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Land use change and the risk of selected zoonotic diseases: observations from a case study in an arid/semi‐arid area in Kenya
Bernard Bett1, Said Mohammed1, Rosemary Sang2, Sally Bukachi3, Johanna Lindahl1, Salome Wanyoike4, Ian Njeru5, Delia Grace1
1. International Livestock Research Institute, Nairobi2. Kenya Medical Research Institute, Mbagathi Way, Nairobi3. Institute of Anthropology, Gender and African Studies, Nairobi4. Department of Veterinary Services, Ministry of Agriculture, Nairobi5. Division of Disease Surveillance and Response, Ministry of Public Health, Nairobi
Oral presentation at the ESPA Annual Scientific Conference, 25th to 26th November 2015 at the Honourable Society of Gray’s Inn, London
Introduction• Climate and demographic changes and food insecurity
• Irrigation – being used to boost food production in arid/semi‐arid areas
• Effects: major trade‐offs in ecosystem services
More food produced (provisioning services) at the expense of biodiversity and regulatory services (disease, flooding, erosion)
Disease transmission contributed by:o Standing water masses associated with irrigationo Human settlements and peri‐urban developmento Livestock diversity – more small that large ruminants
• The effects of irrigation on:oEcosystem changes –diversity of hosts
orisk of vector‐borne and other zoonotic diseases
Irrigated site with stagnant water in the drainage canals – source of water for the people but also breeding grounds for mosquitoes
Objectives
• The study site:• Arid/semi‐arid region in northeastern Kenya
• Two irrigation schemes and pastoral areas around them
• Studies:o Ecological/GIS analyses –secondary data
o Entomological surveyso Sero‐epidemiological surveys in livestock and people
Methods
Study site in Kenya, GIS team, ILRI
20 0 20 40 60 80 Kilometers
N
Open shrubs (65-40% crown cover)Very open shrubs (40-15% crown cover)
Closed herbaceous vegetation onpermanently flooded landOpen to closed herbaceous vegetationon temporarily floodedOpen to closed herbaceous vegetation
Irrigated land / Cropland
CloudsTana River-Waterbodies
Urban and Rural Settements
Open trees on temporarily flooded landTrees and shrubs savannahVery open trees (40-15% crown cover)Open trees (65-40% crown cover)Closed trees
Legenda) 1975 b) 2010
Ecological analyses: Land cover changes between 1975 and 2010
Activities – Field sites
• Mosquito sampling• Livestock and human
samplingo Sample size determinationo Serum samples analyzed
using various ELISA kits
Pathogen Samples used
Rift Valley fever virus, Brucella spp., and Coxiella burnetti
Livestock and people
West Nile virus, denguefever virus, Leptospiraspp
People
Field surveys
Animal sampling, B.Bett, ILRI
CDC light trap for mosquitoes, B.Bett, ILRI
• Ecosystem changes – GIS analysis to determine habitat rarity
• Entomology and sero‐prevalence datao Treated as point‐referenced
datao Analyzed using stochastic
partial differential equation implemented in R INLA
(Rue et al., 2009)o Significance of the spatial
effect ‐‐ DIC
Data analysis
Triangulation in R INLA to capture spatial effects
• Land cover changes
Results
1975 2010 Area Change
(%) Land cover type Area (km2)
Area (%)
Area (km2)
Area (%)
Closed trees 63 1 0 0 -100Open trees 1,138 18 795 12 -30Open trees on temporarily flooded land 171 3 257 4 50Bushlands 765 12 493 8 -36Open shrubs 2,398 37 2,639 41 10Sparse Shrubs 0 0 685 11 11Open to closed herbaceous vegetation 206 3 0 0 -100Herbaceous vegetation on flooded land 881 14 1,301 20 48Cropland 14 0 223 3 1456Water bodies 64 1 47 1 -26Urban and associated rural settlements 0 0 9 0 100Cloud cover 750 12 0 0
• Trends in wildlife and livestock densities
Results
Species PE 1970s
SE 1970s
PE 2000s
SE 2000s d P value
