WestNileVirus-L: Arbovirus control of vectors

From: Richard Lampman <rlampman_at_inhs.uiuc.edu>
Date: January 10 2002

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<div>I apologize for the first posting.  As it was initially held
up, I edited it substantially.</div>
<div><br></div>
<div><br></div>
<div>The exchange about controlling arbovirus vectors has had
several</div>
<div>obvious misstatements and some of these seem to be polarizing
people with different perspectives.<br>
<br>
First, mosquito abatement has been misrepresented as favoring
adult</div>
<div>control as the major line of defense.  Many of the emails
suggest the money for aerial spraying should be used for other more
environmental efforts.  There should be little argument on this
topic because aerial spraying is usually held in reserve after the
other methods fail.  I think few would argue in favor of 
starting with aerial spraying.   If you are dealing with
groups dealing the the control of mosquitoes that feel adulticiding is
the only approach available to protect people from WNV, then you
should question their tactics.   Adulticides are usually
held in cases of emergency (indication of active urban cycles of
transmission), or when larval control is not feasible in an area
(usually due to inaccessibility or size), or if funds and expertise
are inadequate to conduct a proper abatement effort.  Please
refer those groups to the IPM approach championed by many states like
NJ, CA, FL, IL, TX, etc. (although there are cases where ULV spraying
is appropriate and relatively safe).</div>
<div><br></div>
<div> I recommend the following sites to Michael Gochfeld in
order to see an integrated approach as proposed at Rutgers in New
Jersey.</div>
<div>http://www-rci.rutgers.edu/~insects/ipm.htm<br>
http://www.rci.rutgers.edu/~insects/bmpmcnj.htm#appendix</div>
<div>http://www.rci.rutgers.edu/~insects/hisreas.htm</div>
<div><br></div>
<div>Generally mosquito abatement requires - seasonally identifying
and mapping larval habitats of target species in the area under
consideration, seasonal surveillance of those habitats for larvae,
breeding site sanitation and/or manipulation if the habitats produce
the target species, and relatively target-specific biological
larvicides if source reduction of breeding sites is not possible or is
ineffectual.  Synthetic chemical larvicides come next (a few of
which are safe enough to be allowed to be present in drinking water). 
Larval surveillance at the sites gives you some idea of the efficacy
of control efforts and adult surveillance gives you some idea whether
you've identified the major sites.  This is often a major problem
(missing major breeding sites). Adult surveillance coupled with
pathogen detection gives you an idea of relative risk.  With WNV
we also have natural sentinels, the Am. crow and other corvids, which
indicate by an increase in mortality that WNV transmission is
occurring in a general area.  Whether number of dead birds per
unit area per time unit can be used to estimate various levels of
human risk, remains to be seen.  Also keep in mind that it is
very likely that the target vector species varies regionally and
seasonally.  If you have equine cases then you face a risk of
transmission to humans (of course this also varies based on
demographics of the area and time of the year and vector species
involved).</div>
<div><br></div>
<div>If, after all the other treatment options, you are left with an
indication of active transmission in an urban area, then you are
facing an emergency situation.   Assuming you've kept the
public informed and involved throughout the process, the adulticide
option need to be considered (adulticides can be sprayed in a variety
of ways, although the preferred method is by ULV).  Adulticides,
if used properly, can reduce the flying mosquito population which
reduces risk of transmission.  Mosquito abatement is an area-wide
and area-specific problem<font face="Times" size="+1"
color="#000000">. </font><font color="#000000"> Although
generalizations can be made about IPM strategies, they must be
tailored to the physical, biological, ecological, and political
characteristics and restraints within a specific PMU.</font> </div>
<div><br></div>
<div>Some people appear to have misread Dominick Ninivaggi's
statements about mosquito control and concluded he promotes aerial
spraying as the first line of defense.  To me, he seems to agree
that the non-pesticide techniques are the first line of defense, but
pesticides should always remain a viable option.  He's also
addressing the chemophobia many show toward pesticides despite the
overwhelming evidence of their safety.  However, if used
improperly (not following the label), they also pose a risk which is
why pesticide applicators should always be licensed and should
maintain a good, open to review record of pesticide use. 
Unfortunately, most problems with pesticides arise from operational
issues.  For example, I received a newsletter that said one east
coast city changed spraying times from dusk and dawn to two hours on
either side of midnight because they wanted to avoid times when people
were outside.  So, if they were targeting Culex pipiens, then
they switched from spraying at periods of high flight activity to
periods with relatively low flight activity, keeping in mind ULV
adulticiding targets the vector on the wing.</div>
<div><br></div>
<div>A second concept presented by some in this discussion is that
states or communities are wasting money by trying to manage
mosquitoes.  This is a dangerous and poorly informed assumption. 
I look at WNV and see the potential for SLEV-type outbreaks (not to
