Indian River Mosquito Control District

This section will discuss the important mosquito-transmitted viruses that can cause human disease, and that are commonly detected by the IRMCD arbovirus surveillance program. These viruses are West Nile encephalitis virus, St. Louis encephalitis virus and eastern equine encephalitis virus. Though local transmission of dengue viruses has not been detected in Indian River County, dengue will also be discussed since it has recently been re-established in mosquito populations elsewhere in Florida.

West Nile virus, St. Louis encephalitis virus and Eastern equine encephalitis virus

There are three mosquito-transmitted viruses that are of particular concern for residents of Indian River Co. - St. Louis encephalitis virus (SLEV), West Nile virus (WNV) and Eastern equine encephalitis virus (EEEV). These are viruses that are intimately associated with the natural wetlands and agricultural environment of the region. SLEV is a long-term, permanent resident of Indian River Co.; this is its’ “home.” WNV was first detected in Indian River Co. in 2001, and is now a permanent resident of our county, like SLEV. Like the mosquitoes that transmit them, these encephalitis viruses are unlikely to ever be eradicated by man, and will co-inhabit the county with its human population.

St. Louis encephalitis and West Nile viruses are distinct infectious agents that have potential to cause epidemics of human disease in Indian River County. Though different, these viruses are close relatives sharing many characteristics. Both viruses are maintained in nature by two fundamentally different kinds of animal: wild birds and certain (but not all) species of mosquito. SLEV does not cause disease in wild birds, but merely reproduces in their bodies for a short 2-3 day period. WNV differs in that it can cause fatal infections in “Corvids” (the bird family that includes crows and jays) and in horses. In addition, both viruses can cause detectable infections in a wide variety of other animals, including man, without causing serious disease.

Because there is only a fleeting 2-3 day period during which an infected bird produces large quantities of new WNV or SLEV virus particles in its blood stream, there is only a brief window of opportunity for certain mosquitoes to take a blood meal from it and then become infected themselves. About 8-10 days after feeding on such a bird, mosquitoes will likely have developed the capability to transmit newly grown SLEV or WNV virus particles to other birds that they may bite. However, it is only the extremely rare mosquito that meets all the criteria to become infected and survive to infect another creature by bite. Those mosquitoes that succeed are said to serve the role of virus vector … they are one of the essential host animals in the virus life cycle, and their bites are the sole means that virus gets from mosquito to bird (and vice versa).

SLEV and WNV disease in man is quite simply the result of an “accident of nature.” Humans are the only animals in which SLEV infection causes disease (it is harmless to birds and mosquitoes), and they grow so little virus in the blood that infected people are not a concern for spreading SLEV to other mosquitoes. Likewise, WNV infections in man and horse (the two principal non-avian animals displaying serious disease) are also “dead-ends.” You cannot “catch” SLEV or WNV from an infected person (or horse); the virus is not spread by contact or by air.

Why is mosquito-borne encephalitis so important in Florida?

Did you know that in the early 1800’s what is now Indian River Co. was part of a much larger “Mosquito County?” The prevalence of fresh water and coastal wetlands in Florida and the subtropical climate of much of the state were formidable obstacles to its colonization by Europeans. These characteristics made the human inhabitants (and their domestic animals) particularly vulnerable to a variety of mosquito-transmitted pathogens and parasites. Florida has historically suffered from repeated, large epidemics of serious mosquito-borne disease, including yellow fever, malaria, dengue, and encephalitis. Some mosquito-borne diseases are still permanent residents of the state, and have taken on a greater prominence as human development increasingly impinges on the natural habitat of the virus and its hosts. In the past 35 years, St. Louis encephalitis, West Nile encephalitis and eastern equine encephalitis have become increasingly important in Florida. EEEV is infrequently encountered in Indian River County, but SLEV and WNV are cause for local concern every year.

