Very high DDT-resistant population ofAnopheles pharoensisTheobald (Diptera: Culicidae) from Gorgora, northern Ethiopia

Spatiotemporal distribution and bionomics of Anopheles stephensi in different eco-epidemiological settings in Ethiopia

BACKGROUND

Malaria is a major public health concern in Ethiopia, and its incidence could worsen with the spread of the invasive mosquito species Anopheles stephensi in the country. This study aimed to provide updates on the distribution of An. stephensi and likely household exposure in Ethiopia.

METHODS

Entomological surveillance was performed in 26 urban settings in Ethiopia from 2021 to 2023. A kilometer-by-kilometer quadrant was established per town, and approximately 20 structures per quadrant were surveyed every 3 months. Additional extensive sampling was conducted in 50 randomly selected structures in four urban centers in 2022 and 2023 to assess households' exposure to An. stephensi. Prokopack aspirators and CDC light traps were used to collect adult mosquitoes, and standard dippers were used to collect immature stages. The collected mosquitoes were identified to species level by morphological keys and molecular methods. PCR assays were used to assess Plasmodium infection and mosquito blood meal source.

RESULTS

Catches of adult An. stephensi were generally low (mean: 0.15 per trap), with eight positive sites among the 26 surveyed. This mosquito species was reported for the first time in Assosa, western Ethiopia. Anopheles stephensi was the predominant species in four of the eight positive sites, accounting for 75-100% relative abundance of the adult Anopheles catches. Household-level exposure, defined as the percentage of households with a peridomestic presence of An. stephensi, ranged from 18% in Metehara to 30% in Danan. Anopheles arabiensis was the predominant species in 20 of the 26 sites, accounting for 42.9-100% of the Anopheles catches. Bovine blood index, ovine blood index and human blood index values were 69.2%, 32.3% and 24.6%, respectively, for An. stephensi, and 65.4%, 46.7% and 35.8%, respectively, for An. arabiensis. None of the 197 An. stephensi mosquitoes assayed tested positive for Plasmodium sporozoite, while of the 1434 An. arabiensis mosquitoes assayed, 62 were positive for Plasmodium (10 for P. falciparum and 52 for P. vivax).

CONCLUSIONS

This study shows that the geographical range of An. stephensi has expanded to western Ethiopia. Strongly zoophagic behavior coupled with low adult catches might explain the absence of Plasmodium infection. The level of household exposure to An. stephensi in this study varied across positive sites. Further research is needed to better understand the bionomics and contribution of An. stephensi to malaria transmission.

DDT Resistance in Anopheles pharoensis from Northern Cameroon Associated with High Cuticular Hydrocarbon Production

Despite the contribution of secondary vectors to malaria transmission, there is still not enough information on their susceptibility status to insecticides. The present study assesses the resistance profile of Anopheles pharoensis to DDT. WHO tube tests were used to screen mosquito populations collected from the far-north region of Cameroon for susceptibility to 4% DDT. High DDT resistance in An. pharoensis populations from Maga, Simatou and Yangah with mortality rates ranging from 62.79% to 80% was recorded. Direct sequencing (Sanger) of the VGSC gene was undertaken to search for kdr L1014F/S mutations. However, no kdr allele was detected in the resistant samples. We then looked for cuticle alterations and CHC identification and quantitation were undertaken using GC-MS and GC-FID. High production of cuticular hydrocarbon was recorded in the populations of Yangah and Simatou, with 2420.9 ± 265 and 2372.5 ± 225 ng CHCs/mg dry weight, respectively. The present findings are the first ever describing the development of cuticle resistance in An. pharoensis. The data suggest the need to expand surveillance activities on other vector species.

An experimental hut study evaluating the impact of pyrethroid-only and PBO nets alone and in combination with pirimiphos-methyl-based IRS in Ethiopia

Background

Pyrethroid resistance observed in populations of malaria vectors is widespread in Ethiopia and could potentially compromise the effectiveness of insecticide-based malaria vector control interventions. In this study, the impact of combining indoor residual spraying (IRS) and insecticide-treated nets (ITNs) on mosquito behaviour and mortality was evaluated using experimental huts.

