APPENDIX II-BO:  Wrubleski, “West Nile Story: One Year Later,” Conservator, Jan. 25, 2004, p. 1-35.


This appendix is copied from:



BY Dr. Dale Wrubleski.


West Nile virus has moved

across North America in a

wave from east to west, with

regions at the leading edge of

the wave being hit hardest

each year. Once the wave

has passed, the impact of

the virus appears to decline.

Why the virus hits so hard

when it first arrives is not

known. A number of theories

have been put forward but

no one knows for sure

if this pattern will


33 Conservator | 25-1 2004


across North America in 2003, and the number

of human cases increased faster than anyone

would have predicted. Despite this, one groundbreaking

study conducted on the virus in Oakville,

Ont., suggests that many people still are not

taking proper precautions to protect themselves.

The number of Canadians ill with West Nile in 2003 (1,317) was

more than triple that recorded in 2002 (416), although the number

of deaths was lower (10 deaths in 2003, compared to 20 deaths in

2002).The Prairie provinces were hit particularly hard last year,with

141 cases in Manitoba, 774 in Saskatchewan, and 272 in Alberta.A

similar pattern was also seen in the United States, with the western

plains states hit hardest.The five states with the highest number of

human cases were:Colorado (2,477), Nebraska (1,831), South Dakota

(1,013),Texas (598), and North Dakota (507).The total number of

human cases in the United States in 2003 was 8,912, more than

double the 4,156 cases reported in 2002.

West Nile virus has moved across North America in a wave

from east to west, with regions at the leading edge of the wave

being hit hardest each year.Once the wave has passed, the impact

of the virus appears to decline. States that had high numbers of

human cases in 2002 showed much lower numbers in 2003. For

example, Illinois had the highest number of human cases in 2002

with 884, but last year reported 52 cases. Michigan had 614 cases

in 2002, but only 16 cases in 2003. Ontario, which had 308 cases

in 2002, reported 89 cases in 2003. In New York, where the virus

has been present for five years now, the number of human cases has

stayed relatively stable, ranging from 14 to 82 cases each year.

Whether this pattern will continue is not known. If it does, then

states like California will be hit hardest in 2004, and those states

with high human cases last year will show a marked decline in

virus activity this year. British Columbia has yet to find infected

birds or mosquitoes, so it may still be a year away from feeling the

brunt of West Nile.

Why the virus hits so hard when it first arrives is not known.A

number of theories have been put forward but no one knows for

sure if this pattern will continue.The high number of cases in the

west has been attributed in part to a more efficient vector of the

virus. Culex tarsalis has been found in the lab to be a very effective

vector of West Nile, and it has a greater propensity to bite humans

than other mosquitoes that carry the virus. Hot weather in the west

may also have helped increase mosquito numbers last summer.

Another theory suggests that as local bird populations become

exposed to the virus, they develop immunity and thereby slow the

growth of the virus in the bird population, reducing the risk of

spillover into the human and horse population.This may explain


© Robert Tinker (2)

34 Conservator | 25-1 2004

why the first year of exposure to the virus is the worst year, but

then numbers of cases decline in subsequent years. It is more likely

that a combination of factors are responsible for this pattern, and

it may be some time before we fully understand the

dynamics of West Nile in North America.

In 2002, several new and unusual modes

of transmission were documented for

West Nile virus.The most serious was the

revelation that the virus could be acquired

through blood transfusions and organ transplants.

In the United States, an estimated

23 people became infected after receiving

blood products contaminated with the virus.

Blood collection agencies in both Canada and the

United States scrambled to find a test that could

detect the virus in donated blood.Their efforts

were successful, and several testing methods

were ready when mosquito season returned

in the next year.

From late June to mid-September of last year,

2.5 million blood donations in the United States were

tested.Of these donations, 691 donations eventually were

confirmed to be contaminated with the virus. Of the

226,000 blood donations made in Canada, 14 tested positive

for West Nile virus. Although all blood is now tested, the

procedures used are still considered experimental and it

is not yet known how often the tests may fail to detect

the virus. In the United States, for example, there were

two cases of transfusion-related WNV infections

reported in 2003. Because of the high rate of infections in Saskatchewan

last summer, and knowing the sensitivity of the new testing

procedures remained unknown,Canadian Blood Services took the

precautionary measure of recalling all blood donated in the

province during August and destroying it.

Although the numbers of people suffering severe infections of

West Nile virus are low compared to other more common medical

conditions, a recent survey in the United States has found that the

disease still represents a significant impact on the health care system.

