MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies
MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies
MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies
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around 10-15. In this scenario, the lower value of 10 is used. As<br />
in the scenario predicting transmission with nets, it is assumed<br />
that passive case detection can promptly identify and treat 50%<br />
of all infections, and that importation is either 2 or 8 per 1000<br />
people/yr.<br />
Under these conditions, repeated simulations indicate that malaria<br />
would rapidly reemerge in Zanzibar unless other interventions or<br />
health system strengthening take the place of the nets. In this<br />
scenario, we assume that reemergence has occurred when 50 new<br />
infections are identified within two weeks. At this threshold, it<br />
is assumed that drastic action would be required involving focal<br />
IRS, larval control, and other measures. This “failure” point was<br />
reached within 9-13 months, depending upon assumptions about<br />
importation. Such timing is in line with rapid resurgence observed<br />
historically in places with a high R 0, including twice previously<br />
in Zanzibar. Clearly, additional interventions are required if nets<br />
are to be scaled back. As with the nets scenario, simulations were<br />
used to estimate the minimum fraction of cases identified and<br />
promptly treated through passive case detection or the amount of<br />
active case detection that would sustain elimination.<br />
Passive Case Detection<br />
The simulation was run repeatedly at different levels of passive<br />
case detection to determine the level at which local transmission<br />
could be reliably kept below the threshold defined by the WHO.<br />
Results indicate that, without nets, the percent of cases that must<br />
be identified to ensure second generation transmission does<br />
not occur in two consecutive years is 97-98%, depending upon<br />
importation assumptions.<br />
Achieving near perfect infection detection and treatment is<br />
not technically feasible. Even if 100% of individuals with<br />
clinical disease sought treatment at a public health facility,<br />
some infections would remain asymptomatic and thus not seek<br />
treatment. Additionally, diagnostics in the health facility will<br />
always have some error, meaning that some infections will be<br />
misdiagnosed.<br />
Active Case Detection<br />
Alternatively, active case detection (ACD) could be implemented<br />
in lieu of increases in passive case detection (or in addition to<br />
smaller increases). The simulation was run repeatedly at different<br />
levels of active case detection, 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 the<br />
threshold defined by the WHO (Figure 20).<br />
30<br />
FIGURE 20: RELATIONSHIP BETWEEN SIMULATED ONWARD TRANSMIS-<br />
SION AND ACD GIVEN 0% EFFECTIVE ITN COVERAGE<br />
2ND GENERATION CASES PER 1000 PERSON-YRS<br />
14<br />
12<br />
10<br />
8<br />
6<br />
4<br />
2<br />
0<br />
0 200 400 600 800 1000 1200<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 />
Results indicate that ACD alone is insufficient 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, but at 50% detection, nearly<br />
every house in the region must be investigated following each<br />
case to maintain elimination.<br />
Combined Interventions<br />
Increasing both passive and active case detection may result in<br />
making it technically feasible to withdraw net coverage while<br />
still maintaining elimination. Additionally, it may be possible<br />
to decrease importation through border screening. Mainland<br />
Tanzania is currently engaging in a massive effort to scale-up<br />
ITN and ACT coverage around the country, which may also<br />
decrease importation rates.<br />
The fraction of cases that can be identified by passive case<br />
detection is a function of the percent of infections that cause<br />
clinical disease, the percent of febrile individuals that seek<br />
treatment in the public sector, and the fraction of those at public<br />
facilities that are tested and treated for malaria. It may be possible<br />
for Zanzibar to increase the fraction of febrile individuals seeking<br />
treatment in public health facilities greatly over the course of<br />
the decade; as discussed above, the current rate of increase in<br />
the fraction of parents taking febrile children to public health<br />
facilities could reach 90% by 2020. The fraction of infections<br />
that have clinical disease will increase as malaria become rarer;<br />
we assume that 90% of infections will be clinically apparent<br />
in 10 years time. Finally, it is assumed that all fevers are tested<br />
and treated accordingly, but that achieving greater than 95%<br />
sensitivity in diagnosis is not possible. As such, under these<br />
assumptions, passive case detection would identify 90% x 90% x<br />
95% = 77% of infections in Zanzibar.<br />
The estimates of importation used here of 2 or 8 per 1000 people/<br />
yr may also be reduced over time using border screening or due to<br />
reductions in transmission on the mainland. If border screening<br />
can test 90% of individuals entering Zanzibar with a 90% accurate