Changes

==Distribution==

==Distribution==

The virus is currently only present in '''Africa'''. Huge '''epizootics''' develop, affecting up to '''90%''' of a group, usually in '''5-15 year cycles.'''

RVF virus was first identified in 1831 in the Rift Valley in Kenya during an investigation on a sheep farm and has since spread throughout Sub Saharan Africa emerging into North Africa in the 1970’s. the outbreak in Egypt in 1977-78 is considered to be the largest outbreak with 200,000 human cases reported. In September 2000 it was reported for the first time outside of Africa, in Saudi Arabia and Yemen, probably introduced through infected livestock or mosquitoes. The increase in cases in South Africa may be due to the end of an inter epizootic period. Outbreaks are frequently reported though there is no evidence that it has spread to previously uninfected countries in the last 10 years, though it is hard to monitor changes in disease occurrence due to the cyclical occurrence of epidemics. Most recently RVF was reported in Mauritania in November 2012. A map detailing current outbreaks can be found [http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/rvf/rvfmap.htm here]

 

RVF is transmitted by a wide range of '''[[Culicidae |mosquitoes]]''' including ''Anopheles spp.'' and ''Culex spp.'' and also ''[[Hyalomma spp.]]'' '''[[Ticks |ticks]]''' and the '''[[Stomoxys calcitrans |stable fly]]'''. Some trans-ovarial transmission is also thought to occur. Climate and weather play a huge role in the emergence and survival of these vectors and thus also in amplification of the virus. The [[Mosquitoes |mosquitoes]] require cloud cover and regular, significant precipitation.

A number of mosquito species (''Aedes'', ''Culex'', ''Mansonia'', ''Anopheles'') are implicated as vectors of RFV, the most important being ''Aedes'' and ''Culex'' ''spp''. They are responsible for both maintenance and amplification of RVF.

 

The disease follows the '''rainy season''' in most countries.

Mosquitoes can be infected via feeding on infected animals. Vertical transmission can also occur (particularly in ''Aedes spp''); female infected mosquitoes lay virus infected eggs leading to a new generation of infected mosquitoes. Vertical transmission is important in the survival of the virus as the eggs laid by the female can survive for many months in dry conditions, hatching after a period of rain and so increasing spread post rainfall leading to epizootics. Once animal infection has occurred mosquitoes are then responsible for amplifying infection. ''Aedes'' mosquito numbers decrease following rain but ''Culex'' tend to breed in more permanent water sites, hence the continuation of virus spread.

'''''Dambos''''' (e.g. East Africa) – Shallow depressions, often near rivers, that fill with water during the rainy season. Vertical transmission in mosquitoes occurs here.

'''''Semi-arid''''' (e.g. Senegal, Mauritania) – At temporary water points. It is unknown how the virus persists here, presumably either via vertical transmission or reintroduction of virus through visiting herds.

'''''Irrigated areas''''' (e.g. Nile Delta and Senegal River Valley). Yearlong viral transmission occurs here as the permanent water favours ''Culex'' breeding.

'''''Temperate and Mountainous''''' (e.g. Madagascar) – Virus is transmitted by vectors through cattle movement.

No outbreaks have been reported in urban areas.

Zoonotic transmission occurs through direct or indirect contact with infective blood or organs through slaughter, assisting with births and carcass disposal amongst other means. Faecal shedding of virus also occurs, as does spread through nasal and ocular secretions. Aerosol infection has also occurred within laboratory workers. Consuming unpasteurised or uncooked milk has also been associated with infection and seropositivity. Mosquito bites have resulted in infection as well, and blood feeding flies also have the potential to transmit infection. There is no evidence of human to human transmission.

It is currently unknown if there are animal reservoirs of RVF between outbreaks. The namaqua rock rat and bats have been implicated and have been shown to be capable of infection but the potential impact of this is unknown. Low levels of circulation between livestock or wild ruminants and mosquitoes (sylvatic cycle) is also likely to occur.

Outbreaks occur after heavy rain and flooding due to favourable breeding conditions for mosquitoes.

RVF can also be spread by the introduction of infected livestock into previously unaffected areas where mosquitoes are present.

==Signalment==

==Signalment==