This study reports on the successful implementation of an intervention comprising mass radical treatment for P.vivax of a group of 31 individuals from India, resident and working at a factory construction site in a previously malarious area of Sri Lanka. The purpose of this intervention was to mitigate the risk of malaria re-establishment in Sri Lanka. The risk was proven by the occurrence of a P.vivax infection among one member of this group of foreign workers (an index case), and onward transmission from that infection to a Sri Lankan national leading to the first case of introduced malaria in Sri Lanka 6 years after malaria elimination . The intervention of MRT was warranted on the grounds that 1) there remained 31 other members of the group of foreign workers who originated from the same region of India as the index case who, therefore, may have also harbored latent hepatic stage malaria parasites or even sub-clinical and sub-patient blood infections. These could manifest as clinically and parasitologically patent infections at a later time, as did the index case. 2) This coupled with the fact that the area where they were resident had a high prevalence of the primary vector of malaria in Sri Lanka, constituted a risk for resumption of transmission. Even the extremely thorough surveillance and case management system for malaria in place in the country at present [1, 10] and the vector control measures that were implemented at the factory site may not have been sufficient to mitigate the risk of reintroduction because of rampant population movement in the country, as testified by the occurrence of the introduced case.
Apart from alleviating the risk of transmission in the host country, ethical grounds for mass treatment of such groups of migrant labour or refugees, are, we believe, substantial, given the benefits that would accrue to the treated individuals. If there is evidence that a group of individuals originates from a malaria endemic region of a country, and thus are at a moderate to high risk of developing a malaria blood infection while they are in the recipient country, the individual benefits of mass treatment of the group are likely to exceed the risks of treatment, for the following reasons: The stress induced by travel to the country of work, and the often arduous work they have to be engaged in thereafter, increases, considerably, the chances of developing a clinical malaria infection from dormant parasite stages, on arrival in the country. This is supported by the fact that several of the imported malaria cases in Sri Lanka in the past few years have developed malaria within a few days or weeks of their arrival in the country, although they have been malaria-free for many months and even a year prior to arrival. Secondly, if they do fall ill, being foreign citizens, they may not have the same degree of access to diagnosis and treatment facilities in the recipient country as do the nationals. Unless the employing company or agency provides them with a sound health care cover, which is often not the case they may in general not be able to have ready access to malaria diagnosis and treatment. Sri Lanka recently adopted a policy of providing diagnosis and treatment for malaria as in-patients to foreign persons free-of-charge at any government health institution, making an exception to the rule that foreigners are not otherwise eligible for free health care in the government sector. Furthermore, the recently adopted National Migration Health Policy of Sri Lanka  allows for the provision of a defined package of health services to legally imported labour, but illegal migrant labour groups which are many, are not covered by this policy. We contend, therefore, that mass treatment of migrant labour or refugee groups may be justifiable on the grounds that the benefits to the individual are likely to exceed the risks, provided that the chances of them being infected with malaria can be established reasonably well. A history of residence in a malaria endemic country or region, may not, by itself, be a sufficient indication that individuals in a group are likely to harbor a latent malaria infection. The Anti Malaria Campaign is therefore, currently working on validating a serological assay that could serve as a more reliable marker of the degree of exposure of a group of individuals to malaria prior to arrival.
Although in the particular situation described here, MRT was executed and was well tolerated by the individuals with no occurrence of serious adverse events, the procedure faced many operational and technical challenges. Tools to screen for G6PD activity prior to treatment with PQ, the only registered anti-relapse medicine today, were not optimal. Not only were the results inconclusive in a high proportion of the RDTs performed, nearly a third of the results were discrepant between the two tests. These were the only tests that were available and feasible to use in this situation, and neither were quantitative. Therefore, using the approach of an abundance of caution, almost a quarter of the group of individuals had to be excluded from PQ treatment on the grounds that they tested positive for G6PD deficiency on either or both tests. Until a more reliable, quantitative and affordable point-of-care test is available for use in such situations, the strategy of mass treatment of groups at high risk of developing P.vivax malaria may not be feasible in most POR settings.
