Journal of Veterinary Medicine and Health
Open Access

Aedes mosqouito; Epidemiology; Rift valley fever; Transboundary animal diseases

BHK: Baby Hamaster Kidney; CDC: Center for Control and Prevention Disease; CFSPH: Center for Food Security and Public Health; CPE: Cytopathic Effect; CPV: Capripox Virus; DIVA: Potential for Differentiate Infected from Vaccinated Animals; EASAC: European Academic Science Advisor Council; EDTA: Ethylene-Diamine-Tetra Acetic Acid; EVSVRI: Egypt Veterinary Serum and Vaccine Research Institute; FAO: Food and Agriculture Organization; IFN: Interferon; KEVEVAP: Kenya Veterinary Vaccine Producing Institute; MVA: Modified Vaccine Ankara; NABC: National Agricultural Bio Security Center; NS: Non-Structural Protein

Rift Valley Fever (RVF) is a life-threatening disease of domestic ruminants and humans. The causative agent is Rift Valley Fever Virus (RVFV) which belongs to the family Bunyavirridae, genus Phlebovirus [1]. It was first reported among livestock in Kenya in 1931 since then it has been reported as occurring in most African countries. Until recently, it had only been recognized in the African continent. But in 2000 it occurred in the Arab peninsula in Kingdom of Saudi Arabia and Yemen republic [2].

The severity of RVFV, its ability to cause major epidemics among livestock and humans. The lack of efficient prophylactic and therapeutic measures make infection with this pathogen a serious public health concern not only in endemic developing countries, but also, in many non-endemic industrial countries [3].

RVF outbreaks have today be detected in 19 countries there among Kenya, Tanzania, South Africa, Sudan, Egypt, Madagascar, Botswana, Somalia and the Arabian Peninsula. The most recent outbreaks were reported in South Africa (2008, 2009 and 2010) and there was a major outbreak in Kenya and Tanzania 2006-07 [4].

The major parts of Ethiopian highlands do not favor vector survival and multiplication but the lowlands are seasonally flooded as a result of heavy rain in the highlands. These areas also border infected areas in neighboring countries. its geographical proximity to RVF endemic countries like Kenya, Sudan and Somalia, the nature of livestock movements across the international border and the ease with which infected mosquitoes can be moved longer distances by the help of wind can lead to the conclusion that Ethiopia will always be vulnerable to RVF during the epizootic periods of the disease in East Africa. Although, clinical cases of the disease have never been reported in Ethiopia [5].

The world Organization of Animal Health (OIE) asserts RVF as a notifiable disease of multispecies in list A. This list includes those diseases that are defined by OIE as transmissible diseases that have the potential for very serious and rapid spread, irrespective of national borders, that are of serious socio-economic or public health consequence and that are of major importance in the international trade of animals and animal products”. In 2004 the OIE also identified RVF as one of the most global re-emerging zoonotic threats [6].

The disease is currently an economical concern because of the cost associated with preventive measures in endemic areas, monitoring for introduction of disease in neighboring unaffected areas and trade restriction on import and export to and from countries [7].

RVF is one of the most important Transboundary Animal Diseases (TADs) which cross international borders with devastating effect on animal health and food security. It also leads to the jeopardization of international trade, a reduction in confidence, fewer tourists and loss of livelihoods. Huge economic losses are the consequences [8]. The Global Framework for the Progressive Control of Transboundary Diseases also carried out recently a questionnaire around the world to rank the most serious animal diseases. They found that RVF was ranked as of great concern as an animal disease which seriously affects human health. In addition, RVF has been classified as one of those TADs, it captures the attention of FAO and OIE to support concerted efforts to control the disease [9].

There is no specific treatment for RVF, vaccination of susceptible animals in endemic and high-risk areas with safe and cost-effective vaccine during nonepidemic periods remains the only effective method to build sufficient immunity that is able to prevent virus amplification in livestock, break the cycle of transmission and eliminate the main source of human infection.

The objective of this study is to review on:

• Epidemiology, control and prevention of Rift valley fever.
• Socio-economic and public health impact of rift valley fever.