mention equine and avian epizootics).  The statement that WNV is
insignificant because it's not the major cause of meningo-encephalitis
is truly without merit.  That attitude is like</div>
<div>saying we should not invest in studying tire defects because<br>
defective tires are not the major cause of car accidents. 
Such<br>
conclusions about arboviruses are based on a lack of knowledge
about</div>
<div>the history of mosquito-borne diseases in the US, mosquito
biology, and mosquito</div>
<div>IPM.  The truth of the matter is that we have been losing
expertise in medical entomology in the US because we have assumed
vector-borne diseases are only important in tropical areas.  We
still seem to be enamored by the concept of a "silver
bullet", except now it has shifted from the use of a pesticide to
the use of a vaccine or antibiotic. </div>
<div><br>
In the absence of low-cost, readily available, efficacious, and
low-side effect<br>
vaccines or antibiotics, vector management is the only practical<br>
alternative for protecting the population (Breeland et al.
1980). <br>
The rapid spread of WNV, the large number of birds infected with
WNV,<br>
the ability to overwinter in northern and possibly southern
areas,</div>
<div>and the number of equine cases last year all of these tend to
indicate a</div>
<div>potential for human outbreaks in the future.  The statement
that we</div>
<div>should ignore WNV until becomes a significant mortality
factor</div>
<div>among the elderly seems unduly callous.  Furthermore,
assuming it will be a minor problem reminds me of attitudes toward SLE
in 1974, right before a</div>
<div>major outbreak throughout much of the eastern US.  Mosquito
management of vectors does reduce risk and there are numerous examples
throughout the US.</div>
<div><br>
Third, most mosquito abatement districts do maintain a record of<br>
mosquitoes caught in one or more types of traps or by dipping and<br>
they use this to monitor the effectiveness of their treatments,<br>
although this is not necessarily done in a standardized way or in
a<br>
way that directly correlates to clinical cases.  The efficacy
of<br>
various treatments for different mosquito species have been<br>
well-demonstrated, although it is beyond the financial capability
of<br>
most abatement groups to "scientifically" document the
direct impact</div>
<div>of each treatment on target and non-target species. 
However, t<font color="#000000">he cornerstone of managing an
arthropod-borne pathogen is surveillance.  An active monitoring
program of larvae and adults determines when, where, and how to
treat.  It also provides the ecological data to delineate the
proper management strategies for different habitats.  And,
season-long surveillance detects failures in treatment interventions
which may be due to operational problems, resistance,
etc. </font>  If your area does not maintain records, then
someone has missed a fundamental principle of IPM.</div>
<div><br>
Arbovirus transmission cycles are complex and are influenced by</div>
<div>spatial and temporal differences in vector, host, and
pathogen</div>
<div>abundance, biology, and interaction with each of these components
and  with environmental variables. Furthermore, mosquito
abatement is an area-wide and area-specific problem.  Because of
the complexity of arboviral encephalites there will probably never be
a model that allows you to say X% of infected mosquitoes (or wild
hosts) of a specific species will potentially result in Y% of human
infections.  That's why you have listserv subscribers that keep
asking questions like "how many mosquitoes do you need before you
do X intervention" and you end up getting an abundance of less
than satisfying answers. </div>
<div><br></div>
<div>This doesn't mean there hasn't been a considerable amount of
study on the epidemiology of arboviruses.  Take for example St.
Louis encephalitis.  I recommend reading St. Louis Encephalitis,
1980, ed. by T.P. Monath.  There is a fantastic amount of data in
this book (and occasionally West Nile virus is also covered). 
There are also examples in the book of cases where aerial ULV
and</div>
<div>adulticide spraying have successfully controlled vector
species.</div>
<div>One of the problems with vector management is that it optimally
requires a leader that has a solid academic and research background in
the biology of mosquitoes, population dynamics, field ecology,
pathogen transmission cycles, biological and synthetic pesticides,
etc.   Typically, an entomologist is the last person on
everybody's list.  Let me give you an example. Several reports
made it sound like the discovery of Cx. salinarius as a vector of WNV
to humans was a new, unexpected development.   Let's go back
to a chapter from Monath, 1980.-- On page 316, Mitchell, Francy, and
Monath make the following statement - "These data support our
belief that, in the eastern US, Cx. salinarius and Cx. restuans play a
role in enzootic SLE virus transmission second only to Cx. pipiens; in
addition, Cx. salinarius, ... may be involved in transmission to man
and in viral maintenance ..."   </div>
<div><br></div>
<div><br></div>
<div>Richard Lampman</div>

<div>-- <br>
Richard Lampman, PhD<br>
Research Scientist<br>
<br>
Medical Entomology Program<br>
Center for Economic Entomology<br>
Illinois Natural History Survey<br>
607 East Peabody Drive<br>
Champaign, IL  61820<br>
 <br>
Med. Ent. Program: 217-333-1186<br>
Office phone:      217-244-5631<br>
FAX number:        217-333-2359<br>
email:  richlamp@uiuc.edu<br>
<x-tab>      
</x-tab>rlampman@denr1.igis.uiuc.edu<br>
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Received on Thu Jan 10 16:48:28 2002