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RVF has an incubation period of 1-6 days (12-36 hrs in lambs).<ref name="oie" /> Once in the lymph nodes viral replication occurs which leads to viraemia and systemic infection. Spontaneous abortions are seen as the hallmark of RVF outbreaks.<ref name="who"/> Pregnant animals can abort at any stage often with 100% of stock aborting.<ref name="vaccine"/>
RVF has an incubation period of 1-6 days (12-36 hrs in lambs).<ref name="oie" /> Once in the lymph nodes viral replication occurs which leads to viraemia and systemic infection. Spontaneous abortions are seen as the hallmark of RVF outbreaks.<ref name="who"/> Pregnant animals can abort at any stage often with 100% of stock aborting.<ref name="vaccine"/>
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Newborn lambs and kids are highly susceptible to RVF, presenting with pyrexia and anorexia shortly followed by death 24-36hrs after infection.<ref name="vaccine"/> <ref> http://www.fao.org/docrep/006/y4611e/y4611e00.htm#Contents</ref> In newborn lambs hepatocytes of the liver are the predominant target cell with hepatic necrosis being a significant post mortem finding. Other organs affected include the gall bladder (haemorrhage and oedema), gastrointestinal tract haemorrhage, lymph node haemorrhage, cutaneous haemorrhage and haemothorax.<ref>http://www.fao.org/docrep/006/y4611e/y4611e00.htm#Contents</ref> <ref name="oie" />
Newborn lambs and kids are highly susceptible to RVF, presenting with pyrexia and anorexia shortly followed by death 24-36hrs after infection.<ref name="vaccine"/> <ref name="fao"> [http://www.fao.org/docrep/006/y4611e/y4611e00.htm#Contents FAO Recognising Rift Valley Fever]accessed June 23, 2013</ref> In newborn lambs hepatocytes of the liver are the predominant target cell with hepatic necrosis being a significant post mortem finding. Other organs affected include the gall bladder (haemorrhage and oedema), gastrointestinal tract haemorrhage, lymph node haemorrhage, cutaneous haemorrhage and haemothorax.<ref name="fao"/> <ref name="oie" />
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Signs in older lambs, kids, calves and adults vary from acute to subclinical (20-70% mortality), Signs can include fever (lasts 24-96hrs), weakness, bloody diarrhoea, abdominal pain, photosensitivity, anorexia, excessive salivation and decreased milk production. Signs in adult cattle are most often subclinical with less than 10% mortality.<ref name="oie" /> <ref>http://www.fao.org/docrep/006/y4611e/y4611e00.htm#Contents</ref> <ref name="vaccine"/>
Signs in older lambs, kids, calves and adults vary from acute to subclinical (20-70% mortality), Signs can include fever (lasts 24-96hrs), weakness, bloody diarrhoea, abdominal pain, photosensitivity, anorexia, excessive salivation and decreased milk production. Signs in adult cattle are most often subclinical with less than 10% mortality.<ref name="oie" /> <ref name="fao"/> <ref name="vaccine"/>
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Camels display signs similar to those seem with Pasteurellosis infection, though infection can also be subclinical or asymptomatic. Abortions can also occur. During the 2010 outbreak in Mauritania 2 forms of disease were observed in camels; a hyperacute form causing sudden death in <24hrs and an acute form causing fever, ataxia, respiratory signs, icterus, oedema, foot lesions and neurological signs. If haemorrhagic signs were observed death occurred in a few days.<ref> ''Ahmed B. Ould El Mamy, Mohamed Ould Baba, Yahya Barry, Katia Isselmou, Mamadou L. Dia,
Camels display signs similar to those seem with Pasteurellosis infection, though infection can also be subclinical or asymptomatic. Abortions can also occur. During the 2010 outbreak in Mauritania 2 forms of disease were observed in camels; a hyperacute form causing sudden death in <24hrs and an acute form causing fever, ataxia, respiratory signs, icterus, oedema, foot lesions and neurological signs. If haemorrhagic signs were observed death occurred in a few days.<ref> ''Ahmed B. Ould El Mamy, Mohamed Ould Baba, Yahya Barry, Katia Isselmou, Mamadou L. Dia,
==Control==
==Control==
Herd monitoring has been used extensively in Africa as a means of building up a picture of disease spread as well as to identify risk areas. Sentinel herds in representative areas are used for monitoring. Blood is collected initially and, providing no seropositivity is detected, animals are sampled at the beginning of the rainy season and every four to six weeks up to the end of the rainy season. This allows the early detection of any epidemics.<ref>http://www.fao.org/docrep/006/y4611e/y4611e00.htm#Contents</ref> Monitoring is also important as RVF outbreaks in animals precede outbreaks in human populations, as well as identifying increases in vector numbers.
