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Hed. Para ZIT. (MOSKVA), v. 30, pp 643-51. 1961 

(E.I. Martsinovskiy Institute of Medical Parasitology and 
Tropical Medicine, Min. Health USSR, and Institute of 
Malaria, Min. Health, Dem. Rep. Viet Nam) 

STUDIES IN MALARIA EPIDEMIOLOGY IN NORTH VIEJf NAM. 2. TOPOGRAPHICAL 
MALARIOLOGICAL EXPLORATION OF THE THAI MEO AUTONOMOUS REGION (Russian) 

by A. Ya. Lysenko and Nguyen Tien Byu 
(submitted 20/N>. //bl) 







TEOxiISriOA.T-i TRANSLATION 


i 

Distribution of this document is unlimited ! 



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Med. Para ZIT. LM0SKVA), v. 30, pp 643-51, 1961 

(E.I. Martsinovskiy Institute of Medical Parasitology and 
Tropical Medicine, Min. Health USSR, and Institute of 
Malaria, Min. Health, Den. Rep. Viet Nam) 

STUDIES IN MALARIA EPIDEMIOLOGY IN NORTH VIET~NAM. 2. TOPOGRAPHICAL MALARI0L0GICAL 
EXPLORATION OF THE THAI MEO AUTONOMOUS REGION (Russian) 

by A. Ya. Lysenko and Nguyer. Tien Byu 

(submitted 20/Nov/61) 





In comparison with the other provinces of North Viet Nam, the autonomous 
district of Thai Meo which occupies the northwestern, most-alpine part of 
the country (Fig. 1) was explored by malariologists only recently and on a 
very limited scale. 

In Tumanov's monograph on the distribution of the anopheles mosquito in 

Nam (1936), there are no indications for an epidemiological exploration of 
the huge Ihai Meo territory. Pons (1943), while studying the malaria problem 
in North Viet Nam in the ethnographic aspect, pointed to some interesting features 
specific to the spread of malaria in Thai Meo but without sufficient factual 
data. In October, 1956, a detachment of Soviet specialists head by docent G.A. 
Pravikov examined about 2000 inhabitants of the city of Tuan Chou (the center 
of the district) and two villages in its environs; in addition, entomological 
collections were also made. 

The absence of circumstantial data on the distribution of the Anopheles and 
malaria in Thai Meo and also the doubtfulness of the pronouncements by Pons 
concerning the features specific to the populations of this district (Thai, Meo, 
et al.) with respect to their sensitivity to malaria aroused us to undertake a 
reconnaissance investigation of the villages situated along the road from Moc 
Chou to Myong Lai and from Tu?n Ziao to Dien Bien Phu. In this work we found 
3 topographical-malariological zones on Thai Meo territory two of which, mountain- 
river and alpine are similar to the corresponding zones in the Thai Nguyen pro¬ 
vince. (see the report by A. Ya. Lysenko, Dang Van Ngy, Ho Van Hyu, Dang Tung 
Tha, Med. Parazitol. i parazitarn. bol., 1961, No. 3). One is flat-mountainous 
and was identified by us first of all. For purposes of determining more accu¬ 
rately the boundaries of these zones and to make a more penetrating study of 


1 







them, in July and October of 1958, an extensive investigation of the population of 


Thai Meo was undertaken according to a program and along routes especially de¬ 


veloped by us. The investigation was performed by a group of medics from the 


Institute of Malaria of the Ministry of Health of the DRV and the District De¬ 


partment of Public Health (group leader Nguyen Tien Byu), 


ROUTES, TECHNIQUES AND SCOPE OF THE EXPLORATION 


The settlements of the inhabitants of Thai Meo are situated chiefly in the 


narrow valleys along the rivers Song Da and Song Ma and in the path formed by a 


limestone plateau between the ridges Sipsong Cho Thai on the west and Hoang Lien 


Chon and Sa Fin on the east. The river valleys, characterized by a hot climate. 


are inhabited almost exclusively by the Thai nationality, while the poorly 


watered cooler plateaus are inhabited predominantly by the Meo nationality. The 


mountain slopes along the sides of the valley and the inter-oountain synclines 


(Ngia Lc, Cuang Guy, et al.) are inhabited by the other nationalities of the 


district: Myong, da, Man. The routes followed by the exploration were narked 


out in such a way that they passed through inhabited points situated in all of 


the basic topographical areas of Thai Meo (sea Fig. 1). 


