Causes and Risk Factors of West Nile Virus Infection
Infection with the West Nile virus is almost exclusively spread through contact with mosquitoes that carry the virus, although other modes of infection have also been identified. Understanding how this virus is spread is the best way to avoid a West Nile virus infection.The West Nile virus is an RNA virus, classified by infectious disease experts as a member of the Japanese encephalitis virus group. It was first isolated from a blood sample stored in the 1930s from the West Nile area of Uganda.
In recent decades the virus has spread virtually worldwide, and today is found in Africa, the Middle East, Europe, Asia, Australia, and North and South America.1
While it was initially thought to be of no special consequence, the West Nile virus is now known to be responsible for a particularly dangerous form of meningitis and encephalitis in a small proportion of those who become infected.
Common Causes of Infection
West Nile virus is an arbovirus, that is, a virus transmitted by arthropods. It is almost exclusively spread by mosquitoes.1 The virus is acquired by the mosquitoes when they feed on birds, the chief host of the West Nile virus.
Over 60 species of mosquitoes have been shown to be infected with West Nile virus.2 Mosquitoes that spread the virus to humans are usually one of the multitudes of Culex species, insects which are prevalent in many parts of the world. West Nile virus has also been isolated from ticks, but it is not clear that ticks are a vector of infection.
The Role of Birds
Many species of birds have been identified as the hosts which harbor the virus, and are the means by which West Nile virus has spread around the globe. Usually, the birds infected with West Nile virus have high concentrations of the virus in their blood for long periods of time but have no symptoms. This means that an infected bird is able to pass the virus to mosquitoes for a long time.
However, some species of crows, ravens, and jays have had high rates of death from West Nile virus, and several localized areas have experienced widespread bird death.
Furthermore, humans who live in proximity to areas where a lot of birds have died from the virus appear to have a higher incidence of West Nile virus infection.
Other Means of Infection
While by far the major means of human infection is by contact with infected mosquitoes, West Nile virus can also be acquired by contact with blood or blood products from people who have the virus in their bloodstream.
Infection with West Nile virus has been identified as occurring with blood transfusions and with transfusions of red blood cells, plasma, and platelets.3 This form of transmission has been greatly reduced now that universal screening is done in many countries on blood products. This screening is not perfect, however, since it may not detect West Nile virus if it is in very low concentrations.
Rarely, West Nile virus infection has also occurred with organ transplantation from infected donors.3 In these cases screened serum from the donors has been negative for West Nile virus, strongly suggesting that live virus was still present in the donated organs.
There have also been a few cases of congenital West Nile virus infection, caused by the spread across the placenta from the mother to the baby late in the second trimester.4 In these cases, babies developed illness from the virus shortly after birth.
Despite these reports, transplacental transmission of the West Nile virus is thought to be quite rare.
Causes of Symptoms
When West Nile virus enters the bloodstream and begins multiplying, the body’s immune system quickly reacts to get rid of the virus.
Typically, antibodies to the virus appear rapidly. These antibodies bind to the virus particles and cause them to be destroyed. In addition, immune cells quickly adapt to attack the virus. The immune response leads to the production of various interferons and cytokines, which fight the virus but which often produce inflammation, leading to the symptoms characteristic of West Nile fever. By these means, the body’s immune system typically gets rid of the virus within a few days.
In some people, however, the West Nile virus is able to cross the blood-brain barrier and gain a foothold within the nervous system. These people are the ones who develop the most feared consequences of West Nile virus—meningitis or encephalitis.
Any person who is bitten by a mosquito in an area where the bird population carries West Nile virus is susceptible to infection. Since these areas now cover a large portion of the globe, almost any mosquito bite could potentially transmit the virus, to any person. The more mosquito bites you receive, the higher your risk.
Most people who are infected with the West Nile virus suffer only a self-limited illness, or no symptoms at all. However, a small proportion of infected individuals (less than one percent) will develop the serious, life-threatening neurological form of the infection.
While this severe result can affect anyone infected with West Nile virus, some appear to have a higher risk of developing meningitis or encephalitis. Factors that increase this risk include:
In these scenarios, it’s important to work with your doctor if you notice anything out of the ordinary, even if it seems like a typical cold.