Insecticide Resistance in East Africa — History, Distribution and Drawbacks on Malaria Vectors and Disease Control

Malaria is a major contributor to the global disease burden and a significant impediment to socio-economic development in resource-poor countries. In contrast to improved trends of malaria morbidity and mortality in some parts of the world, malaria has remained a life threatening disease in many other regions including East Africa because of factors such as weak health systems, growing drug and insecticide resistance, ecological change, climate anomalies, socio-economic factors and changes in land use patterns. Ongoing malaria vector control strategies rely mainly on the use of indoor residual spraying (IRS) and insecticide treated nets (ITNs) which are the primary intervention strategies to reduce malaria burden. The current success in reducing malaria related morbidity and mortality has led to the optimism that elimination of the disease as a public health problem may be a realistic objective. Efforts during the last decades enabled access to ITNs in sub-Saharan Africa protecting millions of people at risk of malaria. The number of countries that employed IRS as a vector control strategy increased almost by two fold and the percentage of households owing at least one ITN in sub-Saharan Africa is estimated to increase from time to time. Currently, all ITNs are treated with pyrethroids while IRS depends on pyrethroids, DDT and recently on carbamates. Despite IRS and ITNs are known in reducing malaria incidence, insecticide resistance in malaria vectors threatens the success of malaria control program. Resistance to insecticides has occurred in most arthropod vectors with different mechanisms. If the current trends of increased insecticide resistance continue, it may jeopardise the efficacy of current vector control tools. Given the limited choice of available insecticides, i.e., only 12 insecticides belonging to 4 classes of insecticides (organochlorines, organophosphates, pyrethroids and carbamates), resistance to these insecticides has become a limiting factor for current efforts to sustain control. Currently, no other insecticide class with similar efficacy has been approved by WHOPES. The development of insecticide resistance in malaria vectors has been attributed to the prolonged use of insecticides for IRS and high coverage of ITNs/LLINs. The recent use of pyrethroids for indoor residual spraying is likely to have enhanced the selection pressure for insecticide resistance alleles among East African vector populations. Moreover, mosquitoes breeding in agricultural habitats are exposed to sub lethal doses of pesticides used in agriculture. Since currently recommended insecticides for IRS or ITNs were developed with similar active ingredients of pesticides used for agricultural pest control, their extensive and widespread use to boost agricultural productivity is believed to foster insecticide resistance in mosquito populations. There is strong evidence on the emergence of resistance to DDT and pyrethroids in the major malaria vectors in East Africa however, current information on resistance status of the malaria vectors in different areas of the sub-region is scarce. Genes conferring resistance to malaria vectors, including kdr, super kdr and acetylcholinesterase mutations and metabolic resistance are not mapped. The frequency and spatial distribution of East and West African kdr mutations and their association with the phenotypic resistance in East Africa is less understood. The bioassay results after WHO diagnostic tests in different East African malaria vector populations against insecticides used in public health is not well documented. In conclusion, planning and implementing insecticide resistance monitoring and management strategy should be part of the vector control program either for pre-emptive action without waiting for the development of resistance or to slowdown the spread of resistance in malaria vectors in the sub-region

Field and Laboratory Evaluation of Bioefficacy of an Insect Growth Regulator (Dimilin) as a Larvicide against Mosquito and Housefly Larvae

The inhibitory function of Dimilin (Diflubenzuron), mostly a chitin synthesis regulator, on the ecdysis of mosquitoes (Anopheles gambiae s.l., Culex quinquefasciatus) and housefly was evaluated in the field and in laboratory. Three formulations of Diflubenzuron were evaluated in this study: Dimilin, Wettable powder (25%), Dimilin granules (2%), and Dimilin tablets (2%). The laboratory and field evaluation used different rates of concentrations of these formulations. Generally, at higher dosages larvae developments, eggs hatchability and pupation were impossible. The development of mosquitoes was significantly higher in control while highly depressed in different dosages of treatment in both laboratory and field experiments. In houseflies, the adult population decreased sharply after treatment of their breeding sites while pupae mortality was noticed to be high in laboratory-treated samples. Dimilin could be opted as one of the choice of the larval control chemicals to be incorporated in the integrated vector control programmes in urban and rural areas

Chemical Cues for Malaria Vectors Oviposition Site Selection:\ud Challenges and Opportunities

The attractiveness of oviposition site for malaria vector mosquitoes is dependent upon a number of physical and chemical factors. Many aspects of mosquito behavior, including host location and oviposition, are mediated by volatile semiochemicals. It is anticipated that selection of oviposition site by semio-chemicals in the form of attractants or stimulants can be used in oviposition traps to monitor or possibly in combination with insecticides to control gravid mosquito populations for mass trapping. So far, volatile compounds identified as oviposition attractants for mosquitoes include phenol, 4-methyl phenol, 4-ethyl phenol, indole, skatole, and p-cresol from hay infusions; 3-carene, terpinene, copaene, cedrene, and d-cadinene released by copepods; alcohol and terpenoids including p-cresol fromplants; ethyl acetate and hydrocarbon substances, probably released by filamentous algae; 3-methyl-1-butanol identified frombacteria. Research priorities should be directed at identifying more oviposition attractants to determine the properties of these semio-chemicals for possible use in designing control tools. This would aim at luring females to lethal traps or stimulants to increase their exposure to insecticide-impregnated substrates.\ud \u