Preparations for the mass treatment undertaken here were extensive. They included comprehensive planning, communication with the individuals and their employer on the purpose, benefits and risks of treatment, and taking elaborate measures in a remote area of the country to detect and deal with any life-threatening adverse events by way of haemolysis that could have occurred as a result of PQ treatment, despite screening for G6PD activity, albeit with less-than-optimal qualitative tests. All these procedures were costly in terms of time, effort and commodities. It is difficult to imagine that such an intervention can be undertaken on a scale, wide enough to mitigate the risk imposed by the large numbers of migrant labour groups present in the country.
Surveillance, both parasitological and entomological is one of the key strategies used by the AMC to maintain zero transmission since the elimination of malaria in 2012 . With 378 imported malaria cases being diagnosed between 2013–2019, the country remains vulnerable to the re-introduction of malaria. The AMC has thus far been successful in detecting high-risk groups which include foreign labour, asylum seekers from South Asia, fishermen from Sierra Leone , security forces personnel returning from UN Peacekeeping missions in malaria endemic countries [13, 14] and war-displaced Sri Lankan returnees from India and providing treatment to those who are positive. The AMC and Regional Malaria Officers are continuously engaged in identifying new groups of individuals who could re-introduce malaria to Sri Lanka and screening them periodically for malaria. Mass screening and treatment is a costly and labour intensive procedure and not an entirely effective one either, because latent malaria infections are not detected by blood screening, and could, as experience has shown, manifest at a later time. Passive case detection of malaria has also been strengthened in the country, yet it faces major challenges such as delays in diagnosis, owing to malaria being a rare disease in the country [2, 15, 16].
The role of Mass Drug Administration (MDA) in malaria elimination has been given thorough consideration, with policy recommendations from the World Health Organisation (WHO)  and also subjected to extensive debate [18, 19]. A distinction has to be drawn between MDA for malaria elimination and the intervention of mass radical treatment reported here, in that it was used here as a means of preventing the resumption of transmission in an already malaria-free setting and not as a tool for elimination. MRT was undertaken here to address a situation which increasingly faces countries in the tropical belt particularly, but not confined to Asia, which are now eliminating malaria. This is that with massive developmental projects underway in most countries, there is rampant human movement, largely by way of migrant labour between these countries. Those countries in Asia that are now eliminating malaria such as Timor Leste, Bhutan and Nepal are in the tropical belt and they remain highly vulnerable to the re-establishment of malaria due to the presence of malaria vectors, and groups of migrant labour who move freely between them and their neighbouring countries (Indonesia and India respectively) with ongoing transmission [20–22]. They often reside in the recipient country for several months offering a continuous source of parasites. It has been the experience in Sri Lanka during the past six years since elimination, that many imported malaria patients in such groups and even international travellers and refugees, who have often been screened for malaria at various points in time after arrival in the country, have reported negative on screening but developed malaria infection and illness subsequently. This is to be expected given that, particularly in the case of P.vivax and P.ovale, dormant hepatic stages which can later lead to blood infections cannot be detected by screening. More proactive measures to mitigate the risk of re-establishment of malaria from migrant labour and refugee groups beyond active and reactive screening are very much needed and may not be optional strategies in countries which are now eliminating malaria. Although they are expensive and labour-intensive such measures would still cost the country far less than if malaria were to return. Hindsight of Sri Lanka’s experience in the 1960’s  when the disease resurged after near elimination, and persisted for 50 years costing the country dearly in terms of health and human development is testimony to this argument.
Mitigating the risk of re-establishment of transmission is a serious problem in countries in the tropics that eliminate malaria, at least until entire regions become free of malaria . This calls for greater efforts to eliminate malaria from regions in order to make country elimination efforts sustainable and worthwhile.