Herd monitoring has been used extensively in Africa as a means of building up a picture of disease spread as well as to identify risk areas. Sentinel herds in representative areas are used for monitoring. Blood is collected initially and, providing no seropositivity is detected, animals are sampled at the beginning of the rainy season and every four to six weeks up to the end of the rainy season. This allows the early detection of any epidemics.<ref name="fao"/> Monitoring is also important as RVF outbreaks in animals precede outbreaks in human populations, as well as identifying increases in vector numbers.
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During an outbreak, control at slaughterhouses to reduce zoonotic spread. Contact with animals, both direct and indirect, should be avoided. Movement restrictions should be enforced to prevent movement of the disease from diseased areas to RVF free areas; however movement restrictions within endemic countries have had little impact on the spread of RVF.<ref> http://www.fao.org/docrep/006/y4611e/y4611e00.htm#Contents</ref>
During an outbreak, control at slaughterhouses to reduce zoonotic spread. Contact with animals, both direct and indirect, should be avoided. Movement restrictions should be enforced to prevent movement of the disease from diseased areas to RVF free areas; however movement restrictions within endemic countries have had little impact on the spread of RVF.<ref name="fao"/>
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Forecasting is key as outbreaks occur following a period of heavy rain, thus is rain is forecast preventative measures can be implemented beforehand to help prevent/ lessen the impact of an outbreak.<ref name="who"/>
Forecasting is key as outbreaks occur following a period of heavy rain, thus is rain is forecast preventative measures can be implemented beforehand to help prevent/ lessen the impact of an outbreak.<ref name="who"/>
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Mosquito breeding sites should be reduced through drainage and larvicidal measures. Methoprene spraying, larvicidal toxins and controlled burning can be used, though low level aerial spraying has been shown to have little effect.<ref>http://www.fao.org/docrep/006/y4611e/y4611e00.htm#Contents</ref>
Mosquito breeding sites should be reduced through drainage and larvicidal measures. Methoprene spraying, larvicidal toxins and controlled burning can be used, though low level aerial spraying has been shown to have little effect.<ref name="fao"/>
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Education of risk factors and mosquito bite prevention, appropriate clothing, use of insect repellent and mosquito nets should be undertaken. Local populations should be educated as to the risks of eating raw meat and milk products (though a fall in pH destroys the virus so some uncooked meat may be safe).<ref name="oie" />
Education of risk factors and mosquito bite prevention, appropriate clothing, use of insect repellent and mosquito nets should be undertaken. Local populations should be educated as to the risks of eating raw meat and milk products (though a fall in pH destroys the virus so some uncooked meat may be safe).<ref name="oie" />
Modified live attenuated and inactivated virus vaccines are available.
Modified live attenuated and inactivated virus vaccines are available.
