Vector Bionomic Of Culex Species in Possible Japanese Encephalitis Transmitted Areas of Ein-Me Township, Ayayawady Region in Myanmar

Thaung S, Wai KT, Xun L, Tun T, Min Oo SZ, Linn NYY, Tin O, Maung YNM and Htun ZT

Published on: 2024-05-31

Abstract

Japanese encephalitis is a public health problem in many parts of Myanmar. The diseases are transmitted to humans by the biting of vector mosquitoes. The study was conducted in JE suspected cases occurred in villages ThaPhan Pinseit and ThaPyu Pinseit villages in ThaPhan Pinseit RHC and PaukGone and SarPhyusu villages in PaukGone village in PaukGone RHC from Ayeyawady Region were selected for JE vector surveillance from December 2023 to March 2024. 50 houses for the pigs were randomly selected from each village. Mosquitoes were collected in all selected houses and pig farms using WHO sucking tubes. Larvae were collected in and around a 1-kilometer radius from the study sites. The result found that JE main vector Cx. tritaeniorhynchus was abundantly collected in all study villages, and the highest number of vectors, Cx. Tritaeniorhynchus, was collected from SarPhyusu village (2284), followed by ThaPyu Pinseit (1890), and the lowest number of main vectors was collected in ThaPhan Pinseit village (1268). In the overall collected mosquitoes, the highest density was found in 74.18% of the main vector Cx. Tritaeniorhynchus, followed by the suspected vector Cx. Vishnui 10.04%, and the lowest density was observed in Mansonia and An. vagus 0.33% each. Peak biting time of main vector Cx. tritaeniorhynchus was found 9-10 pm. suspected vector Cx. vishnui was also collected in high numbers in all villages. Nine species of mosquitoes were collected from the study villages. Of this, only four species of An. hyrcanus, An. barbirostris, An. vagus and An. tesellatus were collected. These Anopheles species were available in PaukGone and SarPhyusu villages. Anopheles hyrcanus was collected from all villages. Culex tritaeniorhynchus larvae were abundantly collected from rice fields, polluted water pools and stagnant water pools with plenty of hyacinth plants and also larvae were collected in concrete jars, Bago jars and metal drums of water which water was stored for fire water. High density of main vector and suspected vector were collected in all JE suspected villages, pig farms, pigs, and breeding sources of vector mosquitoes were available in the villages and also JE suspected cases are available in the all villages. This study provides information of high collection of main vector adults and larvae on risk of further transmission in the study areas. Environmental and ecological factors are responsible for the spread of JE virus. Therefore, awareness of community to vector borne disease control activities and health education for all family members should be strengthened to prevent JE transmission in the villages.

Keywords

Japanese encephalitis; JE; Vector; Cx. tritaeniorhynchus; Larvae; Habitats; Rice fields; Water pools; Pig farm; Density; Positivity

Introduction

Mosquitoes are responsible for the spread and transmission of several harmful diseases such as Japanese Encephalitis, Lymphatic filariasis, Malaria, Zika, Chikungunya and Dengue. It is known to infect over 700 million people causing 1 million deaths each year especially in developing regions of the world [1]. Japanese encephalitis (JE) is a zoonotic disease caused by JE virus (JEV) which is a mosquito-borne flavivirus spreading to human through the bite of Culex mosquitoes. Culex tritaeniorhynchus is the primary vector. The virus cycles among amplifier hosts such as pigs, wading birds and human [2]. Most of JE virus infection in human is asymptomatic and less than 1% of infected people develop symptoms and the incubation period is 5-15 days. Case fatality rate is approximately 20-30% and among survivors, 30-50% has serious neurologic or psychiatric sequalae [3]. To date, threats of JE outbreaks are mostly found in 27 countries in Asia and the Pacific Regions including Myanmar. In 1974, Myanmar reported its first outbreak of JE and in 2014, there was an outbreak in 46 villages of nine Townships of Rakhine State coupled with lack of awareness of JE transmission and prevention among the villagers [4]. As JE is endemic in many parts of Myanmar, the existence of source, vectors and seed virus outbreak can occur at any time. Cases reported were slightly higher in rural population than in urban population. Simultaneously, VBDC implements awareness raising activities to improve alertness but there are limitations. Department of Public Health indicated a total of 491 JE cases occurred in the whole country especially in Yangon, Bago, Tanintharyi and Ayeyarwady Regions and Kayin, Rakhine and Shan States [5]. In Dawbon Township, Yangon Region JE virus infection was detected in 52.1% of pigs. The known JEV vector mosquito species, especially Cx. tritaeniorhynchus, were found in the study area but no concurrent human JEV infections were elicited [6]. In Myanmar, isolates of virus from pigs in Dike U pig farm were identified as genotype III by PCR [7] and isolates from pig blood sample from Thakeyta Township were found to be Genotype 1 by DNA sequencer [8].

