Emerging Infections of International Public Health Importance
West Nile Virus
Sharon G. Hopkins DVM, MPH
Dr. Sharon Hopkins, DVM, MPH is a public health veterinarian at Seattle King County Public Health. She is also a clinical assistant professor in the Department of Epidemiology at the University of Washington’s School of Public Health and Community Medicine. Her research interests include communicable diseases and emerging infections.
Vector Borne Diseases
Hundreds of disease-causing viruses, bacteria, and parasites require a hematophagous (blood-sucking) arthropod for transmission between vertebrate hosts. There are many arthropod vectors. The arthropod phylum includes insects (such as mosquitoes) and arachnids (such as ticks and mites). Arthropods are jointed body invertebrates with an exoskeleton.
Many of the vector-borne infections are zoonoses, diseases transmitted between animals and humans. Historically, vector-borne diseases were responsible for more human disease and death in the 17th through 20th centuries than any other cause.
This slide shows four examples of arthropods that can spread disease.
There are three main disease classes for vector-borne diseases: bacterial, viral and parasitic. Examples of bacterial diseases are Lyme disease, tularemia, and plague. Viral diseases include West Nile, dengue, yellow fever, and tick-borne encephalitis. Examples of parasitic diseases include malaria and trypanosomiasis (African sleeping sickness, Chagas disease).
Global factors of emergence for vector borne disease
There are several factors that facilitate emergence or resurgence of vector borne diseases:
West Nile Virus (WNV)
West Nile Virus is part of the Japanese encephalitis serocomplex of viruses. This map shows the distribution worldwide of these different viruses which includes St. Louis encephalitis, West Nile, Japanese encephalitis, Murray Valley and others.
The blue color indicates where West Nile is present. It arrived in the U.S in New York City in 1999 and 2000. Prior to that time it was present in parts of Africa, the Middle East and Europe. The black color indicates an area of overlap between Japanese encephalitis and West Nile Virus.
West Nile Virus is primarily a disease of birds.
Human infections are considered ‘incidental’ infections and are not important in the actual lifecycle of this virus. Birds that are most susceptible are the raptors, such as owls, eagles, and hawks. They have a high mortality rate if they become infected. The other highly susceptible group is the corvids, which are the ravens, blue jays, and the crows. These birds also have a high mortality rate when they become infected with West Nile Virus. The flamingo was the first bird West Nile was recognized in when the virus first arrived in the U.S.
The transmission cycle of West Nile Virus occurs between the bird reservoir host and a mosquito vector.
During the course of a season, an increasing number of birds will become infected and infect an increasing number of mosquitoes and the cycle amplifies. At some point, it may spill over to humans, horses and other species. In the U.S., the reservoir bird hosts are not fully known yet. Crows and corvids that have a high death rate and may not be the primary reservoir of this disease. Researchers are looking at other species that may not become as ill or have as high a death rate as corvids or raptors. House sparrows or some other very common birds have been looked at and suggested as primary reserviors, but we really do not fully understand the ecology as yet.
In the U.S, West Nile has been found in more than 150 different species of birds. It has also been seen in deer, bear, wolf, skunk, harbor seal, alligator, bat, squirrel and chipmunk. The most severly affected of the domestic animal species affected are equines (horse, donkey, mule); domestic dogs and cats can get infected but they rarely show clinical illness.
This is a timeline for West Nile.
It was first recognized around the West Nile area of Uganda in 1937, hence the name. It was followed by outbreaks in the Middle East, France, and South Africa. There seemed to be some intensification in the 1990s with recent sporadic outbreaks. It is unclear whether the recent observations are due to surveillance or truly increased number of outbreaks.
This appeared recently in the CDC’s Emerging Infectious Disease online journal.
This brief summary speculates that Alexander the Great may have died of West Nile Virus. He had a two-week febrile illness and ravens were exhibiting unusual behavior and dying at the same time. If true, the disease may have been present since 323 B.C.E.
West Nile Virus in the U.S.
What finally brought West Nile Virus to the U.S.?
The bioterrorism and infected human traveler theories seem unlikely given the epidemiology of the disease in the United States.
Clinical Features of WNV in humans
This is a graph of a series of 45 cases.
You can see that there are low rates of clinical WNV at the younger ages but increases in an almost linear fashion beginning at age 50. There does not seem to be an increase at the younger age groups so it does not have a bi-modal age distribution.
Outcomes among people who are hospitalized with WNV:
Predictors of poor outcome for WNV:
West Nile Virus in Domestic Animals
Because chickens can become infected with WNV but rarely become ill, they are used for sentinel surveillance for this disease. This technique is used in parts of eastern Washington. There is no evidence to suggest that WNV can be transmitted orally to humans by consuming food infected with the virus. It is a vector-borne disease, in this case by the mosquito. West Nile Virus amplifies in nature as mosquito season progresses. Mosquitoes are killed by freezes and only a few that have successfully wintered over hatch out in the spring. You need to have enough mosquitoes biting birds in a continuing cycle to amplify the virus.
Primary route is by the bite of an infected mosquito.
This graph shows three different years of infection with the peak of disease seen in the last weeks of August and first weeks of September.
Additional transmissions modes have been suggested.
This is a table of the WNV case summary for 1999-2003.
In 1999, there were 62 cases and 7 deaths. In 2003, there were 9006 cases and 220 deaths. There were far more equine cases with higher death rates during these same time periods.
