WestNileVirus-L: Vector borne disease control by aerial applicat

From: <drterrymabbett_at_btopenworld.com>
Date: July 11 2002

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<div><font face="Times" size="+1">Read your web site with interest.
Thought you might be interested in the following information written
in co-operation with Micronair (A Division of Micron Sprayers Limited
in the United Kingdom).</font></div>
<div><font face="Times" size="+1">The article reviews a number of
vector-borne disease situations around the world and highlights the
latest developments in nozzles and other technologies, related
to aerial application for the control of vector-borne diseases.
Photographs are available on request.</font></div>
<div><font face="Times" size="+1"> </font></div>
<div><font face="Times" size="+1">Regards</font></div>
<div><font face="Times" size="+1"> </font></div>
<div><font face="Times" size="+1">Dr Terry Mabbett </font></div>
<div><font face="Times" size="+1"> </font></div>
<div><font face="Times" size="+1"> --------------------------<br>
</font></div>
<div><font face="Times" size="+1">Vector-borne disease control by
aerial application<br>
</font></div>
<div><font face="Times" size="+1">By Dr Terry Mabbett*<br>
</font></div>
<div><font face="Times" size="+1">A fast moving and lethal combination
of microbes and parasites, carried and transmitted by invertebrate
animals including insects and molluscs, continually threatens the
health and well being of communities throughout the world.<br>
</font></div>
<div><font face="Times" size="+1">Disease spread and development is
heightened with winged vector insects such as the malaria-carrying<i>
Anopheles</i> mosquitoes and the tsetse fly (<i>Glossinia</i> sp),
which carries and transmits trypanosomiasis (sleeping sickness). Water
is the main distributive effect for those diseases carried and
transmitted by aquatic animals. These include snails (molluscs) which
are vectors of<i> Schistosoma japonicum</i>,<i> S. mansoni</i> and<i>
S. haematobium</i>, the causal agents of various forms of
schistosomiasis (bilharzia).<br>
</font></div>
<div><font face="Times" size="+1">A combination of an air-borne adult
stage and water-borne larval stage in the life cycle of a single
insect vector species complements efficiency and success of the
parasite, while placing even more constraints on control. Mosquitoes
including the<i> Anopheles</i> and<i> Culex</i> vectors for,
respectively,<i> Plasmodium</i> (malarial parasite) and the
encephalitis virus complex (including West Nile Virus - WNV) are
classic cases within this scenario.<br>
</font></div>
<div><font face="Times" size="+1">For many of these diseases the
insect vector is an intermediary host in the transfer of zoonoses from
wild vertebrate animals such as birds and rodents to man. Zoonoses are
diseases transmitted from animals to man. The animals act as
reservoirs of disease. Their movements not only accelerate spread and
development of disease but also add a completely new dimension to its
epidemiology.<br>
</font></div>
<div><font face="Times" size="+1">Venezuelan equine encephalitis
(VEE), which is the most important of the viral zoonoses in tropical
America, is a case in point. VEE is of economic importance for equine
animals and of public health importance in man with a mortality rate
of 1% (80% for horses).<br>
</font></div>
<div><font face="Times" size="+1">During an epidemic, VEE is
transmitted from equine to equine and equine to man by several species
of vector mosquitoes. Between epidemics the disease remains endemic in
rodents and other small mammal populations, within which it is
transmitted by mosquitoes. West Nile virus, currently causing alarm
throughout the United States is spread in the same way by<i> Culex</i>
mosquito vectors from horse to horse and from horses to people, except
that the wild animal hosts are birds which are considerably more
mobile than rodents.<br>
</font></div>
<div><font face="Times" size="+1">In summary, these "arboviruses"
(arthropod-borne viruses) are maintained in nature via transmission
between susceptible vertebrate hosts by blood-feeding arthropods. The
arthropod vectors are mainly mosquitoes but also include sand flies,
ceratopogonids and ticks. The term "arbovirus" is a
descriptive has no importance in the taxonomy (classification) of
different types of virus particles.<br>
</font></div>
<div><font face="Times" size="+1">Dynamics and destruction<br>
</font></div>
<div><font face="Times" size="+1">The consequences of vector-borne
diseases are catastrophic. Anopheline mosquitoes alone are responsible
for over 400 million cases of malaria per year of which 3 million
result in death, mostly in Africa. Out of 360 species of<i>
Anopheles</i>, 60 can transmit the malaria parasite, mainly<i>
Plasmodium falciparum</i> (the most lethal), but also<i> P. vivax, P.
ovale</i> and<i> P. malariae</i>.<br>
</font></div>
<div><font face="Times" size="+1">Dengue and dengue haemorrhagic fever
is spreading rapidly throughout the tropics, via the dengue/yellow
fever<i> Aedes</i> mosquitoes, with dire consequences for local
populations and tourist industries. Even more widespread are a group
of antigenically related human encephalitis diseases (including WNV).