Methods

A Latin Square Design was employed using six experimental huts to collect entomological data. Human volunteers slept in huts with different types of nets (pyrethroid-only net, PBO net, and untreated net) either with or without IRS (Actellic 300CS). The hut with no IRS and an untreated net served as a negative control. The study was conducted for a total of 54 nights. Both alive and dead mosquitoes were collected from inside nets, in the central rooms and verandah the following morning. Data were analysed using Stata/SE 14.0 software package (College Station, TX, USA).

Results

The personal protection rate of huts with PermaNet® 2.0 alone and PermaNet® 3.0 alone was 33.3% and 50%, respectively. The mean killing effect of huts with PermaNet® 2.0 alone and PermaNet® 3.0 alone was 2% and 49%, respectively. Huts with PermaNet® 2.0 alone and PermaNet® 3.0 alone demonstrated significantly higher excito-repellency than the control hut. However, mosquito mortality in the hut with IRS + untreated net, hut with IRS + PermaNet® 2.0 and hut with IRS + PermaNet® 3.0 were not significantly different from each other (p > 0.05). Additionally, pre-exposure of both the susceptible Anopheles arabiensis laboratory strain and wild Anopheles gambiae sensu lato to PBO in the cone bioassay tests of Actellic 300CS sprayed surfaces did not reduce mosquito mortality when compared to mortality without pre-exposure to PBO.

Conclusion

Mosquito mortality rates from the huts with IRS alone were similar to mosquito mortality rates from the huts with the combination of vector control intervention tools (IRS + ITNs) and mosquito mortality rates from huts with PBO nets alone were significantly higher than huts with pyrethroid-only nets. The findings of this study help inform studies to be conducted under field condition for decision-making for future selection of cost-effective vector control intervention tools.

Graphical Abstract

Effect of LongRange™ eprinomectin on Anopheles arabiensis by feeding on calves treated with the drug

Background

Misuse of long-lasting insecticidal nets together with resistance of vectors to most of the insecticides for indoor residual spraying and impregnated nets threaten malaria vector control interventions, requiring search for alternative control methods. Reports have shown that Anopheles mosquitoes die when they feed on endectocidal drugs used to treat humans and animals. A study was designed to investigate the efficacy of LongRange™ (eprinomectin 5%) on laboratory reared Anopheles arabiensis fed on treated calves.

Methods

Anopheles arabiensis from insectary colony was fed on three calves treated with therapeutic dose of LongRange™ eprinomectin (1 ml/50 kg) and on non-treated three other calves as control arm. For the feeding, mosquitoes were placed in paper cups covered with nylon cloth mesh and then allowed to feed on the necks of calves. Subsequently, mosquito survival, fecundity, egg hatchability, larval development and adult emergence were recorded. Data were entered and analysed by using SPSS version 20. The Kaplan–Meier survival analysis and independent sample t-test were used.

Results

All mosquitoes that fed on LongRange™ Eprinomectin treated calves died within 7 days following blood ingestion. The drug also slightly affected fecundity and hatchability of An. arabiensis.

Conclusion

Treating livestock with LongRange™ (eprinomectin 5%) may serve as a supplementary control method for zoophagic An. arabiensis.

Efficacy of the mermithid nematode, Romanomermis iyengari, for the biocontrol of Anopheles gambiae, the major malaria vector in sub-Saharan Africa

Background

The intensive use of chemical insecticides against mosquitoes has led to the development of widespread insecticide resistance. Control of Anopheles mosquitoes in malaria endemic areas of sub-Saharan Africa has become increasingly difficult. There is an urgent need for malaria control programmes to adopt more integrated mosquito management approaches that include sustainable, nonchemical solutions. The mermithid nematode Romanomermis iyengari is one of several natural control alternatives to synthetic pesticides for mosquito suppression. This study evaluated the effectiveness of the nematode R. iyengari for control of Anopheles gambiae.