St. Louis encephalitis virus is normally associated with wild birds and several species of mosquito, most notably Culex nigripalpus in south and central Florida. In the latter half of the 1900s, SLEV became the predominant mosquito-borne disease of man in Florida and was responsible for recurring epidemics in the south and central portions of the state. Major epidemics occurred in 1959, 1961, 1962, 1977, and 1990 and 1997. The 1990 epidemic was largest (223 documented cases) and most widespread (cases in 28 counties), with 11 fatalities. The 1990 epidemic was first recognized in Indian River Co., which also experienced the highest attack rate of any affected county (19 confirmed cases and one fatality, despite a relatively low population). The 1959-1962 outbreaks in the Tampa Bay area involved 55 fatalities amongst 315 cases. In contrast the most recent epidemic (1997) yielded 9 cases throughout the state, with only one fatality.

The true impact of SLEV during epidemics is difficult to assess, since there may be several hundred mild or asymptomatic cases generated for every diagnosed case. The 1977 and 1990 SLEV epidemics resulted in considerable disruption of normal activities of permanent residents, and retarded tourism to the state. Economic loss to the state has not been well documented, but the 1990 epidemic alone is likely to have been responsible for millions of dollars of direct and indirect losses.

The long-term impact of West Nile virus on Indian River Co. is impossible to predict with any degree of accuracy, as this Old World virus only made the jump to North America in 1999. As an “invading species,” WNV is encountering entirely new bird and animal species, some of which will serve important roles in maintaining this virus in nature. To date (August 2010) the greatest number of human cases of West Nile disease in Florida occurred in 2003, a total of 94 cases. Though we detect the presence of WNV virus in Indian River Co. during most years, the county has so far not been the focus of human or horse cases.

What about eastern equine encephalitis?

Eastern equine encephalitis virus (also known as “eastern equine encephalomyelitis virus”) is not a close relative to WNV or SLEV, yet has a generally similar life cycle involving wild birds and mosquitoes. EEEV can be transmitted by a variety of different mosquitoes, most of which are not thought to be important vectors of SLEV. In Florida EEEV transmission is generally most likely to occur in the vicinity of fresh water swamps populated by red maples or bay trees. Transmission of EEEV to sentinel chickens is rare in Indian River Co., being detected only 11 times from 1978 through 2009. Neither human nor horse cases of EEEV have been reported from this county in more than 20 years.

How do we monitor encephalitis virus activity?

Mosquito-borne diseases of man offer special challenges to those trying to prevent or control disease outbreaks. First, with the sole exceptions of yellow fever and Japanese encephalitis, vaccines licensed for use in humans are not available. Equine WNV and EEEV vaccines are available, and fatal encephalitis in horses can be safely avoided by following recommended vaccination schedules. Unfortunately, avoidable horse fatalities are often the result of owner’s failures to keep up with their animal’s vaccination schedules. The lack of human vaccines means that monitoring of local virus activity and local mosquito populations are essential to the management of mosquito-borne disease in Indian River County.

These mosquito-borne viruses have complex life cycles that involve several types of organism, and their disease transmission cycles are not predictable except in a general way, or at best only over a very short time frame. This is in large part due to the significant influence of weather (inherently unpredictable!) on the biology of both virus and mosquito. It has been repeatedly established that emergency measures to reduce encephalitis transmission are largely ineffective if delayed until the first human case appears. For SLEV and WNV, most of the cases to be seen in an epidemic have already been bitten by infected mosquitoes by the time the initial case has been diagnosed as being caused by an encephalitis virus! Timely surveillance data, and quick response to them, is clearly essential.

In “normal” years St. Louis encephalitis virus activity does not result in human cases, but in exceptional years environmental factors come together to produce large numbers of infected birds and mosquitoes within a short period of time. It is at such times that the odds of human infection increase enough to be of concern. In this county, the SLEV surveillance program conducted by IRMCD has two inseparable components. The first component is the use of sentinel chicken flocks to monitor levels of virus transmission from mosquitoes to birds in the county. The second component involves the intensive monitoring of two representative populations of the principal SLEV vector mosquito, Cx. nigripalpus , in different parts of the county. The mosquito analysis is critical in understanding events leading to any newly discovered SLEV transmission. Once a period of excess virus transmission is detected the mosquito analysis can in many cases provide the basis for short term prediction of extraordinarily high risk of transmission to man, or can confirm the cessation of an epidemic period.

