U.S. Department of the Interior

U.S. Geological Survey

Factsheet 2004-3002

August 2003

Effects of West Nile Virus

USGS scientists are embarking on bold new research to better understand 

the consequences this disease may have on 

wildlife and human health

West Nile virus (WNV) has spread 

rapidly across North America, affecting 

thousands of birds, horses, and people 

since it was discovered in the Western 

hemisphere. Since 1999, WNV has 

swept from the New York City region 

to almost all of the continental U.S., 7 

Canadian provinces, and throughout 

Mexico and parts of the Caribbean.

The U.S. Geological Survey (USGS) is 

committed to understanding the effects 

of WNV. To do that and better answer 

the remaining questions surrounding 

the disease, USGS scientists are 

collaborating with many laboratories 

and agencies, including the Centers for 

Disease Control and Prevention (CDC), 

to learn more about the ecology of WNV, 

as well as embarking on bold research 

initiatives to investigate its future effects.

USGS and Public 

Health

Wildlife disease 

scientists at the 

National Wildlife 

Health Center (NWHC, 

Center), the CDC, 

and other federal and state public health 

and wildlife agencies in the national WNV 

surveillance program are monitoring birds 

for the presence of the virus. In addition, 

the NWHC is working with federal and 

state wildlife offi cials in investigating local 

and regional wildlife die-offs potentially 

due to WNV. Surveillence studies, die-off 

investigations, and experimental studies 

in the Center’s biosafety level-3 (BSL3) laboratories have allowed scientists to 

explore, and subsequently understand, 

not only the public health importance of 

the virus’ arrival 

in the Western 

Hemisphere, but 

also its potential 

effect in wild bird 

populations.

USGS geographers 

are contributing 

to the national 

WNV surveillance 

effort by mapping 

WNV cases as 

they are reported 

to the CDC. This 

has allowed the 

CDC and the 

general public to 

monitor the virus’ 

geographic movement as well as seasonal 

trends and clusters of cases throughout 

the continental U.S. In addition, USGS 

and the CDC are investigating the ecology 

of potential WNV vectors and reservoirs 

in Louisiana. Using remote sensing and 

geographic information systems to map 

habitats and environmental factors USGS 

and CDC are seeking to develop better 

models for understanding the associations 

between reservoir hosts, mosquitoes, 

environmental and climate conditions, and 

the virus.

When West Nile virus was fi rst detected 

in New York, local governments began 

increasing insecticide use. USGS quickly 

recognized the need to develop highly 

sensitive tests and analyses to monitor 

these insecticides. This information is 

vital to land managers and vector-control 

agencies to ensure that insecticides are 

safe and effective. On Long Island, 

USGS is testing area waters for several 

insecticides, including resmithrin, 

sumithrin, malathion, and methoprene. 

USGS also monitors for a byproduct 

of methoprene and for the chemical 

piperonyl butoxide, which is used to 

enhance the effectiveness of some of the 

insecticides. These data help offi cials 

manage insecticide application and help 

“In the five years since WNV has 

been discovered in North America, 

we’ve learned a lot about this disease 

and its effects on wildlife. We know 

that it’s maintained in a bird-mosquito 

life cycle, we know many species are 

affected. But there are a lot of things 

we don’t know. Which birds serve as 

important reservoirs? Which mosquito 

species can transmit the virus? How 

has the virus spread so quickly? Are mosquitoes the only 

mechanism in which the virus can over-winter? Are mosquitoes 

the sole significant mode of transmission to birds, humans, 

and horses? Will the geographic extent of the virus continue to 

expand, and if so, what effects will it have on new ecosystems? 

These are important questions that we still need to answer.”

Faces of Science: Emi Saito, WNV Coordinator

What is West Nile Virus?

