Severe infection with West Nile virus is a spreading problem worldwide that disproportionately affects those over age 50. And evidence is mounting that when the virus causes sickness, the long-term heath effects can be serious.

A close follow-up with 172 patients with confirmed West Nile virus infection published in April suggests that people who have symptomatic infections often suffer lifetime consequences that include memory problems, loss of balance and tremors (Vector Borne Zoonotic Dis. 8, 167–174; 2008).

Among those with severe infections, 60% still suffered from persistent symptoms of West Nile infection after the end of the first year of infection. Moreover, the researchers discovered that most, if not all, recovery takes place in the first two years after infection. “If a patient has not recovered by that time, it is very likely they will never recover,” says Kristy Murray of the University of Texas Health Science Center at Houston, the lead researcher on the study. Approximately 40% of patients monitored in the study continued to experience symptoms five years after infection.

Warning signs: Symptoms can linger Credit: IstockPhoto

Approximately 1 in 150 people infected by the West Nile virus develop a severe illness, such as West Nile encephalitis, according to the US Centers for Disease Control and Prevention. When the virus invades the nervous system during severe infections, the risk of long-term effects soars. “Those patients with [West Nile] encephalitis were less likely to recover than those who had meningitis or uncomplicated fever,” says Murray.

There are currently no vaccines approved by the US Food and Drug Administration (FDA) against West Nile, but researchers and companies recognize the threat of the virus. “Protecting the elderly is one important goal of vaccine development,” says Peter Mason, a professor of pathology at the University of Texas Medical Branch in Galveston and senior author on a rodent study published in May hinting that a DNA vaccine might help protect against the West Nile virus (Vaccine 26, 2762–2771; 2008).

The DNA vaccine developed by Mason uses a genetically modified virus that can only survive long enough to infect a single cell, and thus cannot cause disease. The engineered virus simply sticks around long enough to make the proteins needed to induce immune protection. Although promising, the vaccine is based upon an approach that has not yet been approved by the FDA for any human vaccine.

Another vaccine, which uses a slightly more traditional approach, is also in the early stages of development. Researchers at the biotechnology company Acambis, headquartered in Cambridge, UK, have created a candidate vaccine by adding genes from the West Nile virus to a live attenuated virus derived from the yellow fever virus. In a small clinical study of human volunteers, 93% of participants given the vaccine candidate developed signs of an immune response (Proc. Natl. Acad. Sci. USA 103, 6694–6699; 2006).