APPENDIX II-BO: Wrubleski, “
This appendix is copied from:
BY Dr. Dale Wrubleski.
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
of human cases increased faster than anyone
would have predicted. Despite this, one groundbreaking
study conducted on the virus in
Ont., suggests that many people still are not
taking proper precautions to protect themselves.
The number of Canadians ill with
more than triple that recorded in 2002 (416), although the number
of deaths was lower (10 deaths in 2003, compared to 20 deaths in
141 cases in
similar pattern was also seen in the
plains states hit hardest.The five states with the highest number of
human cases were:
human cases in the
double the 4,156 cases reported in 2002.
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
with 884, but last year reported 52 cases.
in 2002, but only 16 cases
in 2002, reported 89 cases
in 2003. In
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
with high human cases last year will show a marked decline in
virus activity this year.
birds or mosquitoes, so it may still be a year away from feeling the
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
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
In 2002, several new and unusual modes
of transmission were documented for
revelation that the virus could be acquired
through blood transfusions and organ transplants.
23 people became infected after receiving
blood products contaminated with the virus.
Blood collection agencies in
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
tested.Of these donations, 691 donations eventually were
confirmed to be contaminated with the virus. Of the
226,000 blood donations made in
procedures used are still considered experimental and it
is not yet known how often the tests may fail to detect
the virus. In the
two cases of transfusion-related WNV infections
reported in 2003. Because of the high rate of
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
conditions, a recent survey in 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
average, it cost $51,826 (
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
monkeys from infection. Human clinical trials of the vaccine are
expected to begin soon.The new vaccine was made by combining
parts of the
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
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
who were infected with
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
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
sample was then tested for
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
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
survey undertaken in
was found to have
Although the proportion of the population exposed to the virus
was similar between
was higher in Oakville.Based on work done in
the initial outbreak of
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
study, the risk of severe infection was one in 84, almost double
the earlier estimate.Unfortunately,the
although most people were aware of the risks of contracting West
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.
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
some indication that people who work with birds may have an
increased risk of infection. A report
workers in a turkey-rearing facility had a much higher incidence
of the virus than the general population living in the same area.
In a related situation in
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
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
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
© Robert Tinker (2)