March 2004

West Nile Virus - Update of an Emerging Infectious Disease in the U.S.

            Attention! West Nile virus is sweeping the Country, this is not a good thing, but realistically, it is inevitable. Once established, the only way to stop the spread of West Nile virus (WNV) is to eliminate birds (the virus multiplier/reservoir) and mosquitoes (the vector/reservoir)...but this will never happen. WNV is here to stay and will spread, probably to all areas of North and South America, wherever there are birds and mosquitoes capable of maintaining and transmitting the virus.

            West Nile virus was discovered and identified in 1934 in Uganda, Africa. Since then WNV has been found to be indigenous to Africa, Asia, Europe and Australia. Human outbreaks were recorded in the 1990's in Europe and the Mediterranean, with the largest incidence occurring in Russia, Israel, and Romania. The virus was recognized in the Western Hemisphere for the first time in 1999 when 59 people in the New York City area became ill with the disease. The viral strain introduced into the U.S. is the same as the WNV strain that was circulating in Israel in 1998. In 2000 and 2001, WNV spread rapidly through the South and Midwest, showing up in mosquitoes, animals, and humans in 28 states. However, large numbers of human cases were not reported. In 2002, WNV spread westward and triggered the largest human, arboviral encephalitis epidemic in U.S. history with 4,156 cases, of which 2,944 were cases with severe neurological disease, and 284 deaths. In 2003, the number of cases more than doubled to 9,006, of which 2,695, involved severe neurological disease, and 220 deaths. The higher incidence of cases in 2003 probably stems from the public’s and medical community’s heightened awareness that their health condition (not feeling well) may be caused by WNV prompting doctor’s to test for WNV rather than assuming their illness was just “the flu.” What will happen in 2004, only time will tell.

            Typically, humans acquire WNV through the bite of an infected mosquito. In nature the virus is maintained in an enzootic mosquito-bird-mosquito cycle that primarily involves Culex mosquito species. About 50 different species in the U.S. have been shown to be able to pick up and maintain WNV. However, only 7 species have been identified as the main vectors of the disease, this research, however, is ongoing. In temperate regions of the U. S., infected adult mosquitoes overwinter in culverts, catch basins, and sewer systems, become active in the spring, infect birds they feed on, and start the disease cycle. Infected birds fly to new areas, are fed on by mosquitoes, the mosquitoes become infected, and then spread the disease in the new area. Epidemics in temperate areas (most of the U.S.) usually occur from July through September. In tropical climates, where mosquitoes are active year-round, the transmission of WNV can happen at anytime. Humans and horses are dead-end or incidental hosts. When they become infected they do not develop sufficient viremia (virus particles per unit of blood) to become infective to other mosquitoes. However, in 2002, human infection of WNV was discovered, for the first time, in person-to-person transmission. It was found that WNV infection could occur from blood transfusions, blood products, organ transplants, from breast milk, and intrauterinely from an infected individual.

            West Nile virus infections appear to be equally distributed among all age groups and in both sexes. However, the incidence of neurological disease and death increases 20-fold among people over 50 years of age. During the 2002 epidemic, approximately 9% of patients with encephalitis or meningitis died. Thankfully, most people infected develop antibodies to the disease, and never have symptoms or feel sick. Approximately one in five or 20% of infected people develop a mild fever and one in 150 infected people develop neurological symptoms. Symptoms generally appear between 2 to 15 days after infection. The majority of symptomatic patients have a self-limiting, febrile illness, occasionally with headache, chills, muscle aches, nausea, vomiting and fatigue...like having a case of the flu. Although the acute illness usually lasts less than 7 days, fatigue can persist for several weeks. Approximately 5% of patients with symptoms (less than 1% of all infected persons) develop more severe neurologic illnesses, such as meningitis, encephalitis, and acute flaccid paralysis, similar to a poliomyelitis syndrome. Those with the most severe encephalitis may progress to coma. Age-related, underlying risk factors are the likely cause of people over 50 developing severe disease when infected. Treatment for WNV, once infected, is supportive.