Etiology

Rift valley fever is caused by Rift valley fever virus, genus Phlebovirus, a member of Bunyaviridae family. RVFV is associated with mosquitoes while other members in the genus are transmitted by phlebotomine sand fly therefore, the name of genus Phlebovirus derived from that. Bunyaviridae family includes many harmful viruses distributed all over the world such as Hantavirus, Nairovirus, Orthobunyvirus, Phlebovirus and Tospovirus. They cause different animal and human diseases and are transmitted by arthropods, except the Hantaviruses which is transmitted by rodent.

Pathogenesis

RVF virus replicates rapidly and to very higher titer in target tissues after entry by mosquito’s bite, percutaneous injury or through the oropharynx through aerosols. Although many components of the RVFV’s RNA play an important role in the virus nonstructural protein S segment (NSs) is the only component that has been found to directly affect the host. NSs is hostile and combative against the hosts Interferon (IFNs) antiviral response, IFNs are essential in order for the immune system to fight off viral infections in a host. This inhibitory mechanism is believed to be due to a number of reasons, the first being, competitive inhibition of the formation of the transcription factor. On this transcription factor, NSs interacts with and binds to a subunit that is needed for RNA polymerase.

After infection the virus spread from the initial site of replication to critical organs such as the spleen, liver and brain which are either damaged by the pathogenic effects of the virus or immunopathological mechanisms, else there is recovery mediated by nonspecific and specific host response. The virus is conveyed from the inoculation site by lymphatic drainages to regulate lymph nodes where there is replication and spin over into the circulation which leads to viremia and systemic infections.

Clinical sign

Host susceptibility depends on age and animal species. In general, breeds that are less susceptible are those indigenous to the tropical and subtropical zones in Africa, while those highly susceptible are the crossbred (hybrid) or imported genotypes exotic to the continent. Sheep and goats, clinical signs in lambs include fever 40°C, mucopurulent nasal discharge, vomiting, anorexia, diarrhea and icterus. Complications include, abortion rate of 100%, per acute hepatic disease in lambs and kids <1 week old, hepatitis, cerebral infection and ocular infection. Case–fatality rate in Lambs <1 week of age may be as high as 100% while in lambs. >1 week of age as high as 20% and in adults 20%-30%.

Cattle, the most severe disease is seen in young animals. The mortality rate in exotic calves of Bostaurus breeds. Some animals up to 6 and even 12 months may be severely ill and debilitated with hepatitis and jaundice for some months. The acute disease is similar to that in sheep. In adult cattle the mortality rate is less than 2%-5% while pregnant cows abort. In extensively ranched cattle, abortions may not be observed and a drop in calving rates may be the only sign recognize.

Humans, patients are suffering from the mild form of the disease that is characterized by sudden fever (37.5°C), face flushing, eye congestion, headache, general muscles and joint pain and photophobia. Severe form of the disease in humans might present with influenza-like illness, jaundice, encephalitis, ocular lesions, permanent loss of vision and fatal hemorrhagic state While RVF was originally associated with mainly the livestock mortality, there is evidence of increased fatality rates in humans during the recent outbreaks.

Pathology

Gross pathology: The characteristic features include severe hepatic lesions and an enlarged liver that is friable, soft and reddish to yellowish-brown in colors with petechial hemorrhage. The contents of the abomasum and small intestine of newborn lambs are chocolate brown. In most animal’s edema and hemorrhages in the wall of gall bladder, as well as enlarged peripheral and visceral lymph nodes were observed all over the body. Accumulation of blood-stained fluids in body cavities and hemorrhagic enteritis were associated with bloody diarrhea.

Histopathology: In the livers of young animals, there are well-defined primary foci of severe coagulative necrosis, which may be centrilobular. These are accompanied by diffuse and massive pan necrosis involving most or all of the rest of the parenchyma. In older animals, the hepatic necrosis may be less extensive and confined to focal areas of individual lobules.