 

 

From May through December each year, IRMCD maintains 8 flocks of 6 carefully raised chickens in cages located throughout the populated eastern half of Indian River County.  Like normal barnyard chickens, the sentinel chickens are readily fed upon by Cx. nigripalpus mosquitoes.  However, sentinels are tested for prior infection with SLEV, WNV and EEE virus before they are placed in the field. Identified by leg bands, each sentinel has an additional blood sample taken each week of the surveillance season.  The blood samples are tested weekly by a Florida Department of Health Laboratory in Tampa, which can determine when antibodies to either of the tested viruses are present.

The presence of virus antibodies in a weekly sample is an indication of a very recent infection, since earlier blood samples will have been shown free of these antibodies.  As soon as virus transmission to a particular sentinel chicken has been confirmed, it is immediately replaced with another uninfected chicken.  This allows us to maintain 48 sentinel chickens in the field at virtually all times.  Sentinel chickens have also proved valuable in monitoring of WNV transmission since its establishment in Florida.  Indeed, the initial detection of WNV in Indian River Co. was in a sentinel chicken.  It is worth noting that sentinel chickens testing positive for one of the encephalitis viruses do not get “sick.”  SLEV, WNV and EEEV are harmless to adult chickens.

The specialized monitoring of sentinel mosquito populations involves the systematic vacuuming of vegetation in known day-time resting sites of the Cx. nigripalpus mosquito. This mosquito travels extensively each night, but seeks shaded and humid resting habitats to wait out the inhospitable daytime hours. Adult mosquitoes accumulate in untold thousands in such places, resting under leaves of succulent vegetation, dead pine needles, or fallen palm fronds (see photo below). The samples of resting mosquitoes provide a nearly ideal cross section of the mosquito population at that instant, and can precisely pinpoint nights of extremely synchronous blood-feeding, egg-laying, or emergence from the immature aquatic stages. Such information is indispensable in interpreting recent SLEV or WNV transmission, and in predicting when future transmissions to sentinels or man are most likely to occur.

 

 

What about the county and state health departments ?

Effective use of encephalitis surveillance information is impossible without significant inter-agency communication and cooperation.  IRMCD makes continuous field observations, and communicates its observations to the Indian River County Health Department which has primary responsibility for local matters relating to public health.  Since SLEV, WNV and EEEV activity often affects several contiguous counties simultaneously, the Florida Department of Health (FL-DOH) has important responsibilities in monitoring and controlling human disease in those circumstances.  Moreover, the FL-DOH provides the specialized laboratory testing of weekly blood samples taken from the IRMCD sentinel chickens.  The county and the state health departments both play critical roles in monitoring potential human  encephalitis cases, and in promoting public awareness during times of recognized risk of mosquito-borne disease.  However,  encephalitis surveillance in Indian River Co. is conducted primarily because of local need, and is funded almost exclusively by local mosquito control tax dollars.

Why is the Culex nigripalpus mosquito so important?

Not all mosquito species that are capable of being infected with St. Louis encephalitis and West Nile  viruses and transmitting by bite in laboratory experiments play an important role in the transmission of virus in nature.  Thus, a particular mosquito species can be a significant public health threat in one part of the USA, but not in other areas where it also occurs.  The mosquitoes Culex quinquefasciatus and Culex tarsalis are important transmitters of SLEV and WNV virus in, respectively, eastern USA north of Florida and in California.  Both species have been shown responsible for large epidemics, but never in Florida (where they also dwell).  In Florida, only Culex nigripalpus has been clearly linked to the repeated SLEV epidemics seen in this state and it is also appears to be the most important vector of WNV.  Why is this?

SLEV and WNV epidemics occur only occasionally, despite the permanent residence of the virus, vector mosquito and numerous bird hosts in Florida.  Epidemics arise from the fortuitous convergence of various biological and environmental factors; in most years these ingredients are to varying degrees “out-of-synch.”  This is why we can readily demonstrate the presence of SLEV and WNV virus most years in sentinel chickens, yet humans are only sporadically infected.

