See Part 1_Status of Anopheles vectors at malaria increasing sites in Khanh Vinh district, Khanh Hoa province in 2023

Results of Anopheles mosquito survey

Composition and density of Anopheles mosquitoes at surveyed sites

In the overall survey of all three communes, Anopheles maculatus predominated over the other species, with a total of 276 individuals. Out of the 16 collected species, four were malaria vectors, including the primary vectors Anopheles minimus and Anopheles dirus, as well as two secondary vectors, namely Anopheles aconitus and Anopheles maculatus.

Figure 2. Number of collected mosquitoes by species at the study sites

Table 4. Species composition and the number of Anopheles mosquitoes collected in Khanh Vinh district

In total, 980 adult female Anopheles mosquitoes were collected at the three survey sites. Among them, the Anopheles mosquitoes collected in Khanh Thuong commune accounted for 38.0% (372 Anopheles specimens), Khanh Thanh commune 38.8% and Son Thai commune 23.3% (228 specimens). In Khanh Thuong commune, Anopheles maculatus was predominant with a collection of 228 specimens, accounting for 61.3% of the total. The least collected species were Anopheles varuna and Anopheles karwari, with o­nly 2 specimens each, representing 0.54% of the total. Similarly, in Khanh Thanh commune, Anopheles aconitus predominated over other species, with 154 specimens, making up 40.53% of the total. In Son Thai commune, Anopheles dirus was the most abundant species.

Figure 3. Number of collected Anopheles mosquitoes by survey method

Cow baited trap collected the highest number of Anopheles mosquitoes compared to other methods. Specifically, the number of mosquitoes collected using the CDC light trap, human landing catch, and cow baited trap were as follows: 15, 51, and 314 Anopheles mosquitoes in Khanh Thanh commune; 37, 28, and 307 mosquitoes in Khanh Thuong commune; and 19, 40, and 169 Anopheles mosquitoes in Son Thai commune.

Table 5. The mean density of Anopheles mosquitoes by collected method in Khanh Thuong commune

In Khanh Thuong commune, a total of 14 Anopheles species were collected. Among these 14 species, there were both of the primary malaria vectors, Anopheles dirus and Anopheles minimus, as well as two secondary vectors, Anopheles aconitus and Anopheles maculatus. Anopheles dirus was collected using all four survey methods, while Anopheles minimus was o­nly collected in cow-bait traps.

Table 6. The mean density of Anopheles mosquitoes by collected method in Khanh Thanh commune

In Khanh Thanh commune, o­nly the primary malaria vector Anopheles dirus was collected, and the density of mosquitoes collecting by outdoor human landing catch was 4.5 mosquitoes per person per night, which is three times higher than indoor method. The CDC light trap o­nly collected mosquitoes indoors, with a density of 2 mosquitoes per trap per night.

Table 7. The mean density of Anopheles mosquitoes by collected method in Son Thai commune

In Son Thai commune, two primary malaria vectors were collected, with Anopheles dirus being collected using all four survey methods. The density of mosquitoes collecting by outdoor human landing catch was 4.33 mosquitoes per person per night, which is nearly double that of indoor method with 2.33 mosquitoes per person per night.

Table 8. Results of malaria mosquito dissections in Khanh Vinh district

Note: Malaria vectors can o­nly be dissected while they are still alive.

The results of mosquito dissection showed that a total of 148 malaria mosquitoes qualified for dissection, meaning they were still alive, and their bodies had not become stiff. Among these 148 mosquitoes dissected, 80 were found to have laid eggs, accounting for 54.1% of the total. The highest parous rate was observed in the primary vector Anopheles dirus at 56% (56 out of 71), while the lowest was seen in the secondary vector Anopheles aconitus at 53.7% (52 out of 101). Examining all three vector species' stomach and salivary glands under a microscope, no malaria parasites were detected.

Results of Anopheles mosquito susceptibility testing with insecticides

Table 9. Susceptibility level of mosquitoes to deltamethrin 0.05%

Anopheles mosquito susceptibility testing with pyrethroid group, including lambda-cyhalothrin 0.05% and deltamethrin 0.05%, was performed. These are insecticides commonly used in the malaria vector control program and are included in the list of approved chemicals for use in Viet Nam.

