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. 2012;6(8):e1808.
doi: 10.1371/journal.pntd.0001808. Epub 2012 Aug 28.

Exploratory space-time analyses of Rift Valley Fever in South Africa in 2008-2011

Raphaëlle Métras  1 , Thibaud Porphyre, Dirk U Pfeiffer, Alan Kemp, Peter N Thompson, Lisa M Collins, Richard G White
Affiliations

Exploratory space-time analyses of Rift Valley Fever in South Africa in 2008-2011

Raphaëlle Métras et al. PLoS Negl Trop Dis. 2012.

Abstract

Background: Rift Valley fever (RVF) is a zoonotic arbovirosis for which the primary hosts are domestic livestock (cattle, sheep and goats). RVF was first described in South Africa in 1950-1951. Mechanisms for short and long distance transmission have been hypothesised, but there is little supporting evidence. Here we describe RVF occurrence and spatial distribution in South Africa in 2008-11, and investigate the presence of a contagious process in order to generate hypotheses on the different mechanisms of transmission.

Methodology/principal findings: A total of 658 cases were extracted from World Animal Health Information Database. Descriptive statistics, epidemic curves and maps were produced. The space-time K-function was used to test for evidence of space-time interaction. Five RVF outbreak waves (one in 2008, two in 2009, one in 2010 and one in 2011) of varying duration, location and size were reported. About 70% of cases (n = 471) occurred in 2010, when the epidemic was almost country-wide. No strong evidence of space-time interaction was found for 2008 or the second wave in 2009. In the first wave of 2009, a significant space-time interaction was detected for up to one month and over 40 km. In 2010 and 2011 a significant intense, short and localised space-time interaction (up to 3 days and 15 km) was detected, followed by one of lower intensity (up to 2 weeks and 35 to 90 km).

Conclusions/significance: The description of the spatiotemporal patterns of RVF in South Africa between 2008 and 2011 supports the hypothesis that during an epidemic, disease spread may be supported by factors other than active vector dispersal. Limitations of under-reporting and space-time K-function properties are discussed. Further spatial analyses and data are required to explain factors and mechanisms driving RVF spread.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Daily number of RVF affected farms in South Africa, between 2008 and 2011.
Figure 2
Figure 2. RVF livestock cases for the years 2008 (A), 2009 (B), 2010 (C) and 2011 (D).
For 2009, both waves are displayed. Provinces are NC: Northern Cape, WC: Western Cape, EC: Eastern Cape, FS: Free State, NW: North West, KN: KwaZulu-Natal, MP: Mpumalanga, GT: Gauteng, LP: Limpopo. The light gray shaded area is Lesotho (no data).
Figure 3
Figure 3. Plot of excess risk attributed to the space-time interactions (D0(s,t)) showing the spatial (distance in km) and temporal (time in days) distances at which clustering occurred in 2009 (wave 1) (A), 2010 (B) and 2011 (C).
The white shaded areas show the space-time distances for which D0(s,t)>1.
See this image and copyright information in PMC

References

    1. Bird BH, Ksiazek TG, Nichol ST, Maclachlan NJ (2009) Rift Valley fever virus. J Am Vet Med Assoc 234: 883–893. - PubMed
    1. Daubney R, Hudson JR, Garnham PC (1931) Enzootic hepatitis or Rift Valley fever. An undescribed virus disease of sheep, cattle and man from East Africa. J Path Bact 34: 545–579.
    1. Gerdes GH (2004) Rift Valley fever. Rev Sci Tech 23: 613–623. - PubMed
    1. Pepin M, Bouloy M, Bird BH, Kemp A, Paweska J (2010) Rift Valley fever virus (Bunyaviridae: Phlebovirus): an update on pathogenesis, molecular epidemiology, vectors, diagnostics and prevention. Vet Res 41: 61. - PMC - PubMed
    1. Anyamba A, Chretien JP, Small J, Tucker CJ, Formenty PB, et al. (2009) Prediction of a Rift Valley fever outbreak. Proc of the Natl Acad Sci U S A 106: 955–959. - PMC - PubMed

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