The Role of Pyrethroids in Disease Vector Control.

Janet Hemingway, Director of the Liverpool School of Tropical Medicine
and CEO of the Bill and Melinda Gates funded Innovative Vector Control
Consortium.

Public health pesticides are often the poor relations of agrochemicals.
In contrast to the agrochemical insecticides, no new insecticides have
been registered for effective mainstream vector control activities for
the last 25 years. The number of insecticides has actually been reduced
over this period with the phasing out of the cyclodienes, and drastic
reductions in the numbers of organophosphates and carbamates being
produced and marketed for large scale vector control. The situation is
sufficiently grave that DDT has been retained purely for use in indoor
residual spraying in malaria control because options for insecticide
choice are so limited. 

Pyrethroids are absolutely essential for disease vector control and any
restriction in their availability could have catastrophic consequences
for control of many of these deadly diseases. Malaria still kills up to
2 million children under the age of 5 every year. The bulk of these
deaths are in sub-Saharan Africa. Additional loss of income through
illness in adults has a major economic impact on many rural communities.
We have two interventions that have a proven impact on reducing disease
transmission – insecticide impregnated bednets and indoor residual
house spraying. Type I and type II pyrethroids are the major
insecticides used in both applications. A sub-set of pyrethroids are the
only insecticides recommended for use on impregnated nets. Their rapid
mode of action, safety for use in close human contact, contact irritancy
and long residual action in Long Lasting Insecticide net formulations
are essential characteristics for insecticides on nets. This combination
is found in no other insecticide class currently available for public
health use.

Indoor residual spraying (IRS) has more options but pyrethroids still
account for more than 70% of this growing market. The only other options
are DDT, organophosphates or a single carbamate. Pyrethroids in the
absence of operational levels of insecticide resistance are the
insecticide class of choice for IRS. The ability to use the second and
third generation pyrethroids at very low dosages make them almost as
cost effective as DDT and new formulations of pyrethroids with a 6 month
or better residual life should eventually allow us to remove that last
vestiges of reliance on DDT for IRS. Resistance to pyrethroids, because
of the different modes of action and metabolic susceptibilities of
different pyrethroids means that it is also possible to maintain the use
of these insecticides by careful insecticide selection when resistance
first arises. Any reduction in the numbers of type I and type II
pyrethroids in this market would be highly detrimental.

In addition to the use of nets and indoor spraying a number of new
pyrethroid formulations on different emanators are currently being
trialled in Mexico and other disease endemic countries. These utilise
the new pyrethroids which more readily vaporise to provide protection to
the residents of houses in which the emanators are placed. Early results
suggest that these should be an effective alternative to nets
particularly against earlier biting malaria vectors (such as Anopheles
culicifacies) and against the vectors of dengue (Aedes aegypti). Again
the characteristics of these insecticides mean that only the pyrethroids
are suitable for use in this format.

While malaria control activities are the major consumers of pyrethroids
it is clear that their large scale use for malaria also has significant
benefits in controlling other vector borne disease in many parts of the
world where these diseases co-exist. The benefits include control of
dengue, leishmaniasis and filariaisis. The global burden of these vector
borne diseases is summarised in table 1. Our ability to control these
diseases is crucially dependent on the availability of a range of
different pyrethroids with characteristics that can be matched to the
application and use setting. We would ask that the EPA consider the
major ramifications in terms of loss of human life and increase in
suffering that any reduction in availability of pyrethroids would cause
in the 100+ countries in which these diseases are endemic before any
global recommendations are made based on theoretical risk analysis in
non-disease endemic countries such as the USA.

Table 1: The current burden of vector borne diseases that are all
transmitted peridomestically (Source: WHO/TDR)

1 World-Wide Disability Adjusted Life Years

Disease	Vector	Disease Burden

DALYS1 (thousands)	Deaths

(thousands)

Malaria	Anopheles mosquitoes	42,280	1,724

Dengue	Aedes mosquitoes	     653	     21

Lymphatic filariasis	Anopheles and Culex mosquitoes	 5,644	      0

Leishmaniasis	Sandflies	 2,357	    59

Chagas disease	Triatomid bugs	    649	   13