            Risk factors associated with infection by WNV includes length of time spent outdoors, failure to apply mosquito repellent with DEET to exposed skin, living in buildings with flooded basements, and the presence of mosquitoes in the home. Presently prevention of getting WNV relies on the operation of comprehensive mosquito control programs and individuals taking measures to avoid mosquito bites. A vaccine against WNV is available for horses now, but a vaccine for humans is still a couple years away.

            Concerning Florida, WNV showed up in June of 2001 in dead crows and horses in Jefferson County. For the year there were over 400 equine cases and 12 human cases with no deaths. During 2002, the state had 28 human cases with 2 fatalities. These cases occurred predominately in the central part of Florida. In late February and early March of 2002, we had our first sentinel bird in South Walton convert for WNV, for the year a total of 14 birds convert. During 2003, 92 human cases and 6 deaths were reported throughout the state. Sixty percent of these cases occurred in the Panhandle Counties of Florida; Escambia had 12, Santa Rosa had 7, Okaloosa had 8, Walton had 1, Bay had 14, Holmes had 2, Washington had 1, Gulf had 4 and Calhoun had 1. For 2003, South Walton Mosquito Control had 69 sentinel birds convert for WNV. This was more than twice the conversion rate for 2002 (we had doubled the number of birds and locations of our sentinel coops for 2003). Conversions to WNV in our birds for 2003 started in July throughout the District and averaged about 14 conversions per month, through November.

            So what does this data tell us? With only two years of data it is hard to say. This year, in 2004, we have already had 4 sentinel birds convert to WNV, this tells me the WNV is here and here to stay. It is in mosquitoes and they are circulating the virus amongst the birds, the normal cycle. This year’s early conversions and no conversions last year until July tells me that there is no seasonality to this virus, that it will cycle amongst the birds provided the environmental conditions are favorable for mosquito activity and there are susceptible birds present.

            So what is the plan to control this disease and keeping it from spilling over into horse and human populations? Because mosquitoes are what transmit and spread the disease we must keep mosquito populations low to minimize the possibility of transmission. Presently we treat all standing water on a routine basis, year-round. We contact horse owners and place public announcements in local papers to remind them to vaccinate their animals. We educate and encourage homeowners to eliminate water holding situations around the home and where they work, use repellents when going outdoors, wear long sleeve shirts, pants and socks, avoid going outdoors during times of peak mosquito activity (dawn & dusk), make sure all screens on the home are in good repair, basically take personal measures to avoid getting bit. When counts go up in our mosquito surveillance traps (collected daily) we adulticide with our spray trucks, get control of the mosquito populations and try to stay there. Basically, as a mosquito control agency we need to stay vigilant and do our job diligently. As individuals we need to develop an awareness and take the personal responsibility needed to avoid the bite of mosquitoes as best as we can.

            The emergence of West Nile virus across America is a wake up call to the people and the governments that serve us, of the potential havoc that a mosquito-borne disease can cause. In the past we have eliminated malaria, dengue, and yellow fever from our country and state, but these are mosquito-man-mosquito cycled, human diseases. The problem with WNV is that it is a mosquito-bird-mosquito cycled, bird disease that causes illness in man when the disease “spills over” out of the normal bird/mosquito cycle. West Nile is a virus, it can mutate, it can become more virulent or less virulent. Luckily, at present, it has a very low mortality rate, less than 1% of those infected. However, the neurological damage it can cause is the major concern. Looking on the bright side, a vaccine should be available soon. Additionally all those infected, whether symptomatic or without symptoms, have been naturally immunized and won’t get the disease again. Over time, as the human population is immunized, either naturally or with a vaccine, the outbreak of epidemics will hopefully cease, the disease will burn out and occur only sporadically. Until then, as a mosquito control agency we must control mosquito populations all the time, as best as we can. As individuals we need to take measures to avoid or minimize the chances of getting bit by mosquitoes.