They are caused by flaviviruses and spread by<i> Culex</i> sp (the
so-called house mosquitoes) and are the cause for much concern in
temperate as well as tropical countries.<br>
</font></div>
<div><font face="Times" size="+1">The potential threat to human and/or
livestock health from many vector-borne diseases is so severe in parts
of Africa that large areas of land, and often the most productive, are
Îout-of-boundsâ. River Blindness (Onchocerciasis) caused by a worm
parasite (<i>Onchocerca volvulus</i>) which is carried and transmitted
by blackfly (<i>Simulium damnosum</i>) is responsible for widespread
disease, including blindness, and massive under-exploitation of
fertile land in the river valleys of West Africa.<br>
</font></div>
<div><font face="Times" size="+1">The consequences of vector-borne
diseases are a reflection of the speed and extent of spread and the
constraints these place on control.<i> Anopheles</i> mosquitoes can
fly for a distance of 2-3 km and have a life span averaging 2-3
weeks.<br>
</font></div>
<div><font face="Times" size="+1"><i>Aedes aegypti</i> arrived in
South America with slave trading ships some four to five centuries
ago. More recently the Asian mosquito<i> Aedes albopictus</i>,
commonly called the tiger mosquito because of its striped appearance,
spread to the United States during the 1970âs as larvae on imported
car tyres. It spread in the same way to Europe, is established in
Italy, entered France in 1999 and now threatens the United
Kingdom.<br>
</font></div>
<div><font face="Times" size="+1">This year French authorities
reported finding another East Asian mosquito (<i>Ochlerotatus
japonicus</i>) breeding in Normandy just across the English Channel.
European entomologists and medical experts are worried that exotic
species from the tropics will establish more easily with increasing
global warming bringing with them lethal arboviruses such as Japanese
Encepahalitis, the vector for which is<i> Culex
tritaeniorhynchus</i>).<br>
</font></div>
<div><font face="Times" size="+1">Similarly, the current spread of
West Nile Virus is giving much Îfood for thoughtâ and considerable
concern. From its origins as a single case in 1937 identified in the
West Nile District of Uganda, this Îtropicalâ disease has spread
through Asia and Europe and is currently causing deaths in temperate
areas of the United States. Over-wintering<i> Culex pipiens</i> (the
Northern mosquito) is believed to be a the root of this
well-entrenched problem, which US experts now regard as Îpart and
parcelâ of everyday pest control practice rather than a temporary
exotic pest and disease problem.<br>
</font></div>
<div><font face="Times" size="+1">Control to match<br>
</font></div>
<div><font face="Times" size="+1">The sheer speed and spread of
vector-borne disease and its consequences for public health means that
rapid-action and emergency control to cover huge areas under threat in
the shortest possible time is the only way to combat serious public
health problems caused by vector-borne diseases. Aerial application is
the only realistic way this can be achieved say John Clayton and Tim
Sander of Micron Sprayers Limited in a recent paper entitled ÎAerial
application for control of public health pestsâ and published in the
journal ÎAspects of Applied Biologyâ.<br>
</font></div>
<div><font face="Times" size="+1">New developments in nozzle
technology, for rotary atomisers, high-pressure hydraulic nozzles and
twin fluid nozzles, related to control of both flying insect vectors
and their aquatic larval stages are providing more targeted control,
say the authors. Increasing use of more environmentally benign
insecticides, including insect growth regulators (IGRâs) and
biopesticides, continue to satisfy the requirements of selectivity and
safety.<br>
</font></div>
<div><font face="Times" size="+1">Speed and spread<br>
</font></div>
<div><font face="Times" size="+1">The essential and irreplaceable role
for aircraft in rapid-reaction, large scale and emergency control
measures is clear. Areas that can be covered by twin engine aircraft
or a small single engine aircraft/helicopter are up to 20 km2 in one
night. These are many times more than by vehicle mounted cold fogger
(225 hectare), motorised knapsack mistblower (30 ha), hand-carried hot
fogger (5 ha) or hand carried indoor generator (5 ha or 250
dwellings).<br>
</font></div>
<div><font face="Times" size="+1">The 2000 outbreaks of Rift Valley
Fever (RVF) in the Middle East are recent examples where aerial
spraying was suddenly required and used. This<i> Aedes</i> mosquito
borne tropical African disease, caused by the Phlebovirus (RVF virus)
and affecting a wide range of common livestock (cattle, sheep goats
