Rift Valley fever virus(Bunyaviridae: Phlebovirus): an update on pathogenesis, molecular epidemiology, vectors, diagnostics and prevention

Abstract

Rift Valley fever(RVF) virus is an arbovirus in the Bunyaviridae family that, from phylogenetic analysis, appears to have first emerged in the mid-19th century and was only identified at the beginning of the 1930's in the Rift Valley region of Kenya. Despite being an arbovirus with a relatively simple but temporally and geographically stable genome, this zoonotic virus has already demonstrated a real capacity for emerging in new territories, as exemplified by the outbreaks in Egypt (1977), Western Africa (1988) and the Arabian Peninsula (2000), or for re-emerging after long periods of silence as observed very recently in Kenya and South Africa. The presence of competent vectors in countries previously free of RVF, the high viral titres in viraemic animals and the global changes in climate, travel and trade all contribute to make this virus a threat that must not be neglected as the consequences of RVF are dramatic, both for human and animal health. In this review, we present the latest advances in RVF virus research. In spite of this renewed interest, aspects of the epidemiology of RVF virus are still not fully understood and safe, effective vaccines are still not freely available for protecting humans and livestock against the dramatic consequences of this virus.

Figure 1.

Schematic diagram of Rift Valley Fever virus (electron micrograph from Linda Stannard [257]). (A color version of this figure is available online at www.vetres.org.)

Figure 2.

(A) Schematic representation of the Rift Valley fever virus genome. The antigenomic sense RNA and the encoded open reading frames (blue box) are represented. For the ambisense S segment, the genome and its open reading frame are represented (below). (B) Schematic diagram of the mRNA transcribed from the segment M of Rift Valley fever virus. (A color version of this figure is available online at www.vetres.org.)

Figure 3.

Rift Valley fever virus M segment maximum a posteriori clade credibility tree, MCMC chain length 9.0 × 10⁷ steps, 2.25 × 10⁶ steps (25%) removed as burn-in. Posterior support values (highest posterior density, HPD) are indicated as integers (i.e., 100% support = 1.0) above each node respectively. The calculated mean times to the most recent common ancestor (TMRCA) are indicated below each respective node and are enumerated as years before the collection date of the last outbreak specimen (May, 2007). The 2006–2007 Kenyan outbreak specimen M segment reassortant (strain #0608) is indicated by an asterisk. Adapted from [21]. (A color version of this figure is available online at www.vetres.org.)

Figure 4.

Minimum spanning networks (MSN) visually describing discrete genetic distance between unique haplotypes of the RVFV M genome segment within the greater east African lineage. Each node represents one nucleotide difference between extant (open circle) or inferred (black filled circle) haplotypes. Proportionally larger open circles or squares represent the relative number of extant haplotypes represented in the network. Generally, squares denote the predicted progenitor haplotype for each lineage, whereas circles indicate progeny haplotypes. Note the greater distance as measured in nucleotide changes (steps) between the Kenya-1 and Kenya-2 lineages than with the prototype Kenyan 1997–1998 RVFV strain. Also note the star-like phylogeny of the Kenya-1 lineage indicating the potential for increases in virus population size or geographic range. An asterisk indicates the relative position of the putative M segment reassortant virus (strain #0608). Adapted from [21].

Figure 5.

Schematic representation of time course of viraemia and antibody responses against RVFV in experimentally-infected animals. (A color version of this figure is available online at www.vetres.org.)