APPENDIX II-BO: Wrubleski, “
This appendix is copied from:
http://www.ducks.ca/conserve/issues/pdf/wnv251.pdf
BY Dr. Dale Wrubleski.
across
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
continue.
33 Conservator | 25-1 2004
across
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
2002).The
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:
(1,013),
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
example,
with 884, but last year reported 52 cases.
in 2002, but only 16 cases
in 2003.
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
states like
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
brunt of
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
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
W
© 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
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.
In the
23 people became infected after
receiving
blood products contaminated with the
virus.
Blood collection agencies in
both
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
for
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
infections in
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
from
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.
1999
2000
2001
2002
2003
35 Conservator | 25-1 2004
THE
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
was collected.The
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
positive for
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
of
County 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.
Impacts of
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
from
workers in a turkey-rearing facility
had a much higher incidence
of the virus than the general
population living in the same area.
infection.
In a related situation in
Prairie
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
Unlimited
of wetlands by educating the general
public to the many positive
values provided by healthy wetland
habitats. A
THE
STUDY
“
”
It may be
some time
before we fully understand
the dynamics of
in
© Robert Tinker (2)