The technique of the explorations W3S basically the same as was mentioned 


in our first report. However, special attention was given to the collection 


of anamnestic data among the inhabitants of Meo nationality. During a period 


of 3 months it was possible for us to investigate the inhabitants of 299 villages 


belong to 65 communities (9 of 13) from the different regions of the district 


(table 1). 


During the course of the investigation, a study was made of the spleens of 


28,908 persons (8.3% of all the inhabitants of the district) and thick-drop blood 


i 




Fig. 1. Hypsometric map of the autonomous district of Thai Meo and the routes 
of the exploration. 


Table 1. Number of inhabitants of different nationalities (ethnic groups) covered 
by the investigation. 


Number Number of 


of Inhabitants 

Nationality Villages Studied 

Meo. 12 080 

Thai. > 8 38? 

Sa. ? 2 205 

Man. 1 988 

Myong. 13 1 396 

Total... 248 26 056 


3 












preparations were made from 28,647. 7102 Anopheles mosquito individuals were 
captured in the dwellings and identified by species. In view of the fact that 
it is difficult to find the site and elevation of the location of some of the 
villages covered by the investigation, data from only 248 villages of the 299 
were processed, (table 2). 

Table 2. Frequency of encounter of A. vagus, A. minimus, and A. jeyporiensis 
as affected by the elevation of the villages studied 


Elevation of 
situation of 
the village, 
m above 
sea level 

Total number 
cf villages 
studied 


of them, with the 

presence of 


A. vagus 

abs. 

number 

% 

A. minimus 
abs. % 
number 

A. jevporiensis 
abs. % 

number 

100 

7 

7 

100.0 

5 

71.4 

1 

14.3 

200 

8 

7 

87.5 

7 

37.5 

5 

62.5 

300 

29 

22 

' 76.0 

28 

96.5 

9 

31.0 

400 

21 

17 

81.0 

17 

81.0 

3 

14.3 

500 

16 

9 

56.6 

10 

62.5 

0 

0 

600 

10 

7 

70.0 

6 

60.0 

I 

10.0 

700 

o 

4 

44.4 

6 

66.6 

1 

11.1 

SOO 

12 

7 

53.3 

3 

25.0 

1 

8.3 

900 

23 

9 

39.1 

J5 

65.2 

1 

4.3 

1000 

18 

12 

66.6 

7 

38.8 

2 

11.1 

1100 

25 

12 

48.0 

4 

16.0 

1 

4.0 

1200 

15 

5 

33.3 

5 

33.3 

1 

6.7 

1300 

8 

1 

12.5 

0 

0 

0 

0 

1400 

9 

2 

22.2 

2 

22.2 

2 

22.2 


4 





Table 2 (cont’d) 


Elevation of 
situation of 
the village, 
m above 
sea level 

Total number 
of villages 
studied 


of 

them, with the 

presence 

of 


A. vagus 

abs. 

number 

% 

A. minimus 

abs. 

number 

% 

A. jevpori 

abs. 

number 

ensis 

% 

1500 

7 

0 

0 

0 

0 

1 

14.3 

1600 

1 

0 

0 

0 

0 

0 

0 

1700 

1 

0 

0 

0 

0 

0 

0 

Total 

219 

133 

60.7 

119 

54.3 

29 

13.2 


RESULTS OF THE INVESTIGATION 

A strict confinement by elevation among the different nationalities of the 
Thai Meo district is to be noted (fig. 2). Different nationalities, even those 
which live on one elevation, select specific micro-terrains corresponding to the 
features specific to their economic activity and mode of life. The basic occu¬ 
pation of the Thai nationality is irrigated rice farming; the villages are rela¬ 
tively large, the houses are big, constructed on poles ("2-story types"), the 
open space beneath the floor of the dwelling is used as a stable for buffaloes 
and in part as a pig sty. The economic and sanitary-hygienic level of the 
dwellings is relatively high; in particular, in most of the houses there are 
canopies (mosquito nets) . The Myong nationality is closest to the Thai nationa¬ 
lity in cultural, level and living habits. They also build 2-story houses and 
use canopies. The overwhelming majority of the population of Myong nationality 
is concentrated in the southern regions of the district. The Thai nationality 
and the the Myong nationality rarely settle at elevations above 500 and never 


