An Overview Of The West Nile Virus Biology Essay


The plus sense single strand RNA virus of the genus Flavivirus, is more commonly known as West Nile Virus. Flaviviruses are found in semi tropical climates, most notably the Bombay region of India. It is an endemic viral disease originating from the country of India and remaining endemic to the region to this day (3). Although cases of West Nile Virus have been seen as far as the United States, most cases of West Nile virus affecting humans are found primarily in the Indian subcontinent (3). The first cases of West Nile Virus were first detected over 1000 years ago in horses originating from sub-Saharan Africa. The virus has a wide range of hosts that it can infect, from avian species to mammals. Its main vector, however is an arthropod mosquito species known as Aedes aegypti and Culex pipiens (1). Species become infected when the mosquito vector bites the animal, thereby passing on the virus through blood. Once infected most individuals exhibit few if any symptoms, however a select few begin to exhibit a high fever followed by encephalitis and meningitis (2). There is no known cure for West Nile Virus, however vaccination studies have been ongoing in order to better combat this viral disease. It is imperative for the people living in the Indian subcontinent that a legitimate vaccine be discovered and administered especially to the young and elderly (3). All across the southwestern region of Asia, the West Nile Virus is an endemic viral disease that causes much distress to the human population of that area(3).

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West Nile Viruses are of the family Flaviviridae and genus flavivirus (1). They consist of a virion size approximately forty-five nm in length and a single strand RNA genomic sequence consisting of about 11,000 to 12,000 nucleotides (4). This makes the West Nile Virus a medium sized RNA virus. Its genes are able to encode about seven non-structural proteins and 3 structural proteins(4). The non-structural proteins of the virus encode the new viral genome and subsequent genomic copies. The structural proteins encode proteins that makeup the viral capsid as well as receptor proteins found on the surface of the virus. The actual shape of the West Nile Virus capsid is an icosahedron that contains two viral glycoproteins coming out of it (4). The entire viral capsid is surrounded by a membrane produced from the host cell that it enters. As the virus enters the host cell it encompasses the host cell's membrane and envelops itself inside of it. The envelope surrounding the virion is made up of a viral protein by the name of E protein, of which there are three separate copies (4). Whichever copy the specific West Nile Virus contains there are over 180 structures surrounding the capsid (4). Each separate strain of West Nile Virus contains a different combination of E protein which affects different antigen present inside the host body. The virion itself within the capsid is spherical with multiple receptor proteins sticking out from it (4). The viral membrane is made up of proteins called M proteins. Both the E and M proteins play a major role in host cell receptor recognition, host cell endosomal membrane fusion, and general virion assembly (4).

Unlike many other virus species, West Nile Viruses are able to undergo vertical transmission in order to survive cold, harsh seasons (6). Female mosquitoes can pass the virus over to their progeny. Adult mosquitoes can also survive the winter season by hibernating in tunnels, caves and sheds (6). Transmission of West Nile Virus occurs when a mosquito carrying the virus, also known as a vector, transfers the disease by sticking its proboscis into a bird, thereby infecting the bird. Birds are known as amplifying hosts because they not only transmit the infection to other birds but also to other mosquitoes (6). The virus itself enters the host's blood supply and begins invading host cells via receptor mediated endocytosis. Plus strand RNA viruses like West Nile Virus are fast replicating viruses. The plus single stranded genome acts as messenger RNA to quickly transcribe both non-structural and structural proteins (6). The genome is released into the host cell when the pH inside the capsid becomes acidic. Then the genome begins to translate at an ORF site also known as an open reading frame site on the viral genome (4). This forms a viral polyprotein which is then cleaved into the non-structural and structural proteins necessary for virion formation (4). Since host cells do not contain an RNA dependant RNA polymerase this is also provided by the virus in order to begin replication. The plus sense strand of RNA works to create minus sense RNA which in turn creates more plus sense strands. In this manner the West Nile Virus can replicate and create many proteins all within a short span of time. Once the genome has been replicated it is repackaged into a newly formed viral capsid and exits the host cell by budding (4). West Nile Virus is able to infect many different species depending on the types of receptors available on the host cell membrane. In its arthropod vector, however, West Nile Viruses do not adversely infect its host. It is simply harboured by the mosquito until a suitable transmission to another species can occur (3).