Cattle 46,919 9,862 40,820 9,354 -0.45 0.326355Camel 1,621 789 4,391 1,082 2.07 0.019226Donkey 926 386 779 314 -0.30 0.488033Shoats 45,348 10,357 71,435 9,614 1.85 0.032157Buffalo 384 235 0 0 -1.64 0.050503Elephant 41 40 0 0 -1.02 0.153864Grant's gazelle 1,341 384 145 77 -3.05 0.001144Gerenuk 618 157 643 144 0.12 0.452242Giraffe 833 230 114 62 -3.02 0.001264Hunter's hartebeest 97 84 83 64 -0.13 0.448283Impala 247 239 0 0 -1.03 0.151505Lesser kudu 209 110 177 59 -0.26 0.397431Oryx 697 209 84 47 -2.86 0.002118Ostrich 629 195 123 47 -2.53 0.005703Warthog 2,567 468 103 43 -5.25 0.000000Burchell's zebra 5,178 1,558 0 0 -3.32 0.000450
Land use change and disease transmission
1
10
100
1000
10000
Aedes spp Anophelesspp
Culex spp Mansoniaspp
irrigated area
non‐irrigated area
Villages
Mosquito species
Log nu
mbe
rof m
osqu
itoes
1
10
100
1000
10000
Aedes spp Anophelesspp
Culex spp Mansoniaspp
irrigated area
non‐irrigated area
Farms
Mosquito species
Log nu
mbe
rof m
osqu
itoes
1
10
100
1000
10000
Aedes spp Anophelesspp
Culex spp Mansonia spp
irrigated area
non‐irrigated area
Villages
Mosquito species
Log nu
mbe
rof m
osqu
itoes
1
10
100
1000
10000
Aedes spp Anophelesspp
Culex spp Mansoniaspp
irrigated area
non‐irrigated area
Farms
Mosquito species
Log nu
mbe
rof m
osqu
itoes
I
Fallow period
Irrigation season
Results: Apparent densities of mosquitoes trapped
Variable Levels All mosquitoes trapped Primary RVF vectorsMean SD Credible interval Mean SD Credible interval
2.50% 97.50% 2.50% 97.50%Land use Irrigation 1.23 0.38 0.46 1.94 1.47 0.19 1.10 1.85
Other 0.00 0.00Rain 0.03 0.00 0.02 0.03 0.03 0.00 0.02 0.03Hyper‐parameters Theta 1 ‐3.03 1.97 ‐6.79 0.95 ‐3.53 3.16 ‐9.75 2.68Theta 2 1.87 1.53 ‐1.23 4.75 2.26 3.16 ‐3.95 8.46DIC 1099.57 641.39
Outputs of a regression model used to analyse the effects of rainfall and irrigation on mosquito densities
Posterior distributions of irrigation and rainfall parameters from the mosquito regression model
Sero‐prevalences of target pathogens in livestock
Pathogen Sero‐prevalence (95% CI)Irrigated areas Non‐irrigated areas
Rift Valley fever 22.2 (20.1 – 24.4) 36.0 (31.7 – 40.5)Coxiella burnetti 14.5 (13.1 – 16.0) 9.5 (7.2 – 12.2)Brucella spp 2.8 (2.0 – 3.7) 5.3 (3.4 – 7.7)
Posterior distribution of the land use parameter
Odd ratios from a regression model used to analyse sero‐prevalences of the zoonotic pathogens in people
‐ Odds of being exposed in an irrigated area compared to pastoral
1.27 1.27
1.13
0.38
0.21
0
0.2
0.4
0.6
0.8
1
1.2
1.4Odd
s ratio
Febrile cases reported in people: Data from hospitals verses cross‐sectional surveys
A total of 1,323 samples collected in cross‐sectional surveys, 481 already screened
0%
10%
20%
30%
40%
50%
WNV Dengue RVF Q fever
Non‐irrigated area
Irrigated area
Pathogen
Sero‐prevalence
Malaria Typhoid
Cases from Bura health centre
Cases from Hola health centre
Malaria Typhoid Brucellosis
Discussion• Irrigation – increased food production at the expense of
habitat fragmentation, biodiversity conservation
• Used multiple pathogens to generate generic lessons
• Irrigation and primary vectors of RVF
• Biodiversity and disease regulation/dilution effect ‐‐‐inconsistent findings o Results from livestock – no significant patterns – movement
across areaso Results from people – higher risk of vector‐borne diseases in
irrigated area and directly transmitted zoonoses in pastoral area
Way forward • Communicate findings:
• Health impacts of land use change/irrigation, biodiversity
• Shifts in suitability of ecologies for multiple pathogens• Health seeking behavior among the poor
• Provision of health services• capacity to diagnose diseases
• Land use and water management•
Acknowledgements
This work is from the project ‘Dynamic Drivers of Disease in Africa: Ecosystems, livestock/wildlife, health and wellbeing: REF:NE/J001422/1” partly funded with support from the Ecosystem Services for Poverty Alleviation Programme (ESPA). The ESPA program is funded by the Department for International Development (DFID), the Economic and Social Research Council (ESRC) and the Natural Environment Research Council (NERC). Other funding was provided by CGIAR Research Program Agriculture for Nutrition and Health