Methods

The nematode R. iyengari was mass-produced, and pre-parasitic stage (J2) were used for laboratory and field experiments. In laboratory experiments, two concentrations of pre-parasitics (5 and 10 J2 per larva) were tested against first- (L1), second- (L2) and third-instar (L3) larvae of An. gambiae. Infected larvae were observed daily to determine their mortality rate and the number of post-parasitic nematodes emerging from dead larvae. In field experiments, 3500, 4000 and 5000 J2/m² were sprayed in separate natural Anopheles breeding sites. After treatment, the larval mosquito density in the breeding sites was assessed every 5–7 days.

Results

Laboratory results showed that larval An. gambiae is susceptible to nematode infection: 100% L1 larvae died within 24 hours post-treatment, and 100% of both L2 and L3 larvae died within 7 days, regardless of nematode concentrations. The average number of post-parasitic nematodes emerging per larva increased with increasing nematode concentration. In field experiments, the monthly applications of 3500 to 5000 pre-parasitic nematodes per m² eliminated larval mosquito development in Anopheles- and mixed breeding sites. Larval mosquito density dramatically decreased five days after the first treatment in all treated sites and was maintained at a very low level during the whole experimental period. Basically, only early instar larva were detected in treated sites throughout the test period. The average number of post-parasitic nematodes emerging per larva collected in treated sites was 1.45, 2, and 5.7 respectively for sites treated with 3500, 4000, and 5000 J2/m².

Conclusions

Malaria mosquito larvae is susceptible to R. iyengari infection in West Africa. Parasitism intensity depends on tested nematode concentrations. Monthly application of 3500 J2/ m² was enough to control effectively larval An. gambiae in wetlands and floodable locations in West Africa.

Dry season occurrence of Anopheles mosquitoes and implications in Jabi Tehnan District, West Gojjam Zone, Ethiopia

Background

Generating evidence on the dry season occurrence of the larval and adult stages of Anopheles mosquitoes helps to design effective malaria vector control strategy as the populations of the vectors is expected to be low.

Methods

Larval and adult stages of Anopheles were surveyed during dry seasons in Mender Meter, Jiga Yehlmidar and Wongie Berkegn villages, Jabi Tehnan District, West Gojjam Zone, Ethiopia. Larvae were surveyed (along the available surface water collections), sampled, identified into genus, counted and late instars of the genus Anopheles identified into species. Indoor-resting adult mosquitoes were collected using insecticide aerosol spray, processed and identified into species. Data was analysed using SPSS version 20.0 to determine frequencies, mean differences and associations.

Results

A total of 3127 Anopheles larvae were collected among which most (91.7%; 2869/3127) were from streams followed by ponds (4.3%; 136/3127) and swamps (3.9%; 122/3127). Anopheles gambiae sensu lato was the most prevalent (84.9%; 921/1085) followed by Anopheles cinereus (7.0%; 76/1085), Anopheles chrysti (3.7%; 40/1085), Anopheles demeilloni (2.8%; 30/1085) and Anopheles rhodesiensis (1.6%; 18/1085). The mean number (mean = 15.3) of An. gambiae from Jiga Yehlmidar was significantly (p = 0.024) higher than the corresponding number (mean = 3.2) from Mender Meter. The mean number (mean = 36.3) of An. gambiae larvae in April 2017 was significantly (p = 0.001) higher than the number (mean = 4.0) in December 2013 and the number (mean = 2.6) in March 2013. A total of 1324 adult Anopheles were collected of which the highest proportion (79.1%; 1048/1324) was An. gambiae, followed by An. chrysti (11.7%; 155/1324), An. demeilloni (6%; 80/1324), An. cinereus (2.6%; 35/1324) and Anopheles coustani (0.5%; 6/1324). The highest proportion (54.3%; 569/1048) of the An. gambiae was collected from Wongie Berkegn followed by Jiga Yehlmidar (23.6%; 247/1048) and Mender Meter (22.1%; 232/1048). The mean number (mean = 7.8) of adult An. gambiae caught in Wongie Berkegn was significantly (p = 0.018) higher compared to the number (mean = 3.0) in Mender Meter. No significant difference was observed in the mean number of adult An. gambiae between the seasons.

Conclusion and implication

Streams were prolific breeding habitats of Anopheles mosquitoes followed by ponds and swamps in the dry seasons. In addition, a high population of indoor resting An. gambiae was caught from indoors. This implies the need for a strengthened vector control during dry seasons using breeding habitat management and improved housing in addition to the existing insecticide (LLINs and IRS) based interventions in Jabi Tehnan District, West Gojjam Zone, Ethiopia.