Several characteristics of Culex nigripalpus contribute to its importance as a vector of WNV and SLEV:

1. Local populations are frequently abundant, and influenced little by normal mosquito control activities directed at other mosquitoes. The 70,000 acres of citrus groves maintained in the county are engineered to contain ditches between rows of trees (see photo below). When flooded by rainfall or irrigation, enormous numbers of Culex nigripalpus and other pest mosquitoes develop in this temporary aquatic habitat.
2. Female Culex nigripalpus feed primarily on the normal animal host of SLEV and WNV (birds), yet commonly feed on man when given the opportunity.
3. Culex nigripalpus is highly susceptible to infection with SLEV virus , and once infected is an efficient transmitter to other animals that it may bite. Local samples of this mosquito have been evaluated in the laboratory, and shown to be competent vectors of WNV virus as well.

What can I do to protect myself and my family from SLEV, WNV and EEEV?

Our local encephalitis surveillance system is designed to identify the arrival of those infrequent periods of exceptional risk of infection by mosquito-borne encephalitis viruses. In normal times, there are no special precautions that are useful in further reducing the already remote possibility of infection. However, when public health officials do indicate that conditions of increased encephalitis risk exist the best protection of all can be provided only by the individual resident… not by IRMCD! Encephalitis warnings and intense application of insecticides alone cannot guarantee that a resident of Indian River County will not suffer from SLEV, WNV or EEEV. Individual residents must also behave responsibly, and take active steps to reduce the exposure of family members to potentially dangerous mosquito bites at night. The virus cannot infect you if the mosquitoes cannot bite you! Take these simple, common-sense precautions during encephalitis alerts:

1. If you need to be out-of-doors at night, apply mosquito repellent containing DEET (a chemical whose full name is N,N-diethyl-m-toluamide) or picaridin to bare skin, following directions on the label. Skin must be covered with a thin layer of repellent; a spot or two on the arm will do no good. Do not disregard the label instructions… they are meant to assure safe and effective use of the product. Public perception to the contrary, Avon Skin-So-Soft® has not been demonstrated to have significant mosquito repellent properties and is not approved for such use. See the web link at the end of this article that provides more information about repellents. Mosquito bites can also be markedly reduced by wearing long-sleeved clothing and trousers rather than shorts or dresses.
2. Use of the following so-called “protections” increases, rather than reduces your exposure to mosquito bites, in part by producing a false sense of security in the absence of real protection! Citronella candles have such limited repellent effect as to be essentially useless, especially outdoors. Electric gadgets, namely various brands of “bug zapper” lights and hand-held “ultrasonic mosquito repellers” have been repeatedly shown to be valueless (except to the seller!). Scientific evaluations of “bug zapper” lights have shown that although they attract and kill moths, beetles and a variety of other stray insects (including mosquitoes), there is no significant reduction of mosquito numbers or attacks when they are in use. In fact, bug zappers appear to attract some mosquito species to the vicinity of people who then become the unfortunate targets of increased biting activity. Although they are widely sold in mail order catalogs and tourist attractions, it is actually illegal to advertise and sell “ultrasonic mosquito repellers” in some states. These are fraudulent devices; it is impossible to demonstrate that mosquitoes are repelled by them.

Of those people actually infected with the SLEV or WNV, only a small percentage will develop any serious symptoms of disease … most experience no recognizable symptoms at all. If symptoms of SLEV or WNV infection ever develop, these typically appear about 10 days after the bite of an infected mosquito. The majority of those people who do become sick will experience only generalized flu-like symptoms, which may include: fever, weakness, dizziness, headache, stiff neck or confusion. These minor cases are unlikely to be diagnosed as due to a mosquito-borne virus (which requires specialized blood tests), but they are self-curing and result in no long-term medical problems. Unfortunately, a small percentage of infected people will develop serious, and potentially fatal symptoms, including “encephalitis” (a swelling of the brain) and coma. The occurrence of severe SLEV or WNV disease is strongly dependent on age, with greatest risk being to those greater than 60 years old. Young children are less likely to become SLE or WN cases. People surviving severe cases sometimes suffer from long-term, residual neurological damage that may include paralysis, memory loss or deterioration of fine motor skills. The disproportionate impact of SLEV and WNV on the elderly is of particular concern in Indian River Co., due to the aggregation of retirees in the community.