Diagnosis

Field diagnosis: Rift Valley Fever (RVF) should be considered when the following group of conditions occurs: Usually the environmental signals of heavy rain, high abortion rates especially in sheep, cattle and other ruminants, high mortalities in young ruminants, severe hepatic necrosis at necropsy of young animals and fetus, flu-like symptoms in humans, high numbers of mosquitoes and rapid spread of disease signals a RVF outbreak. This is always the cases if there is flooding in Savannah or semi-arid area following prolonged rainfall or in irrigated areas, if the mosquito population is high and if there is concurrent illness in human population.

Viral anti gen detection: Fixated tissues from biopsies or after autopsy/necropsy, especially from the liver, are after infection often positive for RVFV antigens which can be visualized using immunostaining techniques. A slower but highly reliable method is the isolation of whole virus particles by adding suspensions of infected tissue or body fluids to a cell culture where the virus can multiply. RVFV infection cause a cytopathic effect (rounding of cells) in many mammalian cell lines followed by cell death 1-2 days after infection.

Cell culture (Viral isolation): The cultures are observed microscopically for 5-6 days. RVF virus induces a Cytopathic Effect (CPE) characterized by slight rounding of cells followed by the destruction of the whole cell within 12-24 hours. Specific identification of RVF virus antigen may be made 18-24 hours after infection by immunofluorescent staining of the cover-slip preparations. Culture remains the gold standard for diagnosis. Virus isolation is considered as the gold standard but it’s very sensitive and specific, they have limitations because of the short duration of viremia which is in general 2-4 days. In addition, manipulation of RVFV generates biohazards.

Antibody detection: Virus neutralization is one of the specific tests to detect RVFV antibodies. It is efficient in endemic areas particularly for the early infection stage. ELISA is widely used to identify the RVF antibodies. A person who experienced RVF in the past may have convalescent immunity for long time, similarly the offspring who could acquire passive immunity from their mother in the first four years of their life’s. Interestingly developed side by side ELISA to be able to differentiate between recent antibodies and the old antibodies resulting from vaccine in both human and animals.

Clinical diagnosis: Affected animals show fever (40°C-42°C), anorexia, depression, weakness, mucopurulent nasal discharge, vomiting, jaundice and hemorrhagic diarrhea.

Treatment

Patients with RVF should be nursed in mosquito-protected premises. There is no specific therapy for infected animals, however, Ribavirin may be efficacyious in humans. It is sometimes used in supportive care. Another approved drug, the proteasome inhibitor Bortezomib which is used against multiple myeloma, has shown to inhibit RVFV infection in cell culture. Other interesting drugs that are being evaluated for anti-RVFV activity is curcumin and favipiravir. Curcumin, an NF inhibitor, has in cell culture been shown to inhibit RVFV infection.

Favipiravir is an RNA polymerase inhibitor with antiviral activity against many RNA viruses. It has been shown to be effective against RVFV infection in both cell culture. It also protects against RVFV induced neurological manifestations as well as prevents lethal disease in hamsters.

Distribution and occurrence

RVFV is endemic to countries of East Africa, South Africa and the Senegal River valley. The virus was first identified in 1931 during an investigation into an epidemic among sheep on a farm in the Rift Valley of Kenya. Since then, outbreaks have been reported in sub-Saharan and North Africa, in 1997-98, a major outbreak occurred in Kenya, Somalia and Tanzania and in 2000, RVF cases were confirmed in Saudi Arabia and Yemen, marking the first reported occurrence of the disease outside the African continent and raising concerns that it could extend to other parts of Asia and Europe.

Dambo areas in East Africa: Dambos are shallow depressions that can be 1 km in length and several hundreds of meters in width and are often located in valleys near rivers. In these areas, a correlation between heavy rainfall events and RVF outbreak occurrence has been clearly demonstrated. Viral transmission from one Aedes mosquito generation to another by vertical transmission and the survival of infected eggs in dry mud for several years, could explain the maintenance of the virus in the field during inter-epizootic periods.

Semi-arid areas of western Africa: Senegal and Mauritania characterized by temporary areas of water. In these areas, a recent modelling study showed that outbreaks could not be directly related to heavy rainfall events, but mostly to abundant regular rainfall occurring throughout the rainy season, which is favorable for successive high density of the main vectors, i.e., Aedes vexans and Culex poicilipes. The persistence of the virus may result from vertical transmission in A. vexans mosquitoes or from the regular introduction of the virus by nomadic herds.

