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 WellbeingLondon, UK

17–18 March 2016

Acknowledgements

Mohammed Said1, Rosemary Sang2, Salome 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, Livestock & Fisheries, Nairobi5. Division of Disease Surveillance and Response, Ministry of Public Health, Nairobi

Dynamic Drivers of Disease in Africa REF:NE/J001422/1”

• 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

• The study site:• Arid/semi-arid region in

northeastern Kenya• Two irrigation schemes and

adjacent pastoral areas• Studies:

o Ecological/GIS analyses –Entomological surveys

oParticipatory studies and socio-economic surveys

o Sero-epidemiological surveys in livestock and people

• Support to policy makers to improve disease surveillance and response

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

o 6pm-6am for 3 consecutive days/site

• Livestock and human samplingo Blood samplingo Serum extraction and storageo 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)

Participatory and socio-economic surveys

Services- Water- Food - Income

Dis-services- Diseases (malaria,

bilharzia)- Exposure to agro-

chemicals

Land use change and disease transmission

Aedes spp Anopheles spp

Culex spp Mansonia spp

1

10

100

1000

10000

irrigated area

non-irrigated area

Villages

Mosquito species

Log

num

ber o

f mos

quito

es

Aedes spp Anopheles spp

Culex spp Mansonia spp

1

10

100

1000

10000irrigated area

non-irrigated area

Farms

Mosquito species

Log

num

ber o

f mos

quito

esAedes spp Anopheles

sppCulex spp Mansonia

spp

1

10

100

1000

10000

irrigated areanon-irrigated area

Villages

Mosquito species

Log

num

ber o

f mos

quito

es

Aedes spp Anopheles spp

Culex spp Mansonia spp

1

10

100

1000

10000

irrigated area

non-irrigated area

Farms

Mosquito species

Log

num

ber o

f mos

quito

es

I

Fallo

w p

erio

d Irr

igati

on se

ason

Results: Apparent densities of mosquitoes trapped

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 0.03Hyper-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

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

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

This work, Dynamic Drivers of Disease in Africa Consortium, NERC project numberNE-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).