Asymptomatic malaria, growth status, and anaemia among children in Lao People’s Democratic Republic: a cross-sectional study
Akiyama et al. Malar J
Asymptomatic malaria, growth status, and anaemia among children in Lao People's Democratic Republic: a cross-sectional study
Takeshi Akiyama takeshiak@nagano‑nurs.ac.jp 0
Yuba Inamine 0
Hisami Watanabe 0
0 Center of Molecular Bioscineces, Tropical Biosphere Research Center, University of the Ryukyus , Senbaru 1, Nishihara, Okinawa , Japan
Background: Asymptomatic malaria can be observed in both stable endemic areas and unstable transmission areas. However, although much attention has been given to acute malaria infections, relatively little attention has been paid to asymptomatic malaria. Nonetheless, because the asymptomatic host serves as a reservoir for the malaria parasite, asymptomatic malaria is now recognized as an important obstacle to malaria elimination. Asymptomatic malaria is also associated with anaemia, a global public health problem with serious consequences on human health as well as social and economic development. In Lao People's Democratic Republic (Lao PDR), malaria, anaemia, and malnutrition are serious public health concerns. However, few studies have focused on the relationship between these variables. Therefore, this study investigated the relationship between asymptomatic malaria, growth status, and the prevalence of anaemia among children aged 120 months old or younger in rural villages in Lao PDR. Methods: In December 2010 and March 2011, data were collected from five villages in Savannakhet province. Anthropometric measurements, blood samples, and malaria rapid diagnostic tests were conducted. The presence of malaria was confirmed with polymerase chain reaction assays for Plasmodium falciparum. Underweight status, stunting, and anaemia were defined according to World Health Organization standards. Results: The mean age of participants (n = 319) was 88.3 months old (Standard Deviation: 20.6, ranged from 30-119 months old), and 20 participants (6.3 %) had an asymptomatic malaria infection, 92 (28.8 %) were anaemic, 123 (38.6 %) were underweight, and 137 (42.9 %) were stunted. Stunted children were more likely to be infected with asymptomatic malaria [odds ratio (OR) 3.34, 95 % confidence interval (CI) 1.25-8.93], and asymptomatic malaria was associated with anaemia [OR 5.17, 95 % CI 1.99-13.43]. Conclusions: These results suggest a significant association between asymptomatic malaria and anaemia in children. Furthermore, stunted children were more likely to have lower Hb levels and to be infected with asymptomatic malaria than children without stunting. However, further studies examining the impact of asymptomatic malaria infection on children's nutritional and development status are necessary.
Asymptomatic malaria; Anemia; Plasmodium falciparum; Malnutrition; Stunting; Underweight; Lao PDR
Asymptomatic malaria can be observed in both stable
endemic areas and unstable transmission areas [1–5].
Although it is difficult to define asymptomatic malaria
because of a lack of standard diagnostic criteria, the most
widely used criteria include the presence of parasites in
peripheral thick blood smears, an axillary temperature
below 37.5 °C, and the absence of malaria-related
symptoms . For an individual, an asymptomatic infection
might be benefit for the host rather than no infection at
all in order to keep the immunity against malaria [7–12].
However, for communities, asymptomatic hosts serve as
a reservoir for the malaria parasite and, therefore,
asymptomatic malaria is recognized as an important obstacle to
malaria elimination .
Asymptomatic malaria has received less attention than
symptomatic malaria in major studies . However, some
studies have also shown that asymptomatic malaria is
associated with anaemia [3, 5, 13–19], a global public
health problem with serious consequences for human
health, as well as social and economic development .
Therefore, because asymptomatic malaria infections can
last up to one year [21–23], they may seriously affect the
host [24, 25] by causing an iron deficiency.
Studies show inconsistent results regarding the
relationship between asymptomatic malaria and the growth
status of children. For example, some studies have
suggested that indicator of malnutrition such as stunting or
underweight was associated with asymptomatic malaria,
while others have not [19, 26–34]. Therefore, examining
asymptomatic malaria and its association with
malnutrition would be important for public health in areas where
malaria coexist .