Irrigated areas: Areas where RVF occurs including the Nile Delta (Egypt) and the Senegal River valley (Senegal, Mauritania), where permanent water bodies favor the development of Culex populations and thus yearlong viral transmission.

Temperate and mountainous areas: Recently demonstrated in Madagascar, where transmission and spread result from local vector-borne transmission associated with specific cattle trade habits.

Affected species

The disease affects a wide range of vertebrates, but the clinical disease is limited to primarily domestic ruminants and humans and non-clinical form of the disease in wildlife. The capacity of RVFV to cause large and severe outbreaks in animal and human populations and to cross significant natural geographic barriers, as exemplified by the virus spread over the Indian Ocean, Sahara Desert and the Red Sea in the past three decades, is great concern for veterinary and public health authorities worldwide. The RVFV is one of the most important emerging zoonotic threats, particularly to vulnerable African communities with low resilience to economic and environmental challenges.

Animals: All ruminants are susceptible to RVF infection, birds are not. Of the livestock species sheep are the most susceptible, followed by goats, cattle and camel and water buffaloes.

Humans: It has been shown that humans (host) get infected by direct contact of infected animals, their aerosol and through arthropods bites. Humans could show either a mild or severe form. Severe form commonly leads to death. The disease incubation period in human varies from 2 to 6 days.

Morbidity and mortality

Animals: Animals, Susceptible animals may develop high viraemia, severe prostration and death. Lambs, calves and kids are highly susceptible to RVFV infection. The mortality rate been reported to be 90%-100% in lambs and kids under a week old and 70% in calve, this acute form is less common in older sheep and goats which have mortality rate of approximately 20%-30%, abortion may be the only marked sign in cattle and mortality in adult cattle is usually less than 10%.

Humans: Historically the disease was responsible from 1% of case fatality. This percentage recently has increased tremendously. The disease has been shown more severe cases with complication in terms of ocular, encephalitis and hemorrhagic features. These complications were considered responsible from many cases of death

Transmission: Rift valley fever is an arbovirus, which is a virus that is transmitted by an arthropod vector, in this case by infected mosquitoes. The vector species can vary between regions but the most common vector is a mosquito belonging to the genus Aedes. These mosquitoes lay eggs in which the first stage larvae develop but then enter a resting phase These mosquitoes, the “floodwater Aedes” mentioned previously have drought-resistant eggs that survive for several years without hatching, this vertical transmission allows RVFV to survive in the environmental heavy rainfall and subsequent flooding stimulates adult emergence Aedes mosquitoes are thought to serve as the major reservoir and vector. However, other species are key secondary vectors capable of biological transmission.

Biological vectors are the Aedes, Culex, Eretmopodites and Mansonia mosquito species. The virus has also been isolated in Culicoides (biting midges) and Simulids (black flies), as in two tick species (Amblyomma variegatum and Rhipicephalus appendiculatus). However, isolation does not necessarily demonstrate that a species is a competent biological vector.

RVFV can be mechanically transmitted by biting flies such as Culicoides, Phlebotomids, Stomoxys calcitrans (stable fly) Simulids and other biting insects. Though livestock are primarily infected by biological vectors, midges and other biting insects play a role in mechanical dissemination of the disease once virus amplification has occurred in an animal host. Other vectors are also likely to transmit RVFV, including other species of Aedes (particularly Ae. cumminsii, Ae. circumluteolus and Ae. mcintoshi), Anopheles, Culex, Eretmapodites and Mansonia. In addition, it is important to note that variation in vector competence has been demonstrated for mosquitoes of the same species from different geographical region.

RVFV can also be transmitted through direct and indirect (fomite) contact. Infected mosquitoes may be transported for long distance in low level winds or air current which may lead to the rapid spread of the virus. The virus circulates between vertebrate hosts and mosquitoes. It does not require continuous vector-host-vector feeding cycle for maintenance.