and camels) as well as humans, caused over 400 deaths in the Middle
East before it was brought under control in a sustained control
programme. This involved the deployment of eight fixed-wing Antonov AN
2 aircraft and six KA-26 helicopters to spray technical malathion
insecticide over an area of 1 million hectares during a 90 day
campaign.<br>
</font></div>
<div><font face="Times" size="+1">In the last four years the
life-threatening West Nile Virus, by making an even bigger jump across
the Atlantic to North America, has triggered a massive spray control
programme in the United States. As the<i> Culex</i> mosquito-borne
virus disease swept up and down the eastern seaboard, City and State
authorities like New York established WNV control programmes to
include the use of aircraft. The disease has now spread into the south
and mid west with infected birds, horses and human cases in Florida,
Texas and Ohio.<br>
</font></div>
<div><font face="Times" size="+1">Logistical situations on the ground
often dictate the use of aircraft. In 1968 following a hurricane on
the Gulf Coast in the United States with thousands of people in
temporary camps an upsurge of western equine encephalitis, transmitted
by<i> Culex tarsalis,</i> was controlled by the emergency aerial
treatment of some 1.4 million hectares.<br>
</font></div>
<div><font face="Times" size="+1">Eleven million square kilometres of
Africa are afflicted by tsetse flies and the trypanosome parasite they
carry and transmit. Tsetse fly control, which was traditionally
carried out by blanket aerial spraying with chemical insecticides,
such as endosulphan and latterly the pyrethroid deltamethrin, has been
the focus of alternative and more environmentally benign control
strategies. Artificial baiting techniques using custom-designed
targets to mimic the host and exploit behavioural responses in the
tsetse fly became the order of the day.<br>
</font></div>
<div><font face="Times" size="+1">Targets are deployed in the field
and comprise blue and black cloth, colours that elicit attractive
responses and landing responses, respectively, from the flies. They
are supplied with odours from sachets of specific semio-chemicals
(octanol and methyl ethyl ketone) in pockets sewn into the cloth which
is sprayed with deltamethrin or alphacypermethrin to provide the
classic Îlure and killâ response. In countries like Botswana,
targets spread over huge areas are managed using GPS (Global
Positioning Systems) and GIS (Geographic Information Systems)
technology.<br>
</font></div>
<div><font face="Times" size="+1">But recent widespread flooding
causing twin problems of higher tsetse infestation and reduced access
on the ground have brought back the need for widespread aerial
spraying. In Botswana over 1 million hectares of forest will be
treated over the next few years with ULV (ultra low volume)
formulations of deltamethrin applied using Micronair AU4000 rotary
atomisers. These rotary atomisers achieve even size spray droplets (30
to 60 µm) released above the forest canopy under inversion
conditions. With optimal particle size for adult tsetse control at 10
to 30 µm, the product formulation contains a volatile solvent
carrier so that droplet size is reduced during flight to that which is
optimal for impact on the adult tsetse fly.<br>
</font></div>
<div><font face="Times" size="+1">Selectivity<br>
</font></div>
<div><font face="Times" size="+1">Winged adult vector insects like
mosquitoes, tsetse flies and houseflies are typically controlled using
ULV aerial application techniques to provide rapid coverage over the
widest possible area with the minimum spray volume. Evaporation
pressures on droplets, typically released at 15m to100m above the
ground, are reduced by the use of non-volatile formulations.<br>
</font></div>
<div><font face="Times" size="+1">Insecticides (adulticides) used in
aerial spraying to control flying adults should have rapid knockdown
action but limited residual activity and persistence in the
environment. Spray is targeted to deliver pesticide directly onto the
airborne adult or close to resting sites and harbourages. They are
timed for early morning and evening when the insects are most active -
mosquitoes enter buildings at 1700 to 21.30 hours and again in the
early hours of the morning with maximum bite activity at and around
midnight. Small discrete droplets (10-30 µm diameter) are required
for maximum airborne time and to be carried by prevailing winds over
considerable distances.<br>
</font></div>
<div><font face="Times" size="+1">This achieves the objective,
different to that of agricultural spraying, of spray delivery through
the area without the droplets actually depositing on the ground.
