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