Anaemia, malnutrition, and malaria are all serious
public health problems throughout Lao People’s
Democratic Republic (Lao PDR) [20, 33] and particularly in the
Savannakhet province located in the southeastern part of
the country. In 2008, this province ranked third among
all provinces in Lao PDR for the overall incidence of
confirmed Plasmodium falciparum (11.5 per 1000 persons)
. Furthermore, wasting (low weight for height) of
children in this province was benchmarked as “serious”
by the World Food Programme .
This study aimed to investigate the relationship
between asymptomatic malaria, growth status, and the
prevalence of anaemia among children aged 120 months
or younger in rural villages in the Savannakhet province
in Lao PDR.
Kao, and Kalouk Mai in Seponne area, December 2010
and Lahanam in March 2011. In Lao PDR, both May and
December are of dry season. Malaria transmission is high
rainy season between May and October , but
transmission is perennial .
As a part of survey for active case detection, all
villagers including adults were invited to the data collection
in each village where interviews were conducted by a
medical doctor, and body temperature,
anthropometric measurements, and blood samples were taken by
nurses. A rapid diagnostic test for malaria was also
conducted, and positive participants were treated with
artemisinin-based combination therapy according to
country protocols. In the Lahanam area, age in months
was calculated from the registered date of birth. In
Seponne, participants’ registered date of birth was not
collected from the local administration office because
of coordination problems. Then children aged less than
one year old were recorded their age in month from the
interview of caregiver. As the caregivers did not know
the exact birth date of their children, children aged one
year old or older were recorded using self-reported age
in completed years. In order to calculate height for age
and weight for age Z-score, the age in months were
estimated by adding six months to age in completed years in
order to minimize the difference from real age in months
of each child.
Informed consent was obtained from the principal
caregiver. The National Institute of Public Health, Ministry
of Health, Lao PDR approved this study in September
Definition of asymptomatic malaria, growth status,
Asymptomatic malaria was defined as being free of
fever (body temperature ≤37.5 °C)  and presence of
malaria parasite infection and absence of any malaria
related symptom at the time of data collection .
Malaria infection was confirmed by polymerase chain
reaction (PCR) assays for P. falciparum in the
laboratory of the University of the Ryukyus. PCR assays were
conducted with the all blood samples from the eligible
children regardless of the results from RDT, including
negative ones. Being underweight was defined as having
a weight-for-age z score ≤−2 standard deviations (SD)
below the World Health Organization’s (WHO) growth
standards . Stunting was defined as having a
heightfor-age z score ≤−2 SD. Anaemia was determined by
haemoglobin (Hb) levels using the HemoCue system®
(HemoCue® AB, Ängelholm, Sweden) with a threshold
of 110 g/L for children aged <5 and 115 g/L for children
aged ≥5 [42, 43]. Anemia was categorized as severe,
moderate and mild .
A total of 784 villagers with all ages (range: 0–70 years
old) participated in anthropometric measurements,
blood sample collection, and malaria rapid diagnostic
tests as a part of active case detection survey. In order
to use the WHO growth standards for weight-by-age,
children and adults aged older than 120 months were
excluded from analysis (n = 427). Children younger than
24 months of age were excluded from
anthropometric measurement due to time and equipment constrains
to measure their length correctly, and take blood
sample (n = 7). As a consequence, the data from children
aged 24 months or older, and 120 months or younger
(n = 350) were included in the analysis. Among those
350 cases, data with missing data for the anthropometric
or body temperature measurements, or blood collection
were excluded from the analysis (n = 26).
Five symptomatic malaria cases were also excluded.
Thus, 319 children were included in the final analysis. As
mentioned above, the age in months among children in
Seponne area were estimated by adding 6 months to age
in completed years (n = 145, 45.5 %).
The demographic characteristics of the participants,
rates of asymptomatic malaria infection, Hb levels, the
prevalence of anaemia, weight, and height were
examined. T-tests were used to compare the differences in
the mean Hb level between boys/girls, underweight/not
underweight, and stunting/no stunting. Odds ratios (OR)
with 95 % confidence intervals (CI) stratified by growth
status were used to examine the association between
anaemia and asymptomatic malaria. A multivariate
linear regression analysis with forced entry was conducted
using Hb level as the dependent variable. The
participants’ age, asymptomatic malaria infection status, and
growth status (stunting/underweight) were included as
independent variables. To avoid multicollinearity, height
and weight were input separately, and two multivariate
regression models were developed: one including weight
and the other including height. The significance level for
statistical testing was set at p < 0.05. IBM SPSS ver. 21
was used for statistical analysis.
