Spatial and temporal epidemiology of clinical malaria in Cambodia 2004-2013

Abstract

Background: Artemisinin-resistant Plasmodium falciparum malaria has recently been identified on the Thailand-Cambodia border and more recently in parts of Thailand, Myanmar and Vietnam. There is concern that if this resistance were to spread, it would severely hamper malaria control and elimination efforts worldwide. Efforts are currently underway to intensify malaria control activities and ultimately eliminate malaria from Cambodia. To support these efforts, it is crucial to have a detailed picture of disease burden and its major determinants over time.

Methods: An analysis of spatial and temporal data on clinical malaria in Cambodia collected by the National Centre for Parasitology, Entomology and Malaria Control (CNM) and the Department of Planning and Health Information, Ministry of Health Cambodia from 2004 to 2013 is presented.

Results: There has been a marked decrease of 81% in annual cases due to P. falciparum since 2009 coinciding with a rapid scale-up in village malaria workers (VMWs) and insecticide-treated bed nets (ITNs). Concurrently, the number of cases with Plasmodium vivax has greatly increased. It is estimated that there were around 112,000 total cases in 2012, 2.8 times greater than the WHO estimate for that year, and 68,000 in 2013 (an annual parasite incidence (API) of 4.6/1000). With the scale-up of VMWs, numbers of patients presenting to government facilities did not fall and it appears likely that those who saw VMWs had previously accessed healthcare in the private sector. Malaria mortality has decreased, particularly in areas with VMWs. There has been a marked decrease in cases in parts of western Cambodia, especially in Pailin and Battambang Provinces. In the northeast, the fall in malaria burden has been more modest, this area having the highest API in 2013.

Conclusion: The clinical burden of falciparum malaria in most areas of Cambodia has greatly decreased from 2009 to 2013, associated with roll-out of ITNs and VMWs. Numbers of cases with P. vivax have increased. Possible reasons for these trends are discussed and areas requiring further study are highlighted. Although malaria surveillance data are prone to collection bias and tend to underestimate disease burden, the finding of similar trends in two independent datasets in this study greatly increased the robustness of the findings.

Figure 1

Cumulative seasonal pattern of malaria cases. A P. falciparum and B P. vivax malaria cases from HIS and average monthly rainfall in Cambodia from 2004 (bottom) to 2013 (top). Each band represents a single year from 2004 (bottom) to 2013 (top). The month with the highest number of cases in each year is indicated by a red dot.

Figure 2

Reported malaria cases and people tested. A Monthly numbers of reported malaria cases and B people tested for malaria in Cambodia HIS from 2004–2013.

Figure 3

Numbers of deaths and percent mortality from P. falciparum malaria in Cambodia from 2004 to 2013.

Figure 4

Geographical distribution of P. falciparum and P. vivax malaria in Cambodia in 2004 and 2013, shown as API per 1000 population.

Figure 5

Reference map of Cambodia showing provinces and neighbouring countries.

Figure 6

Numbers of cases of P. falciparum and P. vivax in each OD in 2004 compared to 2013. The bold diagonal line indicates no change from 2004–2013, points above it represent a decrease in cases from 2004 to 2013 and points below it an increase. It can be seen that in most ODs, P. falciparum has decreased and P. vivax increased.

Figure 7

Number of cases of P. falciparum and P. vivax malaria and proportion of tests positive for malaria in selected regions of western Cambodia. A west, B Pailin Province, C Battambang Province, D Pursat Province, E the Thai-Cambodian border (excluding Pailin) and F Kampot & Kampong Speu Provinces.

Figure 8

Number of cases of P. falciparum and P. vivax malaria and proportion of tests positive for malaria in selected regions of northern and northeastern Cambodia. A northeast Cambodia, B Rattanakiri Province, C Kratie Province, D Steung Treng Province, E Sen Monorom Province, F Preah Vihear OD, G the Vietnam-Cambodian border and H the Laos-Cambodia border.

Figure 9

Malaria control activities and monthly numbers of cases in Cambodia. A ACT courses distributed per year. B proportion of households in high risk areas with ITNs.

Figure 10

Number tested for malaria over time in HIS and in VMW data. A Individuals tested for malaria and B positive cases over time in HIS (solid lines) and HIS plus VMW data (dotted line) with % of villages in Cambodia with a VMW (green line).

Figure 11

Coverage with VMWs by OD in Cambodia from 2003–2013.

Figure 12

Additional monthly malaria cases detected by VMWs (MIS) in Cambodia by species. A P. falciparum and B P. vivax.

Figure 13

Malaria cases and deaths over time in villages with and without VMWs. Cases reported through HIS in ODs A with and B without VMWs. C % mortality in ODs with and without VMWs.

Figure 14

Additional monthly malaria cases detected by VMWs in Pailin by species. A P. falciparum and B P. vivax and C number tested for malaria and % of tests positive.

Figure 15

Additional monthly malaria cases detected by VMWs in the northeast of Cambodia by species. A P. falciparum and B P. vivax and C numbers of positive tests and % of tests positive.