Sanitation and Vector Control - Biological and Environmental Control of Disease Vectors - page 146

sweat, tears and nasal discharge. Water is
essential for fl y survival, and due to frequent
contact with water, food and excreta, fl ies can
act as ef ective mechanical vectors for dif erent
pathogens. The common housefl y and blow fl ies
are associated with the spread of diarrhoeal
disease, while the bazaar fl y, more commonly
found in warm climates, is associated with
trachoma. M. sorbens has, under laboratory
conditions, been found to be an ef ective carrier
of the infective agent of trachoma, Chlamydia
trachomatis (Forsey and Darougar, 1981), and
studies in the fi eld have suggested that M . sorbens
is the principal insect vector of the disease
(Emerson et al ., 2000a).
Flies are an important, although often
understated, vector of diarrhoeal disease (Fig.
8.1). The common housefl y has been found
infected with a wide diversity of enteric
pathogens, including Vibrio cholerae (Fotedar,
2001), Shigella spp. (Khin Nwe et al ., 1989),
Salmonella spp. (Khin Nwe et al ., 1989),
Cryptosporidium parvum and Giardia lamblia
(Szostakowska et al ., 2004). A study in-
vestigating fl y density and diversity in and
around rural households in Thailand, found
roughly 90% of all fl ies caught to be the common
housefl y, with the number of fl ies collected in
kitchens as high as 160 min −1 (Echeverria et al .,
1983). The density of fl ies in this study showed a
strong increase during the hottest and driest
months of the year, which coincided with the
main diarrhoea transmission season. Infection
rates in samples of pooled fl ies collected from

kitchens were much higher for E . coli than for
Shigella fl exneri and non-O1 V . cholerae: 68%
versus 6% and 6%, respectively (Echeverria et
al ., 1983).
Fly control through the use of yeast-baited
fl y traps was found to reduce fl y densities by 64%
in Israeli army camps. Overall diarrhoea
prevalence in soldiers was reduced by 42%,
while shigellosis specifi cally was reduced by 85%
(Cohen et al ., 1991). Work in the North West
Frontier Province of Pakistan showed that
mothers identifi ed diarrhoea as the greatest
health problem for their children and considered
fl ies as a major cause of diarrhoeal disease
(Chavasse et al ., 1996). The use of fl y traps in
Pakistan had limited impact on fl y densities, but
space spraying of households with an insecticide
(deltamethrin, applied at a dose of 0.5-1.0 g of
active ingredient ha −1 ) almost completely
eliminated fl y populations and resulted in a 23%
reduction in diarrhoea incidence (Chavasse et
al ., 1999). Another study, performed in the same
year, investigated the impact of a fl y intervention
on diarrhoea prevalence in The Gambia
(Emerson et al ., 1999). Here, a similar impact of
spraying of villages with 0.175% deltamethrin
was reported with reductions between 57% and
71% in M . domestica populations, and a 22%
and 26% reduction in diarrhoea prevalence in
the wet and dry season, respectively. However,
the decrease in diarrhoea in the dry season was
found not to be signifi cant as a result of the
generally low diarrhoea prevalence during the
dry season (Emerson et al ., 1999).

Fluids

Fields

New
host

Food

Faeces

Flies

Fingers

Fig. 8.1. The F-diagram, showing the different faecal-oral transmission routes (adapted from Wagner
and Lanoix, 1959).