In animals there is no evidence that direct contact transmission plays a significant role in the transmission of RVF. However, in susceptible species, the virus can be transmitted in utero to the fetus, RVFV has also been found in semen. Because transmission can occur through direct contact with blood and other body fluids, it is hypothetically possible that animals could become infected through contact with an aborted fetus or placental membranes which do contain virus, utero transmission of RVFV in a newborn. Though the relative importance of this transmission mechanism is not well understood.

The risk of human-to-human infection through direct contact appears also to be very low. However, in addition to mosquito transmission, humans are easily infected by contact with the body fluids of infected animals through contact with abraded skin, wounds or mucous membranes or by inhalation of aerosols. The slaughter of infected animals, necropsy procedures and laboratory handling of tissues and isolated viruses are activities carrying a high risk of disease transmission. Little or no information is available on the role of wool, bones, skins or other animal fibers in virus transmission. The OIE considers hides, skins, wool and fiber to be safe commodities and recommends not requiring any specific cleaning, disinfection or treatment of these commodities from countries with RVF.

Low concentration of the RVF virus is found in the milk of infected animals and high concentration of RVF virus in the blood of affected animals, that infection can be transferred between animals when they are vaccinated or blood sampled in succession with the same needle during an epizootic. Outbreak of RVF occurs generally when particularly heavy, prolong and often, unseasonal rainfall favors the breeding of mosquito vectors. Epidemics in most of eastern and southern Africa occur in 5-to-20-year cycles, but, in the dry semiarid zones of eastern Africa the periodicity is 15 to 30 year.

Risk factors

The incidence of the disease varies with size of the vector population and it is greatest in season of heavy rainfall. The environmental conditions appear to play a crucial role in the distribution of the arboviruses in any continent, where the activity of the vector is likely to be highest during late summer and early fall. This allows the vector population to breed in surface water in normally dry areas. In contrast, the activity of the vector is usually suppressed during the winter season. The overwintering mechanism for viral transmission is yet to be explained cyclonic winds and warm temperatures determine how far the infection would be transmitted. The vectors can transmit the infection to different continent carried with the wind for hundreds of miles.

The wildlife-mosquito cycling of RVFV could maintain the virus at low levels and might be difficult to detect if the wildlife reservoirs undergo mild or asymptomatic infections. This wildlife-mosquito cycling may involve low-level livestock infections. Animal herdsmen, abattoir workers and other individuals who work with animals in RVF-endemic areas where the virus is present have an increased risk for infection. International travelers increase their chances of getting the disease when they visit RVF-endemic locations during periods when sporadic cases or epidemics are occurring.

Importance of rift valley fever

Economic importance of rift valley fever: The economic impacts of RVF include death of animals and abortion, jeopardization of animal trade, devastating food security and cost of control. Pastoral communities relying on a livestock economy are highly vulnerable to the threat of disease to their livestock such as RVF More over. In the context of the Horn of Africa pastoralists who represent 15-20 million people in Djibouti, Eritrea, Ethiopia, Kenya Somalia and Sudan have turned to a market integration and international trade orientation. This has led to new development opportunities but also to new economic threats by increasing interdependence with the international economy.

The first reported direct socio-economic impact of RVF was on livestock producers due to high levels of mortality and morbidity in animals. This represents an important loss of stock, especially in young ruminants. These effects are perceived over the long term and are subject to the combined influence of other economic mechanisms besides the strict herd dynamics. Livestock exports play a major role as a source of employment, income and foreign exchange. RVF outbreaks may result in the enforcement of embargoes on the exportation of live animals and animal products, as imposed by international sanitary policies. Where the banned export sector has an important economic weight in national trade balance, the ban may significantly affect the national economy.