5 







** y 




Fig. 2. Distribution of the 
strict as affected by elevatf 
of population, B. elevation, 


'-.nef nationalities of the Thai Meo autonomous dis- 
n of situation of their communities. A. percent 
m; C. Thai, D. Meo, E. Myong, F. Sa, G. Man. 



elevation.m . 


Fig. 3. Abundance of A. vagus (i), A. 
affected by elevation of the location 


minimus (2), and A, jeyforiensis 


(3) as 


om the tops of ridges. On the other hand, the Mec population is never found 
in the valleys below 500 m; the Meo prefer to live on open, well-aerated cool 
territories. Meos are good hunters. Their houses are small with dirt floors. 


The economic and sanitary-hygienic levels of their life is nevertheless low 
They do not use canopies, nor do the Sa and Man nationalities who have been 


6 




distinguished until recently by a particularly low ievel of sanitary culture. 

The Thai, Myong, and Sa nationalities live a settled life. The Man and Meo 
periodically move about. Domestic animals are most numerous among the Thai 
and Myong nationalities. There are very few among the Man and Heo. The Man, 
and particularly the Meo who live at greater elevations in small isolated vil¬ 
lages, often go down into the valleys for trading and sometimes in search of 
seasonal work. 

The entomological findings included 10 species of Anopheles among which A. 
vagus was predominant with 76.5%, followed by A. minimus with 21.4%, and A. 
jeyporiensis with 1.4%. The other species were found less often: A. maculatus 
in 0.16%, A. hyrcanus sinensis and A. h. nigerrinus in 0.15% each, A. tesselatus 
in 0.06%, A. kochi in 0.03%, A. barbirostris and A. philippinensis in 0.015% 
each. The frequency of detection of-1 species of Anopheles mosquito or another 
was unequal at different location elevations (see table 2). 

As table 2 shows. Anopheles are found with the greatest consistency at ele¬ 
vations below 600-700 m; less consistently but quite often at an elevation of 
700-1200 u\, and only in isolated villages at an elevation of 1300-1500 n. The 
abundance of Anopheles also depends on the elevation of the village (Fig. 3). 

The comparative abundance of A. minimus (chief vector of malaria in K. Viet 
Nam) in the houses of different nationalities who dwell at similar elevations 
is a matter of interest. The results of the studies were unified by us for this 
purpose into 2 groups. The first group (Fig. 4) includes villages of all 5 
nationalities- routes No. 2,3,6,7 (mountain locale without flat mountain sec¬ 
tions); the second group (Fig. 5) includes only villages of the Thai, Sa, and 
Meo nationalities: routes No. 1, 4, and 5, including portions of the plateau. 


7 



to. A.-... ...li v. 


< X 


- * 


*> 




As Fig. 4 shows, A. minimus dwells in the houses of all nationalities who 
live at elevations of 100 to 800 n; from 900 n anc above, all the villages of 
this group were free of A. minimus. The greatest population density (above i 
mosquito per day resting-place) was noted in villages at elevations of 200 to 
600 c. The average number of ccsquitoes per day resting place vas greatest in 
the villages of the Kyong and Kan (up 4.6-5.2); it was consistently high (core 
than 1 nosquito per day resting place) in Thai and Sa villages; ia Keo houses, 
there were either vary few mosquitoes (at an elevation of 700-90C a.) or none 
at all (at 1000-1500 m). In the second group of villages (Fig. 5) mosquitoes 
were found at all elevations fro^ 200 to 1000 a inclusively. 



