Encephalitis surveillance is impossible without significant interagency 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. As SLE, WN, and EEE activity often affects several contiguous counties simultaneously, theFlorida Department of Health, Bureau of Epidemiology has important responsibilities in monitoring and controlling human disease in those circumstances. The county and the state health departments both play critical roles in monitoring potential human SLE cases, and in promoting public awareness during times of recognized SLE risk. However, SLE surveillance in IRC is conducted primarily because of local need, and is funded almost exclusively by local mosquito control tax dollars.

Not all mosquito species that are capable of being infected with SLE virus and transmitting it 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 SLE virus in, respectively, eastern USA north of Florida and in California. Both species have been shown responsible for large SLE epidemics, but never in Florida (where they also dwell). In Florida, only Culex nigripalpus has been linked to the repeated SLE epidemics seen in this state. It may also prove to be an important vector of WN virus as well. Why is this?

SLE epidemics occur only occasionally, despite the permanent residence of the virus, vector and numerous bird hosts for the virus 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 SLE virus most years, yet humans are only sporadically infected.

Several characteristics of Culex nigripalpus contribute to its importance as an SLE vector:

(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 SLE virus (birds), yet commonly feed on man when given the opportunity.

(3) Culex nigripalpus is highly susceptible to infection with SLE 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 WN virus as well.

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! Encephalitis warnings and intense application of insecticides alone cannot guarantee that no resident of Indian River County will suffer from SLE or WN. 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 bare skin with mosquito repellent containing DEET (a chemical whose full name is N,N-diethyl-m-toluamide) according to instructions 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. Mosquito bites can also be markedly reduced by wearing long-sleeved clothing and trousers rather than shorts or dresses.

2. Use of the following “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 repeatedly been 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 off 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 SLE virus, only a small percentage will develop any symptoms of the disease called “SLE”... most experience no recognizable symptoms at all. If symptoms of SLE virus 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 SLE (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 SLE or WN disease is strongly dependent on age, with greatest risk being to those greater than 60 years old. Young children rarely become SLE or WN cases. People surviving severe cases of SLE sometimes suffer from long-term, residual neurological damage that may include paralysis, memory loss or deterioration of fine motor skills. The disproportionate impact of SLE on the elderly is of particular concern in Indian River County, due to the aggregation of retirees in the community.

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

West Nile Virus and St. Louis Encephalitis  Page 2