Short persistence of bendiocarb sprayed on pervious walls and its implication for the indoor residual spray program in Ethiopia

Background

With the emergence and spread of vector resistance to pyrethroids and DDT in Africa, several countries have recently switched or are considering switching to carbamates and/or organophosphates for indoor residual spraying (IRS). However, data collected on the residual life of bendiocarb used for IRS in some areas indicate shorter than expected bio-efficacy. This study evaluated the effect of pH and wall type on the residual life of the carbamates bendiocarb and propoxur as measured by the standard World Health Organization (WHO) cone bioassay test.

Methods

In phase I of this study, bendiocarb and propoxur were mixed with buffered low pH (pH 4.3) local water and non-buffered high pH (pH 8.0) local water and sprayed on two types of wall surface, mud and dung, in experimental huts. In the six month phase II study, the two insecticides were mixed with high pH local water and sprayed on four different surfaces: painted, dung, mud and mud pre-wetted with water. The residual bio-efficacy of the insecticides was assessed monthly using standard WHO cone bioassay tests.

Results

In phase I, bendiocarb mixed with high pH water killed more than 80 % of susceptible Anopheles arabiensis mosquitoes for two months on both dung and mud surfaces. On dung surfaces, the 80 % mortality threshold was achieved for three months when the bendiocarb was mixed with low pH water and four months when it was mixed with high pH water. Propoxur lasted longer than bendiocarb on dung surfaces, staying above the 80 % mortality threshold for four and five months when mixed with high and low pH water, respectively. Phase II results also showed that the type of surface sprayed has a significant impact on the bio-efficacy of bendiocarb. Keeping the spray water constant at the same high pH of 8.0, bendiocarb killed 100 % of exposed mosquitoes on impervious painted surfaces for the six months of the study period compared with less than one month on mud surfaces.

Conclusions

Mixing the insecticides in alkaline water did not reduce the residual bio-efficacy of bendiocarb. However, bendiocarb performed much better on impervious (painted) surfaces than on porous dung or mud ones. Propoxur was less affected by wall type than was bendiocarb. Studies on the interaction between wall materials, soil, humidity, temperature and pH and the residual bio-efficacy of new and existing insecticides are recommended prior to their wide use in IRS.

Knowledge, attitudes and practice of communities of Wolaita, Southern Ethiopia about long-lasting insecticidal nets and evaluation of net fabric integrity and insecticidal activity

Background

Optimal use of long-lasting insecticidal (LLINs) for malaria prevention depends on mass distribution, the users’ perception and behaviour of local malaria vectors. This study was aimed at assessing knowledge, attitude and practice (KAP) of communities about LLINs and fabric integrities and insecticidal activities of nets under use in Wolaita zone, Sothern Ethiopia.

Methods

Semi-structured interview questionnaires were used to collect data on KAP variables and WHO cone bioassay was used to test the insecticidal activity of sampled nets against an insectary colony of Anopheles arabiensis. Holes and repairs on surfaces of sample nets were counted and categorized following WHO guidelines to assess their fabric integrities. Chi-square (χ²) tests were used to verify associations between the demographic profiles of the respondents and their responses to KAP questionnaires.

Results

Out of a total of 770 household heads interviewed, 323 (42 %) and 447 (58 %) were males and females, respectively. The average family size in the surveyed households was 5.25 (±0.70 SE, range 1–12). The majority (81.1 %) of the households owned at least one LLIN. The average numbers of LLINs being used and sleeping places in the households were 1.61 (0.04 SE, range 0–4) and 2.27(0.03 SE, range 1–6), respectively. Most of the respondents (n = 687; 89.2 %) believed LLINs prevent malaria by killing or acting as physical barriers against mosquitoes. About 21 and 14 % of the respondents considered children under five years and pregnant women as priority groups for sleeping under LLINs. Households use LLINs consistently throughout the year (86.4 %) and tuck nets into bedding materials while sleeping (90.1 %). Physical inspection of fabric integrity of sampled LLINs revealed holes ranging from size 1 (0.5–2 cm) to size 4 (> 25 cm) mostly on lower (“right/left”) surfaces. Moreover, most surfaces of sampled LLINs caused 100 % knockdown and mortality on insectary colonies of Anopheles arabiensis.