Fig. 4. Abundance of A. minimus (A) and nalaria Infestation rate of the popula¬ 
tion—total and childhood spleen indices (•; ) in villages of the different na¬ 
tionalities Thai and Keo as affected by elevation (first group of villages). On 
the right: diagram of altitudinal distribution cf different topographical zones 
1. average abundance of A. minimus ia bouses at the given elevations; 2. child 
spleen index, group from 2 to 8 years, among Thai, Sa, Hyong, and Kan; 3. child 
spleen index among the Meos. A. Thai, 3. 2a, C. Kyong, D. Han, E- Keo, F. two- 
story houses, G- single-storey houses, E. mountain river zone, I. alpine zone, 

A. elevation, a. 

The greatest average population density was noted in the villages at eleva- 
cions of 300 to 700 u; here an especially large number of mosquitoes was found 
in che houses of the Sa. In the high-situated villages of the Keo, as distinguished 


8 







from the first group, the houses were also inhabited by A. minimus, although 
the density of the population was low—as a rule about 0,5 mosquitoes per day 
resting place. It is a matter worthy of attention that the elevation of 800-900 
m is to some extent critical for A. minimus, in the first group of villages it is 



Fig. 5. Abundance of A. minimus (A) and malaria infestation of the population; 
total and childhood spleen indices (5 ) in the villages of different nationalities 
Thai and Meo as affected by elevation (second group of villages). On the right 
is a diagram of the distribution by elevation of the different topographical zonqs. 
A. Thai, B. Sa, C. Meo, D. two-storey houses, E. one-storey houses, F. elevation, 
m, G. mountain-river zone, H. flat-mountain zone, X. alpine zone. 4. average 
abundance of A. minimus in the houses at a given elevation; 5. child spleen index 
(group from 2 to 8 years) among the Thai and Sa; 6 . child spleen index among the 
Meo. 

the upper limit of the range; in the second it is the beginning of the stable but 

* • 

very low population density of the mosquito. Apparently, at this elevation, the 
breeding grounds of A. minimus either disappear (in the first group of villages) 
or are noticeably curtailed (at the transition to plateau in the second group 
of villages). 

An examination of the population allowed the detection of 11453 inhabitants 
with an enlarged spleen and 1082 with parasites in the bloodstream. In 60.9% of 
the cases, P. falciparum was found; in 37.7% P. vivax, and in only 1.4% P. ma- 


9 




lariae. The malaria infestation rate, judging by the results of the investiga¬ 
tion, was high among all 5 nationalities, and only the Meo revealed a low para¬ 
site index (table 3). 

Table 3. Indices of malaria infestation 

Study of Spleen Study of Blood 

Number of Spleen Number of Spleen 


Nationality Subjects Index Subjects Index 

Sa. 2 339 76.3 2 303 9.7 

Man. 1 986 50.2 19 866 7.8 

Thai. 8 976 43.7 8 874 10.7 

Myong ....... 1 651 37.9 1 629 9.1 

Meo . 12 872 31.4 12 817 2.3 

Total . . 27 826 40.6 27 619 6.4 


As a result of the analysis of the age-specific malaria infestation rate, 
an abrupt qualitative difference in it was found among the Meo in comparison with 
the other nationalities (Fig. 6). The curve of the age-specific infestation 
rate of thr: 3a, Man, Thai, and Myong nationalities has the shape of the classi¬ 
cal curve of the infestation rate in hyperendemic foci: a rapid increase of 
the index in the low cige groups and a gradual decrease in these indices in the 
older groups; the indices of -he aduJU*= at* noticeably lower than the indices 
of the 2-8 year-old group of children. , ae ao nationality, a slow incre¬ 
ment in the spleen index whose maximum is reached only among adults is typical. 
The curve of the age-specific parasite index among the Meo is more constant. The 
magnitude of the indices fluctuates between 1.6 and 4.7%. If in the adult group. 


