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)
Distribution of this document is unlimited !
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2130 DELANCEY PLACE, O BOX 3 C3 <3 3 * PHILADELPHIA. PENNSYLVANIA 19:01
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
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
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
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
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
Nationality Villages Studied
Meo. 12 080
Thai. > 8 38?
Sa. ? 2 205
Man. 1 988
Myong. 13 1 396
Total... 248 26 056
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
of them, with the
Table 2 (cont’d)
them, with the
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
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.
Fig. 3. Abundance of A. vagus (i), A.
affected by elevation of the location
minimus (2), and A, jeyforiensis
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
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
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.
to. A.-... ...li v.
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
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
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
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-
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.
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
( . 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¬
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
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
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
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-
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.
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
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
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-
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
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-
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.