Requirements are for extremely low dosage ideally as little as a few
grams of active (5 to 50 g a.i./ha) at spray volumes as low as 50 to
200 ml/ha. Organophosphates such as fenthion and malathion and
synthetic pyrethroids including resmethrin and permethrin are
typically used.<br>
</font></div>
<div><font face="Times" size="+1">Control of aquatic larvae is a
completely different Îball-gameâ. Larvicides need to be deposited
into the water, often through dense vegetation and over relatively
small target areas. For this reason aerial application has so far been
largely confined to applications of granular products by helicopter.
Liquid sprays have been applied to open expansive waterways such as
mud flats using larger droplets (150 to 300 µm) of organophosphates
such as temephos, but with increasing emphasis on the application of
Îsofterâ insecticides like s-methoprene ( an IGR ö insect growth
regulator) and biological insecticides such as<i> Bt</i> (<i>Bacillus
thuringensis israelensis</i>).<br>
</font></div>
<div><font face="Times" size="+1">Nozzle application systems<br>
</font></div>
<div><font face="Times" size="+1">A variety of nozzle systems have
traditionally been used to apply adulticides from the air.<br>
</font></div>
<blockquote><font face="Times" size="+1">Thermal fogs of small
droplets generated from the hot gases of the aircraft engine exhaust.
Less commonly used now due to deleterious effects on some actives and
especially biological insecticides like<i> Bt</i> based on
formulations of living bacterial spores<br>
</font></blockquote>
<ul>
<li><font face="Times" size="+1">Flat fan nozzles, typically the
Spraying Systems 8002 and 8004 types, and less frequently hollow cone
nozzles orientated into the air stream at 45 degrees to maximise air
shear for production of the smallest possible droplet sizes. Wind
tunnel experiments using a Malvern particle size analyser indicate a
typical drop size of 70 to 90 µm vmd (volume median diameter),
depending on nozzle type and airspeed, average droplet size decreasing
with increasing airspeed.<br>
<li>Rotary atomisers, the Îworkhorsesâ for aerial application of
adulticides against mosquitoes and tsetse flies in Africa and Asia and
increasingly in North America as the<i> Culex</i> mosquito vectors of
WNV continue to Îbiteâ. With the ability to produce a much
narrower droplet size spectrum and without large droplets over 60
µm, which fall to the ground to cause waste and contamination, rotary
atomisers are the obvious choice. This is especially so in urban areas
where spotting and staining of cars has become a problem for droplets
over 50 µm.<br>
</font></ul>
<div><font face="Times" size="+1">The Micronair AU4000 and more
recently the Micronair AU5000 rotary cage atomisers have been widely
used for tsetse fly and mosquito control operations. The AU4000
atomiser is usually operated in the 9000-10,000 rpm range, at low flow
rates of 2-4 litres/min with spray droplets of 30 to 45 µm vmd. The
smaller diameter AU5000 rotating at similar speeds gives slightly
bigger droplets (35 to 55 µm vmd) at the point of emission. The
mature particle size will reduce with time, depending on formulation
volatility.<br>
</font></div>
<blockquote><font face="Times" size="+1">Helicopters preferred for
Îlarvicidingâ, usually with granules, and typically featuring two
Îsaddle tankâ hoppers with pneumatically operated outer gates and
either rotating discs or pneumatic spreaders to disperse the granular
formulation. Liquid applications of larvicide can be made using
Micronair type rotary atomisers (e.g. Micronair AU6539 electric drive
atomisers for helicopters) operating at low rotational speeds of 2000
to 3000 rpm for drop sizes in the 100 to 300 µm range.<br>
</font></blockquote>
<div><font face="Times" size="+1"> <br>
</font></div>
<div><font face="Times" size="+1">Most recent developments<br>
</font></div>
<div><font face="Times" size="+1">The Micronair AU4000 atomiser
incorporated on the Micronair Spray Pod system which has recently been
introduced for mosquito control operations in the United States and
elsewhere. Consisting of a 210 litre spray tank with integral 24 V DC
centrifugal pump, flowmeter turbine, diaphragm check valve and
atomiser, the externally mounted and easily removed system offers
increased pilot safety and versatility of aircraft use. Two pods are
mounted on the underside of high wing aircraft including the BN
Islander, Pilatus PC-7 or PC-9, De Havilland DH-2 Beaver and Dornier
DO-228. A control panel installed in the cockpit incorporates controls
for the pumps and electro-magnetic atomiser brakes as well as
application monitors to display flow, volume application rate,