West Nile Virus sequentially moved through the United States. The following slides depict where the disease moved from 1999 through to 2003.
This map shows the distribution of WNV in the US in 1999.
You can see it is primarily concentrated in the New York region where the virus was first introduced.
This shows the distribution in 2000.
Distribution in 2001
Distribution in 2002:
This slide shows WNV in 2003 with the number of cases per state.
The following slides are from the US Geologic Survey, the first two are the human cases and the second two are the veterinary cases for the same years. The red dots represent the cases. The green dots indicate counties that submitted testing but had negative results. Both the human and veterinary cases show the westward progression of the virus.
WNV human cases 2002:
WNV human cases 2003:
WNV veterinary 2002:
WNV veterinary 2003:
Reasons for a decline in cases from one year to the next in a given geographic location are likely to be multifactorial. Mosquito control efforts were undertaken in many areas, which could reduce the mosquito burden. Populations of reservoir birds may have declined. Vaccination of horses may have reduced equine cases.
WNV is evolving epidemiologically. Reported human cases more than doubled in 2003 from 2002 with death rates remaining similar. Different states affected and were seeing more rural cases. Also as the disease moved westward, more recently affected states had higher rates than states affected the year before. Illinois had 884 cases in 2002 and 52 cases in 2003, Michigan was similar with 614 cases in 2002 and 15 cases in 2003. Colorado however had 14 cases in 2002 and 2477 cases in 2003.
Surveillance for WNV
With WNV, the first indicator in an area that there is a problem will be an increase in the dead bird sightings, usually most noticeable in crows. Following that there will be sentinel hosts, such as chickens that become infected. Afterwards, mosquitoes are collected and tested. Usually the next affected are the equines and finally the humans.
This graph demonstrates this surveillance concept based on disease activity and time.
West Nile Virus in Washington State
We have not had a case of WNV in King County yet but have tested about 3000 dead birds. This map shows the counties where WNV cases have been identified in Washington State; three counties have been impacted.
There have been two avian and two equine cases but no human cases to date.
WNV Response in King County
The Public Health Department of Seattle-King County has three components to its response plan:
This is a collaborative effort with the Public Health Department taking the lead and partnering with other agencies. There is a WNV Interagency Work Group is producing a phased response guideline. As part of the detection and control strategies we have also developed a WNV tabletop exercise.
Even with no WNV cases, King County has felt the impact of this disease in the following ways:
WNV Surveillance in King County
2689 dead bird reports
4 horses tested, all negative
Here are pictures of mosquito surveillance techniques
The method on the left is a CO2 trap. It releases CO2 which mimics the respiration of a mammal and attracts the mosquitoes into the trap. A fan blows the mosquito into the net where it is trapped. The method on the right is to have a bucket with standing water and dip into it for mosquito larvae. The larvae are then placed in breeders and hatched. The purpose of these efforts has been to determine the species of mosquitoes found in King County; not all mosquito species are competent vectors of WNV
This is a list of the mosquito species we have found in King County; the names with a star next to them are the primary vectors for WNV
WNV Education Efforts
WNV Control Measures
Begin with public education by accurately communicating the risk and give personal responsibility for property and protection. Then do habitat reduction by reducing standing water—e.g., emptying cans and water troughs when not in use. The third phase is using pesticides, both larvicide (and adult mosquito sprays if indicated by the severity of the epidemic), to reduce the adult mosquito population.
This slide shows some typical mosquito habitats.
These are areas to watch for and take precautions. Removal of old tires and cans (where stagnant water can exist and larva can develop) and any other sources of standing water is important.
This slide shows the life cycle of the mosquito.
The larval stage is the most susceptible; once they reach the pupa stage and hatch, they are no longer susceptible to most types of larvicides.
This slide shows some predators of the mosquito larva.
These can go a long way toward controlling mosquitoes. The larva are a food source for many insects.
Larvicides kill the mosquito larvae. They help to reduce the adult mosquito population in nearby areas. Larvicides can include both microbes (bacillus) and pesticides.
In order to successfully do surveillance and control for this disease, the Public Health Department needs partner agencies. Liaisons with the Department of Natural Resources and Parks, Department of Roads, Department of Ecology, laboratories and others are essential for effective control.
Factors that need to be considered in developing a county-wide response plan:
These are possible response options from the Department of Natural Resources and Parks (DNRP)
The Water and Land Resource Division (WLRD) made the following recommendations:
In practice, before treatment of storm-water ponds was undertaken, there needed to be a way of determining which ponds were to be selected. There needed to be water in the facility and mosquito larva present. The storm-water ponds needed to be near at-risk populations: those with a high density of >50 age groups, near nursing homes, and where morning or evening outdoor events (concerts, sporting events) took place. Also the proximity to mammalian or bird cases would be taken into account.
GIS systems were used to map the storm-water ponds and overlay this with the populations as determined in the first step. The results are seen on the following slides.
Population density of those age 50+ > 1000 per square mile:
Population density of those 50+ where there are greater than 40 people per sq mi.
Storm-water services facilities:
Overlay of storm water facilities with population density of age 50+ and incorporated areas:
As you can see there is only a small area where there is the concentration needed to implement a treatment plan.
This map shows the potential larvicide facilities designated by the at risk populations:
These are marked in yellow and red. Nursing homes are the purple dots. We can see how the use of multiple technologies can be effective in proactively reducing the mosquito population.
Ongoing West Nile Virus issues from 2003 into the future