The virus is now known to be endemic in a large area of Asia with cases being reported from Japan, China, India, the Philippines and Pakistan [9]. There have been no autochthonous cases of JEV in Africa, Europe or the Americas. JEV is the leading cause of human encephalitis in eastern and southern Asia, and currently numerous genotypic forms are recorded in Asia [10].

Japanese encephalitis (JE) is a vector borne zoonosis and one of the world’s leading encephalitic diseases, particularly in the Asia-Pacific region [11]. The disease is endemic in 24 countries in South and Southeast Asia from Pakistan to Japan, northern Australia and Oceania and putting more than three billion people at risk of infection [12]. The annual incidence of JE is about 69,000 cases and annual loss of 709,000 disability-adjusted life years, JE has even a higher disease burden than dengue [13-15].

Background and Justification

The overall incidence of JE is 1.8 per 100 000 per year in endemic countries, it is 5.4 among 1 to 15 years old children. The infection can lead to severe complications with high case fatality [13]. There is no specific treatment till date. However, this disease can be easily prevented by protection from mosquito bite, mosquito control and safe and effective vaccination [2]. Vaccination is the most effective form of prevention. A 30-year retrospective study conducted in Taiwan reported that the vaccine had an estimated effectiveness of 97% against disease incidence, 98% against disease mortality in adult and 19.3% in decreasing the fatality of confirmed JE in 1 to15 year old children [4].

As JE is endemic in many parts of Myanmar, the existence of source, vectors and seed virus outbreak can occur at any time. Cases reported were slightly higher in rural population than in urban population. Simultaneously, VBDC implements awareness raising activities to improve alertness but limited. As of August 2017, data from Department of Public Health indicated a total of 491 JE cases occurred in the whole country especially from Yangon, Bago, Tanintharyi and Ayeyawady Regions and Kayin, Rakhine and Shan States [16]. According to Department of Public Health, Ayeyawady Region was found high prevalence of JE (171 cases) and also observed high risk in Pantanaw, Nyaungdon, Wakema and Hinthada respectively in their Townships [17]. And also, Japanese encephalitis (JE) suspected cases were reported in Ein-me Township Hospital [18]. Moreover, community has poor understanding about the interplay of human health, animal health, changing agricultural practices, environment and socioeconomic factors, and health care infrastructure. There is a need to focus diseases prevention by mitigating the transmission risk through vector surveillance and control programs incorporating one health concept in rural areas of vulnerable sites in Ayeyawady Region. Therefore, the study aims to access the vector bionomics of Culex species in possible Japanese Encephalitis suspected areas of Ein-me Township, Ayeyawady Region in Myanmar.

Materials and Methods

2.1 Study Area and Study Period

The study area of this study was conducted in 4 villages of Ein-me Township, Ayeyawady Region. The selection is based on the 2023-24 Township Hospital Record of JE suspected cases reported (TDOPH 2023-24). Ein-me Township was found high JE suspected cases reported during 2022-2023. Therefore, the study was conducted from January 2024 to December 2024 in ThaPhan Pinseit and ThaPyu Pinseit, PaukGone and SarPhyusu villages in Ein-me Township, Ayeyawady in Myanmar.

2.2 Study Design

A cross-sectional descriptive study design was done.

2.3 Study Population

Study population consisted pig farmers, veterinarians, village authorities, health staff from rural health centers, village volunteers and insect collectors were involved in this study.

Figure 1: Map of the study areas (Ein-me Township, Ayeyawady Region, Myanmar).

2.4 Sample Size Determination and Sampling Procedure

The occurrence of JE vectors and it breeding sites and density were measured in four villages of Ein-me Township, Ayeyawady Region. From the Ayeyawady Region, Ein-Me Township that reported high JE suspected cases in 2023 was selected purposively followed by random selection of ThaPhan Pinseit RHC and PaukGone RHC were selected. In the ThaPhan Pinseit RHC, ThaPhan Pinseit and ThaPyu Pinseit villages and in PaukGone RHC, PaukGone and SarPhyusu villages were selected. ThaPhan Pinseit and PaukGone villages have high density of pig farms and ThaPyu Pinseit and SarPhyusu villages had low density of pig farms. Therefore, this study covered two RHC and four villages.