The mean age of participants (n = 319) was 88.3 months
old (SD: 20.6) and ranged from 30–119 months old
(Table 1). The proportion of male to female
participants was approximately the same (boys = 50.2 %
and girls = 49.8 %). The mean Hb level was 119.0 g/L
(SD: 12.6). A total of 92 participants were classified as
anaemic, and asymptomatic malaria infection was found
in 20 participants. Among anaemic children, few cases
of severe anaemia were found (n = 2), where almost
children were mild or moderate anaemia. There was no
severe anaemia case in the participants infected with
asymptomatic malaria. Stunted children were more likely
to be infected with asymptomatic malaria (OR 3.34, 95 %
CI 1.25–8.93) and there were no significant differences in
growth status or the prevalence of anaemia or
asymptomatic malaria between male and female participants.
Table 2 shows the results for asymptomatic malaria,
anaemia, and growth status. Among the total sample,
the OR for being anaemic and having an asymptomatic
malaria infection was 5.17 [95 % CI 1.99–13.43]. The
relationship remained significant for children underweight
(OR 5.35, 95 % CI 1.26–22.74), and not underweight (OR
5.00, 95 % CI 1.40–17.82) and for children stunted (OR
4.18, 95 % CI 1.31–13.34) and not stunting (OR 6.38,
95 % CI 1.13–36.08).
Table 3 shows the results of the multivariate linear
regression analysis. Being underweight was not
significant in this model whereas stunting was, indicating that
stunting was associated with a decreased Hb level after
adjusting for asymptomatic infection.
Among children in villages in Lao PDR, a significant
association was indicated between asymptomatic malaria
and the risk of anaemia and stunted children were also
more likely to have an asymptomatic malaria infection.
The results of the multivariate analysis also indicated that
asymptomatic malaria correlated with a lower Hb level,
and stunting was associated with the anaemia, even after
adjusting for asymptomatic infection.
Although the results of present study showed that
asymptomatic malaria was significantly associated with
stunting, previous studies have presented inconsistent
findings for this association [26, 28, 33, 44]. For example,
while a previous study conducted in Lao PDR indicated
that wasted children were more likely to be infected
with malaria , in a study of Ghanaian children,
stunting was not significantly associated with asymptomatic
Some studies suggest that children’s malnutrition
may have protective effects against malaria [28, 44].
Conversely, other studies suggest malnourished
children seem to be more susceptible to malaria because of
decreased immune system functioning . For
example, a reduced immunoglobulin G antibody response to
P. falciparum was observed in children with
malnutrition , and the relative risk of morbidity due to malaria
Table 1 Characteristics of the participants (n = 319)
Asymptomatic Pf infection N
infections was higher and more consistent in children
with a low body mass index . In addition, a nationally
representative survey conducted in Equatorial Guinea
indicated that malnutrition had a significant
relationship with malaria parasitaemia in children . However,
complex interactions between malnutrition and the risk
and reaction to malaria infection have been indicated
in studies [45, 47, 49]. An approach targeting not only
malaria parasite but also the comprehensive health status
of host such as immunity level  would be necessary.
In the multivariate analysis in this study, the
association between stunting and anaemia remained even after
adjusting for other factors. However, due to the complex
relationship between malaria and anaemia, a
comprehensive approach is necessary to understand [34, 49, 50]. To
interpret these observations, more information, such as
the duration of infection and mean parasitaemia levels,
should be included in analyses .
Severe anaemia was not found among the
participants infected with aymptomatic malaria, as in a study
conducted in South Ethiopia . In order to study the
longitudinal effect, the severity of anaemia would be
important aspect for the future observation. Although
malaria has been emphasized as acute disease, studies
explore chronical effect of asymptomatic malaria, such as
disability-adjusted life years would be expected.
This study has several limitations. First, the
crosssectional nature of the data precluded an analysis of the
complex, mutual interaction between malaria
infection and malnutrition . This interaction is important
because malaria infection leads to compromised growth
and a compromised nutritional status that in turn leads
to increased susceptibility to malaria infection .
