Malaria vectors in southern of Vietnam

During the 20th century and the early years of the 21st century, malaria is a major public health problem in Vietnam, malaria was circulating in all regions, provinces, and cities in the country. Particularly, in 1991 with more than 1 million infected people, of whom 4,646 people died. Since 1997, implementation of malaria prevention activities has achieved great successes, resulting that none of the malaria outbreaks has occurred for the recent years, the number of malaria deaths has reduced and fluctuated with a few cases annually, malaria has circulated mainly in the highlands and southeastern provinces, border areas, remote and isolated areas. Malaria is mainly transmitted by anopheles mosquitoes, to which 64 species have been identified in Vietnam. Of which, approximately 15 anopheles species are able to transmit malaria, of which, there are 3 primary malaria vectors: An.dirus, An. minimus, An. epiroticus. There are about 42 anophelels species were found in South Vietnam, include An. aconitus, An. annularis, An. argyropus, An. barbirostris, An. barbumbrosus, An. bengalensis, An. campestris, An. crowfordi, An. dangi, An. dirus, An. dravidicus, An. epiroticus, An. gigas, An. hycanus, An. indefinitus, An. jamesi, An. jeyporiensis, An. karwari, An. kochi, An. lesteri, An. letifer, An. maculatus, An. minimus, An. montrosus, An. nigerrimus, An. nimpe, An. nitidus, An. nivipes, An. pampanai, An. pedifinitus, An. peditaenitus, An. philippinensis, An. pursati, An. sawadwongporni, An. separatus, An. sinensis, An. splendid, An. subpictus, An. tessellatus, An. umbrosus, An. vagus, An. whartoni. Thus, in the southern region has the presence of all three major malaria vectors, especially An. epiroticus is only present in the southwest coast area while in other places is not.

Fig 1. Activities collected Anopheles mosquitoes in the field

Mosquito susceptibility to insecticides

Monitoring of Anopheles mosquito susceptibility to insecticides is regularly maintained to detect insecticide resistance in populations of Anopheles vectors for malaria control in the area. The result on sensitivity of Anopheles mosquito to insecticides from 2010 - 2015 show that: (Table 2).

- Alpha-Cypermethrin 30mg/m2: An. aconitus, An. dirus were susceptibility (mosquitoes mortality 100%); An. epiroticus was resistance (mosquitoes mortality 74% - 86%).

- Lamda-Cyhalothrin 0,05%: An. philippinnensis was susceptibility (mosquitoes mortality 100%); An. epiroticus was resistance (mosquitoes mortality 74%) and possible resistance (mosquitoes mortality 91% - 93%); An. sinensis was resistance (mosquitoes mortality 28%).

- DDT 4%: An. epiroticus was susceptibility (mosquitoes mortality 100%).

- Permethrin 0,75%: An. epiroticus was resistance (mosquitoes mortality 82% - 89%).

- Pirimiphos-methyl 0,25%: An. epiroticus was susceptibility (mosquitoes mortality 98% - 100%); An. sinensis was resistance (mosquitoes mortality 5%).

- Malathion 5%: An. epiroticus was susceptibility (mosquitoes mortality 100%).

- Propoxur 0,1%: An. epiroticus was susceptibility (mosquitoes mortality 100%).

In 2018, the result of Anopheles sensitivity tests at several provinces in South Vietnam as follows (Table 1):

Table 1. Results of susceptibility tests on Anopheleles to insecticides

N0

Provinces

Anopheles

Alpha-Cypermethrin

30mg/m2

Lamda-Cyhalothrin 0,05%

The mosquito mortality (%) after a holding period of 24 hours (60 minutes exposure)

1

Ca Mau

An. epiroticus

73

84

2

Bac Lieu

69

47

3

Soc Trang

84

80

4

Tra Vinh

93

86

5

Ben Tre

67

64

6

Long An

An. subpictus

98

66

Note: The current recommendations are: Mortality ≥ 98 - 100% (Susceptibility); Mortality between 90% - 97% (Possible resistance); Mortality ˂90% (Resistance).

  • Alpha-Cypermethrin 30mg/m2: An. epiroticus collected in Ca Mau, Bac Lieu, Soc Trang, Ben Tre showed resistance (mosquitoes mortality 73-87%) and An. epiroticus collected in Soc Trang that was possible resistance (mosquitoes mortality 93%). An. subpictus collected in Long An which was susceptibility (mosquitoes mortality 98%).
  • Cyhalothrin 0,05%: An. epiroticus and An. subpictus collected in 6 provinces (Ca Mau, Bac Lieu, Soc Trang, Ben Tre, and Long An) showed resistance (mosquitoes mortality 47 - 86%).

Fig 2. Activities of Anopheles mosquito susceptibility tests in the field

Table 2. Results of susceptibility tests on Anopheles to insecticides (from 2010 - 2015).

Province

Time

Anopheles

Alpha-Cypermethrin

30mg/m2

Lamda-Cyhalothrin

0,05%

DDT

4%

Permethrin

0,75%

Pirimiphos-methyl 0,25%

Malathion

5%

Propoxur

0,1%

The mosquito mortality (%) after a holding period of 24 hours (60 minutes exposure)

Binh Phuoc

2010

An. philippinnensis

-

100

-

-

-

-

-

2010

An. aconitus

100

-

-

-

-

-

-

2010

An. dirus

100

-

-

-

-

-

-

Ben Tre

2015

An. epiroticus

-

-

-

-

98

-

-

HCM

2013

An. epiroticus

78

74

100

89

-

-

-

Bac Lieu

2014

An. epiroticus

78

-

-

-

-

-

-

2015

An. sinensis

-

-

-

-

5

-

-

Ca Mau

2010

An. epiroticus

86

91

-

-

-

-

-

2011

-

93

-

82

-

-

-

2012

74

-

-

-

-

-

-

2013

-

93

100

82

100

100

100

2014

83

-

-

-

-

-

-

2015

-

-

-

-

100

-

-

Tra Vinh

2013

An. epiroticus

82

-

-

-

-

-

-

An Giang

2010

An. sinensis

-

28

-

-

-

-

-

Note: The current recommendations are: Mortality ≥ 98 - 100% (Susceptibility); Mortality between 90% - 97% (Possible resistance); Mortality ˂90% (Resistance).