Rift Valley Fever

Rift Valley Fever

1. 1.       The disease 

Rift Valley Fever (RVF) is a zoonotic (affects humans) insect-borne viral disease caused by a Phlebovirus. It is transmitted by mosquitoes and other insect vectors. The disease is endemic in Africa and the Middle East. In endemic regions, which include some Southern African Development Community (SADC) countries, serious outbreaks occur when climatic conditions favourable to the explosion of the insect vectors prevail. Major outbreaks of the disease occur in cycles (every 5 – 7 years) and generally result in high mortalities and abortions in large and small ruminants (cattle, sheep and goats). Concurrent disease in people may result in fatalities.

1. 2.       Historical Overview of RVF

Although a disease fitting the description of RVF was first described by Montgomery in Kenya, RVF virus was in fact first isolated and characterised in 1931 close to Lake Naivasha in the Kenyan Rift Valley. Until recently, the disease was recognised only with the African continent, until 2000 when it occurred in the Arabian Peninsula. A historical timeline of the known occurrence of RVF is summarised below:

• 1910-13: disease compatible with RVF described in lambs (European breed) in Rift Valley, Kenya.   (Daubney et al., 1931)
• 1944: isolation of RVF virus in Semliki forest Uganda (no livestock or humans in vicinity) – hence RVF assumed to be endemic in forests with spread to grasslands after heavy rains. Subsequent recognition of presence of virus in many sub-Sub-Saharan countries.
• 1977-1978: appearance beyond sub-Saharan Africa – in Egypt.
• 1979: recognition of RVF in Madagascar
• 1987: large outbreaks in Mauritania/Senegal
• 1997-1998: large outbreak n-e Kenya/Somalia/Tanzania
• 1998-1999: Large outbreak South Mauritania, also Senegal
• 2000-2001: appearance of RVF beyond African region in the Arabian Peninsula (Saudi).
• 2010 break in Mauritania (Clear case of introduction into an uninfected area)
• 2010-2012: Outbreaks in South Africa, Namibia and Botswana

1. 3.       Impacts of RVF

Outbreaks of RVF often cause substantial socio-economic and public health impacts.

Socio-economic Impacts

–          Loss of animals due to high mortalities and arbortions

–          Reduced productivity and trade due to high morbidity

–          Subsequent reduction in livelihoods and food security

Public health impacts

Zoonosis (Human fatalities)

–          Egypt (1997): 200000 human cases, 600 reported fatalities

–          Mauritania (1987): over 300 fatalities

–          Sudan (2007-2008): 738 human cases, 230 deaths

–          South Africa (2010): 242 lab-confirmed human cases with 26 deaths

–          Mauritania (2010): 63 human cases, 13 deaths

–          Potential agent for bio-terrorism

1. 4.       Control of RVF

Epidemics of RVF principally result from a synergy of at least three factors which can vary considerably: (i) the presence and circulation of the phlebovirus within the mosquito population (ii) the number of mosquito breeding sites and hatching frequency, two parameters that are both highly dependent on environmental factors, particularly rainfall events and (iii) the availability and distribution of susceptible hosts vulnerable to increased vector contacts.

Vector control is essentially difficult to implement, whilst the cyclical nature of the disease (variable inter-epidemic periods) complicates targeted control options. The use of sentinel animals, to detect infection at early stages, depends on good diagnostic tools that are not always available.

The control of RVF is therefore reliant on effective surveillance and vaccination. The RVF Smithburn vaccine currently in use offers the best effective means to control RVF. However, other modified live and molecular derived RVF antigens are under development and not yet available for field use.

Table 1: Currently available RVF vaccines *Click to view table

Table 2: RVF situations and control options *Click to view table

Current challenges in the control of RVF in Africa

• Costs of yearly vaccination campaigns
• Safety concerns: often under external communal grazing systems, it is difficult to determine the physiological stages of pregnant animals with subsequent inadvertent abortions in vaccinated animals.
• Irregularity of outbreaks: inter-epidemic periods vary considerably, leading to lapses in surveillance and vaccination programs.
• Policy aspects: Most countries in Sub-Saharan Africa, vaccination against RVF is not covered by government except in exceptional circumstances where explosive outbreaks occur.