Presentation by Bernard Bett at a symposium on 'One Health for the Real World: Zoonoses, Ecosystems and Wellbeing', London, UK, 17–18 March 2016.

1. Irrigation and the risk of Rift Valley fever
transmission: A case study from Kenya
Bernard Bett, International Livestock Research Institute
One Health for the Real World: Zoonoses, Ecosystems and Wellbeing
London, UK
17–18 March 2016

2. Acknowledgements
Mohammed Said1, Rosemary Sang2, Salome Bukachi3, Johanna Lindahl1, Salome Wanyoike4,
Ian Njeru5, Delia Grace1
1. International Livestock Research Institute, Nairobi
2. Kenya Medical Research Institute, Mbagathi Way, Nairobi
3. Institute of Anthropology, Gender and African Studies, Nairobi
4. Department of Veterinary Services, Ministry of Agriculture, Livestock & Fisheries, Nairobi
5. Division of Disease Surveillance and Response, Ministry of Public Health, Nairobi
Dynamic Drivers of Disease in Africa
REF:NE/J001422/1”

3. • RVF:
• Mosquito-borne viral
zoonosis
• High and persistent rainfall
• Would irrigation promote
endemic RVF?
• Irrigation and trade offs in
ecosystem services
 Water and food
 Risk of vector-borne diseases
Irrigated site with stagnant
water in the drainage canals
– source of water for people
but also breeding grounds
for mosquitoes
Rift Valley fever case study

4. • The study site:
• Arid/semi-arid region in
northeastern Kenya
• Two irrigation schemes and
adjacent pastoral areas
• Studies:
oEcological/GIS analyses –
Entomological surveys
oParticipatory studies and
socio-economic surveys
oSero-epidemiological surveys
in livestock and people
• Support to policy makers to
improve disease surveillance and
response
Methods
Study site in Kenya, GIS team, ILRI

5. 20 0 20 40 60 80 Kilometers
N
Open shrubs (65-40% crown cover)
Very open shrubs (40-15% crown cover)
Closed herbaceous vegetation on
permanently flooded land
Open to closed herbaceous vegetation
on temporarily flooded
Open to closed herbaceous vegetation
Irrigated land / Cropland
Clouds
Tana River-Waterbodies
Urban and Rural Settements
Open trees on temporarily flooded land
Trees and shrubs savannah
Very 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

6. Activities – Field sites
• Mosquito sampling
o 6pm-6am for 3 consecutive
days/site
• Livestock and human sampling
o Blood sampling
o Serum extraction and storage
o Sample screening using ELISA
kits
• Data analyzed using
geostatistical models to
account for spatial effect
Field surveys
Animal sampling
(photo credit: ILRI/Bernard Bett)
CDC light trap for mosquitoes
(photo credit: ILRI/Bernard Bett)

7. Participatory and socio-economic surveys
Services
- Water
- Food
- Income
Dis-services
- Diseases (malaria,
bilharzia)
- Exposure to agro-
chemicals

8. Land use change and disease transmission
1
10
100
1000
10000
Aedes spp Anopheles
spp
Culex spp Mansonia
spp
irrigated area
non-irrigated area
Villages
Mosquito species
Lognumberofmosquitoes
1
10
100
1000
10000
Aedes spp Anopheles
spp
Culex spp Mansonia
spp
irrigated area
non-irrigated area
Farms
Mosquito species
Lognumberofmosquitoes
1
10
100
1000
10000
Aedes spp Anopheles
spp
Culex spp Mansonia spp
irrigated area
non-irrigated area
Villages
Mosquito species
Lognumberofmosquitoes
1
10
100
1000
10000
Aedes spp Anopheles
spp
Culex spp Mansonia
spp
irrigated area
non-irrigated area
Farms
Mosquito species
Lognumberofmosquitoes
I
FallowperiodIrrigationseason
Results: Apparent densities of mosquitoes trapped

9. Variable Levels All mosquitoes trapped Primary RVF vectors
Mean 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 0.00
Rain 0.03 0.00 0.02 0.03 0.03 0.00 0.02 0.03
Hyper-parameters
Theta 1 -3.03 1.97 -6.79 0.95 -3.53 3.16 -9.75 2.68
Theta 2 1.87 1.53 -1.23 4.75 2.26 3.16 -3.95 8.46
DIC 1099.57 641.39
Outputs of a regression model used to analyse the
effects of rainfall and irrigation on mosquito densities

10. Analysis of sero-prevalence data from people
Variable Level Rift Valley fever sero-prevalence
Odds Ratio P> |Z |
Estimate 95% CI
Fixed effects
Gender Male 1.85 1.28 – 2.66 0.00
Female 1.00
Age (years) <9 -
9 - <18 0.10 0.02 – 0.48 0.00
>18 - <30 0.64 0.42 – 0.98 0.04
>30 1.00
Occupation Farmer 0.44 0.21 – 0.92 0.03
Pastoralist 1.00 -
Student 0.32 0.05 – 2.03 0.23
Other 0.85 0.47 – 1.54 0.60
Household size <10 1.00 -
>10 1.81 1.20 – 2.73 0.01
Site Irrigated 1.77 0.85 – 3.92 0.12
Riverine 1.83 0.85 – 3.92 0.11
Pastoral 1.00
Random effects
ICCc: Household | Village
Log likelihood -343.87

11. Discussion
• Irrigation – increased food production but more habitat
fragmentation and less biodiversity
• Primary vectors of RVF found in drainage canals. This
implies increased risk of RVF
• Seri-prevalence in livestock and people– higher in
irrigated area but not significant. Surveillance for active
infections required
• To manage vector-borne diseases -- better irrigation
technologies instead of flood irrigation should be
considered

12. This work, Dynamic Drivers of Disease in Africa Consortium, NERC project
number
NE-J001570-1, was funded with support from the Ecosystem Services for
Poverty Alleviation (ESPA) programme. The ESPA programme is funded by the
Department for International Development (DFID), the Economic and Social
Research Council (ESRC) and the Natural Environment Research Council
(NERC).