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Equine Encephalitis
Eastern equine encephalitis (EEE)
and western equine encephalitis (WEE) virus infections occur in horses
and other equines in Louisiana. Eastern equine encephalitis (EEE) is
more common and tends to occur in outbreak form. These diseases are
transmitted to horses and humans by mosquitoes that have fed on
infected wild birds. The EEE virus was first identified in mosquitoes
in Louisiana in 1951 near Ponchatoula in Tangipahoa Parish. The virus
is active in horses to some degree every year. EEE virus attacks the
central nervous system of its host. Unvaccinated horses are
particularly susceptible to the infection. The disease appears within
five days after mosquitoes transmit the virus to the horse. Onsets of
clinical signs of EEE are abrupt, and affected horses die within three
days. Signs of EEE in horses include fever; a sleepy appearance ; some
muscle twitches of the head, neck, shoulder and flank; and a weak,
staggering gait. Affected animals are soon down, unable to stand. There
is no effective treatment. The fatality rate is 90 percent or higher.
An animal that survives may have permanent brain damage.
Where does EEE come from?
EEE occurs in nature in a wide
variety of songbirds. Blood samples from New Jersey birds, where most
of the research on EEE has been done, indicate that the blue jay,
tufted titmouse, chickadee, catbird and cardinal are most often
infected. Although these birds do not develop the disease, they
maintain high levels of the EEE virus. They are considered reservoirs
in the disease cycle.
~Culiseta melanura~
is the primary mosquito species that transmits the virus from bird to
bird. This mosquito inhabits marshes and wooded wetlands and rarely
feeds on people or horses. Consequently, the EEE virus normally exists
in a rather secluded environment where it does not pose an immediate
threat to horses or people.
Are emus susceptible to EEE and WEE?
An exotic species sometimes grown
in Louisiana, the emu is susceptible to EEE and WEE. The emu infected
with EEE virus develops a fatal intestinal disease characterized by
bloody diarrhea. Emu owners need to be aware that an infected emu is a
source of virus to be spread by mosquitoes. A person should not skin an
infected emu or contact the bloody stool. The emu develops a nervous
system illness when infected with WEE virus. Emus should be vaccinated
annually for EEE and WEE.
What causes of EEE outbreak in horses?
There are two possible
explanations for EEE outbreaks in horses, and both are probably
responsible in part during epidemic years. One factor involves Mother
Nature, and the other is related to health management of the horses.
Transmission of EEE from birds to
horses and people can occur when mosquito populations are high.
In this situation, mosquito species other than Culiseta melanura become
involved in the transmission cycle of the EEE virus. Since these other
mosquitoes are more likely to feed on people and horses, it gives the
EEE virus a means of escape from the bird reservoir and from the more
secluded habitats of Culiseta melanura. Horses are usually the first to
become infected because they are normally housed in rural areas and
they have a much greater exposure to mosquitoes. The potential for an
outbreak of the disease in horses may correspond to the number of
unvaccinated animals present. It is unusual to see EEE in a horse that
has received the initial vaccine series and timely boosters. During an
out-break, most people have their animals vaccinated but then fail to
follow up with annual booster shots the next year or so. Two or three
years after the last outbreak, the population of unvaccinated animals
has increased considerably, and another outbreak is possible.
How do horses get EEE?
Horses become infected when a
mosquito carrying the EEE virus bites them.
What time of the year does EEE infect horses?
EEE cases in horses usually begin
to appear in mid to late summer and increase into the fall. In 1998,
following a warm winter, infected horses were seen in south Louisiana
beginning in early spring. The outbreak continued to spread north
during the summer. In Louisiana, outbreaks of EEE in horses tend to
occur at two- to four-year intervals.
What is the best way to protect my horse?
