Global malaria importations
Authors: Karin Leder, Lin H Chen
Journal: The Lancet Infectious Diseases (2016)

In The Lancet Infectious Diseases, Andrew J Tatem and colleagues1Tatem et al.The geography of imported malaria to non-endemic countries: a meta-analysis of nationally-reported statistics.Lancet Infect Dis. 2016; (published online Oct 21.)http://dx.doi.org/10.1016/S1473-3099(16)30326-7Google Scholar describe the global assembly of nationally reported imported malaria statistics over 10 years (2005–15), describing broad geographic patterns of malaria importation. This study marks the first worldwide compilation of data for imported malaria in 20 years. Tatem and colleagues aimed to quantify international patterns of malaria movements from endemic to non-endemic areas looking at case numbers, worldwide connectivity, and species distributions. To achieve this, the researchers used publicly available nationally reported statistics on imported malaria from 40 non-endemic countries, comprising more than 50 000 individual cases of malaria. The researchers aggregated cases exported from every endemic country, examined networks of connectivity between countries, used modelled parasite prevalence maps from the Malaria Atlas Project, and obtained data for migrations between each pair of endemic and non-endemic countries to permit comparisons to be made with the imported malaria statistics.2Moyes CL Temperley WH Henry AJ Burgert CR Hay SI Providing open access data online to advance malaria research and control.Malar J. 2013; 12: 161Crossref PubMed Scopus (35) Google Scholar Unsurprisingly, the investigators noted that infection movements were highly heterogeneous, reflecting high traffic travel routes that have been established as a result of air networks, and historical, economic, language, and cultural ties. For example, the west African region accounted for 56% of all imported cases to non-endemic countries, especially to France, the UK, and the USA. Other notable connections were recorded between the USA and India, the USA and Haiti, Australia and Papua New Guinea, and the UK and Pakistan. They also noted strong spatial clustering of malaria species, with Plasmodium falciparum dominating African and Caribbean sources whereas Plasmodium vivax dominated cases originating in central and south America, Asia, and Oceania. As the researchers state, increased air travel means that geographical distance is less of a barrier to pathogen dispersal, with a potential effect both on the spread of drug-resistant parasite isolates and on the goal of global malaria eradication. Therefore, it is ever more important to understand importation patterns to prioritise mitigation strategies. Although the results are fairly intuitive and unsurprising, the relative novelty of this study comes from the fact that it used robust methods to overlay endemic malaria data with travel and migration patterns and species breakdowns to provide a contemporary overview of malaria importations. The method used could be applied to analyse other infectious disease importation and predict likelihood of disease acquisition. Several factors can explain the patterns recorded in cases of imported malaria, including the transmission intensity in locations visited, the number of travellers going to each area, demographic and ethnic background of travellers, activities undertaken during the trip, type of accommodations, and adherence to prophylaxis among visiting populations.3Freedman DO Chen LH Kozarsky PE Medical considerations before international travel.N Engl J Med. 2016; 375: 247-260Crossref PubMed Scopus (80) Google Scholar The scale-up of malaria intervention, urbanisation, increasing wealth, and better health systems in the past decade in malaria-endemic regions have probably contributed to changing prevalence and species composition, making comparisons between years difficult. Additionally, nuances in traveller's exposure might reveal differences in risks of malaria acquisition. However, assessment of the driving factors behind the patterns in malaria notifications and describing any trends over time were beyond the scope of the study. Despite limitations of national notifiable disease surveillance data in terms of incomplete case capture, variations in health system reporting mechanisms, and differing diagnostic capacities, data for imported cases can provide valuable information about the epidemiology of malaria in endemic regions where surveillance systems are suboptimal. In recognition of this role, national notification data can now be supplemented by specific traveller surveillance networks, the largest of which is GeoSentinel, a network that uses specialised travel and tropical medicine clinics to diagnose and report on imported illnesses reported in travellers to detect geographical and temporal trends.4Leder K Torresi J Libman MD et al.GeoSentinel surveillance of illness in returned travelers, 2007–2011.Ann Intern Med. 2013; 158: 456-468Crossref PubMed Scopus (333) Google Scholar Compilation of various data sources, including the notifiable statistics analysed by Tatem and colleagues, recognises that travellers can serve as sentinels to provide information about disease activity in areas visited. Furthermore, travellers can be crucial in the spread of infectious diseases, posing a potential risk to areas that have the anopheles vector but have eliminated endemic malaria. Additionally, assessment of aggregated data compiled from notification and surveillance activities offers up-to-date epidemiological information about disease activity, which can be used to refine appropriate pre-travel preventive advice and to prioritise diagnostic testing when assessing the ill returned traveller. KL is a member of the GeoSentinel leadership group, and both KL and LHC are GeoSentinel directors. LHC is also advisor to Shoerland Inc. The geography of imported malaria to non-endemic countries: a meta-analysis of nationally reported statisticsThe architecture of the air network, historical ties, demographics of travellers, and malaria endemicity contribute to highly heterogeneous patterns of numbers, routes, and species compositions of parasites transported. With global malaria eradication on the international agenda, malaria control altering local transmission, and the threat of drug resistance, understanding these patterns and their drivers is increasing in importance. Full-Text PDF Open Access