Malaria situation in Namibia: A study of vector species and effectiveness of the past and current control strategies in selected parts of Namibia

Abstract

The distribution of malaria in Namibia is mainly confined to the northern parts of Namibia. Although the malaria vectors were presumed to be Anopheles arabiensis, a member of the Anopheles gambiae group of morphologically similar species, no scientific study had been undertaken to identify the species in Namibia. Given the variable behaviour of this species, in an area where the mainstay for vector control has been indoor residual house spraying (IRS) with 75% DDT wettable powder since the 1960’s, reassessment of the vector species composition and its resting and biting behaviour are absolutely important. In addition to insecticide house spraying, the malaria control included case management with Chloroquine, as the first line treatment for uncomplicated malaria. The general objective of the study was to determine the vector species in malaria transmission, seasonal abundance, behaviour of malaria vectors and efficiency of the diagnostic and treatment procedures and the overall malaria control scenario in Namibia. Two sites were selected for this study based on the demographic, epidemiological and climatic conditions that are believed to represent the various malaria endemic areas in northern Namibia. Malaria vector species were collected from Kalimbeza village, Katima Mulilo district in northeast of Namibia by means of exit window traps of the Muirhead-Thomson design for a year. In addition, Pyrethrum Spray Catches (PSC) were used to collect mosquitoes in Calueque, southern Angola and northern Namibia. Species identification was by means of Polymerase Chain Reaction (PCR). Thick and thin blood smears and body temperatures were collected from suspected malaria patients in Bukalo and Mahenene health centres in order to determine malaria parasite prevalence and malaria episodes caused by Plasmodium falciparum. The therapeutic efficacy study was carried out over 28 days’ follow-up period, in line with the World Health Organization (WHO). Retrospective malaria data for inpatient and outpatient were obtained from the Ministry of Health and Social Services. They were analysed and graphically presented on a map. Analysis of the retrospective malaria statistics in Namibia from 1995 to 2003 showed that, children under the age of five years were more affected by malaria as compared to those above five years. This pattern is similar in all malaria endemic areas because of lack of immunity among the children under the age of five years. Immunity due to malaria is known to build up with age. In non-malaria endemic areas on the other hand, there is usually no significant difference between the different age groups. The highest mean incidence ratio of malaria among the two age groups was observed in Kavango (3.4:1) and Caprivi (3:1) regions. Furthermore, the overall malaria morbidity and mortality rates were exceptionally higher in Kavango regions followed by Omusati and Ohangwena regions. There are two explanations for this finding. Firstly the areas receiving more rains and/or are in proximity to rivers have more malaria than others. The same is true if the areas border with neighbouring countries where there are no prevention interventions in place as a result of either a damaged health system or general lack of resources. This was the case for the three regions. It is therefore important for neighbouring countries to strengthen border coordination for malaria control and prevention as malaria knows no borders. The identification of mosquito specimen by means of Polymerase Chain Reaction (PCR) revealed that the most common vector species in northern Namibia is An. arabiensis (89%). This was the only member of the An. gambiae group of species reported from the present study. Other malaria vector species included An. funestus, which accounted for 1% and the remaining 10% comprised of non-malaria vector species. The study further revealed that there are more An. funestus (36%) in unsprayed areas of Calueque, in southern Angola than An. arabiensis which only accounted for 30%. This confirmed the high endophilic tendency (preference for indoor-resting) of An. funestus, which makes it more amenable to vector control interventions such as indoor residual house-spraying and/or insecticide-treated bednets (ITNs). More An. arabiensis (77%) were identified during the wet season in Andara and Kalimbeza villages than during the dry season (13%). To the contrary, An. funestus species were more abundant during the dry season (6%) compared to 3% collected during the wet season. A chi-square statistic of 67.3, P = 0.000 with 2 degrees of freedom (n = 444) indicate that the two variables (season and species) are not independent. The seasonal distribution of An. arabiensis therefore coincided with the malaria peak season which is during the wet season. Any vector control intervention must therefore be applied just before the wet season to have an impact on the malaria transmission. Out of a total of 1294 mosquitoes belonging to An. arabiensis caught in window traps, only 31% were fully blood fed. These results suggest that some mosquitoes could penetrate into huts but could not rest on walls or roofs of sprayed huts to find sufficient time to land on humans for a blood meal. The possible explanations for this are variable. It could possibly be that the insecticide on the walls (DDT) irritated and/or inhibited feeding of the mosquitoes. A high proportion of