EEEV causes a different pattern of human disease than SLEV or WNV. Epidemics of EEEV are rare, with single human cases being the norm. Unfortunately, the mortality rate is very high (50% or more) and survivors of EEEV may suffer from long-term neurologic damage. Also unlike SLEV and WNV, children and the elderly are equally likely to become victims of EEEV.

 

Dengue Fever

What is dengue and why is it a local concern?

Dengue viruses (DENV) are the cause of one of the most important mosquito-borne diseases throughout the world. Unlike the encephalitis viruses (SLEV, WNV and EEEV), dengue viruses have only two hosts… man and mosquito. Birds or other vertebrate animals have nothing to do with the DENV life cycle. Dengue was a disease that historically plagued Florida until 1934, which until 2009 was the last epidemic year within the state. Since 1934, imported cases have been documented yearly in Florida. These are cases where Florida residents were infected by mosquito bites while visiting other countries with active DENV transmission, but they did not become ill until returning to Florida. Two species of mosquito are important transmitters of DENV, and one or both species occur throughout Florida. We have always known that there was potential for re-establishment of locally transmitted DENV viruses in Florida as a consequence of the imported cases that were returning to the state each year. By 2009, it became clear that dengue had established a new foothold in Key West. Along with the continued discovery of new dengue cases in Key West during 2010, at least one additional focus of local DENV transmission has been discovered to date (August 2010) in Broward County. It is no longer hard to imagine that dengue could become a problem in Indian River Co., though that is so far not the case.

 

How is dengue different from the diseases caused by encephalitis viruses, and what does that mean for mosquito control agencies, health departments, and citizens?

For a simple explanation of dengue and what you need to know to avoid becoming a victim, click on this link, “Dengue fever. A guide to preventing infection.”, for a printable information sheet. For more details, keep reading…

The diseases collectively referred to as “dengue” are actually caused by any of four closely related viral serotypes (the equivalent of virus “species”). These are named DENV-1, DENV-2, DENV-3 and DENV-4. Typical of virus diseases, recovery from a DENV infection results in lifelong immunity. Unfortunately, the immunity is only to that specific DENV serotype; a person can theoretically be infected with all four dengue viruses in their lifetime! Also unfortunate is the fact that safe and effective dengue vaccines do not exist despite decades of extensive effort. Classical dengue fever (sometimes described as “break-bone fever”) is a painful, debilitating febrile disease that is rarely fatal. Dengue hemorrhagic fever (DHF) is a group of severe hemorrhagic symptoms that can lead to death, particularly in children. If identified in time for treatment, 3% of DHF cases have fatal outcomes. The fatality rate for untreated DHF is much higher, about 50%. Though DHF can result from in initial infection with any of the four dengue viruses, it is more commonly the result of a second infection (often years later) by one of the other DENV serotypes. Infants born to mothers who experienced a dengue infection in the past are at increased risk of developing DHF if they are infected, since their reaction to a first-time infection can be similar to a secondary infection. However, large dengue epidemics involving thousands of secondary infections at times result only in milder classic dengue disease. For more details about dengue disease, see the links at the end of this discussion.

Two Florida mosquitoes are the most important transmitters of dengue viruses throughout the world; none of the other 75 mosquitoes known to occur in Florida can serve as DENV transmitters. Aedes aegypti (“the Yellow Fever” mosquito) is highly domesticated, and almost exclusively utilizes artificial, water-holding containers as larval habitats. Aedes aegypti is adept at dwelling within and around homes. In contrast, Aedes albopictus (the “Asian tiger mosquito”) is fundamentally a treehole- and leaf axil-dwelling species that is secondarily an artificial container dweller. However, it is generally artificial container habitats (such as discarded tires) that generate locally pestiferous Ae. albopictus populations in North America. Aedes albopictus is more common and abundant in Indian River Co. than Ae. aegypti. Neither species has a long flight range, so if you are bothered by either of these mosquitoes at your home, you are likely producing them on your own property!

