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Harvard's Jessica Cohen: 'Zanzibar gains could be erased in months'

April 10, 2013 - 20:50 -- Bart G.J. Knols

Harvard University organised a mini-symposium on malaria on 5 April titled 'Defeating malaria, from the genes to the globe'. It was the first in a series examining global public health problems like malaria. Noteworthy in that regard are the views that were expressed during this symposium regarding the malaria situation on Zanzibar. Assistant Professor Jessica Cohen, who reportedly advised the government of Zanzibar on how to move forward with its fight against malaria made some pretty remarkable statements.

Cohen's predictions showed that malaria on Zanzibar could be eliminated in just 5 years if everyone on the island (more than a million people) would sleep under bednets. Moreover, she noted that if 'only' 65% of the population would use nets, it would take 22 years. The bad news followed: If usage rates drop to 50% she predicted an increase in prevalence to 5% in just 3 months, up from the 2% prevalence now. Worse, if it dropped to just 35%, malaria would strike back and prevalence would rise to 18% in just 3 months.

She concluded that 'these gains can be erased in months'...

I am amazed by such statements from an expert that provides advise to the Zanzibar government. No doubt these data originated from a fancy and well-thought-through model (yet another model) but let's put this into perspective.

1) Suppose Cohen's data are correct, than she implies that the hope of eliminating malaria on Zanzibar is virtually nil. Sustaining high coverage rates (i.e. people sleeping under nets) in a place that is hot most of the year (making sleeping under nets very uncomfortable) and where malaria is 'almost' gone is a sure recipe for failure. Usage of nets is bound to go down, with only one hope remaining and that is that nuisance biting by culicines may lead to sustained use. Culex mosquitoes may save the day for malaria control...

2) Cohen is of the opinion that bednets alone can eliminate malaria on Zanzibar. This is quite amazing too. Does her model take into account the fact that outdoor biting can sustain as much as 20% of the total transmission of malaria? And that mosquito vectors adapt to outdoor feeding and shift their biting activity to earlier in the evening before people retire under their nets? Did Cohen take notice of the fact that just last week massive resistance to pyrethroids was reported from Zanzibar?

Even if bednet usage would remain very high, there will still be problems in the real world that no model can capture. It needs to be repeated time and again: if we rely on biocides only to do the job we're heading for the wall.

3) Cohen stated (according to the Harvard article) that parasites can remain in the liver for a long time. That is likely a reference to P. vivax. But this parasite (according to the report 'Malaria elimination in Zanzibar: A feasibility assessment' published in 2009, does not occur in Zanzibar. Admittedly, the report mentions a low prevalence of P. malariae which does have such characteristics.

If the HarvardScience report quoted Cohen correctly than this means bad news for Zanzibar. I already blogged about this last week (here), but this reduces hopes that Zanzibar is ever going to succeed even further. The optimism that the 2009 report portrayed is vanishing. And after all that can be published on it has been reported by us scientists the only group that will pay the price is the group we started the whole campaign for in the first place...

Bednets are a fabulous tool - but on their own, most medical entomologists will agree, they will not help us to eliminate malaria anywhere in Africa. Regretfully, beyond nets and indoor residual spraying, those that develop fancy models overlook the obvious, simple, additional tools that could be sustainable and keep malaria on Zanzibar really low or even eliminate it. Where is the mention of larval source management? Where are statements on house improvement (e.g. through screening of eaves and windows), where do we read about environmental management?

Regretfully, we too often seem to forget that malaria is not just a disease, but that it is a vector-borne disease. Unless we get serious, and I mean really serious, about hammering vectors on Zanzibar, Cohen and myself will finally agree that the success of controlling malaria on Zanzibar will go up in smoke. Mark Benedict mentioned the successful elimination of tsetse flies on Zanzibar now 13 years ago. And yes I know, the reproductive capacity of tsetse is much lower than that of mosquitoes, but elimination of anophelines elsewhere has even proven that wrong can be done.

Bummer that WHO's position paper on larval control will likely be put on the table to justify that such intervention should not be incorporated in the activities on Zanzibar. A lost opportunity...

(picture by Kris Snibbe)



Submitted by Anonymized User (not verified) on

Professor Cohen's website suggests her background is economics (PhD (Economics) Massachusetts Institute of Technology, 2007) so perhaps the differences between Pf and Pv aren't her focus, but surely someone in the department could assist her??!!

It's also quite possible Pv is in Zanzibar, albeit at at low levels:

Any thoughts on this, Dr. Knols?

Bart G.J. Knols's picture
Submitted by Bart G.J. Knols on

It could be, although the 2009 report that describes the feasibility of malaria elimination on Zanzibar only mentions falciparum and malariae...

Submitted by Anonymized User (not verified) on

I am taking my first steps in this field (currently writing my doctoral thesis in medical entomology) and so am still piecing together the detailed history of malaria control initiatives (how else to learn from our mistakes?) The interesting editorials on Malaria World help to give me some perspective of how control strategies are viewed by experienced professionals and the politics involved, and I found it fascinating and disheartening to hear how techniques such as larval source management are unwanted in Zanzibar.

As I am only starting my career in medical entomology, I don’t feel I have the experience to critique the situation; but I am increasingly concerned by what seems to be the marginalisation of medical entomologists in developing the strategies that are needed to cope with physiological and behavioural resistance in the mosquito vectors of malaria. I would never dream of ‘simplifying’ the economics involved, yet is seems the complex ecology of the disease vectors (and perhaps the disease itself in this case) is often ‘simplified’ to its most basic elements.

We cannot rest on our laurels; I hope my coming career in medical entomology can be spent tackling the issues, rigorously and creatively, rather than shouting at a world that will not listen.