Tropical Health Matters

When the international effort to eradicate guinea worm started in 1987 the focus was on only 20 currently endemic countries.  Tayeh and Cairncross report the status of these countries at the end of 2008 –

• 6 have interrupted transmission and been certified as free of guinea worm
• 8 have interrupted transmission and certification is under way
• 6 are still reporting cases, of which three are a major focus

Guinea worm is a highly focal disease that does not move too easily. The differences in transmission and the sheer number of endemic countries sets malaria off clearly from guinea worm. But are there lessons to learn?

Of concern here is the process of certification that transmission has stopped.  Looking at malaria’s pathway to elimination, we realize that most countries are in the scale-up phase of control and have a long way to go to sustain control and enter pre-elimination, let alone elimination and then prevention of re-introduction.  At some point in the elimination phase, which could be 20 years from now, countries will need to be certified free of malaria.

Tayeh and Cairncross address the realities that larger scale eradication programs like polio take a regional approach to certification, as a country-by-country approach would be too costly.  They also discuss the dangers of re-introducing a disease to places that are free of transmission, which corresponds well with the final phase of the malaria elimination pathway.

Another issue is the challenge of identifying cases when transmission becomes very low. The system of rewards to citizens may not be most appropriate for malaria, but the ability to keep people engaged in eradication efforts when they don’t see a reason will be difficult.

What our colleagues don’t mention is the challenge of false positives. We found that local beliefs in guinea worm endemic areas can lead to high false positive rates on surveys – that swelling on his leg, that ache in her stomach, those rashes on his body.  Local beliefs about malaria presentation abound, too.  It is good, therefore, to start considering what surveillance and certification of malaria elimination will look like now, so a good system is in place when the time comes. In fact we may get good opportunities to practice in places like Zanzibar.

Malaria researchers in Niger have been observing malaria transmission and mosquito populations in two villages over two years and discovered major differences even though the settlements are only 30 km apart. Finding a 10-fold difference in mosquito density, the authors conclude that, “The highly focal nature of malaria in the Sahel makes detailed representation necessary to evaluate village-level risks associated with hydrology-related vector population variability.”

Other factors affecting transmission differentials include seasonal availability of nutrients and predators. Ultimately the authors recommend, “Topography, vegetation, soil type differences as well as shallow groundwater behaviour must all be incorporated at appropriate scales in order to accurately evaluate malaria transmission at the village scale using coarse resolution climate models.” These issues should inform ‘malaria early warning systems’ at a more focused level, the village.

The researchers were concerned about, “The sudden appearance of widespread but temporary water pools gives rise to a rapid increase in mosquito populations clustered around human habitation, resulting in the highly focal malaria transmission that is a characteristic of Sahel villages.”  While these local variations are known, the problem arises when computer models lump together areas of tens to hundreds of kilometers in size together, missing the local differences that imply different control strategies.While the overall climate and vector behavior were not different, local hydrology was. The researchers specifically modeled differences in “local topography, distributed land cover type, and subsurface hydrology environment.”

Such differences in transmission dynamics have also been reported for urban malaria. For example, in Luanda, Angola transmission was more than four times greater in areas 15 km or more beyond the city core, compared with the central areas.  In Dakar, Senegal transmission was highly focal, and “no mean figures for transmission would provide a comprehensive picture of the situation; risk evaluations should be conducted on a local scale,” even for areas only a few kilometers apart. Urban agriculture provides another example of very focal transmission.

A the present moment when countries are focusing on large efforts to ‘scale up for impact’ the small scale or focal differences might be ignored. As we move along the pathway toward elimination, more focal surveillance and intervention will be essential for mopping up transmission and monitoring against reinfestation. Now is the time to build those surveillance systems so that countries and communities will not be caught unaware when malaria attempts to make a comeback some years hence.

Non-endemic does not mean no malaria.  The Caribbean Epidemiology Centre (CAREC) explained that, Jamaica has been malaria-free for over fifty (50) years, although a number of imported malaria (Plasmodium falciparum) cases were identified each year.” That said, Rawlins and colleagues observed that there were 897 reported malaria cases in the Caribbean Epidemiology Centre (CAREC) Member Countries (CMCs) between 1980 and 2005 with Jamaica accounting for 38.4%.

Then the situation changed briefly. WHO posted information in 2007 that, “The Ministry of Health of Jamaica has confirmed 280 cases of malaria due to Plasmodium falciparum on the island between 6 November 2006 and 3 February 2007. Of these reported cases, 264 have occurred in Kingston, 12 in St Catherine, 3 in St Thomas and 1 in Clarendon. There have been no reported deaths due to malaria.”

WHO recommended that while “Jamaica is a non-endemic country for malaria … The occurrence of this outbreak highlights the importance of conducting surveillance in countries that are non-endemic to malaria.”

[LEFT: malaria free but not alligator free]

Likewise Rawllins et al. warned that, “All the essential malaria transmission conditions–vector, imported malaria organism and susceptible human host–now exist in most CMCs. A call is now made for enhanced surveillance, vector control and anti-malaria skills to be established in CMCs.”

We cannot turn our backs on malaria – malaria carrying mosquitoes do not mind whether they bite us in front or in back.