Conclusions

The overall knowledge, attitude and practice about LLINs were satisfactory in the study areas. However, the family size net ratio must be narrowed and attention needs to be given to malaria vulnerable groups (children < 5 years of age and pregnant women). Continuous monitoring and evaluation of use and durability (fabric integrity and insecticidal activity) of LLINs should be conducted to avoid misuse and associated attrition of nets before the intended period.

Distribution and utilization of vector control strategies in a malarious village of Jabi Tehnan District, north-western Ethiopia

BACKGROUND

Adequate coverage and proper use of long-lasting insecticidal nets (LLINs) and indoor residual spaying (IRS) reduce density of indoor-resting mosquitoes, man-mosquito contact and malaria infection. However, distribution, ownership and usage of the interventions may vary among households in a malarious area, which in turn limits the impact of interventions on the transmission of malaria. A study was undertaken to assess distribution and utilization of LLINs and IRS in a malarious village of north-western Ethiopia.

METHODS

A total of 352 randomly selected households in Jiga Yelmdar village, Jabi Tehnan District of north-western Ethiopia were interviewed using a structured questionnaire. The most important questions included distribution and utilization of LLINs/IRS and knowledge by the households of malaria and the interventions.

RESULTS

More than 99% of the respondents had information about malaria. About 97% of the households had at least one LLIN and 89.3% of houses had been treated with IRS within the previous six months. Only 58.2% of the LLIN-owning households had used the nets the previous night. Not being a malaria transmission season was the main reason cited by 69.7% of the households for not using their LLINs the previous night. The most preferred malaria control strategy in the village was LLINs (cited by 71.1%) followed by IRS (cited by 14.5%). About 29% of the households had a history of malaria within the previous six months and the great majority of them (86.3%) sought treatment at the Jiga Yelmdar Health Post or Jiga Health Centre.

CONCLUSION

Residents of Jiga Yelmdar village were knowledgeable about malaria and the control strategies of the disease such as LLINs and IRS. Although LLIN use is their most preferred strategy, the compliance rate was low which probably contributed to the 29% of household-level malaria infection in the village within the previous six months. This indicates the need for improved compliance to LLINs and IRS in the village.

Blood meal sources and entomological inoculation rates of anophelines along a highland altitudinal transect in south-central Ethiopia

BACKGROUND

The role of anophelines in transmitting malaria depends on their distribution, preference to feed on humans and also their susceptibility to Plasmodium gametocytes, all of which are affected by local environmental conditions. Blood meal source and entomological inoculation rate of anophelines was assessed along a highland altitudinal transect in south- central Ethiopia.

METHODS

Monthly adult anopheline sampling was undertaken from July 2008 to June 2010 in Hobe (low altitude), Dirama (mid altitude) and Wurib (high altitude) villages located at average elevations of 1800 m, 2000 m and 2200 m, respectively. Anophelines were collected using CDC light trap, pyrethrum space spray catches (PSC) and artificial pit shelter methods. Upon collection, females were categorized according to their abdominal status and identified to species. Their human blood index, sporozoite rate and entomological inoculation rate was determined.

RESULTS

A total of 4,558 female anophelines of which Anopheles arabiensis was the most prevalent (53.3%) followed by Anopheles demeilloni (26.3%), Anopheles christyi (8.9%), Anopheles pharoensis (7.9%) and Anopheles cinereus (3.6%) were caught and tested for blood meal source or sporozoite infection depending on their abdominal status. The proportions of human fed and bovine fed An. arabiensis were generally similar. In the low altitude village, there were 0.3% (1/300) and 0.2% (1/416) Plasmodium falciparum infected An. arabiensis among the CDC trap catches and PSC respectively. The percentage of Plasmodium vivax infected An. arabiensis were 3% (9/300) and 0.7(3/416) among the CDC and PSCs respectively in the village. In addition, there were 1.4% (1/71) and 50% (1/2) P. vivax infected An. pharoensis from the CDC light trap and PSCs, respectively. In the mid altitude village, 2.5% (1/40) and 1.7% (1/58) from among the CDC and PSCs of An. arabiensis respectively carried P. vivax sporozoites. Among the CDC light trap catches; there were 3.7 and 0 P. falciparum infective bites per year per household for An. arabiensis in the years July 2008 to June 2009 and July 2009 to June 2010 respectively in the low altitude village. The corresponding numbers for P. vivax infective bites for An. arabiensis were 33 and 14.5 in the same village. Space spray catches revealed 0.32 P. vivax infective bites per household for An. pharoensis during the first year in the low altitude village.