10 











Age Groups 


Fig. 6. Comparative age-specific malaria infestation rates of the different 
nationalities. I. under 1 year; II. 1-2 years. III. 3-4 years; IV. 5-8 years; 
V. 9-16 years; VI. 17-64 years, a. 1. spleen index of the Sa, 2. spleen index 
of the Man; 3. parasite index of the Man; 4. parasite index of the Sa, b. 1. 
spleen index of the Thai, 2. spleen index of the Myong, 3. parasite index of 
the Thai, 4. parasite index of the Myong, c. 1. spleen index of the Heo, 2. 
parasite index of the Meo. 


such a low parasite index can be explained by the increasing immunity among the 
repeatedly infested inhabitants, then in the younger age group it testifies to 
low opportunities for children to become infested with malaria. This contra¬ 
diction can be explained in only one way: the adults are often and repeatedly 
infested outside of the local foci with a low endemic degree. We find con¬ 
firmation for this statement when we compare the infestation rates of popula¬ 
tions of different nationalities who dwell for some time at different elevations 
with different densities of A. minimus in their dwellings (see fig. 4 and 5). 

In the villages of the first group (see fig. 4) the curve of the malaria 
infestation rate (spleen index of the child group) is kept at a high .level: 
more than 50% below the elevation of 700 m inclusively where the average A. 
minimum density is quite high (1 mosquito per day resting place and more). At 
an elevation of 800 to 900 m where the A. minimus density falls to 0.6 mosquitoes 


11 





KHP A 



( . 5 ) tlsoCKQirpjst it»a D 

mn lMCCrota;.Hai soul & 

Fig. 7. Diagrammatic map of the topographical-nalariological zones of Thai 
Meo. A. China, B. Laos, C. Mountain-river zone, D. flat-mountain zone, E. al¬ 
pine zone. 

• 

per cky resting place, and also above this, where the mosquitoes of this species 
were not found, the infestation race of the Thai, Sa, and Man populations is 
noticeably decreased (spleen index 34%), and the Meo nationality is apparently 
not infested, (index below 10%). Thus, the villages of the first group belong 
to a single endemic mountain-river zone whose elevation limit has a maximum 
at an elevation of 800-900 m. 

Above this is the nonendemic alpine zone (see tig. 4-5). 

In the villages of the second group (see Fig. 5), the population infesta¬ 
tion curve is maintained at a very high level (70% and above) up to au altitude 
of 800-900 m. The Meo who live at this elevation are infested sufficiently in¬ 
tensively but considerably more weakly than other nationalities (spleen index 


12 



26% as opposed to 70%). From this it follows that there is no justification 
for considering the Meo a nationality with any particular susceptibility to 
malaria as Pons suggests. In the all cases when the Meo living at the same 
elevations as other nationalities (see Fig. 4) elev. 600-900 m; see Fig. 5, 
800-100), their infestation rate was relatively lower, but the clinical picture 
of the disease and the character of the parasitemia in the Meo children are 
similar to those seen in children of the other nationalities. 

Although the malaria infestation rate among the Meo nationality is noted 
to an elevation of 1600-1700 m, the local morbidity (detection of parasites 
in the children who had never descended to the villages below) was observed 
to be no higher than at an altitude of 1200-1300 m. Some of the adults who live 
at elevations of up to 1200—1300 and all of those who live above this were 
infested with malaria in the villages situated below during their periodic visits 
to them. 

Thus, one might speak of two endemic zones in the second group of villages: 
the mountain-river zone extending to the elevation of 800-900 m, and the flat- 
mountain zone occupying the territory at the elevation of 1000-1300 m (see Fig. 5). 
The comparative indices of area and location of the 3 topographical-malariological 
zones of the autonomous district of Thai Meo are shown on Fig. 7. In the plan for 
the elimination of malaria in Thai Meo, different measures had to be used for 
the specific mountain-river and flat-mountain malariogenic zones. The data from 
this investigation permit the boundaries of the zones to be more accurately de¬ 
lineated and the extent of the necessary measures which previously had been 
orientatively planned. 


13 



DISCUSSION OF RESULTS 


The first large malariological exploration of the autonomous district of 
Thai Meo (northwestern part of the DRV) revealed some supplementary facts con¬ 
cerning the epidemiology of malaria in North Viet Nam. The presence of A. min¬ 
imus was established at such elevations as 1200-1500 m which are significant for 
this species and which are nearly twice the elevation at which it is found in 
the regions to the east of the Hoang Lien Chon and Sa Fin mountain ridges. 