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Although West Nile Virus is seen to be a harsh and debilitating disease, many humans affected with the virus have asymptomatic infections (4). They do not show any physical signs of disease. West Nile Virus has a relatively low level of viremia during its infection; therefore, it is not seen as infectious to other humans. Human to human transmission has only been detected in blood transfusions and organ donation procedures (5). Another outcome of the virus is a mild fever that occurs when the host body begins to combat the virus by employing its immune system to fight off the infection. An outcome of this immune response is a fever, in which the body releases cytokines to attack West Nile Virions and allow macrophages to remove them from the body through endocytosis (7). Fevers are the body's way of attacking and neutralizing the infecting virus. At this stage West Nile Viruses can cause headaches, excessive sweating, chills and overall weakness. This stage is known as the viral incubation stage which can last up to a week. West Nile Viruses can also cause gastrointestinal problems, joint pain, and swollen lymph nodes (7). These symptoms tend to last anywhere from a week up to two months. A very limited amount of patients end up with the most virulent form of West Nile Virus, which is known as a neuroinvasive disease. West Nile Meningitis or West Nile Encephalitis can cause the host to lapse into a coma. It is hard to diagnose which patients will end up at this stage since the early symptoms of meningitis and encephalitis are similar to the early febrile stage (7). Once in a coma the patient's survival rate dramatically decreases and their chance of death is very high. The body's immune system continues to combat the virus; however, once the virus's replication level exceeds that of the host's cytokine attack, the patient ends up losing their life.

Unfortunately, no cure for West Nile Virus is currently on the market. Scientists have been manipulating mosquito saliva mosquitoes somehow inoculate themselves and become immune to the virus (2). However, after many studies conducted by the CDC it was observed that mosquito saliva, while beneficial to its own species, serves to speed up the replication process when mixed in with human blood (2). The saliva can actually exacerbate the situation and cause even more serious symptoms of the disease to appear (2). Since no vaccine is available, people of this region can best combat the disease by preventing its initial entry into their body. Perhaps the best form of prevention is not exposing oneself to mosquitoes (7). People show wear long clothing when going outside and use any type of insect repellent they can find. In third world countries such as India, many people cannot afford to spend their money on insect repellent. They should instead try to remove any still water that is around their homes and businesses. Still water is the breeding ground for these mosquito species. Keeping a clean and healthy environment and home is also extremely important. Do not remain outdoors after sunset and use mosquito nets to surround the bed so as not to get bit while asleep (7). Once bit, however the only treatments available are to soothe the symptoms and not cure the infection itself (7). Pain medication and rehydration are the two main forms of easing a patient's distress. Without any medication or medical help most patients can survive since the primary result of West Nile Virus is an asymptomatic infection that leaves the body within a couple of weeks. Many patients may not even know they were infected with West Nile Virus. In the most severely affected patients, however, survival is minimal. Once a patient's brain begins to swell it is only a matter of time before death follows.

West Nile Virus is especially deadly to those with a weakened immune system. This includes the elderly, the young and patients susceptible to opportunistic infections such as HIV patients. However, those people that live in high risk areas, or work during high risk times such as after dusk are also at risk. Anyone can be bit by an infected mosquito at any time, but these are the demographics of people most at risk. Recently West Nile Virus has been found to enter the United States as of 1999 (6). Now West Nile Viruses are found in areas of Texas, Wisconsin and New Jersey. West Nile infected Europeans as far bas as 1960, when it arrived in France through Egypt (6). However, the region most impacted by the virus remains to be the Indian subcontinent. Economically, the people of India have lost wages, and are unable to continue their education. The young may be forced to leave school and start jobs at a much earlier age in order to provide for their families and care for their sick relatives. Families can be forced into poverty and whole villages can be erased if a West Nile Virus outbreak occurs.

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West Nile Virus is endemic to the region of southwest Asia. In most cases it is an asymptomatic infection with a relatively low risk of death. However, in more serious cases it can cause encephalitis which brings the patient into a coma and eventually death. This single strand RNA virus is a plus sense virus that can replicate immediately upon host cell entry. It is a member of the family Flaviviridae and is a type of flavivirus. West Nile Viruses are slowly becoming endemic in other areas such as Europe, Africa, and North America. There is no known cure for the virus, however scientists do recommend preventative measures to combat the disease. It is important to wear long sleeved clothing and insect repellent when entering areas known for containing mosquitoes. Once infected the only medications available are for easing the symptoms caused by the virus and not the virus itself. Rest and plenty of fluids to prevent dehydration are some methods to stay healthy (7). West Nile Viruses use arthropod vectors to infect mammal species such as humans. Although many cases of the disease are asymptomatic, it is imperative to the people of India that a vaccination be discovered and implemented. Many inhabitants of India are wary of doctors and hospitals, so their chances for survival are even fewer. The high levels of death due to West Nile Virus infections have a negative impact on the Indian economy. Many people tend to lose their jobs after infection which hinders company advancement and production levels. Overall, West Nile Virus is a debilitating viral pathogen that affects the lives of many humans all over the world, but especially in India.