Insecticidal activity of the essential oil from fruits and seeds of Schinus terebinthifolia Raddi against African malaria vectors

<p>Abstract</p> <p>Background</p> <p>Alternative insecticides for the control of malaria and filarial vectors are of paramount need as resistance is increasing among classes of insecticides currently in use in the public health sector. In this study, mosquitocidal activity of <it>Schinus terebinthifolia </it>essential oil against <it>Anopheles gambiae </it>s.s., <it>An. arabiensis </it>and <it>Culex quinquefasciatus </it>was assessed in laboratory, semi- field and full- field conditions</p> <p>Method</p> <p>Twenty third instar larvae of both <it>Anopheles gambiae </it>s.s. and <it>Cx. quinquefasciatus </it>were exposed to different dosages of plant extract in both laboratory and semi- field environments. Observation of the mortality response was assessed at intervals of 12, 24, 48 and 72 hours. Adult semi- gravid female mosquitoes were exposed to papers treated with <it>S. terebinthifolia </it>and compared with WHO standard paper treated with alphacypermethrin (0.05%).</p> <p>Results</p> <p>Gas chromatography, coupled to mass spectrometry, identified 15 compounds from <it>S. terebinthifolia </it>extracts, the most abundant identified compound was δ-3-carene (55.36%) and the least was γ-elemene (0.41%). The density of the oil was found to be 0.8086 g/ml. The effective dosages in the insectary ranged from 202.15 to 2625.20 ppm and were further evaluated in the semi- field situation. In the laboratory, the mortality of <it>Cx. quinquefasciatus </it>ranged from 0.5 to 96.75% while for <it>An. gambiae </it>s.s it was from 13.75 to 97.91%. In the semi- field experiments, the mortality rates observed varied for both species with time and concentrations. The LC₅₀and LC₉₅value in the laboratory was similar for both species while in the semi- field they were different for each. In wild, adult mosquitoes, the KT₅₀for <it>S. terebinthifolia </it>was 11.29 minutes while for alphacypermethrin was 19.34 minutes. The 24 hour mortality was found to be 100.0% for <it>S. terebinthifolia </it>and 75.0% for alphacypermethrin which was statistically significant (<it>P </it>< 0.001).</p> <p>Conclusion</p> <p>The efficacy shown by essential oils of fruits and seeds of <it>S. terebinthifolia </it>has given an opportunity for further investigation of individual components of these plant extracts and to evaluate them in small- scale field trials.</p

Impact of climate change on human health and health systems in Tanzania: a review

Climate change (CC) has a number of immediate and long-term impacts on the fundamental determinants of human health. A number of potential human health effects have been associated either directly or indirectly with global climate change. Vulnerability to the risks associated with CC may exacerbate ongoing socio-economic challenges. The objective of this review was to analyse the potential risk and vulnerability in the context of climate-sensitive human diseases and health system in Tanzania. Climate sensitive vector- and water-borne diseases and other health related problems and the policies on climate adaptation in Tanzania during the past 50 years are reviewed. The review has shown that a number of climate-associated infectious disease epidemics have been reported in various areas of the country; mostly being associated with increase in precipitation and temperature. Although, there is no single policy document that specifically addresses issues of CC in the country, the National Environmental Management Act of 1997 recognizes the importance of CC and calls for the government to put up measures to address the phenomenon. A number of strategies and action plans related to CC are also in place. These include the National Biodiversity Strategy and Action Plan, the National Action Programme, and the National Bio-safety Framework. The government has put in place a National Climate Change Steering Committee and the National Climate Change Technical Committee to oversee and guide the implementation of CC activities in the country. Recognizing the adverse impacts of natural disasters and calamities, the government established a Disaster Management Division under the Prime Minister&rsquo;s Office. Epidemic Preparedness and Response Unit of the Ministry of Health and Social Welfare is responsible for emergency preparedness, mostly disease outbreaks. However, specific climate changes associated with human health issues are poorly addressed in the MoHSW strategies and the national health research priorities. In conclusion, CC threatens to slow, halt or reverses the progress the country has made or is making to achieve its national and millennium development goals. It is therefore important that Tanzania prepares itself to appropriately address CC impact on human health. It is equally important that policy makers and other stakeholders are engaged in a process to update and adapt priorities, mobilize resources and build interdisciplinary research and implementation capacity on climate change and its mitigation

Role of cattle treated with deltamethrine in areas with a high population of Anopheles arabiensis in Moshi, Northern Tanzania

