Each year when SYMVCD sprays adulticides over
populated areas officials make some variation of this statement to
the press: "When a certain threshold of the virus is reached we have
to spray to break the transmission cycle." Another phrase we heard
this year is "tamp down the virus." While both are catchy phrases,
which make it sound as if they are actually accomplishing something
and saving people from a serious threat, we urge people to insist on
answers to the following questions:
1. Now that the previous
trigger has been abandoned, which involved the virus in
mosquitoes, birds, humans, horses, and sentinel chickens, what exactly is
the formula or mechanism for determining that the spray
threshold has been reached?
is the scientific justification for this threshold?
exactly does "break the transmission cycle" or "tamp down the
is the scientific evidence that this spray program does that,
whatever it means?
We have queried officials over the years about
this trigger, and the responses have always been very murky.
Based on this experience we suggest that the actual answers to the
above four questions are as follows:
1. The formula or mechanism must be
"spraying by the seat of their pants." A perusal of
the SYMVCD website reveals no trace of the previous index, for
which we have never been given any scientific basis, and the
current discussion makes it difficult to tell how this threshold
is now determined. It appears that birds and mosquitoes may
have replaced humans in the definition of epidemic.
is none – no scientific justification has ever been
supplied, for what they did before or what they are doing now.
3. This means nothing.
As mathematical modeling and data collection show, a two or three
day spray program knocks down the numbers of mosquitoes
temporarily, but the numbers quickly rebound to their previous
levels. No effect on status of the virus in the birds has
been reported, and the birds are a critical component in the
spread of the virus.
of good scientific evaluation about the level of infection in
the birds makes the assessment of efficacy of any kind of
control program almost impossible to justify scientifically
because the most important data is absent. The mathematical modeling suggests that
a small, 6%, increase in mortality in larval mosquito
populations early in the season has the same effect as a 90%
reduction of the adult population in July. This modeling would
justify an early treatment date for larvaciding but not
insecticides applied against the adult mosquitoes.
4. There is none – no cycle of any
kind is broken by this. There is neither
scientific nor anecdotal evidence to support suppression of the
virus long enough to actually halt transmission of the virus to
humans. Short of actually breaking the transmission cycle,
effective control can be obtained if at least 90% of the adults
can be killed, as Cornell entomologist David
Pimentel notes. However,
local officials report cumulative kill of only about 50% of the
important vector, Culex
tarsalis, during three days of spray in the sprayed
area. The current two day protocol could be expected to kill
less than 40% of the adult mosquito population. And, both
mosquitoes and birds will move within and into the sprayed
region to help boost the virus back to pre-spray levels.
It seems critical to insist on answers to the
four questions in any discussions with vector control
officials. Many people seem to assume that the spray program
is effective and then concern themselves with the tradeoff of
putting more toxins in the environment for protection from a
disease. Since the spray program is ineffective, the only
thing accomplished is to add toxins to the environment – we are
assuming a certain level of risk for zero gain – spraying
adulticides puts us in a lose-lose situation.
June 26, 2012 Note: After we recently posted the above
analysis, a belated 2012
West Nile virus Activity page appeared on the SYMVCD website.
Observe that the totals for the year include the
components used in risk assessment in previous years: humans,
horses, sentinel animals, mosquitoes, and birds. However, a
perusal of the cited West
Nile Risk Assessment for 6/22/2012 reveals the use of
different components and not humans: environmental conditions,
mosquito abundance, virus infection in mosquitoes, sentinel
chickens, and birds. Also, the assessment appears to be a
district-wide one, so it is unclear how a specific spray area can be
targeted. We still have been given no scientific justification
for any of this, and it thus seems to remain "seat of the
pants." Perhaps it is time to begin to press officials for the
reason they spray somewhere in the Sac/Yolo district at least once a
year – is it out of a genuine concern for the public health or is it
out of concern for maximizing upcoming SYMVCD budgets by giving a
highly visible illusion that something is being accomplished?
Whatever the motivation, the complete lack
of evidence of efficacy for the spray program means that
SYMVCD is indeed synonymous with Seat
of the Pants Vector Control.