The study in this thesis was to follow the guidelinesof the Individual Training Grant received from the Special Programme for Research and Training in Tropical Diseases (TDR) under the framework of the Institute Strengthening Grant given to Chiang Mai University, Thailand. This was aimed strengthening /esearrh abilities in the field needed for improving the prevention and control of six TDR- targeted tropical diseases, including malaria, in Northern Thailand.
2.2 Northern Thailand.
Most of the areas in this region are mountainous with many types of forest covering about 47% o f the total area. It has the second poorest socioeconomic status in the country after the northeast region. There are limited fertile lowland areas for growing agricultural crops. Large numbers of the rural population have to earn their living from growing upland crops, trading or becoming farm/non-farm wage workers (Singhanetra-Renard, 1986). The control of malaria in northern Thailand is under the responsibility of the Malaria Centre of Region II in Chiang Mai, covering 8,053,613 population in 13 provinces in the North (Malaria Division, 1991).
2.2.1 Mortality and morbidity
In 1949, before DDT spraying, a survey in Chiang Mai found about a 90% spleen rate. This sharply declined to only 1% in 1959 (WHO/SEA, 1960). In 1987, the malaria mortality w as 3.1 per 100,000 population (Table 2.1). It decreased to 2.1 in 1990. The annual parasite incidence (API) was 3.65 in 1986, increased to 3.84 in 1987, decreased to 3.20, 2.74 and 2.64 in 1988,1989 and 1990 respectively.
The high malaria mortality and morbidity in the region have been confined to the border areas with Myanmar (Figure 1.3 and 1.5).
2.2.2 Parasitology
About 58% of cases were falciparum malaria in 1986. This increased to 67% in 1990. P.vivax. P.malariae and mixed infections were 32%, 0.1% and 0.8%
respectively in 1990 (Malaria Division, 1991).
Table 2.1 Malaria mortality and morbidity in the north, 1986-1990.
P o p u la tio n B l.e x a m . C ase s % A B E R % S P R A PI M R .
1986 7 ,7 6 6 ,4 4 6 2 ,1 5 0 ,9 4 4 2 8 ,3 7 9 2 7 .7 1.32 3.7 n .a .
1987 7 ,8 9 0 ,7 8 0 2 ,4 7 9 ,1 2 7 3 0 ,2 7 9 3 1 .4 1.22 3.8 3.1
1988 7 ,9 5 8 ,8 3 0 2 ,4 7 8 ,4 7 1 2 5 ,4 7 3 31.1 1.03 3.2 2 .6
1989 8 ,0 3 5 ,2 1 1 2 ,3 5 9 ,7 7 7 2 1 ,9 9 8 2 9 .4 0.93 2.7 2.1
1990 8 ,1 2 2 ,2 7 4 2 ,2 5 3 ,8 6 1 2 1 ,4 5 5 2 7 .8 0.95 2 .6 2.1
%ABER - Annual blood examination rate (%)
%SPR - Slide positive rate (%)
API - Annual parasite rate (cases/1,000 persons/year) M.R. - Mortality rate (deaths/100,000 population/year)
2.2.3 Entomology
Before starting DDT spraying, the major vectors in the north were found to be an endophilic and endophagic An.minimus (Ministry of Public Health, 1957). In 1959, an independent appraisal team from WHO came to evaluate the situation at Chiang Mai province in the north. They reported the satisfactory eradication of An.minimus in the plain areas by DDT spraying (WHO/SEA, 1960).
However, they found transmission still occurring in the hill villages and explained this by the number of An.minimus and An.dirus (known as An.balabacensis then) found.
Later, the role of An.dirus. in malaria transmission in forested areas, was confirmed in eastern Thailand (Scanlon and Sandhinand, 1965).
In the forest and forested fringe areas of Pitsanulok province, northern Thailand, Ismail et al (1974, 1975, 1978) reported the effects of DDT spraying on malaria transmission. They found that transmission was perennial.
An.dirus was maintaining transmission all the year round inside the deep forest; and
31
only in the monsoon season in the forest fringe and its surrounding area. An.minimus was maintaining transmission during the dry season, late in the year, and appeared to be more exophagic than An.dirus. but both seemed to be exophilic. The vectorial capacity of, as well as the impact of DDT spraying on, An.dirus was higher than that of An.minimus. DDT spraying alone, though it had a considerable effect on the reduction of the vectorial capacity of the vectors, was not enough to interrupt transmission in the area. After DDT spraying, both vectors bit earlier, especially An.minimus during the dry season, and more outdoors than before the spraying.
Forest clearance seemed to lead to the disappearance of An.dirus and favoured the prevalence of An.minimus. With the development of cleared areas in the foothills, An.minimus alone was expected to maintain transmission, though at a much lower level.
However, 10 years later, Baimai et al (1988) reported the geographical distribution of the An.dirus complex in Thailand. It was noticed that the vector varied both in numbers and types of species throughout the country. In the north, the prevalence of An.dirus along the Thai-Myanmar border were less than those areas eastward. This confirmed the finding by Harbach et al. in 1987, reporting the predominance of An.minimus and the scarcity of An.dirus in the mountain of Tak province, close to Thai-Myanmar border. An.maculatus was also common but no sporozoites were found in it. They concluded that An.minimus may be the major vector in the area.
2.2.4 Sociological aspects
Population movements were common in the north, both within and across the forest border areas (Singhanetra-Renard, 1986). These are due to socioeconomic reasons and political conflicts. Villagers in the area are used to moving into the forest hilly areas for growing upland crops. Many socioeconomic development projects in the area also cause a high level of labour movements, from both within the country and abroad, into the area. When there is fighting along the Thai-Myanmar border between ethnic minorities and the Myanmar government, many refugees flee into Thai territory.
32
These movements have resulted in a high malaria incidence, due to three main factors.
1) The exposure of immigrants to malaria vectors in forested areas, especially at night.
2) Contact between infected Burmese, Karen or Shan from across the border with uninfected, non- or semi-immune people inside Thailand.
3) Control measures designed for the general population are not effective among highly mobile populations engaging in illegal activities.
As described above, the malaria problems in the north are located at Mae Hong Son province close to Thai-Myanmar border. Hence, the southern part of the province, i.e. Mae Sariang district and Sob Moey subdistrict, were chosen for the studies. Below are some details of Mae Hong Son province and the study area.
2.3 Mae Hong Son province
The province is situated between 17 - 20 degree» north and 97 - 99 degreeieast. It ha<i about 167,000 population in 1990, spread over the total 13,184 square kilometres of forest hilly areas. Only about 1% of the area is used for agricultural purposes. The main crop is rice. There are 5 districts and 1 subdistrict, spread along the Thai-Myanmar border. It borders with Myanmar in the north and west, with Chiang Mai province in the east and with Tak province in the south (Figure 2.1). There are flights from Chiang Mai or Bangkok to Mae Hong Son and back everyday. Apart from this, the way to reach the province is by road, from Chiang Mai and Tak province. It is about 400 and 900 kilometres from Chiang Mai and Bangkok respectively. Before 1991, there was only one road from Chiang Mai to the main town of Mae Hong Son. It takes about 8-9 hours, passing through the Mae Sariang district in the south of the province. Since 1991, a new road from Chiang Mai, passing through the north of the province, has been completed, and it takes about 5 hours travelling time. The altitude of the main towns is about 211 - 267 metres above the sea level. Most of the people in this province are of the Karen ethnic minority. Their socioeconomic status is poor, although the area is rich in mineral deposits of wolfram, manganese, fluorite and tin, as well as forest products.