 

Dengue virus transmitters - Aedes aegypti (left) and Aedes albopictus (right)

Puerto Rico and other Caribbean islands experience DENV cases every year. Despite this DENV outbreaks tend to grow “silently,” and may not be recognized by local health personnel until large numbers of cases have already appeared. Identification of human dengue cases is the only way that epidemics are recognized, since symptoms of classic dengue fever overlap with symptoms of other diseases (like influenza). There is no equivalent of a “sentinel chicken” that can be used to anticipate periods of elevated risk to man. Monitoring local mosquitoes for dengue infection has also proven impractical for managing dengue in other areas. This means that timely identification of initial local dengue cases is critical to avoid the unchecked development of an outbreak within an unsuspecting community. Minimal surveillance in Florida currently involves: (1) annually notifying physicians and public health authorities of the possibility of dengue cases in Florida; (2) helping physicians clinically differentiate these exotic diseases from more common febrile illnesses; and (3) encouraging physicians to immediately submit sera from all suspect cases to the FL-DOH laboratories for confirmation.

What can Indian River Mosquito Control District do to protect me from dengue? What must I do?

Upon recognition of a local focus of DENV transmission, IRMCD mosquito control personnel would do their best to minimize risk of further transmission within the county. However, it is important to recognize the special challenges in controlling DENV transmission, and the limited impact that even the best mosquito control operations are able to achieve. Reducing exposure of residents to biting Ae. aegypti and Ae. albopictus mosquitoes by any means necessary should be the goal. This involves treatment, or removal, of all container habitats found in and around the home. That is and will always be the responsibility of the individual homeowner; IRMCD does not have the capability of inspecting every household in Indian River County!

Experience in other parts of the world has shown that community involvement is critical to management of dengue epidemics. IRMCD is not a public health agency, and the Indian River Co. Health Department and the FL-DOH would be the lead agencies in organizing a response to a local dengue incident. However, IRMCD would be actively collaborating with those agencies in encouraging residents to take appropriate personal protection measures, and do their part to eliminate artificial container habitats around their homes. Personal protection measures include the use of mosquito repellents (see the link at the end of this discussion). It is important to emphasize that the mosquitoes responsible for transmission of DENV are primarily active during the day, not at night like most other mosquitoes. The most important times to use repellents for protection from dengue infection are dawn-daytime-dusk. This is in contrast to the mosquitoes responsible transmission of WNV, SLEV and EEV, where the important times to use repellents are dusk-nightime-dawn.

What about spraying insecticide to kill adult mosquitoes? IRMCD routinely uses truck-mounted, computer- and GPS-controlled spray equipment to apply an insecticide (permethrin) to kill night-flying mosquitoes when appropriate. The District also occasionally arranges for aerial application of another insecticide (naled) when there are county-wide, massive populations of pestiferous or disease-transmitted mosquitoes; these aerial applications also occur during the night. Unfortunately, neither truck nor aerial insecticide spraying is very effective if attempted during the daytime when dengue transmitters can be impacted. The microscopic spray droplets rise and evaporate in the daytime thermal environment, and have little chance of impacting flying mosquitoes close to the ground. Of course, outdoor spraying will not impact any mosquitoes resting in houses or other structures. The best way for residents to reduce their exposure to mosquitoes capable of transmitting dengue virus is to make sure that they are not raising these mosquitoes on their own property, and to use repellents when needed to further eliminate bites.

For more information about mosquito-transmitted disease…..

IRMCD printable dengue brochure: “Dengue fever. A guide to preventing infection.”

World Health Organization fact sheet on dengue and dengue hemorrhagic fever: http://www.who.int/mediacentre/factsheets/fs117/en/index.html

Florida Department of Health mosquito-borne disease page: http://www.doh.state.fl.us/Environment/medicine/arboviral/index.html

Centers for Disease Control and Prevention dengue page: http://www.cdc.gov/dengue/

Centers for Disease Control and Prevention West Nile page: http://www.cdc.gov/ncidod/dvbid/westnile/index.htm

Mosquito repellents: http://www.cdc.gov/ncidod/dvbid/westnile/RepellentUpdates.htm

Florida Medical Entomology Laboratory, University of Florida, Vero Beach, FL: http://fmel.ifas.ufl.edu/

Mosquito Information Website, FMEL, University of Florida, Vero Beach, FL: http://mosquito.ifas.ufl.edu/