Biology Reference
In-Depth Information
THE
GLOBAL LIMITS OF TRANSMISS
ION
In common with most parasitic infections, the transmission of
A. lumbricoides is constrained by temperature and humidity. This is
because the eggs excreted by female worms require a period of embry-
onation in the external environment prior to being picked up and ingested
by humans. Experimental studies suggest that maximum rates of
embryonation of Ascaris ova occur at temperatures between 28 and 32
C,
with embryonation arresting below 5 and above 38
C.
22
Similarly, at low
humidity (atmospheres less than 80% saturation) Ascaris ova do not
embryonate.
23
These differing rates of development and survival will
influence parasite establishment in the human host and hence observed
limits of transmission, which can be observed at global and country
levels.
Figure 13.1
A presents the global limits of A. lumbricoides, based on
spatial analysis of available data and a range of environmental factors.
19
This analysis shows that high and low land surface temperature and
extremely arid environments limit A.
lumbricoides transmission.
In
particular, the prevalence of A. lumbricoides is generally
4% in areas
where maximum land surface temperature exceeds 35
C, and drops to
<
<
1% by 40
C. Prevalence is also
2% in areas classified as “arid” (as
defined by an aridity index which combines data on annual total rainfall
and the ability of the atmosphere to remove water through evapo-
transpiration) and
<
0.1% in “hyper-arid” areas.
Within these biological limits local endemicity will be mediated by
settlement patterns and urbanization. Interestingly, the prevalence of
A. lumbricoides is higher in urban and peri-urban than in rural settings,
especially in Africa.
19
Such differences may reflect variations in inade-
quate water and sanitation or higher population density in peri-urban
and urban areas compared to rural areas.
<
A S
HRINKING GLOBAL DISTRIBUT
ION
A further factor influencing the contemporary distribution of
A. lumbricoides is local control efforts. Historically, A. lumbricoides, as well
as Trichuris trichiura and hookworm, was prevalent in parts of Europe,
Japan, South Korea, Taiwan, the Caribbean, and the southern states of
America, but sustained control efforts and economic development helped
to eliminate intestinal nematodes from these countries.
17,24
e
28
An example
of such sustained control is illustrated for South Korea in
Figure 13.2
.In
the early 1960s, A. lumbricoides and other intestinal nematodes were highly
prevalent and in response to this situation, the Korean government began
a national parasite control program, supported by the Parasitic Diseases
