There have been significant declines in locally acquired malaria cases from 2000 in Aseer (Fig. 3). The case incidence is now exceptionally low, but not zero. It is notable that aberrations can occur with small rises in case incidence in some years, for example, 2007 and 2015. Reasons for these outbreaks are not clear, but cases are detected early and treated effectively. Large-scale onward transmission, to create seasonal epidemics typical of the 1990s, does not happen and there have been no deaths from malaria for over 25 years. While Aseer has not achieved elimination it has, nevertheless, achieved a public health success.
An important feature of this region over the last decade is that, against a background of sustained malaria control activities, there has been an equally significant increase in economic development. The landscape of paved roads, electrification of rural sectors, urbanization, and expansion of universal education and primary healthcare is completely different in the 2000s compared to the preceding decade. The impact of broader economic development cannot be uncoupled from malaria control success. Analysis of its direct contribution is beyond the scope of the present paper but warrants a more detailed investigation.
In concert with the notion of a changing receptive environment for low, stable P. falciparum transmission, a feature of Aseer a decade earlier, is that there continued to be a constant influx of imported malaria infections into the region (Fig. 3). These imported cases comprised both P. vivax and P. falciparum and originated from adults of non-Saudi nationality (Table 1) who presumably acquired their infections while in their native countries. The cases shown in Fig. 3 and Table 1 are those that have presented to PHCC diagnostic centres; because of their number, it is reasonable to assume that equivalent, if not more, infections are brought into the region by asymptomatic carriers who have an acquired clinical immunity. Despite this constant source of imported infection, the reported local residence of these cases (Fig. 5) appears not to correspond directly with the foci of locally acquired cases (Fig. 6), and equally there was no locally acquired P. vivax case in the region despite 57% of all imported cases harbouring this parasite. These observations are hard to explain based on the epidemiology of transmission. Vectors are not refractory to vivax in this area, and focal vivax epidemics have been reported previously. Addresses provided by imported cases may not be reliable, and it is possible that these cases migrate on further immediately after treatment. The lack of any obvious correspondence between imported infections and local infections may be a result of the overall change in social and land use ecology, resulting in a very different malariogenic potential or imprecision in the locality and duration of residence of imported cases.
The data presented here required a considerable degree of spatial confirmation. Cases reported within the catchment of one malaria-reporting centre had their household residence in another catchment area. This extended to cases detected in Aseer who were residents of villages in Jazan, and it was not possible to consider the reverse where cases reported in Jazan had residences in Aseer. Boundaries between malaria centres and between administrative regions (Jazan and Aseer) have also changed with time. A complete geo-coded census of all villages in either region is not available to the malaria programmes of these regions. Each residence requires confirmed coordinates each time a case presents to one of the PHCCs. These realities make the classification of area-specific incidence hard to define and interpret (Fig. 4). In addition, without an accurate, spatially configured denominator of all villages, meaningful analysis of hotspots cannot be undertaken [20–22], which would provide insights into putative risk factors [23–25]. Presently, and importantly when case incidence is so low, all that is possible is to map individual case residences within Aseer for those reported in Aseer. However, the residence of the host may not always be the same as the mosquito origin of the infection. Very detailed travel histories are required to establish precise locations where individuals may have slept over periods extending back at least ten days before they present with clinical symptoms. There may be social reasons why respondents might be reluctant to provide accurate information, including recreational travel. Even with reliable travel histories of those who have been locally mobile there is no guarantee that an infection can be linked to the travel location or the usual residence, even with current genotype barcodes of the parasite [26, 27]. Where genotyping parasites will become valuable is in the mapping of onward transmission. Being able to classify F1 and subsequent generations of new infections in an area would provide useful information for a programme to understand how foci evolve [28].
The malaria programmes in Aseer and Jazan have slightly different procedures for detecting cases; in Jazan, for example, annual mass blood surveys are undertaken in areas of previous foci. In Aseer, reactive ACD is mounted during the year an imported or locally acquired case is detected, however, during the following year communities in these localities are not screened again. The degree to which locations where cases are identified are temporally interrogated determines the reliable classification of active, residual non-active and cleared foci [28]. These definitions were proposed as metrics for targeted elimination in Aseer and Jazan during the Kingdom’s 2004 plan of action [29]. These definitions, based on case data, mass blood surveys and entomological investigations are not strictly adhered to and are not mapped in a way that allows time and space to be included in the 2 year cut-off periods between foci classifications. Nevertheless, with the available resources, and in the absence of a universal geographic information system (GIS), the notion of foci are used as part of timely notification of malaria cases from public and private sector health facilities, leading to every locality being investigated for the characteristics of transmission to mount intensified control. This pragmatic approach continues to serve Aseer well, and case incidence remains low. To reach a position where complete confirmation of the absence of transmission, for a certification of elimination, would however require a substantial increase in use of GIS, increased 2 year longitudinal, active surveillance of sites where cases have been detected, and a more detailed, coordinated interrogation of travel and residence histories within Aseer and with neighbouring Jazan.