The presence of A. minimus at such great elevations can be explained by the 
fact that for the topography of the Thai Meo district, large flat mountain masses 
with streams and brooks suitable for breeding—not too swiftly flowing—are 
characteristic. The great elevation at which A. minimus was found has caused 
the presence of malaria foci which are situated very high (1200-1300 m). The 
Meo nationality lives here exclusively. These foci which make up in toto the flat 
mountain zone were distinguished by a low level of malaria transmission and can 
be characterized as hypoendemic. 

The hypoendemic nature of the foci in the flat mountain zone is caused 
in our opinion, by two basic factors: the low density of A. minimus and the 
cooler continental climate. The relatively high infestation rate of adults in 
the foci of this zone is explained by the unilaterial periodic migration of these 
tribesmen into the valleys in conjunction with trade and for seasonal labor. 

During the course of the investigation, no factors were found which were 
considered to be evidence of race-specific features in malaria susceptibility 
among the Thai Meo inhabitants. Quite the contrary, a complete parallelism 
between the degree of population infestation among the different nationalities 
and the density of A. minimus in their houses 7 and thereby with the physicogeo- 


14 





graphical conditions of life of the different nationalities^was established. 

The infestation rate of the Meo population who dwell at lower limits of their 
inhabited area (600-800 m) is considerably less than the infestation rate of the 
Thai and Ran nationalities who live at the sane elevations as a result of the 
confinenent of their inhabitation to poorly-watered open sections of the terrain. 
The Sa nationality which lives on the sane elevation is nore intensively nalaria 
infested than the other nationalities, chiefly because of the close location of 
the settlements to mountain brooks and the low sanitary culture. Tne nalaria 
infestation rate of the Thai nationality is only slightly dependent on television 
since the Thai live at all elevations in open clearings directly next to rivers 
and brooks, cultivate irrigated rice, and use covers (canopies). Their suscepti¬ 
bility to malaria is just as high as that of the other nationalities: the in¬ 
tensity of increasing immunity by age group is fundamentally no different from its 
intensity among the Man, Sa, and Myong nationalities. 

There is no basis tc consider, as Pons affirmed, that the susceptibility of 
the Thai nationality is highly different from the susceptibility of the other 
nationalities and tht the Thai, so to speak, the race best adapted to live 
in malarial localities. 

CONCLUSIONS 

1. A. minimus on the territory of the autonomous district of Thai Meo to 
the west of the ridges Hoang Lien Shon and Sa Fin is distributed as high as 
1300-1500 m above sea level, almost twice as high as the altitudes known for 
its distribution to the east of the same ridges. The cause of this difference 
is explained chiefly by the presence on the territory of Thai Meo of large flat 
mountain masses on which, despite, the great elevation of their situation, there 


15 




are favorable breeding conditions for A. minimus. 

2. Foci of local nalaria in Thai Meo were found at an elevation of 100 to 
1200-1300 = . Two groups of foci are identified on the basis of the intensity of 
population aalarial infestation: hyperendenic foci situated at an elevation of 
up to 80G-900 n, and hypoendemic foci, situated at an elevation of 900-1000 n to 
1200-1300 a. 

3. Hyperendenic foci of nalaria of Thai Meo are confined to river valleys 
and their side slopes inhabited by Thai, Myoag, Sa, and Man nationalities. The 
totality of foci of this type sake up the eountain-river topographical-nalario- 
J.ogical zone. 

4. The hypoendenic foci of nalaria are distribured chiefly on the flat- 
mountain terrain of the district inhabited almost exclusively by the Meo nationa¬ 
lity. The totality of foci of this type make up the fls : mountain zone first 
identified in Thai Heo and encountered, apparently, nowhere else in Xortn Viet 
:ias. 

5. The differences found in the population nalaria infestation rates of 
the different nationalities of Thai Meo depend not on the racial classification, 
but rather on the confinement of their settlements to different topographical- 
malar iological zones.either due to custom.or economic reasons, and the sanitary- 
hygienic level. 

6. The topographical malariological invescion of Thai Heo permits a well- 
founded and economical plan for the elimination of malaria in the district and 
its prevention, after this improvement in sanitary conditions has been completed, 
to be projected. 


16