Short Report: Forest Malaria in Central Vietnam Nguyen Hong Sanh, Nguyen Van Dung, Nguyen Xuan Thanh, Trieu Nguyen Trung, Truong Van Co, and Robert D. Cooper* Institute of Malariology, Parasitology, and Entomology, Qui Nhon, Vietnam; Military Preventive Medicine Centre, Ho Chi Minh City, Vietnam; Military Institute of Hygiene and Epidemiology, Hanoi, Vietnam; Australian Army Malaria Institute, Enoggera, Queensland, Australia Abstract. Studies were conducted in a village in central Vietnam to explain the existence of a forest malaria cycle of transmission external to the village. The findings suggested no malaria transmission in the village because of the absence of a suitable vector, but suggested evidence for transmission in villagers when attending garden plots in the forested hills surrounding the village. A sizeable population residing near these garden plots, the presence of Anopheles dirus (a highly efficient vector), and a degree of malaria immunity within the inhabitants created suitable conditions to sustain malaria transmission outside the village. Anopheles dirus s.l. is the primary vector of malaria in South- east Asia (Bangladesh, Burma, Thailand, Cambodia, Laos, and Vietnam). 1 Its exceptional longevity, anthropophilic nature, and carriage of human malaria parasites make An. dirus an important vector species. Additionally, its exophilic behavior makes it difficult to control by conventional intervention measures such as indoor residual spraying and insecticide- treated nets. 2–5 Such characteristics make An. dirus a danger- ous vector even at low densities. This mosquito species is associated with forested hills, which include native rainforests and secondary regrowth or rubber or fruit tree plantations that provide shade for its pre- ferred breeding sites. 2,5,6 The combination of An. dirus, Plas- modium spp. parasites, and forested hills defines the term forest malaria, which has been used to describe transmission of malaria in communities living in or near forested hills that provide suitable habitats for An. dirus. 4,5 In these communi- ties, stable malaria transmission is more dangerous and diffi- cult to control than in communities not associated with for- ested areas. 7 In Vietnam, the terms internal and external malaria are used to differentiate transmission that occurs in the village (internal) compared with malaria infections that are acquired outside the village (external) and are brought back into the village. This terminology is related to forest malaria because it is believed that persons from the village go into the forest for various activities (food gathering, hunting, crop cultiva- tion, timber cutting, gem mining), become infected with ma- laria because An. dirus is present in the forest, and bring malaria back into the community. There is still this association between forest, An. dirus, and malaria, but the original concept of forest malaria has been modified and is now applied to persons who do not live in (or near) the forest but who visit it for various reasons and for varying periods. However, little is known as to how the malaria cycle is maintained in the forest. 7 The role of An. dirus is such that any communities that have high levels of malaria transmission should have an association with this spe- cies and the forest, but too often there is only a vague asso- ciation between persons, the forest, malaria, and An. dirus. 8 Erhart and others 9 associated sleeping overnight in the forest while cutting timber and gardening with a statistically signif- icant increase in the risk of acquiring malaria. However, how these activities, particularly those as transient as timber cut- ting, provide a stable host population that supports transmis- sion of the parasite is not known Because of the lack of any direct evidence demonstrating how malaria transmission can be maintained in populations moving into and out of the forest, we evaluated forest malaria in a village in south central Vietnam. Dong Thong village in the Phuoc Chien Commune is lo- cated in Ninh Thuan Province. The land surrounding the village is largely unsuitable for crop cultivation and is used mainly for grazing cattle and goats. Farmers practice slash-and-burn clearing of the surrounding hill sides to grow staple crops such as corn and cassava. Each of the five villages in the Phuoc Chien Commune has a designated crop-growing area in the surrounding hills. Erhart and others 10 showed that villagers in Dong Thong had a high rate of malaria parasite positivity (32.1% in November–December 2003), and villagers who slept overnight in the forested hills while tending their gardens were associated with a significant increase in malaria infection risk. These workers composed 23.3% (1,003 of 4,306) of the study population, which included inhabitants from Dong Thong who spent nights in the forest. The median number of nights spent in the forest was 12 per month (range  1–30 nights). This information raises two questions. First, would this situ- ation create sufficient human/vector contact to maintain the malaria cycle? Second, do persons who go into the hills exist as a community or are they scattered in isolated groups throughout the hills? Bearing in mind that the flight range of An. dirus is approximately 1.5 km, 6 one would expect that a large human population within close proximity over sufficient time would be needed for malaria transmission to be main- tained. In September–October 2006, we conducted a malaria sur- vey in Dong Thong village (total population  1,153). The rate of parasite positivity was 15.5% (48 of 310: 8 persons 0–5 years of age, 12 persons 6–14 years of age, and 28 persons > 15 years of age) on the basis of a mass blood survey and thick film microscopy. We also conducted a night biting mosquito catch (from 6:00 PM to 6:00 AM, with one indoor and two outdoor collectors collecting for 50 minutes each hour) and cattle shed collec- tions in which for five nights anophelines were collected from three cattle sheds (all located within the village) over a * Address correspondence to Robert D. Cooper, Australian Army Malaria Institute, Gallipoli Barracks, Enoggera, Queensland 4051, Australia. E-mail: bob.cooper@defence.gov.au Am. J. Trop. Med. Hyg., 79(5), 2008, pp. 652–654 Copyright © 2008 by The American Society of Tropical Medicine and Hygiene 652