epizootic cycles are quite
independent:
- The enzootic (Culex spp.) versus epizootic (Psorophora
spp., Aedes spp. and others) cycles of transmission
utilize different species of mosquitoes.
- The strains of enzootic VEEV that circulate among rodent
reservoirs are distinct from the epizootic strains of VEEV that
infect and cause disease in horses and humans.
- Experimental infection of horses with enzootic strains of
VEEV produces only mild fever and leukopenia. Likewise, human
disease with the enzootic strains appears to be limited, although
people living in endemic regions are often sero (+).
- It remains unclear how the epizootic strains develop and
are maintained. Do they simply circulate among horses or some
other unknown reservoir, or do they arise de novo in each epizootic
by mutation from an enzootic strain? Results of recent sequence
analyses suggest the later, i.e., that the epizootic strains
arise by mutation from particular enzootic strains. In particular,
mutations that introduce positively-charged amino acids in the
E2 protein may be associated with transformation from an enzootic
to a pathogenic epizootic strain biotype for horses.
HORSES ARE NOT DEAD-END HOSTS FOR VEEV LIKE THEY ARE
FOR EEEV AND WEEV! HORSES ARE, IN FACT, THE KEY RESERVOIR SPECIES
FOR THE EPIZOOTIC STRAINS OF VEEV THAT CAUSE CLINICAL DISEASE
IN BOTH HORSES AND HUMANS.
- People replicate virus to high enough titer to infect mosquitoes
as well, but they have never been implicated as being epidemiologically
important to the spread of the virus in nature.
VEEV clinical disease:
Infections with VEEV may present, in both humans and horses,
as either encephalitic disease or as simply a febrile disease
without profound neurologic signs. Horses may die after a very
acute course, even without any neurologic signs, but mortality
in humans is generally low.
VEE in South America, 1995:
- 55,000 human cases (and countless cases in horses) occurred
in Venezuela and Colombia in 1995. This outbreak was attributed
to low rates of vaccination of horses against VEEV, thus allowing
epizootic strains to replicate widely in horses and subsequently
spill-over into the human population.
Prevention of equine encephalitis virus transmission:
- drainage of standing water to reduce mosquito replication
- use of insecticides during times of high mosquito populations
- sentinel pheasants (pheasants are susceptible to infection
with EEEV) or light traps (and subsequent virus isolation from
mosquitoes that are caught) to alert public health officials
to the presence of virus in a given geographic area
- vaccination of horses
- This is particularly critical from a public health point-of-view
for VEEV because of the role of horses as reservoir hosts for
the virus.
- For EEEV and WEEV, vaccination is conducted to prevent clinical
disease in horses themselves. Vaccination should be conducted
in the spring, prior to the mosquito season, to optimize protection.
In most parts of the U.S., horses are vaccinated only against
EEEV and WEEV, but in the southern U.S., horses may also be vaccinated
against VEEV to protect against transmission via mosquitos from
Central America.