unfed female mosquitoes could also be due to lack of insemination during mating before they entered the sprayed huts. Answers to these questions however, were not part of the initial objectives of the present study. The results of the relationship between fevers and clinical malaria on the other hand, revealed no correlation between fevers and clinical malaria. The correlation coefficient between having fever and having a positive blood smear was 0.10 (p = 0.71), indicating that the difference is not statistically significant. Moreover, the Chi-square statistic for reports of fevers and a positive blood smear was also not statistically significant different. For example 72% of the patients who had fever were in fact not having malaria. This means that the statistical test reveals that most patients that were diagnosed to have fever and hence given malarial treatment did not have clinical malaria on the basis of microscopic examination (clinical malaria). The current study therefore indicates that health facilities in Namibia, like elsewhere are wasting a lot of meagre resources through over-diagnosis of patients as malaria. The situation is made worse when patients have to be treated with the more expensive Artemisnin-based combination therapies (ACTs) due to chloroquine and supposedly Sulfadoxine Pyrimethamine failures. This finding calls for the need to use microscopic examination to improve malaria diagnosis in health facilities so that only those positive for malaria are treated with antimalarial drugs. The results of the chloroquine efficacy study demonstrated that Treatment Failure rate was not significantly lower than 25%, which is the cut-off point for detecting resistance. With these results it is clear that chloroquine can no longer be used as a first line drug for the treatment of uncomplicated malaria in Namibia. Based on the findings of the current study and its implications to improve the vector control and case management interventions employed by the National Vector-borne Disease Control Programme in Namibia, the following recommendations are pertinent: 1. There is a need to further explore the presence of An. funestus along the border of Kavango and Angola and that of Caprivi and Zambia. Further investigation is needed to establish whether or not this species is indigenous. The possibility of introduction from Angola and Zambia cannot be completely ruled out thus calling for closer malaria border coordination activities. 2. The proportion of surviving adult blood fed mosquitoes caught in exit window traps is worrying as they can still transmit malaria even in the presence of insecticide residues. The challenge remains for the National Vector-borne Disease Control Programme to further monitor the proportion of vectors resting in-doors and out-doors and based on the respective findings, be able to respond appropriately. 3. The insecticides currently in use (DDT, deltamethrin and permethrin) by the National Vector-borne Disease Control Programme are effective against the local malaria vectors - An. arabiensis and An. funestus group. To ascertain their continued use in Namibia would require the strengthening of the control programme current capacity to monitor insecticide resistance in well chosen sentinel sites across the malaria endemic regions of the country. 4. Malaria is highly over-diagnosed. The MoHSS needs to seriously consider introducing diagnostic facilities at peripheral health facilities to minimize over-diagnosis and wastages of drugs - especially as the current trend is to move towards the more expensive artemisinin-based combination therapy (ACT) as first line treatment for uncomplicated malaria. In non-sprayed villages of southern Angola malaria prevalence is higher than in the neighbouring villages of Namibia where spraying was done. It is important that border coordination is strengthened with neighbouring countries – including the area of vector control. 5. Because of the reduced efficacy of chloroquine, the need to change the treatment policy to combination-based therapies is more imminent than ever before for reasons of efficacy and compliance. 6. Analysis of the retrospective malaria data in the country demonstrated that malaria incidence was highest in Kavango region followed by Omusati, Ohangwena, Oshikoto, Caprivi, Kunene, Otjozondjupa and Oshana respectively. On the other hand, the analysis of mortality data revealed that Kavango, Omusati, Oshikoto, Ohangwena, Oshana and Caprivi regions experience the highest rates in descending order. The data also revealed that the under five-age group in Kavango and Caprivi were three times more frequently affected than the five years and above age groups. This is an indication that malaria is relatively more stable in Kavango and Caprivi regions as compared to other malaria endemic regions. 7. Appraising the role of vector control as one of the key strategies for malaria control in Namibia, it is important that the national capacity to plan, implement, monitor and evaluate the different vector control interventions is strengthened by allocating adequate resources for capacity in entomology and vector control and resources for physical infrastructures and for operations. 8. With the general trend of most malaria endemic countries to decentralize their health services to the lowest administrative levels, it will be necessary for the MoHSS to determine the administrative level to which vector control services could be devolved while ensuring that the impact of key vector control interventions are not compromised.