The Anopheles mosquitoes used for susceptibility testing were alive and healthy individuals collected from cow bait net trap and human landing catch in the field. Insecticide susceptibility bioassays were conducted following the World Health Organization's 2018 protocol.

The results showed that both An. maculatus and An. aconitus mosquito species are susceptible to the insecticides-treated papers in Khanh Vinh district (a mortality of 100%).

Conclusions

- In terms of the malaria situation in 2023, Khanh Vinh district is experiencing a complex and significant increase in malaria cases compared to the same period in 2022. Specifically, in Khanh Thuong commune, a few malaria cases occurred sporadically between 2018 and 2022. However, from the beginning of 2023 to July 25, 2023, Khanh Thuong commune has reported a high number of confirmed cases, totaling 36.

- Concerning malaria vector control measures, despite the increasing number of cases and the presence of mosquito bed-nets and hammocks at health stations, community-based vector control measures have not yet been implemented, including residual spraying.

- Regarding the habitats and habits of the local population in the survey areas, the investigation sites are characterized by their flowing streams and occasional rainfall, and residents stay overnight in temporary field-huts. Currently, the people are in the period of harvesting acacia tree and clearing vegetation, which has led to the formation of small, rudimentary cluster camps to stay overnight, facilitating malaria transmission in the community.

- Concerning the mosquito species composition, a total of 16 Anopheles mosquito species were collected in the three communes, including 2 primary vector species and 2 secondary vector species. In Khanh Thuong commune, 14 mosquito species were collected, including both primary forest species, An. dirus and An. minimus, and 2 secondary species, An. aconitus and An. maculatus. Similarly, 10 species were collected in Son Thai commune, including 2 main vector species and 2 secondary vector species. In Khanh Thanh commune, the surveyed teams collected 11 species but o­nly captured the main vector species, An. dirus, and two secondary vector species. Therefore, the presence of both main vector species increases the risk of malaria transmission in the near future, especially as the number of malaria patients has increased significantly in the first half of July 2023.

- Regarding the behavior of the main vector in the survey areas, An. dirus is frequently collected during surveys conducted by the Entomology Department (IMPE-QN) and the local habitat in Khanh Vinh district is favorable for the year-round development of An. dirus. During the surveys, An. dirus is predominantly active outdoors, with mosquito catches starting from 20:00 in Son Thai and Khanh Thanh communes, while in Khanh Thuong commune, mosquitoes are caught later, starting at 9:30. For malaria mosquito dissection results, the overall parity rate is 54.1%, and no malaria vectors infected with sporozoites were found.

- In term of the susceptible level of malaria mosquitoes to insecticides: Insecticide sensitivity testing was conducted o­n An. aconitus and An. maculatus collected in the field using two types of insecticides, lambda-cyhalothrin 0.05% and deltamethrin 0.05%. The results indicate that An. aconitus and An. maculatus are still susceptible to both lambda-cyhalothrin 0.05% and deltamethrin 0.05%.

Recommendations

- Health Stations: Health stations that currently have bed-nets and hammocks in stock should soon distribute additional bed-nets and hammocks to households, especially those going into the forest, encourage them to use mosquito nets even when going to the fields. Furthermore, these health stations, particularly in Khanh Thuong commune, promote health education and communication through the local radio station about the malaria situation in Khanh Vinh district. The aim is to raise the people?s awareness about malaria and encourage them to take proactive measures against mosquito bites and seek medical attention when having a fever.

- Khanh Vinh District Health Center: Directs the health stations to proactively monitor and detect malaria cases and promptly supply additional bed-nets and hammocks to households going into the forest; Provides mosquito repellent creams to groups of people working in the forest. For farmhouses with walls near the fields, it is necessary to apply indoor residual spraying of insecticides o­n the walls to protect people;

- Khanh Hoa Provincial Center for Disease Control: Supplies the communes with anti-malarial drugs; especially, those with increasing cases like Khanh Thuong commune, should be provided technical guidance and support in monitoring malaria vectors and using bed-nets. Deploysmalaria control measures to protect the local population, considering the presence of both primary vector species as discovered by the investigative team.

- IMPE-QN: Enhances the technical and expertise support for areas with an increasing number of malaria cases, including screening and treating patients, assessing the risk in these areas. Additionally, supports the localities in Khanh Hoa province in monitoring malaria parasites, malaria vectors, and providing recommendations for local malaria prevention and control efforts.

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