Rift Valley Fever in Africa
Kariuki Njenga, DVM, PhDCenters for Disease Control & Prevention, Nairobi, Kenya
L. Baringo
L.
Bogoria
Legend: • Livestock confirmed cases
+ Sublocations with confirmed human cases
Solonchak soils
Density map of human and animal cases on
solonchak soil
Munyua et al., Amer J Trop Med Hyg 83 (Supp2), 58-64,2010
Transmitted by infected mosquito bites
– Various species of mosquito
– other biting insects can transmit
Affects sheep, cattle, camel, goats
– Abortions
– bleeding
RVF Disease: Transmission in Livestock
RVF Disease: Transmission in humans
o Direct contact with blood, meat, milk and other
body fluids of infected animals
o Can occur during slaughtering, skinning
o Bite of infected mosquito
Aerosol transmission?
– Generated during slaughter of infected livestock
What are risk factors of acute RVF in humans?
• Touching an aborted fetus [OR = 3.83]
• Consuming or handling products from sick animals
[OR = 2.53]
• Being a herdsman [OR = 1.77]
• Exposure to mosquitoes was not an independent
risk factor
Amwayi et al., Amer J Trop Med, 83 (supp2), 14-21, 2010
19
40
19
50
19
60
19
70
1980
19
90
20
00
20
10
Egypt
Egypt
Mauri
tania
Ma
urita
nia
KS
A/Y
em
en
Egypt
Su
da
nR
SA
/Com
oro
s
RVF activity in East Africa*
RVF Epidemics (1932 – 2011)
RS
AM
adagascar
RS
A
Recent Severe Human RVF OutbreaksYear Countries Estimated
Cases
Reported
Cases
Deaths
1977 Egypt 200,000 18,000 598
1987 Mauritania 1500 300 224
1993 Egypt 1500 ? 20
1997-98 Kenya, Somalia,
Tanzania
89,000 ? 4581
1998 Mauritania 300 150 7
1999 Mauritania ? ? 2
2000 KSA, Yemen 1500
2000
516
1087
87
121
2003 Egypt 300 148 27
2006-07 Kenya, Tanzania,
Somalia
? 700
309
107
158
144
54
2007 Sudan ? 125+ 60+
2008, 2009 Madagascar, RSA,
Comoros
? ? ?
2010,2011 RSA ? 255 26
1Data available from Kenya only
RVF Risk Mapping: Set upRVF – epizootics occur under favorable and persistent eco-climatic conditions
Can be mapped – either as rainfall or
vegetation – NDVI integrates all the required conditions
Algorithm:
– Mapping of potential epizootic areas –
based on literature survey and climate
variable thresholding= potential epizootic
area mask (PEAM) – (C. J. Peters & K.J.
Linthicum in Handbook of Zoonoses)
– NDVI anomaly calculation -- + anomalies >
0.025 threshold (desert calibration) over 3
month period
– Persistently + anomalies must have three
month mean > 0.1
– All “pixels” that meet this criteria and are
within the PEAM are mapped to have
conditions necessary for the occurrence
of RVF activity
Epidemic prediction: Relies on field dataTeamwork
Capacity Building
1
12
21
2
3
17
10
7
6
9
15 8
5
22
14
20
16
4
19
18
11
13
6-
7
14
20
16
19
18
11
13
1
12
21
2
3
17
10
7
6
9
15 8
5
22
14
20
16
4
19
18
11
13
6-
7
14
20
16
19
18
11
13
1951-1955 1997-2007
Murithi et al., Epi Infect 18, 1-9, 2010
Kenya: RVF Spread Over 52
Last 2 epizootics affected 6 of 8 provinces, 38 of 69 districts
Prediction vs. Outbreak Timing: Epi-Curves: 2006 - 2008
Anyamba et al (In Review AJTMH)
What is the inter-epizootic period (IEP)?
IEP
IEP
IEP
IEPMean IEP = 3.6 years
Murithi et al., Epi Infect 18, 1-9, 2010
Based on Kenya Dept Vet Svs Data – 1910 - 2007
Where is RVF virus maintained ?
1. Transovarially in Aedes mosquito spp
2. Endemicity. Cycling between wildlife, livestock and possibly humans within a permissive
ecosystem, mediated by the appropriate vectors.
Cummulative percent of positive samples at each titer level
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
1:12
80
1:64
0
1:32
0
1:16
01:
801:
401:
201:
10
Titer level
% o
f p
osit
ive s
am
ple
s o
f
sp
ecie
s Buffalo
Black rhino
Other
In wildlife
Evans et al, Epid Inf 8, 1-92007
Negative: Lions, giraffes, common zebras
9 of 17 species tested
were positive
- Buffalos, Black rhinos,
- Thompson gazelles,
- Impalas, Elands, Kudus
Rostal et al, Amer J Vet Res 71, 522-6, 2010
In livestock : Kenya during the 1999 – 2006 IEP
Where is RVF virus maintained ?
