Epidemiology of Arboviruses in Florida
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Hello, I am Rebecca Shultz, the Arbovirus Surveillance Coordinator for
the State of Florida. Today I am going to discuss the epidemiology of
Arboviruses in Florida.
The word arbovirus is shortened from Arthropod-borne virus. Of these,
diseases transmitted by mosquitoes tend to get the most attention.
However, ticks, sand fleas, and biting midges are also arthropods that
can transmit diseases to humans.
The ecology of Florida supports arbovirus circulation and transmission.
We have a warm climate, with regular rainfall, both of which facilitate
mosquito breeding. We also have identified in Florida a number of the
mosquito vectors that are known to transmit disease. We have a bird
population which is suitable for amplification of the virus. In
addition, historical data show a long history of arboviral diseases in
Arboviruses are RNA viruses, and the main ones that we do surveillance
for here in Florida fall into 2 genera: flavivirus and alphavirus. The
flaviviruses include St. Louis Encephalitis (SLE), West Nile (WN) virus,
and Dengue virus. The alphaviruses include Eastern Equine Encephalitis (EEE)
virus, and Highlands J (HJ).
These Arboviruses are distributed throughout the U.S. EEE is seen in
many eastern and southern states, as far west as Texas. The California
serogroup viruses have a distribution that covers almost the entire
U.S., except for a few northwest states. Western Equine Encephalitis
(WEE) is seen west of the Mississippi River, with Highlands J covering
the southern states east of the Mississippi. SLE and WN are distributed
throughout the country.
The transmission cycle for SLE, EEE, and WN involves mosquitoes as
vectors, and birds as the hosts. An infected mosquito will bite a bird
causing it to become infected. Another mosquito will bite the same bird.
The second mosquito will become infected and may continue to spread the
virus to each subsequent bird that it feeds on. Occasionally, an
infected mosquito will bite a horse or a human instead of a bird. The
horse or human may become infected, but the level of virus in their
system will remain too low to contribute to the transmission cycle.
Therefore, humans and horses are considered dead-end hosts.
Birds and humans are at risk for SLE. Horses infected with SLE do not
develop clinical disease.
The clinical symptoms of SLE can depend on the severity of the disease.
In mild cases, a person would experience a slight fever and a headache.
In more severe cases, symptoms can include high fever, disorientation,
headache, convulsions, or coma. Symptoms generally appear between 5-15
days of being bitten by an infected mosquito. The case fatality rate is
estimated to be between 3% and 30%.
In Florida, SLE outbreaks tend to occur every 15 years or so. Outbreaks
were seen in 1958-1960, 1977, and 1990. Since 1990, few cases have been
seen in Florida. In fact, only one case has been reported since 2000 (in
September and October are the peak months for transmission of SLE to
In general, the incidence of SLE in humans increases with age, with
the highest incidence reported among those 80 years old and older.
This map shows the distribution of human SLE cases in the United States
between 1964 and 2003. During that time 4,632 confirmed or probable
human SLE cases were reported. The only states that did not report a
case during that time are: Maine, Vermont, New Hampshire, Massachusetts,
Rhode Island, South Carolina, Alaska, and Hawaii.
EEE also affects birds and humans, but unlike SLE, EEE can cause
clinical illness in horses which is often severe.
People with EEE develop an acute central nervous system illness that
can range from mild aseptic meningitis to encephalitis with coma,
paralysis, and death. Of the people who develop the disease,
approximately 35% will die. Of those who survive, approximately 35% will
have some kind of neurological deficit.
Since 1989, 27 human EEE cases have been reported in Florida. Normally,
between 0 and 5 cases are seen each year.
The peak transmission period for EEE is a little earlier than that for
SLE. Most human cases experience symptom onset between June and August,
with the highest number becoming infected in July.
Also unlike SLE, EEE occurs more often in younger people. Of the 77
cases seen in Florida since 1957, over half were in people under 20.
Nearly a third of cases were in children under 9 years old.
Of the 24 cases among children between 0 and 9 year of age, one third
(8 cases) occurred in children under one year of age.
Even though West Nile virus is relatively new to the Western
hemisphere, it has been around in other parts of the world for a long
time. It was first isolated in the blood of a feverish woman in Uganda
in 1937. As mentioned before, West Nile belongs to the flavivirus genus.
This genus includes the Japanese Encephalitis Antigenic complex. This
complex includes several diseases, all of which are mosquito-borne. Many
can also cause febrile, sometimes fatal, illnesses in humans.
The first recorded West Nile virus epidemic was in Israel in the
1950s. West Nile is one of the most widespread of the flaviviruses.
