Sanitation and Vector Control - Biological and Environmental Control of Disease Vectors - page 148

America and the Middle East. People suf ering
from active trachoma serve as the reservoir of
the disease and carry the bacteria in tears and
other human secretions to a new host through
four suggested transmission routes: (i) direct
contact during play or sharing a bed; (ii) fi ngers;
(iii) eye-seeking fl ies; and (iv) coughing and
sneezing (Emerson et al ., 2000b). The relative
importance of each of these transmission routes
might vary in time and place, but it is unlikely
that a single transmission route is responsible
for all the transmission within a given area.
The provision of improved sanitation
through, for example, basic latrines, seems to be
an ef ective way to control M . sorbens
populations, as no adult fl ies emerged from pit
latrines in Botswana, Tanzania, and The Gambia
(Curtis and Hawkins, 1982; Emerson et al .,
2000a). This hypothesis was explored further in
a study that provided ventilated improved pit
latrines in The Gambia (Emerson et al ., 2004).
The latrine provision signifi cantly reduced the
number of fl ies caught from children's faces by
30% in intervention villages, but the reported
29.5% reduction in trachoma infection was
found to be non-signifi cant as result of a low
baseline prevalence (Emerson et al ., 2004).

8.3.2 On-site sanitation

(Traditional) Pit latrines

In the developing world, it is generally recognized
that installing Western-style sewers and
treatment plants is often too costly, particularly
in rapidly growing urban communities.
Sanitation for many of the urban and rural poor
therefore often takes the shape of a pit latrine,
with an estimated number of pit latrine users
worldwide of 1.7 billion (Cranfi eld University et
al ., 2007).
Pit latrines consist of a dug pit covered by a
slab with a squat hole and a superstructure (Fig.
8.2a). The pit is often unlined, though,
depending on household income, bricks, oil
drums or stones might be used. The depth of the
pit varies, and often depends on the soil type,
depth to groundwater, and space within or
around the household compound. Shallow pits
are often dug in areas with high groundwater
tables, rocky ground conditions or sui cient
space to dig new pit latrines when the old one is
full. The covering slab might be constructed of
wooden beams covered with either soil or
concrete. The squat hole might be constructed

(a)

(b)

Fly screen

Air movement

Latrine superstructure
(optional)

Vent pipe

Ground level

Flies

Fig. 8.2. (a) Pit latrine and (b) ventilated improved pit latrine schematic.