Border malaria associated with multidrug resistance on Thailand-Myanmar and Thailand-Cambodia borders: transmission dynamic, vulnerability, and surveillance

Abstract

This systematic review elaborates the concepts and impacts of border malaria, particularly on the emergence and spread of Plasmodium falciparum and Plasmodium vivax multidrug resistance (MDR) malaria on Thailand-Myanmar and Thailand-Cambodia borders. Border malaria encompasses any complex epidemiological settings of forest-related and forest fringe-related malaria, both regularly occurring in certain transmission areas and manifesting a trend of increased incidence in transmission prone areas along these borders, as the result of interconnections of human settlements and movement activities, cross-border population migrations, ecological changes, vector population dynamics, and multidrug resistance. For regional and global perspectives, this review analyzes and synthesizes the rationales pertaining to transmission dynamics and the vulnerabilities of border malaria that constrain surveillance and control of the world's most MDR falciparum and vivax malaria on these chaotic borders.

Figure 1

Complex epidemiological settings of border malaria. (a) A hilly 2202 km long Thai-Myanmar border where most investigations center on epidemiology, surveillance, and monitoring of MDR-associated BM in Tak province, Thailand, as one important area of studying MDR malaria in Southeast Asia region. (A) Numerous refugees revisiting either refugee camps temporarily or nearby pocket villages outside camps play important role in malaria transmission. (B) Schoolchildren including 12-year-old boy infected with P. vivax are at risk of malaria infections in endemic villages. (C) Breeding site of potent malaria vector, Anopheles minimus, is commonly found in endemic villages with irrigation and agricultural practices. (b) Similar to the endemic settings on or surrounding Thai-Myanmar border, a 798 km long Thai-Cambodia border where human settlements have extended to pocket villages with agricultural intensifications on plantations of rubber trees and fruit orchards. (A) Border crossings at immigration checkpoints are easier for migration of cross-border people due to geographical uplands, hills, hillside slope areas, and valleys. (B) Myanmar migrant workers including 32-year-old rubber plantation worker infected with P. vivax are at risk of malaria infections in endemic villages; both Myanmar and Cambodian migrant workers play key role in border malaria transmission. (C) Breeding site of potent Anopheles dirus and Anopheles maculatus is commonly close to human inhabitations with fruit orchards or rubber plantations.

Figure 2

Trend of foreign migrant workers (Myanmars, Laos, and Cambodians) subject to the registration of work permits, according to provincial prorata demands in Thailand, 2001–2009. Regarded as the Section 13 of the Working of Alien Act, B.E. 2551 (2008), these cross-border migrant workers as illegal immigrants can apply for the engagement in officially permitted works as notified with regard to national security and social impacts in the government gazette by the Council of Ministers. Among foreign migrant workers, dynamic movements of Myanmar migrant workers are likely to be forced by some push effects. Data were modified from the Office of Foreign Workers Administration, Department of Employment, Ministry of Labor, Thailand (http://wp.doe.go.th/).

Figure 3

BM transmission dynamics and vulnerability. (A) Any cross-border person carrying malaria infection during an incubation period is exposed to multiple bites of Anopheles vectors at multiple locations on or close to the border due to occupational and/or behavioral risks and, vice versa, can spread malaria during a prodromal period until seeking treatment. (B) In MDR-associated BM setting, it is possible that any cross-border person carries geographically prone MDR malaria that can be epidemiologically linked to the ecotypes of Anopheles vectors carrying geographically prone MDR malaria in certain transmission areas on or close to the border.