2.5 Study Tools/ Data Collection Methods

For objective 1: Mosquitoes were collected from fixed mosquito catching stations between pig farm and human households of      

selected ThaPhan Pinseit, ThaPyu Pinseit, PaukGone and SarPhyusu villages of both field areas of ThaPhan Pinseit and

PaukGone RHCs using Kanda big mosquito nets (330 x 330 x 180 cm) (K-net) for animal bait and CDC light traps method was used for indoor and outdoor mosquito collection, Pig farms and cow shed collection in the selected areas of four villages with WHO sucking tubes from 18:00 to 06:00 hours of the next day. All collected mosquitoes were kept in individual labeled paper cups.

2.6 Larva Collection

For identification of breeding sites, larval surveys were conducted in polluted water sources in and around three kilo-meters radius from the study site, such as ponds, rice fields, slowly running water, streams, creeks foot prints, irrigation ditches, hyacinth vegetation pond, water pools, and all different types of water holding places were examined for larvae by 3 Dips /water holding place with WHO dipper [19]. The captured larvae and pupae were put in labeled plastic bags and brought back to the laboratory for species identification and colonization.

2.7 Susceptibility test

Susceptibility of Culex adult mosquitoes was done with WHO test kit as Deltamethrin 0.05%, Cyfluthrin 0.15% and Pyrethrin 0.75% [20].

2.8 Morning resting collection

Morning resting mosquitoes were collected in indoor and outdoor of the household, pig farms and cow shed from 6:00-7:00 am.

2.9 For Objective 2

Map of the location of mosquito collection sites, vector breeding sites, pig farming sites and human settlement areas in the study villages were drawn by using Global Positioning System GPS device (GPSMap16 Garman, 18x-5HZ) software interface application method by expert person.

2.10 Identification of Mosquitoes

Collected adult mosquitoes and adult emerged from larva survey were identified by species according to different identification keys [21-24].

2.11 Data Analysis

Data from various sources were triangulated for meaningful interpretation. Larval density was calculated by larvae/dip and percentage was computed for adult mosquito density per village by Excel software. Map of the study areas were drowning by GPS device (GPSMap16 Garman, 18x-5HZ) software interface application method by expert person.

Results

Table 1: Distribution and density of collected mosquitoes in 4 different villages in Ein-me Township.

Sr. No.

Species

ThaPhan Pinseit

ThaPyu Pinseit

PaukGone

SarPhyusu

Total

No. of collected

Density%

No. of collected

Density%

No. of collected

Density%

No. of collected

Density%

No. of collected

Density

%

1

Cx. Tritaeniorhynchus

1268

87.33

1890

88.24

1470

85.76

2284

90.21

6914

88.08

 

Density%

18.34

 

27.34

 

21.26

 

33.03

 

100%

 

2

Cx. Vishnui

184

12.67

252

11.76

244

14.24

248

9.79

936

11.92

 

Density%

19.66

 

26.92

 

26.07

 

26.5

 

100%

 

Total

1452

100%

2142

100%

1714

100

2532

100

7850

100

Total Density%

18.5

 

27.29

 

21.83

 

32.25

 

100%

 

Table 1. Shows that total collected mosquito species and densities in the villages. A total of 9320 mosquitoes were collected from pig farms, cow sheds and households. Three groups of mosquitoes as Culex, Mansonia and Anopheles were collected. Of these 4 species of Culex mosquitoes i.e. Cx. tritaeniorhynchus, Cx. vishnui, Cx. gelidus, Cx. quinquefasciatus, one species of Mansonia and 4 species of Anopheles as An. hyrcanus, An. barbirostris, An. vagus and An. tesellatus. Were collected. Highest density of mosquitoes was collected from SarPhyusu village (35.75%) followed by ThaPyu Pinseit (24.41%) and lowest density was observed in ThaPhan Pinseit village (16.18%). Main vector of JE Cx. tritaeniorhynchus was collected in high density (74.18%) followed by Cx. vishnui (10.04%) in all collected villages lowest density was found Mansonia and An. vagus (0.33%). Highest density of main filarial vector Cx. quinquefasciatus was collected from SarPhyusu village (3.83%) followed by PaukGone village (3.78%) and lowest density was observed ThaPhan Pinseit village (1.26%).

Table 2: The distribution of JE vector and suspected vectors in four villages in Ein-me villages.

