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West Nile Virus & Encephalitis
General Information
West Nile Virus, also called West Nile Fever, is a new disease to North America. It is caused by a Flavivirus of the Japanese encephalitis antigenic complex that also includes diseases with exotic names such as Cacipacore, Koutango, Kunjin, Murray Valley Encephalitis (Australia), Rocio, Usutu, Yaounde and of course our own St. Louis encephalitis. It is one of the 100 or so arboviruses (arthropod borne viruses) that infect humans, and except for Urban Yellow Fever and Dengue fever, humans are accidental hosts. Western equine encephalitis, (WEE), Eastern equine encephalitis (EEE), and Venezuelan equine encephalitis (VEE) also occur in the Americas.
As the following photos show, birds act as a natural reservoir for these viruses, and mosquitoes of the Culex genus transmit them when the feed on infected birds, then on healthy birds. Eventually, the infection amplifies to a point that infected mosquitoes begin to transmit the virus to humans, horses and other animals called dead-end hosts.
West Nile Virus is an old world disease described from such far flung regions as Africa, Europe, the Middle East, Asia and Oceana. It first appeared in the US in the New York City area in 1999. How it arrived here is a mystery that will probably never be solved. A tremendous number of people and associated cargo arrive in the New York City area each year. It could have arrived with an infected mosquito, an infected person, or an infected bird. Once the virus is transmitted to resident birds and mosquitoes, it is established in the new world.
The virus is spreading down the East Coast and westward, mostly by migrating birds that are infected with the virus. It will continue to spread as infected birds move into other areas of the US and infect local mosquito populations. As of this writing (August 2001), it has been identified in states from New England down the east coast to Florida, and west to Ohio,
Indiana and Louisiana.
The photos that follow were kindly provided by Dr. Mike Sinsko, Senior Entomologist at the Indiana State Department of Health (ISDH). We appreciate the use of these photos, and hope that this web page will provide an educational outreach regarding these biological hazards, and highlight the ongoing bird monitoring and mosquito control programs that reduce the risk of arborvirus outbreaks.
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In the 1970’s, the midwestern USA was struck with an outbreak of mosquito born equine encephalitis. This and the following diagram show the number cases of this disease reported in each county in Indiana in 1975. |
Figure 1 - Cases of equine encephalitis in Indiana - 1975
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Figure 2 - Cases of equine encephalitis in S. Indiana - 1975
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Note that the number of cases are highest in urban areas where human populations are greatest. The mosquitoes that transmit this virus require stagnant water contaminated with organic matter to breed. These conditions commonly occur in populated areas where leaking septic tanks and other organic matter produced by man’s activities pollutes bodies of water, lowering dissolved oxygen levels to zero.
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Not all species of birds are involved. Some 86 species of birds have been shown capable of harboring the virus. The birds associated with man, such as the House Sparrow, can be prime reservoirs of this disease. In the case of West Nile Virus, this virus is fatal to corvids such as Crows and Jays. |
Figure 3 - Not All Bird Species
Carry the Virus
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Figure 4 - The Mosquito, Culex pipiens
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The virus is transmitted to man and other animals by certain species of mosquitoes that feed on infected birds then bite man and other animals. Morbidity and mortality are greatest in young children and older age groups.
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The St. Louis strain is transmitted by mosquitoes of the genus Culex (Fig. 4). Mosquitoes feed on infected birds and transmit the virus to other birds. This cycle results in amplification of the virus. The greatest danger occurs in the warm summer months when the virus and the mosquitoes are active. Even then, however, birds that carry the virus are only infective for a period of 4 days.
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Figure 5 - SLE Virus Infection Cycle
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Figure 6 - SLE Virus Spillover to Man
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As this amplification cycle continues, and the mosquito population increases, infected mosquitoes bite man and other animals. The virus then spills over to “dead-end-hosts” such as man, horses and other mammals. |
The Michiana region of Indiana and southern Michigan is particularly noted for outbreaks of Eastern Equine Encephalitis (EEE). The unique geological history of this area produced sandy soils with depressions known as kettles.
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Figure 7 - Map of Michiana Region
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Figure 8 - Sphagnum Bog in
Michiana Region
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These kettles are the sites of sphagnum bogs that are ideal breeding sites for the mosquito Culiseta melanura, the prime vector in transmission of EEE in this region The dead vegetation, wet soils and shallow roots shown at the base of the tree in this photo provides an ideal breeding site for these mosquitoes.
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Eastern Equine Encephalitis (EEE) is particularly virulent, with a 60% fatality rate in humans, and a 100% fatality rate in horses. Fortunately, this region of sphagnum bogs is relatively remote from human population centers. The community of birds that occur here are different as well, and include many species of Neotropical migrants.
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Figure 9 - Eastern Equine
Encephalitis Cycle
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Figure 10 - Mist Nets Are Used to Catch the Birds
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Since the 1970’s, the Indiana Department of Health has monitored the birds in Indiana to detect and provide early warning in those areas where the these viruses are found. During the summer season, when the threat of virus transmission greatest, mist nets are used to catch and monitor local birds to detect the infection. |
Birds are caught and carefully removed from the mist nets, then identified and measured. |
Figure 11 - Removing a Bird from the Net
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Figure 12 - Banding the Bird
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Each bird receives a serially numbered metal band. This will serve to identify this individual each time it is caught, and allows the scientists to follow the infection cycle in individual birds throughout the summer season. |
A blood sample is taken from each individual bird, and prepared for a laboratory analysis to detect the presence and amount of the encephalitis virus. The birds are then released unharmed. |
Figure 13 - Taking the Blood Sample
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Figure 14 - Monitoring Locations in N. Indiana
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As staff and budget permit, the bird
populations in as many counties as possible are monitored. In northern and central
Indiana, monitoring efforts are concentrated in urban areas where conditions are most
favorable for transmission of the virus.
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In Southern Indiana, urban areas are also targeted to maximize the effectiveness of the monitoring program.
If the birds in an area are found to be infective, health warnings are issued through the local news media to advise residents to make special efforts to avoid being bitten by mosquitoes.
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Figure 15 - Monitoring Locations in S. Indiana
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What can you do?
Reduction of the mosquito population is an efficient, cost effective way to minimize the danger of an arbovirus outbreak in your neighborhood. Check your neighborhood to locate and remove containers of standing water that act as breeding sites for mosquitoes. Old tires, cans, plastic containers, pet water dishes, bird baths, poorly drained ditches, roadside swales and many other sites can hold water long enough to breed mosquitoes.
Work with your local health officials to identify and eliminate mosquito breeding sites, especially areas of contaminated water.
If you would like to learn more about this and other diseases, and things you can do to protect yourself and your community, visit the web site of the
Center for Disease Control or contact your State of local health department.
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All images are courtesy of CWBO. All image copyrights are owned by CWBO.
Any use of these images must have permission of CWBO. |
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