Rift Valley Fever - epidemiological update and risk of introduction into Europe

Abstract

Rift Valley fever (RVF) is a vector-borne disease transmitted by a broad spectrum of mosquito species, especially Aedes and Culex genus, to animals (domestic and wild ruminants and camels) and humans. Rift Valley fever is endemic in sub-Saharan Africa and in the Arabian Peninsula, with periodic epidemics characterised by 5-15 years of inter-epizootic periods. In the last two decades, RVF was notified in new African regions (e.g. Sahel), RVF epidemics occurred more frequently and low-level enzootic virus circulation has been demonstrated in livestock in various areas. Recent outbreaks in a French overseas department and some seropositive cases detected in Turkey, Tunisia and Libya raised the attention of the EU for a possible incursion into neighbouring countries. The movement of live animals is the most important pathway for RVF spread from the African endemic areas to North Africa and the Middle East. The movement of infected animals and infected vectors when shipped by flights, containers or road transport is considered as other plausible pathways of introduction into Europe. The overall risk of introduction of RVF into EU through the movement of infected animals is very low in all the EU regions and in all MSs (less than one epidemic every 500 years), given the strict EU animal import policy. The same level of risk of introduction in all the EU regions was estimated also considering the movement of infected vectors, with the highest level for Belgium, Greece, Malta, the Netherlands (one epidemic every 228-700 years), mainly linked to the number of connections by air and sea transports with African RVF infected countries. Although the EU territory does not seem to be directly exposed to an imminent risk of RVFV introduction, the risk of further spread into countries neighbouring the EU and the risks of possible introduction of infected vectors, suggest that EU authorities need to strengthen their surveillance and response capacities, as well as the collaboration with North African and Middle Eastern countries.

Keywords: Rift Valley Fever; introduction; livestock; mosquitoes; transmission; vectors.

Figure 1

Steps for the MintRisk approach to assess the overall risk of pathogen/disease introduction into the EU

Figure 2

Temporal distribution of reported RVF outbreaks in animals and humans from 2006 to October 2019 (OIE and WHO data)

Figure 3

Number of years of reported presence of RVF and species affected in the outbreaks reported between 2006 and 2019 (OIE and WHO)

Figure 4

Amended map of CDC classification of the countries where RVF has been confirmed: (i) blue: countries reporting endemic disease and substantial outbreaks of RVF; (ii) green: countries reporting few cases, periodic isolation of virus, or serological evidence of RVF; and (iii) grey: RVF status unknown or not reported. (source: CDC 7)

Figure 5

RVF reported outbreaks in Mayotte in 2018 and 2019 in ruminants (cattle, sheep, goats) as notified to the ADNS and provided by the French Veterinary Authorities

Figure 6

Countries (Algeria, Iraq, Iran, Turkey, Tunisia and Western Sahara), where RVF seropositive results have been detected, with data available through publications or reports

Figure 7

Cycle of transmission of the Rift Valley Fever virus. Vectors (mainly Aedes and Culex spp.) are able to transmit the virus to domestic and wild animals, as well as humans. Direct transmission is possible among animals and from animals to humans. Vertical transmission has been described in animals and vectors. The role of vertical transmission for maintaining the virus during inter‐epizootic periods is still under discussion (modified from Balenghien et al., 2013)

Figure 8

Principal movement pathways of live ruminants across North African and Middle East countries. Adapted from information reported by Bouslikhane (2015), (Di Nardo et al., 2011), Bouguedour and Ripani (2016), Jenet et al. (2016)

Figure 9

Number of outbound civil flights in 2016–2018 from countries that have reported RVF outbreaks in the same period

Figure 10

Rate of entry of RVF for the vector pathway (for country codes see Glossary)

Figure 11

Probability of establishment of RVF in each MS for animal and vector pathway

Figure 12

Overall risk of introduction: estimated number of no. epidemics/year of RVF in the MSs by MINTRISK

Figure 13

Overall introduction rate for the four EU regions