researchPublished 2007
West Nile Virus now at peak prevalance in North America?
West Nile Virus has caused more than 19,000 cases of human illness in the USA since it was first documented in this country in 1999. The virus circulates in birds, from which it is transmitted by mosquitoes; humans and other mammals are typically dead-end hosts, although they can develop serious symptoms such as encephalitis.
A known West Nile
vector. Image courtesy
of the Public Health
Image Library; image
no 4408.
The viral lineage that caused the first outbreak in 1999 — the NY99 strain — was closely related to strains found in Israel. But in 2002, another lineage was detected; this WN02 strain is now widespread while the NY99 strain has disappeared.
Researchers have now reconstructed more details of the recent population history of West Nile Virus in North America, and conclude that it may have reached peak prevalence.
Specifically, changes in genetic diversity of viral genomes isolated from birds between 1999 and 2005 suggest that:
- The WN02 strain arose a few years before it was first detected.
- The two viral lineages grew at different rates: the WN02 population size grew faster, on average, than the NY99 strain.
- Viral poplation growth rates were not linear: the fastest growth occurred after the lineage first appeared. The period of peak growth of WN02 (in 2002 and 2003) coincided with a peak in reports of human infections.
- The growth rate of the WN02 strain has slowed lately; there is no evidence that its population size is currently growing.
These findings are detailed in a 2007 issue of Journal of Virology.