GRF 2nd One Health Summit 2013: Presentation by KIMARO, Dr. Calvin Sindato, Southern African Centre for Infectious Disease Surveillance-Sokoine University of Agriculture, Morogoro Tanzania

1. SOUTHERN AFRICAN CENTRE
FOR
INFECTIOUS DISEASE SURVEILLANCE
SPATIAL AND TEMPORAL PATTERNS OF RIFT
VALLEY FEVER OUTBREAKS IN TANZANIA:
1930-2007
Calvin Sindato, Esron Karimuribo, Fredrick Kivaria, Bett
Bernard, Leonard Mboera, George Dautu, Dirk Pfeiffer,
Janusz Paweska and Mark Rweyemamu

2. Rift Valley Fever (RVF)
•
•
Spatial and temporal pattern of
the past outbreaks is poorly
understood
Caused by RVF virus
(RVFV); genus
Phlebovirus ; family
Bunyaviridae
•
Recognized in Tanzania in 1930
Mosquito-borne viral
disease
Survive in sporulated
eggs of Aedes
mosquitoes
•
Acute disease in sheep,
cattle, goats, buffalo,
camels and humans
•
Cryptic infection in
humans and ruminants

3. Study objectives
To determine and describe over the past 80 years in
Tanzania:
• (i) Spatial and temporal pattern of RVF outbreaks
• (ii) Clustering of RVF cases during past outbreaks
• (iii) Determinants for clustering of cases

4. Sources of Data

5. Data analysis and mapping of distribution of outbreaks
• STATA 12: descriptive and logistic regression
• Epidemic curves: to identify the monthly temporal
pattern of cases
• Space-time permutation in SatScan 9.1 to detect
clustering of cases
• Spatial distribution of outbreaks was mapped using
ArcGIS 10.2.

6. Results and discussion
No. Districts and Villages per
outbreak wave
Figure 2a
Proportion (%) of animal spp per
outbreak wave
Figure 2b

7. Figure 3a. Distribution of village-level space-time clusters of RVF cases from 1947 to 1978. There were no
clusters detected in 1930; from 1947 to 1978 three primary clusters were persistently detected in Ngorongoro
district, each involving one village. An asterisk represents the centre of cluster that involved more than one village;
relative risk for each cluster is displayed (RR) along with the buffer (circle) size in kilometres (km).

8. Figure 3b. Distribution of village-level space-time clusters of RVF cases in domestic ruminants
from 1997 to 2007. Asterisks represent the centre of clusters; relative risk for each cluster is
displayed (RR) along with the buffer (circle) size in kilometres (km).

9. Figure 4. Distribution of the village-level space-time clusters of RVF cases in humans and domestic
ruminants (livestock) during 2006/2007 outbreak wave. Between January and February 2007 there was
an overlap of livestock and human primary clusters in the same location. Asterisks correspond to
villages that were included within human space-time clusters; Relative risk for each cluster is displayed
(RR) along with the buffer (circle) size in kilometres (km).

10. Figure 2. The curves showing the pattern of total monthly rainfall, maximum and minimum monthly tempratures
and RVF outbreak waves (where the actual month of occurence was mentioned) in domestic ruminants (livestock)
[1930 to 2006/2007] and humans [2006/2007].

11. Table 1. Results of multivariable logistic regression examining the association
between the geoecological and anthropogenic factors and RVF clusters .
Variable
Soil type and texture
Soil type
Others
Chernozems
Phaeozems
Alisols
Soil texture
Sandy
Clayey
Loamy
Land cover and Cattle density/km2
Cattle density
≤37
>37
OR
95% CI
1*
6.9
5
3.49
1.49, 31.96
1.03, 24.24
1.38, 8.86
1*
4.95
3.75
1.05, 23,37
0.57, 24.63
1*
6.57
3.03, 14.25

12. Table 1…….
Land cover
OR
Others
95% CI
1*
Closed broadleaved deciduous forest 3.9
1.23, 12.31
Herbaceous cover, closed-open
4.23
1.93, 9.29
Tree cover, broadleaved, evergreen
5.19
1.00, 26.96
Tree cover, broadleaved, deciduous,
open
2.64
0.72, 9.68
Cultivated and managed areas
0.17, 2.10
0.6
Elevation
≥ 1445
1*
944-1444
2.64
1.21, 5.74
≤ 943
2.84
1.23, 6.53

13. Table 1…….
Rainfall pattern
OR
Unimodal rainfall
1*
Bimodal rainfall
10.45
95% CI
3.55, 30.75
Total annual rainfall (mm)
≤777.8
1*
>777.8
4.81
1.48, 15.61
Total outbreak wave rainfall (mm)
≤551.3
1*
>551.3
0.07
0.02, 0.300

14. Conclusion
• Successive outbreaks tend to cover increasing areas
of the country
• Once the disease had been introduced into an area
it was likely to be involved in future outbreaks
• Factors associated with the clustering of RVF cases:
–
–
–
–
–
–
high cattle density;
soil types in particular phaeozems, chernozems, alisols;
clayey and loamy soil texture;
lower elevation;
total annual rainfall, bimodal rainfall pattern;
closed broadleaved deciduous forest, Herbaceous Cover,
closed-open & Tree cover, broadleaved, evergreen

15. ACKNOWLEDGEMENT
Grant WT087546MA