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article imageEssential Science: West Nile virus causes memory loss

By Tim Sandle     Jun 27, 2016 in Science
One of the lasting effects of West Nile virus, for some sufferers, is neurological problems including memory loss. A new study links this to an immune reaction.
In essence, the new research indicates that the long-term memory loss that afflicts some people who contracted West Nile virus is the result of the patient's own immune system turning on the body and destroying parts of the patient's own neurons.
West Nile virus causes West Nile fever, a mosquito-borne infection. Despite the name, the disease occurs in many parts of the world including: Africa, west and central Asia, The Middle East, Europe, particularly southern Europe, North, Central and South America, The Caribbean and Australia. In terms of its name, the virus was first detected in the West Nile district of Uganda in 1937.
The virus is an arbovirus of the Flavivirus kind. Currently, no vaccine against the infection is available. Diagnosis is achieved through antibody tests.
In most of those bitten by an infected insect (around 8 out of 10 people) the disease is asymptomatic. In the remaining 20 percent, symptoms include fever, headaches, muscle pain, nausea, and rashes.
In one percent of those infected, serious neurological diseases can result. Here the central nervous system is affected. A range of different diseases may result:
West Nile encephalitis, triggering inflammation of the brain,
West Nile meningitis, inflammation of the protective membranes that cover the brain,
West Nile meningoencephalitis, inflammation of the brain,
West Nile poliomyelitis, which causes a polio-like spinal cord inflammation.
These diseases can be fatal. They can also cause permanent side-effects. Globally, ten thousand West Nile survivors live with long-term neurological problems including fatigue, weakness, difficulty walking, and memory loss. Estimates suggest this number goes up by about 1,000 each year.
Mosquito  carrier of dengue fever
Mosquito, carrier of dengue fever
With memory loss, the reasons have perplexed medical staff for years. Now it seems the reason is a type of immune response. The connection has been revealed through the use of an animal model.
Using mice to replicate West Nile encephalitis, researchers began studying the viral infection of the hippocampus, the brain region important for memory. It was noted that four weeks following the mice recovering from the infection, their ability to navigate a maze was poor even though the maze was an area previously made familiar to the rodents.
Initial tests showed the hippocampal neurons had not been destroyed by the virus. Further tests showed microglial cells (a brain-based immune cell) had become clustered around the neurons at the site of infection and remained active. In addition, an immune protein called complement was also found at an atypically high concentration. This was shown to be affecting the synapses, which control how information is passed from cell to cell.
The researchers speculate that the viral infection sent the immune system in the brain into overdrive, resulting in the destruction of important synapses. This causes cognitive dysfunction and leads to memory problems.
It is thought that the same effect occurs with a low number of West Nile survivors who are unable to create new synapses. As well as memory, emotions called also be affected, as tweeted by GEN (@GENbio) - "Researchers discover how the most severe forms of West Nile #virus cause memory loss and mood disorders."
Commenting on the findings, lead researcher Dr. Robyn Klei stated in a research note: " My thinking has been, if we can determine what triggers this brain damage, maybe we can prevent this from happening or stop it afterwards."
Hence, going forwards, the research suggests that intervening in the immune response could prevent brain damage or, alternative, be used as a treatment step to help patients recover. There are currently no medications to treat the disease. Treatment for severely affected patients includes hospitalization, intravenous fluids, and respiratory support. The disease is managed through mosquito control.
The research is published in the journal Nature. The study is titled "A complement–microglial axis drives synapse loss during virus-induced memory impairment."
In related news, affected the U.S. mosquitoes collected in north suburban Evanston have tested positive for West Nile virus and a dead crow found at Smith Ranch Road and Yosemite Road in San Rafael, tested positive for West Nile virus.
This article is part of Digital Journal's regular Essential Science columns. Each week we explore a topical and important scientific issue. Last week we explored how a limitation with wearable technology — being brittle — could be overcome a new self-healing material. The week before we examined a possible connection between the neurodegenerative disease and certain infections.
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