The Impact Of Climatic Variability And Malaria Control Interventions On Malaria Parasite Prevalence And Vector Abundance In Western Kenya

ABSTRACT

The World Health Organization estimated annual global malaria mortality to be at 405,000 deaths in 2018 making malaria one of the most important diseases in the world. In Kenya, malaria control measures have been put up, however, the ongoing climate change and climatic variability is proving to be a great challenge. The aim of this study was to investigate the impact of climatic variability and malaria control interventions on malaria parasite prevalence and vector abundance in western Kenya. This was a longitudinal study where the same population was studied over a continuous period of time. Six sites; Iguhu, Emakakha, Marani, Kombewa, Rae and Miwani were selected for the study. Indoor resting mosquitoes were collected monthly from 2012 to 2014. Outdoor mosquito collections were done using rotator traps every two hours between 6pm-8pm during the same time period. Human behaviour study was also conducted in the study sites to show when the study participants stayed outdoors. Polymerase chain reaction was done to confirm species and Enzyme linked immunosorbent assay was done to confirm sporozoite infectivity. Bed net ownership in the study sites was analysed. Malaria parasitological surveys were conducted in 100 randomly selected children aged between 6-15 years in 2002-2003 and 2012-2015 in Iguhu to determine parasite prevalence and densities of sexual and asexual stages. Age-dependence in parasite infections was compared between the pre and post intervention period. Climate and clinical data were analysed from Iguhu and Marani from 1980 to 2015. Data on vector abundance, various malaria control interventions and asymptomatic malaria cases was collected from the study sites. The effects of temperature and rainfall on malaria parasites and vectors was shown using Poisson regression analysis. The results revealed 58.2% Anopheles gambiae were collected and 41.8% Anopheles funestus. Fifty percent of the study population stayed indoors in the study sites between 08.00pm-06.00am. Bed net ownership in 2012 averaged 87%. Plasmodium falciparum parasite prevalence among children ≤ 15 years of age reduced from 43.5% in the pre intervention period to 11.5% in the post intervention period, a 73.7% reduction in the post intervention period. Parasite densities per ml of blood of P. falciparum between the pre and post intervention period increased by 71.2%. The geometric mean gametocytaemia in Iguhu was 106.4 parasites/µl, in the post intervention period. Prevalence and density of P. falciparum showed no age-dependency during post-intervention period. Maximum temperature and rainfall were highly associated with the density of An. gambiae (χ 2=2.44, d.f = 1, P= 0.015). Malaria cases showed a positive significant relationship with a 4-month moving average of the total monthly rainfall despite control. In conclusion, insecticide treated bed net use led to early feeding of malaria vectors before dawn and before dusk. The use of both bed nets and Artemisinin combination therapy for a prolonged period of time reduced parasitological resistance and transmission blocking immunity. Increased rainfall and maximum and minimum temperature led to the increase in malaria transmission even in areas where interventions were in place. Sustained use of insecticide treated nets as a component of integrated vector management is recommended to reduce early feeding of mosquitoes and prevent malaria transmission in children. Continuous surveillance of Plasmodium parasite transmission is also recommended to identify new malaria hotspots due to climatic variability.