MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies

DEFINING IMPORTATION RISK
As illustrated in Figure 5, the control measures that are required
to keep overall malariogenic potential in check and thus ensure
sustainable elimination will differ depending upon the level of
importation risk that exists. For this reason, understanding the
amount of malaria being transported to the islands is as essential
as understanding the risk of onward transmission.
Importation risk involves the probability of infectious individuals
or mosquitoes carrying malaria parasites into Zanzibar. Because
Zanzibar is comprised of islands, the risk of importing malaria
is far higher in human hosts than mosquitoes, and we ignore
the latter risk here. Calculating importation risk quantitatively
involves considering each of the following factors:
�� The entry routes through which people travel to Zanzibar,
such as ferry, airplane, or informal boats;
�� The number of people who travel into Zanzibar through each
of these entries by traveler type, such as tourists, migrant
laborers, or Zanzibar citizens;
�� The probability of travelers being infected with malaria
parasites, which will vary with the traveler type and where
they stayed prior to arrival in Zanzibar; returning Zanzibar
residents will have different probabilities of bringing parasites
back with them depending on where on the mainland they
stayed; and
�� The probability of infected individuals within those groups
infecting a mosquito in Zanzibar, which depends upon how
long and where they stay.
In this section, we consider approaches to quantifying each
of these factors, though limited data availability increases the
uncertainty surrounding the estimates derived from information
obtained on air and ferry passenger numbers, and evaluations on
tourism to Zanzibar. A novel approach using mobile phone use
data obtained from the Zantel mobile phone company allowed
for more precise estimates, especially for Zanzibari residents
traveling to the mainland. Careful assessment of each component
must occur during the next several years as Zanzibar plans its
strategy for eliminating malaria and ensuring that it does not
return.
IMPORTATION RISK ANALYSIS BASED ON ROUTINE TRAVEL/
TOURISM DATA
Routes of Importation
Because Zanzibar is comprised of islands, the majority of
incoming people and consequently, parasites, arrive through
specific gateways, including ferry ports and airports. A country
sharing many land borders with neighbors would have far more
points of entry, greatly complicating these calculations. Records
exist of some passenger traffic via ferries and airplanes; however,
no statistics exist at present on the specific numbers of passengers
involved in informal boat traffic between the islands and the
mainland (although landing points are known), so future surveys
and surveillance on informal movements will be necessary to
refine migration estimates.
40,000
35,000
30,000
25,000
20,000
15,000
10,000
5,000
1 | Technical Feasibility
Air Transport
As tourism to Zanzibar continues to increase, so do the number
and capacity of incoming flights (Tanzania Ministry of Natural
Resources and Tourism, 2007). Figure 6 shows that December-
January and July-August represent the peak incoming passenger
seasons, and that numbers increased significantly over the course
of 2006 and 2007. This increase does not necessarily indicate
an accompanying growth in infection importation risk because
many, if not most, of the flights may be carrying tourists (often
taking prophylactic drugs) traveling from non-endemic countries
who have had to change flights in Malaria Endemic Countries
(MEC) to get to Zanzibar (see tourism section below). As such,
while data on passenger origins and purpose of visit are required
for precise estimates, it can be safely assumed that air travel
represents a minority of the risk of parasite importation.
FIGURE 6: NUMBERS DISEMBARKING IN ZANZIBAR FROM DOMESTIC
FLIGHTS (TANZANIA MAINLAND INCLUDED) IN 2006 AND 2007
J F M A M J J A S O N D J F M A M J J A S O N D
MONTHS
Ferry Transport
Through collation and analysis of available ferry timetable
data, Figure 7 shows the major ferry routes and daily capacities.
These figures give a useful indication of maximum expected
numbers of visitors and movements between islands, but do
not indicate where passengers have typically traveled from.
Nevertheless, the data shown in Figure 7 provide useful figures
for estimating the volume and location of entry of passengers
entering Zanzibar through the ferry system. Figure 8 provides
additional information on actual numbers of ferry passengers
and how these have varied through 2006 and 2007. The data
shows some seasonal fluctuations and also some significant
inter-annual variations. However, this data does not distinguish
between numbers arriving or leaving the islands, which islands
are involved and where passengers have come from–these are all
features that should be included in future surveys.
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