Its been found in humans, birds, and other vertebrates in Africa,
Eastern Europe, West Asia, and the Middle East. West Nile virus first
showed up in the Western Hemisphere in New York City in 1999. The
transmission cycle involves mosquito vectors and bird hosts, the same
mechanism that was discussed earlier for EEE and SLE.
As mentioned previously, West Nile virus affects birds, horses, humans,
and other vertebrates. Animals with reported infections include bats, a
chipmunk, a skunk, a squirrel, and a domestic rabbit.
West Nile virus outbreaks have occurred all over the world. From 1951
to the present, outbreaks have been recorded in Israel, France, South
Africa, Romania, Italy, Russia, and the United States.
When West Nile was first seen in New York City in 1999, there were
several prominent hypotheses to explain this introduction. The first is
that it was brought in by an infected human host. This is unlikely
because humans do not produce enough circulating virus to infect a
mosquito and continue the transmission cycle. The second idea is that it
came from a vertebrate host introduced by a human either legally or
illegally. This is possible, given the size of the exotic pet trade. A
third hypothesis is that a human-transported vector was involved. This
is also possible, and may be the most likely. Its possible that an
infected mosquito stowed-away in someones luggage, or that cargo ships
or barges coming into the U.S. had infected mosquitoes aboard. A fourth
idea is that a storm-transported vertebrate host introduced the virus to
NYC. This would be a bird that was displaced from its normal fly-way due
to storm conditions, and is a possibility. Finally, some originally
hypothesized that the virus was introduced intentionally, as a
bioterrorism weapon. This has been pretty much ruled out. The cycle of
West Nile and its transmission indicate that this is a naturally
occurring outbreak. In addition, looking at West Nile from a
bioterrorism perspective, it really does not cause large numbers of
deaths when compared to other agents. So its highly unlikely that a
potential bioterrorist would use West Nile virus as their weapon of
This map illustrates the rapid east to west spread of West Nile virus
through the United States from 1999 to 2002. In 1999, only a handful of
northeastern states had activity. By 2000, activity had spread through
most of the northeastern states, and as far south as North Carolina. In
2001 the virus spread south to Georgia and Florida, and west to
Louisiana, Arkansas, Missouri, and Iowa. By 2002, only Arizona, Utah,
Nevada, and Oregon had not seen activity.
This graph shows the number of West Nile virus cases in Florida since
the first case was seen in 2001. The number of cases increased from 11
in 2001 to 35 in 2002. The number of cases jumped again to 94 in 2003
before decreasing to 42 in 2004 and again to 21 in 2005. This
upside-down U-shaped curve is pretty typical for an emerging disease. A
disease will come into a new area and spread pretty quickly for a few
years before a certain level of immunity is established in the host
population. The numbers for 2004 and 2005 may be more typical of what we
would see in a normal year.
Clinical symptoms of West Nile virus can manifest in one of two forms.
The first is West Nile Neuroinvasive disease. This is defined as an
arboviral infection that results in a febrile illness with neurological
symptoms (such as aseptic meningitis, encephalitis, confusion,
paralysis, sensory deficits, convulsions, abnormal movements, or coma of
a varying degree). The other form is the milder West Nile fever. This is
defined as an arboviral infection that results in a febrile illness
without neurological symptoms. Persons suffering from West Nile fever
may experience headache, nausea, vomiting and rash. Of all the people
that are infected with West Nile virus, only an estimated 20% develop
symptoms. The majority of these people have WNF. A small percentage
(about 1% of total infections) experience neurological symptoms.
In order for a case to be confirmed, laboratory criteria must also be
met. A confirmed case will have:
Fourfold or greater change in serum antibody titer
Isolation of virus from or demonstration of viral antigen or genomic
sequences in tissue, blood, cerebrospinal fluid (CSF), or other body
Specific IgM antibody by enzyme immunoassay (EIA) antibody captured in
CSF or serum. Serum IgM antibodies alone should be confirmed by
demonstration of IgG antibodies by another serologic assay (e.g.,
neutralization or hemagglutination inhibition (HAI)
The Florida Department of Health utilizes several surveillance tools to
detect West Nile virus and other arboviral activity in the state. The
first is veterinary surveillance, which includes non-avian animals. The
second is live wild or captive birds. The third is dead bird reports.
The fourth is the collection and testing of mosquito pools. The fifth is
our sentinel chicken surveillance program; and the sixth is human cases.
When looking at disease activity over time, this graph represents when
each surveillance system should come into play. An increase in dead bird
reports should be the first indicator of an increase in activity. As
disease activity continues to increase, we should see activity in
sentinel chickens, followed by mosquito pools and veterinary cases.
Finally, when activity is near peak levels, human cases should be seen.
The point of utilizing all these different surveillance systems is so we
can detect increases in activity and put out the appropriate prevention,
education, and control information before a human case is seen.
