Rift Valley Fever is a mosquito-borne viral disease that affects livestock and humans in Africa. The document summarizes key information about Rift Valley Fever, including its transmission between animals and humans, risk factors for human infection, mapping of outbreak areas based on environmental conditions, and details major outbreaks that have occurred across Eastern and Southern Africa between 1940-2011.

1. Rift Valley Fever in Africa
Kariuki Njenga, DVM, PhD
Centers for Disease Control & Prevention, Nairobi, Kenya

3. Density map of human and animal cases on
solonchak soil
Legend: • Livestock confirmed cases
+ Sublocations with confirmed human cases
Solonchak soils
L. Baringo
L.
Bogoria
Munyua et al., Amer J Trop Med Hyg 83 (Supp2), 58-64,2010

4. RVF Disease: Transmission in Livestock
Transmitted by infected mosquito bites
– Various species of mosquito
– other biting insects can transmit
Affects sheep, cattle, camel, goats
– Abortions
– bleeding

5. 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

6. 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

7. 1940
1950
1960
1970
RVF activity in East Africa*
Egypt
1980
RVF Epidemics (1932 – 2011)
Mauritania
1990
Egypt
Mauritania
2000 KSA/Yemen
Egypt
Sudan
RSA/Comoros
Madagascar
2010 RSA
RSA

8. Recent Severe Human RVF Outbreaks
Year Countries Estimated Reported Deaths
Cases Cases
1977 Egypt 200,000 18,000 598
1987 Mauritania 1500 300 224
1993 Egypt 1500 ? 20
1997-98 Kenya, Somalia, 89,000 ? 4581
Tanzania
1998 Mauritania 300 150 7
1999 Mauritania ? ? 2
2000 KSA, Yemen 1500 516 87
2000 1087 121
2003 Egypt 300 148 27
2006-07 Kenya, Tanzania, ? 700 158
Somalia 309 144
107 54
2007 Sudan ? 125+ 60+
2008, 2009 Madagascar, RSA, ? ? ?
Comoros
2010,2011 RSA ? 255 26
1Data available from Kenya only

9. RVF Risk Mapping: Set up
RVF – 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

10. Epidemic prediction: Relies on field data
Teamwork
Capacity Building

11. Kenya: RVF Spread Over 52
Last 2 epizootics affected 6 of 8 provinces, 38 of 69 districts
1951-1955 1997-2007
10
12
21
22
19
19 2
20
7 20
7
14
16
14
16
1
11
18 11
13 4 18
15 13 8
17
3
9
5
6
- 6
Murithi et al., Epi Infect 18, 1-9, 2010

12. Prediction vs. Outbreak Timing: Epi-Curves: 2006 - 2008
Anyamba et al (In Review AJTMH)

13. What is the inter-epizootic period (IEP)?
Based on Kenya Dept Vet Svs Data – 1910 - 2007
Mean IEP = 3.6 years
IEP
IEP IEP
IEP
Murithi et al., Epi Infect 18, 1-9, 2010

14. 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.
In wildlife
Cummulative percent of positive samples at each titer level
9 of 17 species tested
100.00 were positive
% of positive samples of
80.00
60.00
- Buffalos, Black rhinos,
- Thompson gazelles,
species
Buffalo
40.00 Black rhino - Impalas, Elands, Kudus
Other
20.00
0.00
80
0
0
0
80
40
20
10
64
32
16
-20.00
12
1:
1:
1:
1:
1:
1:
1:
1:
Titer level
Evans et al, Epid Inf 8, 1-92007
Negative: Lions, giraffes, common zebras

15. Where is RVF virus maintained ?
In livestock : Kenya during the 1999 – 2006 IEP
Sheep Goats
Year
Percentage Percentage
of No. tested No. tested
positive positive
birth
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
Rostal et al, Amer J Vet Res 71, 522-6, 2010

16. Northeastern Province of Kenya
December, 1997 December, 2006

17. Sequential Epidemics in Kenya, Somalia, and Tanzania [1997-8; 2006-7]

18. Reported Human RVF Cases (Deaths) in Kenya
NEP
333 of 700 (45%)
Wajir
26(12)
cases
ISIOLO
Baringo
13(0)
174(13)
Garissa
Baringo
Nakuru 176(59)
1(1)
174 of 700 (24.8%)
Ijara
Cases
Kirinyaga
131(27)
5(2)
Kajiado Tana-River
Maragua 10(6) 21(8)
4(0)
Taita-Taveta
Malindi
15(4)
Kilifi
Thika
15(1) 73 of 700 (10.4%)
2(0) Kilifi
Nairobi 73(20) cases
5(2)

19. Cases
0
2
4
6
8
10
12
14
16
18
20
21/11/2006
28/11/2006
05/12/2006
12/12/2006
19/12/2006
NEP
26/12/2006
02/01/2007
09/01/2007
16/01/2007
Kilifi
23/01/2007
30/01/2007
Date of Onset
06/02/2007
13/02/2007
Kenya 2006/07(N=340)
20/02/2007
Baringo
27/02/2007
06/03/2007
13/03/2007
Alive
Dead
20/03/2007
Confirmed and Probable Rift Valley Fever Cases ,
27/03/2007

20. RVF Outbreaks in Somalia
First case confirmed 20th January, 2007
Total:107 cases, 54
(50.5%) deaths

21. RVF Outbreak in Tanzania
First case confirmed Feb 1st, 2007
Total: 309 cases, 144 deaths

22. 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

23. How do epizootics spread?
Multiple lineages of RVF virus during one epidemic indicate spontaneous
activation of resident virus
Kenya 2
Kenya 1b
Kenya 1a
Tanzania 1
Nderitu et al., J Inf Dis 203, 2011; Bird et al, 2008

24. 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

25. South Africa: RVF Infection Map, April 2011
Source: NICD, South Africa

26. RVF in Madagascar
1st Epizootic / Epidemic 1990 -few
cases
Antananarivo
Another in 1991 #
Perinet Forest
Epizootic / Epidemic 1990
Epizootic / Epidemic 1991
26

27. RVF in Madagascar
Last cases in March 2009
Antananarivo
Primary Forest of Perinet #
Epizootic / Epidemic 1990
Epizootic / Epidemic 1991
Epizootic / Epidemic 2008
Epizootic / Epidemic 2008-2009
Last cases in 2009
27

28. Major Interventions (Eastern Africa)
Social mobilization
Slaughtering ban
– Major religious holiday (Idd festival)
Livestock quarantine
Livestock vaccination
Vector control (minimal)

29. RVF Outbreak Response 2006 – A Step Behind
Livestock vaccination Public Education

30. Challenges
Poor accessibility: Flooding
Timing: Occurred in peak holiday season
Limited health services in the region
Coordinating surveillance between
livestock and human health