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West Nile Virus

West Nile virus (WNV) is a virus of the family Flaviviridae. It is part of the Japanese encephalitis antigenic complex of viruses and is found in both tropical and temperate regions. It primarily infects birds but can infect humans, horses, dogs, cats, bats, and other mammals. Humans are generally infected through the bites of mosquitoes and about 90% of West Nile Virus infections are without symptoms.

The virion is 45-60 nm and covered with a relatively smooth protein surface. It is similar to the dengue fever virus. The genetic material is a positive-sense, single strand of RNA, which is between 11,000 and 12,000 nucleotides long. WNV is either asymptomatic, a mild febrile syndrome, or a neuroinvasive disease termed West Nile meningitis or encephalitis. The ratio of each of these happening is 110:30:1.

The second stage has an incubation period of 2 to 8 days followed by fever, headache, chills, diaphoresis, weakness, lymphadenopathy, drowsiness, pain in the joints and symptoms like those of influenza or the flu. Sometimes there is a truncal rash and some patients experience gastrointestinal symptoms including nausea, vomiting, loss of appetite, or diarrhea. Symptoms generally resolve within 7 to 10 days although fatigue can persist.

Dangerous encephalitis is characterized by similar early symptoms but also a decreased level of consciousness, sometimes approaching near-coma. Recent outbreaks have resulted in a deeper study of the disease and other, rarer, outcomes have been identified.

There is no way to measure the number of worldwide cases, however, in the United States there were 663 cases in 2009. Texas had the most cases with a 104 total. The mortality rate in 2009 was 30 deaths of the 663 reported serious cases. That is a 4.5% casualty rate. 80% of cases have no symptoms.

The virus is transmitted through mosquito vectors, which bite and infect birds. The birds are amplifying hosts, developing sufficient viral levels to transmit the infection to other biting mosquitoes that go on to infect other birds. American Robin and American crow are the most common carriers. The virus doesn’t multiply as quickly in mammals, which tends to make them dead end infections, meaning mosquitoes won’t pick up enough of the virus to spread it from biting a mammal.

The US outbreak revealed transmission methods such as blood transfusion, organ transplant, intrauterine exposure, and breast feeding. Since 2003 blood banks do screenings for the virus amongst donors.

The most severe outcomes from WNV infection are associated with advancing age and a patient history of organ transplantation and diabetes. Sometimes there is also a genetic factor that can increase susceptibility. It has also been shown that mosquito saliva may alter the immune system in an advantageous way for the virus. It is unknown if the mosquito receives any benefit from this.

There is no vaccine for humans although one for is one for horses and some birds have received vaccinations with unknown effects. Dogs and cats don’t really show any sign of infections although they can become infected.

WMV was thought to have emerged as a distinct virus around 1000 years ago. The initial virus split into two distinct lineages. It was possible that WMV was the cause of Alexander the Great’s early death. It was first isolated from a 37 year old woman at Omogo in the West Nile District of Uganda in 1937. It first appeared in the western hemisphere in 1999 with encephalitis reported in humans, dogs, cats, and horses and then spread in the United States.

The American outbreak started in New York City and later moved to New Jersey and Connecticut.

The virus is controllable through control of the mosquito population.

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West Nile Virus


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