Sr. No.

Species

Cattle sheds

Pig farms

Indoor LT.

Outdoor LT

House holds

Morning Resting

Total

Density

1

Cx. tritaeniorhynchus

1246

2789

782

894

766

437

6914

74.18

2

Cx. Vishnui

251

443

56

93

48

45

936

10.04

3

Cx. Gelidus

21

34

6

9

6

0

76

0.82

4

Cx. quinquefasciatus

98

123

26

19

21

11

298

3.2

5

Mansonia

11

17

0

3

 

 

31

0.33

6

An. Hyrcanus

493

310

13

25

5

0

846

9.08

7

An. Barbirostris

68

56

2

8

1

0

135

1.45

8

An. Vagus

15

9

2

4

1

 

31

0.33

9

An. Tesellatus

26

14

3

8

2

0

53

0.57

 

Total

2229

3795

890

1063

850

493

9320

100

 

Percent

23.92

40.72

9.55

11.41

9.12

5.29

100

74.18

Table 2. shows that the distribution of JE vector and suspected vector in four villages in Ein-me villages and found that the highest total density of both main vector Cx. tritaeniorhynchus and suspected vector Cx. vishnui was observed in SarPhyusu village 2532 (32.25%) and followed by ThaPyu Pinseit village 2142 (27.29%) and lowest density was found 1452 (18.50%). When compared with Cx. vishnui, the density of main vector Cx. tritaeniorhynchus was higher over 85% in all villages than suspected vectors of Cx. vishnui species. Of this the highest density of main vector was collected from SarPhyusu village (33.03%) and followed by ThaPyu Pinseit village (27.34%) and lowest density was collected from ThaPhan Pinseit ThaPhan Pinseit village. In suspected vector, the high density of suspected vector Cx. vishnui was collected from ThaPyu Pinseit (26.92%), SarPhyusu (26.50%) and PaukGone villages (26. 07%) followed by ThaPhan Pinseit village (19.66%).

Table 3: Mosquito collected by different collection methods in Ein-me Township Ayeyawady Region.

Sr. No.

Species

ThaPhan Pinseit

ThaPyu Pinseit

PaukGone

SarPhyusu

Total

No. of collected

Density

No. of collected

Density

No. of collected

Density

No. of collected

Density

No. of collected

Density

1

Cx. tritaeniorhynchus

1268

84.08

1890

83.12

1470

66.16

2284

68.93

6914

74.18

2

Cx. vishnui

184

12.2

252

11.21

244

11.21

248

7.48

936

10.04

3

Cx. gelidus

16

1.06

10

0.44

30

1.58

14

0.45

76

0.82

4

Cx. quinquefasciatus

18

1.26

68

2.99

84

3.78

124

3.83

298

3.2

5

Mansonia

0

0

10

0.57

18

0.81

-

0

31

0.33

6

An. hyrcanus

12

0.86

24

1.19

220

10.04

580

17.59

846

9.08

7

An. barbirostris

8

0.53

0

0

104

4.68

22

0.69

135

1.45

8

An. vagus

0

0

0

0

20

0.9

10

0.33

31

0.33

9

An. tesellatus

0

0

10

0.48

19

0.86

22

0.69

53

0.57

 

Total

1508

100

2275

100

2222

100

3315

100

9320

100

 

Density

 

16.18%

 

24.41%

 

23.84%

 

35.57%

 

100%

Table 3 shows that mosquito collection by different methods and found that highest number of mosquitoes were collected in pig farms 3795 (40.72%) followed by cow shed 2229 (23.92%) and lowest number of mosquitoes were collected by morning resting collection. Main vector of Cx. tritaeniorhynchus and suspected vector of Cx. vishnui were also highest in pig farms and second most was Cow shed collection. All the collected mosquitoes were found higher in Cow shed and Pig farms than other collection methods. CDC light traps collection found that higher mosquitoes collected by Outdoor light trap collection 1063 (11.41%) than indoor light traps collection 890 (9.55%). Indoor Households collection was found 850 mosquitoes and morning resting collection was found 493 mosquitoes is a lowest collection of mosquitoes by different mosquito collection methods. Higher number of main vectors Cx. tritaeniorhynchus and suspected vector Cx. vishnui were collected in all method. It may be due to high density of both mosquitoes are available in these areas

Figure 1: shows that hourly collection of main JE vector Cx. tritaeniorhynchus and suspected vector Cx. vishnui mosquitoes in house hold.

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