atomiser rpm and other parameters. Application monitors have been
integrated with agricultural GPS track guidance systems to record
application parameters for use within GIS software packages. Most
recently two of these systems were fitted to BN Islander aircraft as
replacements for ageing DC-3 aircraft and gave reduced operating costs
- $650 down to $250 per<br>
hour.<br>
</font></div>
<blockquote><font face="Times" size="+1">A new high pressure hollow
cone nozzle system developed in the United States for use on
helicopters and fixed wing aircraft uses a high pressure pump
delivering 2500 psi for liquid atomisation through small stainless
steel nozzles. Droplet size at the point of emission is just 20 to 30
µm vmd, sufficiently small to ensure that the droplets stay airborne
when released over the target area. Other Florida operators have
developed twin fluid atomisation systems by utilising bleed air from
turbine engines, and cooled in a heat exchanger, to atomise liquid at
the nozzle. These high pressure and twin fluid atomisation systems
have been fitted to Hughes 500 helicopters and Shorts Skyvan twin
engine fixed wing aircraft.<br>
</font></blockquote>
<ul>
<li><font face="Times" size="+1">Recent studies have reinforced the
role and importance of droplet size. Comparisons were made between
standard flat fan spray nozzles and a new high pressure hydraulic
nozzle system using assessments of ground deposition and mortality of
mosquitoes in cages suspended near the ground over distances up to 5
miles downwind from the point of spray emission. The novel high
pressure system gave a very small spray deposit on the ground but
achieved much higher mosquito mortality, compared with the flat fan
nozzle system, showing the crucial importance of droplet size control
to minimise ground deposits and potential water contamination. The
increasing use of high-speed rotary atomisers, twin fluid or high-
pressure atomisation systems with the production of smaller droplets
is improving efficacy and productivity, reducing dose rates and
avoiding the deposition of insecticide into water resources. This is
vital to avoid the destruction of non-target aquatic fauna such as the
fiddler crabs of Florida.<br>
<li>Computer models, simulating spray deposits from different droplet
sizes released from aircraft, indicate that spray application should
be optimised to account for spray spectrum, formulation
characteristics and meteorological conditions, so that spray delivery
is maximised within the target zone. When sprays are emitted at
heights of 30 to 100m then offsets of 500 to 1500 and upwind of the
target area are required and a series of spray runs on the same flight
path are now considered more appropriate than conventional sequential
parallel swathes.<br>
</font></ul>
<div><font face="Times" size="+1">At the point of contact with flying
insects, droplet size should be approaching 10 to 30 µm. With this
achieved the greater number of smaller spray particles increase the
chances of impact while minimising Îfall outâ and ground
deposition with consequent waste and contamination of water resources.
New advanced aerial application techniques, which ensure optimal small
droplet size, and continued focus on more selective insecticides are
improving target selection and selectivity and maintaining
environmental integrity. Aerial spraying of liquid larvicides is an
increasingly attractive proposition using helicopters over smaller
target areas, Îsofterâ chemicals and biological insecticides and
increased droplet size to buffer the filtering effect of vegetation on
spray droplets. Aerial spraying continues to provide the only real
option for rapid-reaction and large-scale, emergency control
operations in the face of highly mobile vector insects and potentially
lethal human diseases.<br>
</font></div>
<div><font face="Times" size="+1">Further information on the
technology of aerial spraying can be obtained from:<br>
</font></div>
<div><font face="Times" size="+1">John Clayton and Tim Sander. Micron
Sprayers Limited. Bromyard Industrial Estate. Bromyard. Herefordshire.
HR7 4HS. United Kingdom. Tel. +44 (0) 1885 482397. Fax. +44 (0) 1885
483043. E-mail: micron_at_micron.co.uk URL: http://www.micron.co.uk<br>
</font></div>
<div><font face="Times" size="+1"><i> <br>
</i></font></div>
<div><font face="Times" size="+1">* Dr Terry Mabbett Consultants. 2
Albemarle Avenue. Potters Bar. Hertfordshire. EN6 1TD. United Kingdom.
Tel/Fax: +44 (0) 1707 644953. E-mail:
DrTerryMabbett@btinternet.com<br>
</font></div>
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Received on Thu Jul 11 11:34:02 2002

Vector borne disease control by aerial application