Sheep Goats
Year
of
birth
No. testedPercentage
positiveNo. tested
Percentage
positive
1999 12 25 5 0
2000 27 11 7 14
2001 17 29 7 0
2002 19 11 10 0
2003 17 18 17 0
2004 29 24 10 10
2005 63 17 15 0
2006 4 0 4 0
Total 188 18* 2/75 3
Northeastern Province of Kenya
December, 1997 December, 2006
Sequential Epidemics in Kenya, Somalia, and Tanzania [1997-8; 2006-7]
Reported Human RVF Cases (Deaths) in Kenya
Wajir
26(12)
Garissa
176(59)
Ijara
131(27)Tana-River
21(8)
Kilifi
73(20)
Malindi
15(4)Taita-Taveta
15(1)
ISIOLO
13(0)
Kajiado
10(6)
Baringo
174(13)
Nakuru
1(1)
Kirinyaga
5(2)
Maragua
4(0)
Thika
2(0)Nairobi
5(2)
NEP333 of 700 (45%)
cases
Baringo174 of 700 (24.8%)
Cases
Kilifi73 of 700 (10.4%)
cases
Confirmed and Probable Rift Valley Fever Cases ,
Kenya 2006/07(N=340)
0
2
4
6
8
10
12
14
16
18
2021/1
1/2
006
28/1
1/2
006
05/1
2/2
006
12/1
2/2
006
19/1
2/2
006
26/1
2/2
006
02/0
1/2
007
09/0
1/2
007
16/0
1/2
007
23/0
1/2
007
30/0
1/2
007
06/0
2/2
007
13/0
2/2
007
20/0
2/2
007
27/0
2/2
007
06/0
3/2
007
13/0
3/2
007
20/0
3/2
007
27/0
3/2
007
Date of Onset
Ca
se
s
Alive
DeadNEP Kilifi
Baringo
RVF Outbreaks in Somalia
First case confirmed 20th January, 2007
Total:107 cases, 54
(50.5%) deaths
RVF Outbreak in Tanzania
First case confirmed Feb 1st, 2007
Total: 309 cases, 144 deaths
Sequential Epidemics: How does the virus spread?
2006-2007 Epidemic
Region 1: Nov 26, 2006
Region 2: Dec 26, 2006
Regions 3,4: Jan 7, 2007
Region 5: Feb 5, 2007
Region 6: Jan 27, 2007
Region 7: Feb 1, 2007
Nderitu et al., J Inf Dis 203, 2011
Ken
ya
2
Ke
nya
1b
Ken
ya
1a
Ta
nzan
ia 1
How do epizootics spread?
Multiple lineages of RVF virus during one epidemic indicate spontaneous
activation of resident virus
Nderitu et al., J Inf Dis 203, 2011; Bird et al, 2008
Number of confirmed RVF cases and deaths by
province, South Africa, 2010- 2011 (latest April 10, 2011)
All cases occurred mid-February – May each year
Province Cases Deaths 2010 2011 2010 2011
Eastern Cape 17 13 1 0
Free State 123 3 11 0
Gauteng 0 0 0 0
KwaZulu-Natal 0 0 0 0
Limpopo 0 0 0 0
Mpumalanga 0 0 0 0
Northern Cape 74 2 11 0
North West 9 0 2 0
Western Cape 4 9 1 0
Total 228 27 26 0
Source: NICD, South Africa
South Africa: RVF Infection Map, April 2011
Source: NICD, South Africa
1st Epizootic / Epidemic 1990 -few cases
Another in 1991
26
#
Antananarivo
Perinet Forest
Epizootic / Epidemic 1990
Epizootic / Epidemic 1991
RVF in Madagascar
Last cases in 2009
Last cases in March 2009
#
Antananarivo
27
Epizootic / Epidemic 2008-2009
Primary Forest of Perinet
Epizootic / Epidemic 1990
Epizootic / Epidemic 1991
Epizootic / Epidemic 2008
RVF in Madagascar
Major Interventions (Eastern Africa)
Social mobilization
Slaughtering ban
– Major religious holiday (Idd festival)
Livestock quarantine
Livestock vaccination
Vector control (minimal)
RVF Outbreak Response 2006 – A Step BehindLivestock vaccination Public Education
Challenges
Poor accessibility: Flooding
Timing: Occurred in peak holiday season
Limited health services in the region
Coordinating surveillance between
livestock and human health