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The live attenuated '''Smithburn''' vaccine only requires one dose but may cause spontaneous abortion in pregnant stock. This vaccine has adverse effects in newborn kids and lambs and teratogenic effects or abortion in pregnant cows, ewes and goats.<ref> ''Lagerqvist N, Moiane B, Bucht G, Fafetine J, Paweska J.T., Lundkvist Å and Falk K.I. 2012''. Stability of a formalin‐inactivated Rift Valley fever vaccine: evaluation of a vaccination campaign for cattle in Mozambique. Vaccine 30(46):6534‐40.</ref><ref>''Botros B, Omar A, Elian K, Mohamed G, Soliman A, Salib A, Salman D, Saad M, Earhart K.'' (2006) Adverse response of non-indigenous cattle of European breeds to live attenuated Smithburn Rift Valley fever vaccine. J Med Virol. 2006 Jun;78(6):787-91.</ref><ref name="vaccine"/> Protection is conferred to offspring via suckling. Antibody titres post vaccination are higher in sheep than cattle.<ref name="vaccine"/> One dose will protect for three years.<ref>http://www.fao.org/docrep/006/y4611e/y4611e00.htm#Contents</ref>
The live attenuated '''Smithburn''' vaccine only requires one dose but may cause spontaneous abortion in pregnant stock. This vaccine has adverse effects in newborn kids and lambs and teratogenic effects or abortion in pregnant cows, ewes and goats.<ref> ''Lagerqvist N, Moiane B, Bucht G, Fafetine J, Paweska J.T., Lundkvist Å and Falk K.I. 2012''. Stability of a formalin‐inactivated Rift Valley fever vaccine: evaluation of a vaccination campaign for cattle in Mozambique. Vaccine 30(46):6534‐40.</ref><ref>''Botros B, Omar A, Elian K, Mohamed G, Soliman A, Salib A, Salman D, Saad M, Earhart K.'' (2006) Adverse response of non-indigenous cattle of European breeds to live attenuated Smithburn Rift Valley fever vaccine. J Med Virol. 2006 Jun;78(6):787-91.</ref><ref name="vaccine"/> Protection is conferred to offspring via suckling. Antibody titres post vaccination are higher in sheep than cattle.<ref name="vaccine"/> One dose will protect for three years.<ref name="fao"/>
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The formalin inactivated virus vaccine (with Aluminium Hydroxide adjuvant) requires multiple doses to achieve immunity, and annual boosters are needed as it is less immunogenic.<ref> ''Lagerqvist N, Moiane B, Bucht G, Fafetine J, Paweska J.T., Lundkvist Å and Falk K.I.'' 2012. Stability of a formalin‐inactivated Rift Valley fever vaccine: evaluation of a vaccination campaign for cattle in Mozambique. Vaccine 30(46):6534‐40.</ref>It is safe to use in pregnant ewes. Biosecurity is also a consideration when using this vaccine as reversion to virulence is a possibility.
The formalin inactivated virus vaccine (with Aluminium Hydroxide adjuvant) requires multiple doses to achieve immunity, and annual boosters are needed as it is less immunogenic.<ref> ''Lagerqvist N, Moiane B, Bucht G, Fafetine J, Paweska J.T., Lundkvist Å and Falk K.I.'' 2012. Stability of a formalin‐inactivated Rift Valley fever vaccine: evaluation of a vaccination campaign for cattle in Mozambique. Vaccine 30(46):6534‐40.</ref>It is safe to use in pregnant ewes. Biosecurity is also a consideration when using this vaccine as reversion to virulence is a possibility.