Annual vaccination of all horses
and other equines is recommended. Horses vaccinated for the first time
must receive an injection of EEE-WEE combined vaccine followed by a
second dose of the vaccine three to four weeks later. This two-dose
series is essential to establish an effective immune response. A
booster shot should be given in the spring each year thereafter. If an
outbreak of EEE or WEE occurs in the area in late summer or fall, a
second booster is advised. Foals should be vaccinated at 4, 5 and 6
months of age (three doses total) to ensure protection. Another booster
should be given in six months and annually thereafter. Vaccines are
available from various sources. Proper handling and administration of
vaccine are essential to ensure adequate protection. An overall health
program for horses is recommended. It would include vaccinations
administered by a veterinarian.
What should I do if my horse develops symptoms?
It is important to find out the
cause of the illness. Other diseases and poisoning may have similar
symptoms or clinical signs. The veterinarian may want to obtain blood
samples to send to a diagnostic laboratory to test for EEE. Rabies,
equine infectious anemia (EIA), equine leucoencephalornalacia (moldy
com poisoning), liver failure, equine protozoan myelitis (EPM),
botulism and other conditions must be considered. If the animal dies, a
thorough autopsy may be necessary to confirm the cause of illness. In
who infection evidence to done, individuals proximity transmits some
cases; removal of the brain for laboratory analysis may be all that is
needed. EEE in horses does not require a quarantine. It is important to
confirm the diagnosis of EEE so other horse owners can be told of the
need to vaccinate their animals.
Can humans contract EEE directly from infected horses?
No, the EEE virus is not passed
from horses to humans by contact with body fluids or in other ways.
Furthermore, it is proven WNV is not spread from horse to horse or
people either. EEE-infected horses generally do not have enough virus
in their blood to infect mosquitoes. The horse is described as a
''dead-end host"! EEE is acquired only from mosquitoes that have
fed on infected birds. An infected horse does not pose a disease threat
to its human owners. It does indicate that the virus is present in the
local bird population and that local mosquitoes are transmitting the
disease.
How do humans get EEE?
Humans can become infected when a
mosquito carrying the EEE virus bites them.
Does EEE represent a serious health threat to humans?
Human cases of EEE are rare, but
they can occur during an outbreak year. The disease produces serious
illness, and the probability of recovery is less than 50 percent. In
overt cases (cases where infection actually develops into disease), the
virus produces an illness that begins with headache and stiff neck. As
the disease progresses, the patient can fall into coma, with death as a
likely outcome. Recovery is possible, but individuals who recover
usually have brain damage. Children appear to be more susceptible than
adults. The good news is that most humans who are bitten by infected
mosquitoes abort the infection in the early stages and recover with no
evidence that they ever had the disease. The overt to unapparent ratio
of encephalitis in New Jersey, where most of the research on EEE has
been done, is estimated at one overt case for every 23 individuals
bitten by infected mosquitoes. People who live near acid water swamps
and in close proximity to ponds filled with emergent vegetation are at
increased risk from mosquitoes that transmit EEE.
What is the best way to protect my family if my horse
contracts EEE?
No human vaccine is available for
routine use, so avoiding mosquitoes is the best protection where EEE is
present. In parishes with mosquito control districts, these agencies
should be made alert. Mosquito control personnel are familiar with the
EEE cycle and have the expertise to reduce the mosquitoes that transmit
the disease. Homeowners as well as horse owners can take measures to
reduce the threat of EEE. First, avoid mosquito-infested areas and use
insect repellent when exposure is unavoidable. Eliminating
water-holding containers from your property (buckets, tires and other
receptacles) will reduce mosquito breeding in the immediate vicinity.
Horse troughs provide excellent mosquito breeding habitat and should be
flushed at least once each week to reduce mosquitoes near the paddock
area. The use of residual insecticides for treating mosquito-resting
areas around homes and livestock premises is helpful, too. In some
cases, removal of the brain for laboratory analysis may be all that is
needed. EEE in horses does not require quarantine. It is important to
confirm the diagnosis of EEE so other horse owners can be told of the
need to vaccinate their animals.
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