West Nile Virus Term Paper

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West Nile virus was first detected in the Western Hemisphere in 1999 and during the last few years has spread cross the North American continent into all forty-eight continental states, seven Canadian provinces, throughout Mexico (West). Moreover, the virus has been detected in Puerto Rico, the Dominican Republic, Jamaica, Guadeloupe, and El Salvador (West). According to the United States Centers for Disease Control and Prevention, more than 15,000 people in the United States have tested positive for the virus since 1999, resulting in over five hundred deaths (West). Most experts believe that many more people have most likely been infected with the virus, but have experienced mild or no symptoms, thus have gone unreported (West). An individual's risk of contracting the West Nile is statistically low, and less than 1% of those infected develop serious illness (West). The elderly population and those with compromised immune systems are at the highest risk for developing serious illness, however, anyone at any age may develop the illness, thus it is important for everyone to take precautionary measures against mosquito bites to minimize the risk of infection (West).

The microbe that causes the West Nile infection belongs to a group of disease-causing viruses known as flaviviruses, which are generally spread by ticks and/or mosquitoes, and are known to cause other diseases such as yellow fever, Japanese encephalitis, dengue, and Saint Louis encephalitis (NIAID).

The majority of people infected with the virus have no signs or symptoms, and roughly 20% of humans develop a mild infection referred to as the West Nile fever (West1). According to the Mayo Clinic, common signs and symptoms of the West Nile fever include, fever, headache, muscle aches, backache, lack of appetite, nausea, vomiting, diarrhea, skin rash, and swollen lymph glands (West1). Less than 1% of infected individuals will develop more serious neurological infections, such as encephalitis, meningitis, and paralysis (West1). Signs and symptoms of these diseases include, high fever, severe headache, stiff neck, disorientation, confusion, stupor, coma, tremors, muscle jerking, incoordination, convulsions, partial paralysis, and signs and symptoms of Parkinson's disease or Parkinsonism (West1). Symptoms of the fever will generally last a few days, however symptoms of encephalitis or meningitis may last several weeks and particular neurological effects, such as paralysis, may be permanent (West1). Although most cases occur in elderly individuals or those with impaired immune systems, there have been recent cases that have shown that the virus can be transmitted through blood transfusions or organ transplants from virus-infected donors (NIAID). Moreover, experts believe that it is possible to transmit the virus from a mother to her unborn child, and also through breast milk (NIAID).

In addition to West Nile fever, meningitis, or encephalitis, some individuals who become infected with the virus can develop "acute flaccid paralysis," which is the sudden onset of weakness in the limbs and/or breathing muscles (West2). In most people, acute flaccid paralysis is due to the development of West Nile poliomyelitis, which is an inflammation of the spinal cord that causes a syndrome similar to that caused by the poliovirus (West2). West Nile poliomyelitis was first widely recognized in the United States in 2002, and persons with this may develop sudden or rapidly progressing weakness (West2). Most often the weakness tends to affect one side of the body more than the other, and may involve only one limb, however it is usually not associated with any numbness or loss of sensation, yet may be associated with severe pain (West2). In severe cases, the nerves going to the muscles that control breathing may be affected, resulting in sudden respiratory failure, thus it is important to recognize that this weakness may occur in the absence of meningitis, encephalitis, or even fever or headache (West2). In fact, there may be few clues that indicate that the weakness is due to West Nile virus infection (West2).

The first step of the transmission cycle occurs when a mosquito bites an infected bird or other infected animal, and crows are especially associated with the virus because they are highly susceptible to infection (NIAID). More than 138 bird species that can be infected have been identified by scientists, and along with more than 43 mosquito species than can transmit the virus (NIAID). The virus primarily cycles between mosquitoes and birds, however infected female mosquitoes can also transmit the virus through their bites to humans and other incidental hosts, such as horses (NIAID). Thus, due to such susceptible hosts to amplify the virus and such a variety of mosquitoes to transmit it, West Nile has been able to spread rapidly throughout the United States (NIAID).