Malaria control measures were initiated from in October 2005 to August 2006 in the Lower Moshi irrigation schemes, Tanzania. This manuscript reports on the entomological evaluation of the impact of pyrethroid-treated cattle in reducing the population of the Anopheles arabiensis for selected houses in the Lower Moshi irrigation scheme. Cattle were sprayed with the pyrethroid (deltamethrin) acaricide. Grazing and non-grazing cattles were compared and assessed for difference in knockdown resistance (kdr) time using cone or contact bioassay and residual effect (mortality). In experimental huts, mortality was compared between the huts with treated and untreated cattle. Results from contact bioassays of cattle treated with deltamethrin showed a knockdown effect of 50% within 21 days for grazing cattle and 29 days for non-grazing cattle. Residual effect at 50% was achieved within 17 days for grazing cattle compared to 24 days for inshed cattle. In discussing the results, reference has been made to the exophilic and zoophilic tendencies of An. arabiensis, which are conducive for zooprophylaxis. Experimental studies in Verandah huts at Mabogini compared An. arabiensis and Culex spp collected from huts with different baits, i e. human, untreated cow and treated cow. Results indicate higher mortality rates in mosquitoes collected from the hut containing the treated cow (mean = 2) compared to huts with untreated cow (mean = 0.3) and human (mean = 0.8). A significantly higher number of Culex spp. was recorded in huts with treated cows compared to the rest. This study has demonstrated the role of cattle treated with pyrethroid in the control of malaria and reduction of vector density. It showed that, in areas with a predominant An. arabiensis population, cattle should be placed close to dwelling houses in order to maximize the effects of zooprophylaxis. Protective effects of cattle can further be enhanced by regular treatment with pyrethroids at least every three weeks. This paper demonstrates that cattle can be considered as Insecticide-Treated Material (ITM) as long as acaricide treatment is conducted regularly

Larvicidal efficacy of monoterpenes against the larvae of Anopheles gambiae

AbstractObjectiveTo evaluate the larvicidal efficacy of eight volatile components of essential oils against 3rd instar larvae of Anopheles gambiae s.s.MethodsLarvicidal effects of each compound were evaluated in both laboratory and semi-field trials. Stock solution was prepared and serial dilutions were made in six concentrations for each compound. A total of 20 larvae were exposed to larvicides for each replicate and monitored at intervals of 12, 24, 48 and 72 h. Larvae monitoring was done on basis of dead and live larvae in all intervals.ResultsAll assayed compounds were larvicides and presented varying degrees of larval toxicity, with LC50 values ranging from 1.28 to 1938.92 mg/L depending on the treatment time (12, 24, 48 or 72 h). (−)-Perillyl alcohol presented the strongest larvicidal activity towards Anopheles gambiae larvae, with LC50 values of 73.60, 18.36, 1.72 and 1.28 mg/L after 12, 24, 48 and 72 h of exposure, respectively. The next strongest were (−)-isopulegol (LC50 = 135.10, 49.39, 34.39 and 20.22 mg/L) and (−)-carvone epoxide (LC50 = 168.86, 124.74, 80.84 and 23.46 mg/L). After 12, 24 and 48 h of treatment, hydroxydihydrocarvone was the least toxic compound, with LC50 values of 1938.92, 1172.18 and 401.03 mg/L, respectively.ConclusionsThe data obtained in this study suggest that all evaluated monoterpenes, especially (−)-perillyl alcohol, have remarkable larvicidal effects and may be considered as potential sources for the development of suitable natural larvicides for mosquito management programs. Further small-scale field trials should be conducted

Activity of Cinnamomum Osmophloeum Leaf Essential Oil Against Anopheles gambiae s.s

The increasing status of insecticide resistant mosquitoes in sub-Saharan Africa is a threatening alert to the existing control efforts. All sibling species of An. gambiae complex have evolved insecticide resistance in wild populations for different approved classes of the insecticides currently in use in the field. An alternative compound for vector control is absolutely urgently needed. In this study, the larvicidal activity and chemical composition of the Cinnamomum osmophloeum leaf essential oils were investigated. C. osmophloeum leaf essential oils were extracted by hydrodistillation in a Clevenger-type apparatus for 6 hours, and their chemical compositions identified using GC-MS. These oils were evaluated against An. gambiae s.s. in both laboratory and semi-field situations. The WHO test procedures for monitoring larvicidal efficacy in malaria vectors were used. The composition of C. osmophloeum leaf essential oil has been found to have 11 active compounds. The most abundant compound was trans-cinnamaldehyde (70.20%) and the least abundant was caryophyllene oxide (0.08%). The larvicidal activity was found to be dosage and time dependant both in laboratory and semi-field environments with mortality ranging from 0% to 100%. The LC50 value was found to vary from 22.18 to 58.15 µg/ml in the laboratory while in semi-field environments it was 11.91 to 63.63 µg/ml. The LC90 value was found to range between 57.71 to 91.54 µg/ml in the laboratory while in semi-field environments was 52.07 to 173.77 µg/ml. Mortality ranged from 13% to 100% in the laboratory while in semi-field environments it ranged between 43% to 100% within mortality recording time intervals of 12, 24, 48, and 72 hours. The larvicidal activity shown by C. osmophloeum leaf essential oil is a promising alternative to existing larvicides or to be incorporated in integrated larval source management compounds for An. gambiae s.s control. The efficacy observed in this study is attributed to both major and minor compounds of the essential oils. \u