More recently a naturally attenuated virus vaccine has been developed (Clone 13) that is marketed in South Africa and Namibia.<ref>''Kortekaas J, Zingeser J, de Leeuw P, de La Rocque S, Unger H, Moormann RJ.'',2011 Rift Valley Fever Vaccine Development, Progress and Constraints. Emerg Infect Dis. 2011 Sep;17(9)</ref><ref>http://www.rr-africa.oie.int/docspdf/en/2012/RVF/MBAO01.pdf</ref> Testing in pregnant ewes and lambs demonstrated safety and it has been shown to be as effective as the Smithburn vaccine in conferring immunity.<ref>''Dungu B, Louw I, Lubisi A, Hunter P, von Teichman BF, Bouloy M'' (2010). Evaluation of the efficacy and safety of the Rift Valley Fever Clone 13 vaccine in sheep., Vaccine. 2010 Jun 23;28(29):4581-7</ref> <ref>''von Teichman B, Engelbrecht A, Zulu G, Dungu B, Pardini A, Bouloy M'' (2011). Safety and efficacy of Rift Valley fever Smithburn and Clone 13 vaccines in calves. . Vaccine. 2011 Aug 5;29(34):5771-7</ref> The vaccine has however shown to cause neurological disease and paralysis in some vaccinated mice.<ref>''Vialat P., Billecocq A., Kohl A., and Bouloy M.'' (2000). The S segment of Rift Valley fever phlebovirus (Bunyaviridae) carries determinants for attenuation and virulence in mice. J Virol 74:1538‐1543.</ref>
More recently a naturally attenuated virus vaccine has been developed (Clone 13) that is marketed in South Africa and Namibia.<ref>''Kortekaas J, Zingeser J, de Leeuw P, de La Rocque S, Unger H, Moormann RJ.'',2011 Rift Valley Fever Vaccine Development, Progress and Constraints. Emerg Infect Dis. 2011 Sep;17(9)</ref><ref>http://www.rr-africa.oie.int/docspdf/en/2012/RVF/MBAO01.pdf</ref> Testing in pregnant ewes and lambs demonstrated safety and it has been shown to be as effective as the Smithburn vaccine in conferring immunity.<ref>''Dungu B, Louw I, Lubisi A, Hunter P, von Teichman BF, Bouloy M'' (2010). Evaluation of the efficacy and safety of the Rift Valley Fever Clone 13 vaccine in sheep., Vaccine. 2010 Jun 23;28(29):4581-7</ref> <ref>''von Teichman B, Engelbrecht A, Zulu G, Dungu B, Pardini A, Bouloy M'' (2011). Safety and efficacy of Rift Valley fever Smithburn and Clone 13 vaccines in calves. . Vaccine. 2011 Aug 5;29(34):5771-7</ref> The vaccine has however shown to cause neurological disease and paralysis in some vaccinated mice.<ref>''Vialat P., Billecocq A., Kohl A., and Bouloy M.'' (2000). The S segment of Rift Valley fever phlebovirus (Bunyaviridae) carries determinants for attenuation and virulence in mice. J Virol 74:1538‐1543.</ref>
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A live attenuated vaccine MP12 has been shown to be safe in newborns, lambs and pregnant cows and ewes. Malformation has occurred when the vaccine was administered to sheep during the first trimester (days 35-56)<ref>''Hunter P, Erasmus BJ, Vorster JH.'' (2002) Teratogenicity of a mutagenised Rift Valley fever virus (MVP 12) in sheep,. Onderstepoort J Vet Res. 2002 Mar;69(1):95-8.</ref> and viral shedding has been documented in macaques following MP12 vaccination.<ref> ''Lagerqvist, N,'' Rift Valley fever virus vaccine strategies, Karolinska Institutet 2013</ref> Colostrum from vaccinated ewes gives temporary immunity to lambs.<ref>http://www.fao.org/docrep/006/y4611e/y4611e00.htm#Contents</ref>
A live attenuated vaccine MP12 has been shown to be safe in newborns, lambs and pregnant cows and ewes. Malformation has occurred when the vaccine was administered to sheep during the first trimester (days 35-56)<ref>''Hunter P, Erasmus BJ, Vorster JH.'' (2002) Teratogenicity of a mutagenised Rift Valley fever virus (MVP 12) in sheep,. Onderstepoort J Vet Res. 2002 Mar;69(1):95-8.</ref> and viral shedding has been documented in macaques following MP12 vaccination.<ref name="vaccine"/> Colostrum from vaccinated ewes gives temporary immunity to lambs.<ref name="fao"/>
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An R566 strain has been developed from MP12 and Clone 13 and has shown to confer immunity in laboratory experiments. <ref name="vaccine"/>
An R566 strain has been developed from MP12 and Clone 13 and has shown to confer immunity in laboratory experiments. <ref name="vaccine"/>