Reporting of dead bird mortality still remains an effective way of
detecting a possible increase of disease activity in an area. Currently,
the Florida Fish and Wildlife Conservation Commission (FWCC) maintain a
dead bird reporting website, located at www.myfwc.com/bird. Citizens can
log onto this website and fill out bird reporting form. The form asks
for the location, species, and condition of the bird, as well as the
estimated date of death. Once the report is submitted, it appears in the
FWC database. Florida Department of Health monitors the database, and
forwards the reports to the affected county health departments. It is up
to the county health department to decide if a reported bird should be
picked up for testing.
Next is the sentinel chicken surveillance program. A little over half
of Floridas 67 counties maintain sentinel chicken flocks. Each week,
the flocks are bled and the samples are sent to the Department of Health
State Laboratory in Tampa. Here they are tested for antibodies to SLE,
EEE, and WN viruses. Positive samples indicate recent transmission in
the area of the flock. If the number of positive samples changes
compared to historical, or background, levels, action is triggered in
the form of increased public education and possibly mosquito control
Mosquito surveillance is conducted by local mosquito control districts
in conjunction with the Department of Agriculture and Consumer Services
(DACS). Mosquitoes are collected in traps and can be sent to the DOH
State Laboratory in Tampa for testing. This is a very useful service,
especially during the wake of a hurricane. Hurricane rains and standing
water can provide additional breeding grounds for mosquitoes, and it is
important to know if the growing mosquito population is carrying any
In the context of Arboviruses, veterinary surveillance most often
refers to horses. The Department of Agriculture and Consumer Services is
responsible for testing horse serum or tissue. They then report any
positive results to the Department of Health.
This is an example of a map that we create on a weekly basis to
illustrate arboviral activity in the state. The counties are colored in
various shades of green depending on how many sentinel chickens tested
positive for an arbovirus this week. This map is available on our
website. We also maintain an arbovirus hotline to provide statewide
information. This can be reached at 1-888-880-5782.
Unlike the previous map which only showed a weeks worth of activity,
this map illustrates activity from the beginning of the year. A small
red mark shows which areas have seen West Nile activity, and depending
on the shape of the mark, you can tell if it was a chicken, a horse, or
a wild bird that was positive. This map also shows which counties are
currently under medical alert or advisory. The counties shaded in green
are under medical alert, and those shaded in orange are under an
advisory for mosquito-borne illness. These maps are also updated weekly
and can be located on our website.
This chart demonstrates the flow of information. Up on the top, you see
the five surveillance tools that we use. Sentinel chickens and mosquito
pools are shaded in blue because they are considered active
surveillance systems. This means that Arboviruses are being actively
sought in these populations via routine laboratory testing. The
remaining systems are passive, meaning that cases are reported when they
Information about each of these surveillance systems is gathered by
different agencies. Humans are reported by hospitals, clinics, and
county health departments (CHDs), dead birds are reported to county
health departments, and sentinel chicken info comes via CHDs and
mosquito control programs. Horse information comes from veterinarians,
and mosquito pool information from mosquito control programs. Specimens
are tested through the laboratories of each of these respective
agencies, and results are reported back to the submitter, as well as to
the Department of Health (DOH). The DOH submits the information to CDC
via ArboNet, and also to the public via medical alerts, our website, and
mosquito control. The CDC also posts information to their website that
can be accessed by the public.
So by collecting and disseminating all this information, our ultimate
goal is to reduce the risk of human arboviral transmission in Florida.
The best way to do this is to reduce contact between humans and
mosquitoes. We strongly advocate personal protection measures, and
behavioral modifications on the part of the individual. Another way to
reduce human-mosquito contact is through mosquito control efforts. These
operations are effective and efficient, and conducted in an
environmentally proper way. Again, its very important to target
resources in regions where our other surveillance systems indicate an
elevated risk for human transmission. This needs to be done in advance
of human cases.
To reduce the chance of becoming ill, we recommend practicing the 5Ds
- Don't go outdoors between DUSK & DAWN when mosquitoes are most active.
- Keep a physical barrier between you and biters. DRESS so your skin is
covered with clothing.
- Keep DOORS & SCREENS closed to keep mosquitoes out of your home.
- Use a mosquito repellent containing DEET. Picaridin and oil of lemon
eucalyptus are other recently approved repellent options.
- DRAIN stagnant water and empty containers outdoors so wrigglers can't
hatch out to become adult biters
Contact information for the Arbovirus Surveillance Program staff is as
Rebecca Shultz, M.P.H.
(850) 245-4444 x2437
(850) 245-4444 x2994
(850) 245-4444 x2829
Carina Blackmore DVM, PhD.
Thank you for your time! Please dont hesitate to contact program staff
with any questions you may have.