The virus, first isolated in Uganda in 1937, is commonly found today in Africa, West Asia, Europe, and the Middle East, and was found in the Western Hemisphere for the first time in 1999 in the New York City area (NIAID). By early spring of 2000, it had appeared again in birds and mosquitoes and began to spread to other part of the eastern United States, so that by 2004, it had been found in birds and mosquitoes in every states except Alaska and Hawaii (NIAID).

Health officials are not certain how the virus got into the United States, but most likely it arrived through the importation of an infected bird (West1). According to the Centers for Disease Control and Prevention, from January 01, 2004 to January 11, 2005, there were some 2,470 cases of the virus, including 88 deaths, and human cases have now been reported from throughout the Untied States and in Canada (NIAID). At this time, there are no specific vaccines or treatments for the West Nile virus, and confronting the potential of the disease spreading rapidly across the United States, scientists and public health officials are accelerating research on the development of new tools to prevent and treat the disease (NIAID).

The complex interactions between the West Nile virus, mosquitoes, birds and other animals, and the environment have influenced the pattern of the virus emergence and distribution across the United States, yet certain factors contributing to the emergence of the virus are still not completely understood, and knowledge of these principles is vital to planning strategies for the prevention, treatment, and control of this disease (NIAID).

The main route of infection is through the bite of an infected mosquito (West1). When a mosquito bites a bird infected with the virus, the virus enters the mosquito's bloodstream and circulates for a few days before finally settling in its salivary glands, then when the infected mosquito bites a human or an animal, the virus then enters the host's bloodstream, where it may cause serious illness (West1). The exact mechanism of illness is still unknown, however most experts believe that the virus probably enters the host's bloodstream, multiplies and moves on to the brain, crossing the blood-brain barrier, which is a barrier that separates the blood from the central nervous system (West1). And once it crosses that barrier and infects the brain or the linings of the brain, an inflammatory response occurs and symptoms arise (West1).

It is mostly during warm weather when the virus transmission occurs, a time when mosquito populations are active (West1). The incubation period, that is the period between the time an individual is bitten by the infected mosquito and the appearance of signs and symptoms of the illness, can range from three to fourteen days (West1). Within the United States, wild and domestic birds, such as crows and jays, are the primary reservoirs for the virus (West1). Bird migrations distribute the virus to new area, where mosquitoes can bite the infected birds and thus spread the virus to humans (West1). Although humans and horses are not the primary hosts for the infection, they are incidental or accidental hosts (West1). Experts claim that the virus cannot be transmitted from person to person by a mosquito, nor can it be spread through kissing or touching, and there is no evidence that the virus can spread by caring for an infected horse or eating infected game meat, as long as the meat is cooked properly (West1).

As mentioned above, there are other routes of transmission by which the virus can spread (West1). In 2002, four people developed the virus after receiving organs from an infected donor, and 23 people were infected after receiving blood products from 16 infected blood donors (West1). This led to national blood donor screening for the virus beginning in 2003, and also to refusal of blood donations from anyone who had fever and headaches during the week before they arrived to donate blood (West1). Although there were still two cases of the virus from blood transfusion occurred in 2003, the donor screening as certainly reduced the potential risk of infection by removing hundred of units of infectious blood products donated by individuals who were unaware that they had the virus (West1).

In 2002 in New York, a woman in her last trimester of pregnancy contracted West Nile virus, and when her baby was born five weeks later, doctors discovered…

Sources Used in Document:

Work Cited

Guidelines for Investigating Suspect West Nile Virus Cases in Equine. The Animal and Plant Health Inspection Service. Retrieved November 09, 2005 from:


NIAID Research on West Nile Virus. National Institute of Allergy and Infectious

Disease. National Institutes of Health. U.S. Department of Health and Human Services. Retrieved November 09, 2005 from:

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