The city of Dallas, Texas, declared a state of emergency this week in what has become the year’s worst outbreak of West Nile virus (WNV) in the United States. So far, there have been 700 cases in Texas and 14 deaths, the heaviest toll reported since 2004.
West Nile virus is a mosquito-borne disease. The authorities in Dallas have authorized private companies to increase aerial spraying of mosquito-control pesticides, because there is neither a vaccine nor specific treatment for the disease.
More than 80% of people exposed to West Nile Virus have no symptoms. Almost all the rest experience flu-like symptoms, headaches or skin rashes. But in its most severe form, West Nile Virus invades the nervous system, causing inflammation of the brain, high fever, vision loss, paralysis and even death. Elderly and immunocompromised individuals are most susceptible, but it remains unknown why only a small percentage of those exposed to the virus develop symptoms, whereas most are not affected.
“Infection of the central nervous system is one of the most serious aspects of WNV disease,” says Tom Hobman, a virologist at the University of Alberta in Edmonton, Canada. His research group recently published findings in PLoS ONE that provide insight into how the West Nile virus breaches the blood–brain barrier.
The virus destroys key proteins that restrict the movement of materials, including viruses, into brain tissues. This is crucial to understanding how the disease breaks through the body’s defences (see this Nature Medicine article). Only a few viruses can do this, and knowledge of the mechanism may be the key for a vaccine or drugs to treat the disease.
There is now no West Nile virus vaccine available for humans, although a number of candidate vaccines are being tested. DNA vaccines, engineered to include some West Nile virus proteins that provoke an immune response in humans, may be safer than vaccines made up of virus particles. There is a DNA-based West Nile vaccine for horses, but a similar vaccine is not yet approved for human use. Scientists are also working on comparing the genetic sequences of non-lethal strains with lethal strains of WNV to determine what factors make specific strains of the virus so deadly. These findings hold promise for new vaccine development.
In addition to preventive vaccines, treatment options for West Nile viral infections are also being explored. An antiretroviral drug previously tested for use in HIV infections has shown promise against WNV because the drug can cross into the central nervous system. So far, it has been shown to interact with a crucial receptor in the blood–brain barrier of mice, thereby allowing specific T cells to attack the virus in its most lethal form, inside the human brain.