West Nile virus first appeared in the Western hemisphere in 

1999 and quickly spread through North America. It can infect 

at least 48 species of mosquitoes, over 250 species of birds, 

and at least 18 species of mammals, including humans. The 

virus is a member of the Flavivirus family, and is transmitted 

through infected mosquitoes.
However, the effect of WNV on wildlife 

populations, particularly wild birds, 

a major reservoir of the virus, is less 

understood. The NWHC is working on 

three new ambitious research studies to 

investigate WNV in wild birds.

The fi rst involves the Hawaiian Amakihi. 

Amakihi are part of a family of birds 

known as Hawaiian Honeycreepers, a 

diverse and successful avian species 

before the arrival of humans on the 

islands exposed them to invasive species 

and diseases. Today, at least 12 different 

species of Honeycreeper are endangered. 

The Center’s Amakihi study will help 

assess the potential effect WNV could 

have on Hawaiian birds. Scientists are 

testing the birds’ susceptibility to WNV 

infection, while investigating how well 

the virus is transmitted naturally via 

mosquitoes.

A second WNV study, done in close 

partnership with the USGS Fort Collins 

Science Center, investigates the effects of 

the disease on kestrels, a raptor prominent 

along the Front Range of Colorado. While 

nationwide monitoring of crows and blue 

jays continues, raptors and other passerine 

species have yet to be adequately studied. 

Scientists are monitoring the kestrels 

by taking tissue samples from adults, 

nestling, and fl edgling birds over the 

course of between 12 and 30 months. The 

results of this study will help scientists 

understand the effects WNV is having on 

kestrel populations.

The third study looks at fi ve non-native 

and one native Hawaiian bird species 

to determine their susceptibility to the 

virus, as well as testing the natural 

transmissibility of the virus. Of particular 

interest is the native golden plover, which 

occasionally migrates to Hawaii from the 

continental U.S, and could potentially 

answer broader questions needed for 

environmental risk assessment of how 

these insecticides move through our 

environment.

USGS is also studying the role migrating 

birds play in disseminating West Nile 

virus. Investigators have captured and 

sampled more than 12,000 birds over the 

past 3 years at 8 different study locations 

along the east coast and 5 study locations 

along the Mississippi River. Based on 

the detection of WNV specifi c antibodies 

in these birds, scientists are seeing an 

increasing number of birds that were 

exposed to and have survived infections 

with WNV. Scientists have also detected 

WNV infections in 19 birds, primarily 

along the east coast in fall of 2003. While 

this represents very few of the birds 

sampled, all these birds were sampled at a 

critical time of daily southward migration. 

If these birds remained healthy and 

migrated southward shortly after sampling, 

they could have transported WNV along 

the migration corridor. More analysis is 

needed to understand the implications of 

these fi ndings. 

USGS New Research

Much work has been done to understand 

the public health effects of this disease 

since its arrival in New York City. 

For more information on West Nile virus 

projects, please contact Emi Saito, USGS, (608) 

270-2456, Erik Hofmeister, USGS, (703) 648-4071, 

Stephen Terracciano, USGS, (631) 736-0783 

x118, or Steve Guptill, USGS, (703) 648-4520.

spread WNV to the islands. We will also 

study the Japanese white-eye because it 

shares habitat with many native Hawaiian 

forest birds as well as Java sparrows, 

Laceneck and Zebra doves, and Mynah 

birds. This study will help scientists 

further understand the impacts of WNV, if 

it reaches Hawaii.

Other studies are planned to determine 

the susceptibility of indicator reptile, 

amphibian, and mammalian species to 

WNV, to monitor the changing genetic 

structure and virulence of the disease, 

and to determine the effects of WNV in 

pelican populations.

These studies, together with the 

collaborative programs and monitoring 

USGS is conducting with the CDC and 

other local public health and vector 

control agencies, paint a picture of a 

comprehensive research strategy to 

better understand WNV in wildlife and 

ecosystems.

West Nile Virus: Activity Over Time in North America

Confirmed WNV activity

1999 2000 2001 2002

1999 2000 2001 2002

Crows perch in an NWHC BSL-3 laboratory

A USGS scientist draws blood from a sparrow.