CONCLUSION

Anopheles arabiensis was the most prevalent vector of P. vivax and P. falciparum malaria in the low and mid altitude villages followed by An. pharoensis. Annual entomological inoculation rates showed that vivax malaria transmission was higher than that of the falciparum and both decreased with increase in altitude.

Abundance and dynamics of anopheline larvae in a highland malarious area of south-central Ethiopia

Background

Malaria is a public health problem in Ethiopia, and increasingly so in highland areas, possibly because of global warming. This study describes the distribution, breeding habitat and monthly dynamics of anopheline larvae in Butajira, a highland area in south-central Ethiopia.

Methods

A study of the abundance and dynamics of Anopheles larvae was undertaken at different sites and altitudes in Butajira from July 2008 to June 2010. The sites included Hobe (1817 m.a.s.l), Dirama (1995m.a.s.l.) and Wurib (2196m.a.s.l.). Potential anopheline larval habitats were surveyed once per month in each village. The recorded characteristics of the habitats included habitat type, pH, surface debris, emergent plants, algae, substrate, turbidity, temperature, length, width, depth, distance to the nearest house and anophelines. The Spearman correlation coefficient and Mann–Whitney U test were used to calculate the degree of association between the density of anopheline species and key environmental factors.

Results

Among the different types of habitat surveyed, the Odamo, Akamuja and Assas streams and Beko swamp were positive for anopheline larvae. A total of 3,957 third and fourth instar larvae were collected from the three localities, and they represented ten species of anophelines. These were: Anopheles cinereus (32.5%), An. arabiensis (31.4%), An. chrysti (23%), An. demeilloni (12.2%), An. pretoriensis (0.6%), An. azaniae (0.1%), An. rufipes(0.1%), An. sergentii (0.06%), An. garnhami (0.06%) and An. pharoensis (0.03%). The density of anopheline larvae was highest during the dry months. An. arabiensis was widely distributed, and its density decreased from the lowest elevation in Hobe to the highest in Wurib. The density of An. arabiensis larvae was correlated positively with larval habitat temperature (r = 0.33, p < 0.05) and negatively with depth of larval habitat (r = −0.56, p < 0.05).

Conclusion

Ten species of anophelines were identified, including two known vectors of malaria (An. arabiensis and An. pharoensis), along streams in Butajira. Larvae of An. arabiensis were found in streams at 2200m.a.s.l. This possible expansion of the malaria vector to highland areas indicates an increasing risk of malaria because a large proportion of the Ethiopian population live above this altitude.

Potential of the bush mint, Hyptis suaveolens essential oil for personal protection against mosquito biting

We studied the potential of the essential oil extract from the bush mint, Hyptis suaveolens, for use against mosquito biting under both laboratory and field conditions. In the laboratory, the repellency of various concentrations (1-6%) of the essential oil was assessed against Anopheles gambiae, based on a 15-min landing and biting on treated forearms of volunteers. In the laboratory, the percentage of mosquitoes landing on the forearm was 42, 33, 23, 23, 9, and 2 for 1%, 2%, 3%, 4%, 5%, and 6% essential oil concentration, respectively; and 92 and 91 for the solvent (isopropanol) and untreated control, respectively. The percentage of mosquitoes taking a blood meal was 22, 12, 13, 12, 5, and 3 for 1%, 2%, 3%, 4%, 5%, and 6% essential oil, respectively; and 52 and 51 for the solvent and control, respectively. In the field, the 6% essential oil repelled all mosquitoes immediately postapplication; this activity declined to 75% after 5 h. The repellent action of the 8% essential oil concentration was higher, 97% after 5 h. Based on these data, the essential oil of H. suaveolens appears to be a good candidate for use in the integrated management of mosquito vectors of disease.

