The (Petri) Dish: From the West Nile to the Colorado River
September 4, 2012
In my July 4 (Petri) Dish article on mosquitoes, I briefly touched on West Nile Virus (WNV). Little did I know that WNV would be all over the news in August due to a large outbreak in Dallas County, Texas that has infected 733 people and killed 30 (as of Aug. 30; Centers for Disease Control). The epidemic has led to a state of emergency in Dallas and the declaration that this is the worst WNV outbreak the country has ever seen. To date, there have been 1,590 reported cases and 65 deaths nationwide. While 23 WNV cases have been reported in Colorado, there have fortunately been only two deaths.
The history of WNV offers an important lesson about the spread of infectious disease in the jet age. The virus was first identified in the late 1930s in the West Nile region of Uganda. It was subsequently found to range throughout east Africa and in west India, Australia, Egypt and the Middle East. However, the virus was not found in the Americas – that is, until it was introduced into the U.S. in the 1990s. The first signs came in 1999 when a veterinary pathologist at the Bronx Zoo in New York, Dr. Tracey McNamara, noticed numerous dead and sick crows around the zoo’s grounds. She became particularly alarmed when many of the zoo’s exotic birds, including its flamingo population, started to die off. After performing necropsies on some of the birds, she found that they had died from brain hemorrhages and heart lesions. At the same time, in nearby Queens, two elderly patients were dying of undiagnosed encephalitis that appeared to be caused by a “new” virus. The Centers for Disease Control got involved, and subsequently two research teams confirmed that the virus that killed both the birds and people was WNV. The final toll of confirmed infections was 59 humans, 103 birds and 12 horses. Since then, the virus has become endemic in the U.S. and is responsible for yearly bird kills. Human cases and fatalities have risen sharply, presumably due to the geographic spread coupled with increased surveillance and improved diagnostics. The virus has also spread throughout Canada, Mexico, the Caribbean and Central America.
WNV is primarily a bird virus, causing epidemics in various bird species. It is transmitted by mosquitoes that bite infected birds and then pass on the virus to uninfected birds. Transmission to humans, and other non-avian species, represents a dead-end for the virus – making us unfortunate collateral damage from an epidemiological standpoint. This is primarily because the virus is not well adapted to humans and does not establish high enough levels in the blood for transmission back to mosquitoes. The poor adaption of the virus to humans means most human WNV infections are either mild or asymptomatic. The more severe cases of infection and mortality are generally found in individuals with weakened immune systems (such as transplant recipients, diabetics and the very elderly).
How did WNV initially get to New York? Since human infection is a “dead end” for the virus, it had to have been brought to the U.S. via either an infected bird or mosquito. While birds certainly migrate long distances, the sudden appearance in New York did not fit with longstanding migratory patterns. This points a finger at the ease of travel in the jet age and our global interconnectivity. The most likely explanation is that WNV-infected exotic birds smuggled into the U.S. introduced the virus to the local New York mosquito population. In fact, genetic analysis of the North American strain suggests it is most closely related to a WNV strain from the Middle East, specifically one isolated from a goose in 1998. Once in the U.S., local mosquitoes probably picked up the virus and introduced it into native bird populations, establishing endemic infection.
Geographic distribution is uneven and the location of disease outbreaks tends to vary each year. For example, after experiencing a major outbreak in 2003 (2,947 cases and 63 deaths), Colorado has been mostly spared in the current outbreak. Fortunately, Summit County has never experienced any WNV activity, suggesting that the virus is not endemic in the region. The reasons are unclear, but may be related to climate and the relatively low numbers of mosquitoes. However, this may change in the future. And given the fact that WNV is now a wide-ranging problem, there needs to be greater attention to developing vaccines and treatments that can complement ongoing mosquito control efforts. In this regard, a 2013 Keystone Symposia conference on the family of viruses of which WNV is a member – positive-strand RNA viruses – may contribute to advances in controlling this disease.
David L. “Woody” Woodland, Ph.D. is the chief scientific officer of Silverthorne-based Keystone Symposia on Molecular and Cellular Biology, a nonprofit dedicated to accelerating life science discovery by convening internationally renowned research conferences in Summit County and
worldwide. Woody can be reached at (970) 262-1230 ext. 131 or firstname.lastname@example.org.