The Centers for Disease Control and Prevention (CDC) found

that it cost at least $200 million (US) to treat the 4,156 people suffering

from West Nile encephalitis or meningitis in 2002. On

average, it cost $51,826 (US) to treat each patient. Costs varied

according to the severity of infection, ranging from $2,000 per

patient (medical examination and testing without hospitalization)

to as much as $250,000 (US).The most severely affected patients

spent weeks in coma, had difficulty breathing, and relied heavily

on long-term supportive care.

There are some bright spots on the horizon. Researchers at the

National Institute of Allergy and Infectious Diseases (NIAID) in the

United States have created a West Nile virus vaccine that protects

monkeys from infection. Human clinical trials of the vaccine are

expected to begin soon.The new vaccine was made by combining

parts of the West Nile virus and a distantly related virus, creating a

hybrid virus vaccine. NIAID has also begun trials of an experimental

treatment for patients infected with the virus. Currently,

Above: A map showing the movement of West Nile Virus reveals a

continued western advance in North America.The spread of West

Nile Virus reached new territory in 2003, including Alberta,Nevada,

Utah and Arizona. Since its inception in North America,WNV has

spread outward each year,with the brunt of its force being felt

on the virus’ advancing western fringe.






35 Conservator | 25-1 2004



In the spring of 2003, a survey was done in southern Ontario’s

Oakville area to determine the number of people living in the region

who were infected with West Nile virus.This area was selected because

it had the highest incidence of serious human cases in 2002.

There were six cases of encephalitis (all hospitalized), five cases of

meningitis (one hospitalized) and eight cases of West Nile fever (one

hospitalized), in a population of 30,467 people (18 years and over).

The purpose of the survey was to select a sample of the human

population and determine what proportion had been exposed to the

virus last year.To do this, researchers selected random households in

two postal code areas (L6L and L6K) and contacted each by telephone.

One individual was selected from the household and a blood sample

was collected.The sample was then tested for West Nile virus antibodies

to determine if the individual had been exposed to the virus.

Of the 1,505 people who participated in the survey, 46 (3.1%) tested

positive for West Nile antibodies.These people had been bitten by an

infected mosquito, but were unaware that they had been exposed to

the virus. If this number is extrapolated to the entire population for

the area, it is estimated that 670 to 1,219 residents of the area were

likely exposed to the virus in 2002.The proportion of the population in

Oakville exposed to the virus is comparable to that found in a similar

survey undertaken in New York City in 1999, when the first outbreak

of West Nile virus took place, but lower than another survey in Cuyahoga

County in Ohio,where an estimated 4.0% to 6.4% of the population

was found to have West Nile virus antibodies.

Although the proportion of the population exposed to the virus

was similar between Oakville and New York City, the risk of severe illness

was higher in Oakville.Based on work done in New York during

the initial outbreak of West Nile, it has generally been reported that

the risk of a severe infection was one of 150 infections.That is, for

every 150 people infected with the virus, there will be one severe

infection requiring medical treatment and hospitalization.In the Oakville

study, the risk of severe infection was one in 84, almost double

the earlier estimate.Unfortunately,the Oakville survey also found that

although most people were aware of the risks of contracting West

Nile virus, half the respondents did nothing to prevent mosquito bites.

patients can only be offered supportive care, as there is no treatment

for the infection. In this study, patients will be given antibodies to

the virus to determine if they will be better able to fight off the

virus and prevent the severe health problems that can result.

Impacts of West Nile virus on wildlife are still being assessed.We

know that corvids (crows,magpies, blue jays) have been impacted

severely by the virus, but impacts on other bird species are not yet

known. Concerns have been raised about shrikes, raptors and

grouse, but more information is needed before we will know if

these species are also at risk from West Nile virus.There is also

some indication that people who work with birds may have an

increased risk of infection. A report from Wisconsin found that

workers in a turkey-rearing facility had a much higher incidence

of the virus than the general population living in the same area.

Turkeys in the production facility had a very high rate of West Nile


In a related situation in Prairie Canada, several waterfowl banders

also became infected with the virus last fall. Exactly how these

workers became infected is not clear, but laboratory studies have

found that birds that appear healthy can shed the virus in their saliva

and feces. Further work is needed to determine if handling infected

birds poses an increased risk of exposure to the virus.

Finally, concerns regarding widespread mosquito control and

drainage of wetlands to combat West Nile have eased somewhat in

the last year.Control efforts remain focused on Culex mosquitoes,

which prefer small aquatic and container-type habitats and are not

produced in large numbers by natural wetlands. In addition, Ducks

Unlimited Canada’s efforts have also helped to reduce degradation

of wetlands by educating the general public to the many positive

values provided by healthy wetland habitats. A




It may be some time

before we fully understand

the dynamics of West Nile

in North America.



© Robert Tinker (2)