Insecticide resistance in Anopheles arabiensis (Diptera: Culicidae) from villages in central, northern and south west Ethiopia and detection of kdr mutation

Background

Anopheles arabiensis is the major vector of malaria in Ethiopia. Malaria vector control in Ethiopia is based on selective indoor residual spraying using DDT, distribution of long lasting insecticide treated nets and environmental management of larval breeding habitats. DDT and pyrethroid insecticides are neurotoxins and have a similar mode of action on the sodium ion channel of insects. It was therefore necessary to verify the insecticide susceptibility status of An. arabiensis, to better understand the status of cross-resistance between DDT and the pyrethroids in this species as well as to detect a resistant gene.

Methods

Standard WHO insecticide susceptibility tests were conducted on adults reared from larval and pupal collections from breeding sites at three villages namely: Sodere in the Rift Valley, Gorgora in the north and Ghibe River Valley in the south west of Ethiopia. The occurrence of cross-resistance between pyrethroids and DDT was determined using a DDT selected laboratory colony originally collected from Gorgora. Phenotypically characterized mosquitoes were tested for the presence of knockdown resistance (kdr) alleles using the standard polymerase chain reaction assay.

Results

All An. gambiae s.l. specimens assayed by PCR were identified as An. arabiensis. The knockdown and mortality results showed An. arabiensis resistance to DDT in all villages, resistance to deltamethrin and permethrin in the Ghibe River Valley and permethrin resistance in Gorgora. Bioassay susceptibility tests also indicated the presence of cross-resistance between DDT and permethrin, but not between DDT and deltamethrin. The knockdown resistance (kdr) mutation of leucine to phenylalanine in the sodium ion channel gene was detected in populations from Gorgora and the Ghibe River Valley.

Conclusion

Since An. arabiensis shows high levels of resistance to DDT in all villages tested and varying pyrethroid resistance in Gorgora and the Ghibe River valley, precautionary measures should be taken in future vector control operations. Moreover, the status of resistance in other locations in Ethiopia and the spread of resistant gene (s) should be investigated.

Field investigation on the repellent activity of some aromatic plants by traditional means against Anopheles arabiensis and An. pharoensis (Diptera: Culicidae) around Koka, central Ethiopia

A study was undertaken to evaluate the impact of traditional application methods of mosquito repellent plants in the reduction of the human-vector contact of malaria vectors in central Ethiopia. The plants (Corymbia citriodora, Eucalyptus camaldulensis, Ocimum suave and Ocimum basilicum) were tested by thermal expulsion and direct burning on traditional stoves in the field against two important malaria vectors in Ethiopia (Anopheles arabiensis and An. pharoensis). A Latin-square design was applied for randomly assigning the treatment plants and control to experimental houses over different nights. The percentage repellency of each candidate plant by both application methods was estimated from the catches of mosquitoes in the treatment and control houses. On direct burning of the plants, O. basilicum showed the highest percentage repellency (73.11%, P<0.001) and E. camaldulensis the least repellency (65.29%, P<0.001) against An. arabiensis. By the same method of application, C. citriodora on the other hand gave the highest repellency (72.87%, P<0.001) while E. camaldulensis was still the least repellent plant (66.60%, P<0.001) against An. pharoensis. On thermal expulsion, C. citriodora exhibited the highest repellency (78.69%, P<0.001) while E. camaldulensis was the lowest repellent plant (71.91%, P<0.001) against An. arabiensis. Against An. pharoensis, C. citriodora gave the highest repellency (72.9%, P<0.001) while E. camaldulensis still gave the least repellency (72.2%, P<0.001) on the same method of application. All the tested plants by both methods of application gave partial but significant protection (>65%) against the house-entry and biting of two important malaria vectors in Ethiopia, and thus have a potential to be used at least as supplements to other control methods. However, feasibility and actual impact on disease transmission need to be known on these and other potentially useful plants.