Furthermore, malnutrition impairs the function of innate
and adaptive immunity, which is important for defense
against parasitic infection , and repeated malaria
Table 2 Asymptomatic malaria, anemia and growth status
Table 3 Multivariate linear regression analysis on
hemoglobin level (g/dL)
Total sample (N = 319)
No underweight (n = 196)
Underweight (n = 123)
Asymptomatic Pf infectiona,*
Asymptomatic Pf infectiona,*
infections might lead to anaemia and chronic ill health
. Therefore, longitudinal studies are needed to help
clarify this issue.
Second, the sample was collected by convenience
sampling, and as this study was conducted during dry season,
only a small proportion of the participants had an
asymptomatic malaria infection. Thus, this might not be
representative of other populations.
Third, data on other parasitic diseases than P.
falciparum were not collected due to limited resources and time
to make samples for PCR analysis and Hb level in the
field. However, one study has shown that other parasitic
diseases, such as hookworm infection, may be stronger
predictors of Hb levels than sex, malarial parasitaemia,
and Ascaris lumbricoides infection , while
Plasmodium vivax infection has also been suggested as a
predictor of anaemia . Therefore, future studies should
include other parasites to further explain the relationship
between asymptomatic malaria and anaemia.
Fourth, children aged 120 months or younger, being
asymptomatic on the day of the data collection were
eligible for inclusion. Therefore, patients with anaemia caused
by a recent malaria episode that had cleared by the day
of the data collection  may have been included in the
Finally, registered age in months could not be used
due to coordination problems and insufficient memory
of caregivers. The date of birth is important
information to evaluate the growth of children and this can be
solved asking approximate date with the help of event
calendar during the interview with caregiver .
However, such interviews could not be done due to limited
time and other resources. This will limit the validity and
generalization of the results of this study. Despite these
limitations, this study also has several strengths. For
example, although PCR is limited because
deoxyribonucleic acid fragments remain in the blood for a short time
and observations using the PCR technique would capture
only living parasites [57, 58], light microscopy is not able
to assess submicroscopic infections . Because
submicroscopic malaria infections can contribute to the
prevalence of anaemia , and submicroscopic P. falciparum
gametocyte carriers could be an infectious reservoir in
areas of seasonal transmission [60, 61], PCR-based
methods, rather than light microscopy, are a useful and
important tool for the detection of the malarial parasite given
their sensitivity to detect an infection [62, 63].
In addition, most of the major studies on asymptomatic
malaria have been conducted in African regions while
epidemiological information on asymptomatic malaria in
Asian countries has been limited [13, 33, 47, 55, 63–68].
Given that anaemia, malnutrition, and malaria are public
health concerns in Lao PDR, the results of this study are
clinically important for understanding the health status
and nutritional problems among children in this country.
In this study, asymptomatic malaria was shown to be
associated with anaemia in children, and stunted
children were more likely to have lower Hb levels. However,
further studies are necessary to understand the impact of
asymptomatic malaria on the health status of children in
CI: confidence interval; Hb: haemoglobin; Lao PDR: Lao People’s Democratic
Republic; OR: odds ratio; PCR: polymerase chain reaction; SD: standard devia‑
tions; WHO: World Health Organization.
TA carried out the field research, analyzed the data, and wrote the manuscript.
TT supervised and conducted the field and laboratory tests. YI and RT carried
out the laboratory tests. TW supervised the laboratory tests. HN and KM super‑
vised the field activities. SK, TP and SP supervised the field and laboratory
tests. HW supervised the field and laboratory tests as well as the manuscript
writing. JK supervised the manuscript writing. All authors read and approved
the final manuscript.
We thank all the staff of National Institute of Public Health, Ministry of Health,
Lao PDR for their participation and cooperation in this study. This work was
supported by The Grant for National Center for Global Health and Medicine
(22A‑3, 25A‑2, 28A‑4).
The authors declare they have no competing interests.
Availability of data and materials
Our datasets and data definition are available from University of the Ryukyus’
data archive to other research groups upon reasonable request.
Ethics approval and consent to participate
Informed consent was obtained from the principal caregiver. The National
Institute of Public Health, Ministry of Health, Lao PDR approved this study in
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