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There are three mosquito-transmitted viruses that are of particular concern for residents of Indian River Co.: St. Louis encephalitis (or“SLE”) virus, West Nile (or “WN” )virus, and Eastern Equine encephalitis (or “EEE”) virus. These are viruses that are intimately associated with the natural and agricultural environment of the region. 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. SLE virus doesnot cause disease in wild birds, but merely reproduces in their bodies for a short 2-3 day period. WN virus differs in that it often causes 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. SLE virus is a long-term, permanent resident of Indian River Co.; this is its’ “home.” WN virus was first detected in Indian River Co. at the time of writing (Nov. 2001), and has also been detected in nearby counties. WN virus will undoubtedly soon become a permanent of our county, like SLE virus.

Because there is only a fleeting 2-3 day period during which an infected bird produces large quantities of new 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 can become infected themselves. About 8-10 days after feeding on such a bird, mosquitoes will likely have developed the capability to transmit newly grown SLE or WN 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).

SLE disease in man is quite simply the result of an “accident of nature.” Humans are the only animals in which SLE virus 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 SLE to other mosquitoes. Likewise, WN virus infections in man and horse (the two principal non-avian animals displaying serious disease) are also “dead-ends.” You cannot “catch” SLE or WN from an infected person (or horse); the virus is not spread by contact or by air.

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 and eastern encephalitis have become increasingly important in Florida. Eastern encephalitis virus is infrequently encountered in Indian River County, but St. Louis encephalitis virus is cause for local concern every year.

SLE 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 this century, SLE has become the predominant mosquito-borne disease of man in Florida and is 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 relative 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. Only two human SLE cases were documented in Florida during 1998.

The true impact of SLE 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 SLE 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 WN 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,” WN virus is encountering entirely new bird and animal species, some of which will serve important roles in maintaining this virus in nature. We can only assume that WN virus will cause some periodic episodes of human disease in our area. The listed weblinks can provide additional background regarding the extent and severity of WN encephalitis in Florida during 2001, and throughout the known distribution of the virus in the US to date.

Eastern equine encephalitis virus (also known as “eastern equine encephalomyelitis virus”) is not a close relative to WN or SLE viruses, yet has a generally similar life cycle involving wild birds and mosquitoes. EEE virus can be transmitted by a variety of different mosquites, most of which are not thought to be important vectors of SLE virus. Though not a hard and fast rule in Florida, EEE virus transmission is generally most likely in the vicinity of fresh water swamps populated by red maples or bay trees. Transmission of EEE virus to sentinel chickens is rare in Indian River Co., last being detected in 1994. Neither human nor horse cases of EEE have been reported from this county in more than 20 years.

Mosquito-borne diseases of man offer special challenges to those endeavoring to prevent or control disease outbreaks. First, with the sole exceptions of yellow fever and Japanese encephalitis, commercial human vaccines are not available. An experimental WN vaccine for horses was rushed into wide use in Florida during 2001, but to date there are no conclusive evaluations of the vaccine’s effectiveness and safety. Even more important, all of these pathogens and parasites have complex life cycles that involve several types of organism. Like the weather that influences them in diverse ways, these disease transmission cycles are not predictable except in a general way, or at best only over a very short time frame.

It has been repeatedly established that emergency measures to reduce encephalitis transmission are largely ineffective if delayed until the first human case appears. For SLE and WN, 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.

SLE virus is detected nearly every year in Indian River County. In “normal” years SLE 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 SLE 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 “sentinel” populations of the principal SLE vector mosquito, Cx. nigripalpus in different parts of the county. The mosquito analysis is critical in understanding events leading to any newly discovered SLE 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 IRC. Like normal barnyard chickens, the sentinel chickens are readily fed upon by Cx. nigripalpus mosquitoes. However, sentinels are tested for prior infection with SLE, WN or 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. Once an virus transmission to a particular sentinel chicken is confirmed, that bird is immediately replaced with another uninfected chicken. This allows us to maintain 48 sentinels in the field at virtually all times. Sentinel chickens have also proved valuable in monitoring of WN virus transmission in Florida during 2001. Indeed, the initial detection of WN virus in Indian River Co. was the seroconversion of a sentinel chicken. It is worth noting that sentinel chickens testing positive for one of the encephalitis viruses do not get “sick;” SLE, WN and EEE viruses 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 SLE transmission, and in predictingwhen future transmissions to sentinels or man are most likely to occur.