MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies

the same time, it is expected that the fraction of infections that
are clinically symptomatic will increase as immunity to malaria
decreases with transmission. These two trends combined may
make it possible to reach the 70-80% requirement estimated here
and maintain elimination as long as net coverage is kept in place.
Increases of this magnitude will require active strengthening of
the health system as discussed in Chapter 2.
Active Case Detection
It will take many years before passive surveillance alone can reach
the high levels described above. Complementing the passive
surveillance system with targeted active surveillance activities will
help prevent imported cases from leading to reintroduction of
transmission. Active surveillance would likely be most effective
if focused on key high-risk areas. At present, it appears that the
northern ends of Pemba and central Unguja comprise areas
of particularly high risk. However, there may be very specific,
identifiable clusters, such as areas around vector breeding sites
that will be identified as residual foci during the elimination
campaign. Active case detection teams that visit the homes and
neighbors of identified cases who have not traveled outside of
Zanzibar and who thus, likely represent local transmission may
prove to be an efficient means of halting onwards transmission.
As malaria is eliminated, Zanzibar should maintain a GIS
database of case locations in order to identify spatial clusters–these
locations will indicate where interventions should be particularly
focused on the way to elimination, as well as where active case
detection might usefully continue following elimination. Such
clustering may also suggest key vector habitats that would
benefit from environmental management or larviciding, as well
as indicate high-risk groups where screening could potentially
occur (e.g., “hot-spots” around agricultural areas may signal the
need for migrant worker screening). The relative importance of
active surveillance is likely to increase over time as overall malaria
prevalence continues to approach zero.
In the simulation, we modeled an active case detection (ACD)
program in lieu of the previously modeled increases in passive
case detection. Simulated ACD teams visited the houses of cases
identified through passive surveillance that did not have a history
of traveling outside of Zanzibar, testing family members and all
neighbors in a defined number of closest houses. The simulation
was run repeatedly while varying the number of neighbors
visited by the ACD teams but keeping the fraction of infections
identified by passive case detection at 50%, to determine the
level at which local transmission could be reliably kept below
the threshold defined by the WHO (Figure 19). Results indicate
that performing ACD on approximately 100 neighboring houses
around each identified case appears to be sufficient to prevent
reemergence of malaria if no increases in passive case detection
are assumed. The required amount of ACD will decrease as
passive case detection increases.
1 | Technical Feasibility
FIGURE 19: RELATIONSHIP BETWEEN SIMULATED ONWARD TRANSMIS-
SION AND ACD GIVEN 75% EFFECTIVE ITN COVERAGE
2ND GENERATION CASES PER 1000 PERSON-YRS
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0 50 100 150 200
HOUSES INVESTIGATED AROUND EACH IDENTIFIED INFECTION
The green line indicates predictions for importation = 2/1000
while the blue line is for 8/1000; the dotted line indicates the
approximate threshold below which WHO criteria for prevention of
reintroduction are met.
Simulating Post-elimination with No ITN Coverage
If the R C for Zanzibar can be maintained permanently below
one through sustained use of nets or IRS, malaria will not spread
effectively in the islands if it is introduced. However, as described
previously, R 0 in Zanzibar–or the rate that transmission would
occur if control measures were stopped–remains quite high. As
such, maintaining such a low R C would require permanently
high coverage by interventions like effective ACT treatment, and
protection by measures like LLINs or IRS. Over the long term,
it will be desirable for both operational and financial reasons for
interventions like IRS and LLINs to be scaled back. After malaria
is no longer an apparent threat, it may be difficult to convince
individuals to continue to use their nets or to allow their homes
to be sprayed. Additionally, the ongoing costs of sustaining net
distribution or widespread IRS will place considerable financial
pressure on the program (see Chapter 3). Finally, long-term
use of insecticides will eventually lead to the development of
resistance by mosquitoes and parasites.
As long as the innate risk of malaria remains high, however,
scaling back safely can only occur if importation risk is
minimized and effective surveillance and response capabilities
can identify imported cases and prevent them from resulting in
local transmission. Other countries that have scaled back these
protective measures without adequately considering the risk of
importation or ensuring the existence of sufficient surveillance
and response capacity have seen severe epidemics cause high
mortality and morbidity.
With no nets, it is expected that R C will increase until it reaches
R 0. As estimated previously, R 0 in Zanzibar is estimated to be
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