Public health importance of rift valley fever: Persons working close to RVFV infected livestock, such as veterinarians and slaughterers and people ingesting raw milk from infected animals are at high risk of getting RVF. In addition, humans can get RVFV by bites of infected mosquitoes. The incubation period is 4-6 days and the viremia last at least 4 days. Most commonly, RVFV infection only give rise to a mild febrile illness sometimes accompanied by muscle pain, nausea and abdominal irritation. Eye symptoms such as redness and irritation are also seen occasionally. Approximately of RVFV develop severe symptoms such as encephalitis, neurological symptoms is found in around 1% of RVFV patients most likely due to direct damage from viral infection in the CNS. Ocular disease and hemorrhagic fever with liver impairment. The liver damage caused by many hemorrhagic fever viruses can result in defect production of coagulation factors which in combination with uncontrolled inflammation, platelet activation and endothelial dysfunction can lead to severe hemorrhagic symptoms.

The estimated mortality rate of RVFV is around 1%, but there are large variations in the reported case fatality rates. where some studies describe a death rate of over 30%. There could be many explanations for this variation. Fatality rates describe the deaths among often very sick patients and are not representative for the overall population. The lack of availability to reliable diagnostic tools could also lead to under reporting and thus an overestimation of fatality rates.

Prevention of people in endemic areas should protect themselves against mosquito bites by use of mosquito repellents and sleeping under mosquito bed nets. Contact with sick animals, especially aborted animals, blood and other body fluids or tissues should be avoided. Samples that are suspected to be RVFV positive should be handled with caution and protective gear, such as gloves, aprons and mouth and eye protection, should be used to minimize the risk of infection.

Control and prevention of rift valley fever

The preventive and control measures for RVF is mainly through effective vaccination of livestock before the onset of an outbreak, commonly using vaccine that confers lifelong protection in animals. Other preventive and control measures include public education, mosquito vector control and restriction of animal movements. To control occupational hazards, protective gear such as gloves and other appropriate protective clothing should be worn and care taken when handling sick animals, patients, their tissues or any other suspected biological materials. By following up the international RVF early warning alerts of heavy rains, flooding, mosquito blooms and RVF events, it is likely that preventive initiatives are implemented timely.

Outbreaks of RVF in animals can be prevented by a sustained program of animal vaccination. Both modified live attenuated virus and inactivated virus vaccines have been developed for veterinary use. As outbreaks of RVF in animals precede human cases, the establishment of an active animal health surveillance system to detect new cases is essential in providing early warning for veterinary and human public health authorities.

Quarantine measures: Enforcement quarantine of animals is difficult in Africa; it is not generally an effective control measure. However, movement of animals from epizootic situation should be restricted to prevent the further spread of RVF.

Movement control refers to activities regulating the movement of people, animals, animal products, vehicles, and equipment in an area subject to certain criteria. Movement control is accomplished through a permit system that allows entities to make necessary movements without creating an unacceptable risk of disease spread. Quarantine refers to imposing restrictions on entering or leaving a premise, area or region where disease exists or is suspected. Quarantine stops the movement of infected animals, contaminated animal products and fomites from infected, contact and suspect premises. Infection can be introduced in to an area free of RVF by infected animals, animal products and insects (Aedes).

Vector control

Mosquitoes are the most important way that RVF is spread. It is only the female mosquito that feeds on blood as she needs the protein to produce eggs. Mosquitoes will lay their eggs on or near the edge of water. Ways in which to control the spread of RVF involve control of the vector and protection against their bites. Larviciding measures at mosquito breeding sites are the most effective form of vector control if breeding sites can be clearly identified and are limited in size and extent. During periods of flooding, however, the number and extent of breeding sites is usually too high for larviciding measures to be feasible.

Vaccinations

There is no specific treatment for RVF. However, different vaccines are available and are commonly used for control of RVF in endemic countries:

Smithburn vaccine: Currently produced in OBP and KEVEVAPI in freeze-dried form. The vaccine administered via subcutaneous route for the immunization of sheep, goats and cattle. The vaccine can cause abortion or fetal malformation in a small percentage of animals, particularly sheep, as well as a slight febrile reaction that may occur on the second to fourth day following inoculation. The use should be restricted to non-pregnant animals above six months of age before or at the mating season so as to ensure maternal antibodies and to avoid abortion as well. Despite these adverse outcomes, it has been widely used for many years as the major prevention measure as a cost-effective vaccine in most endemic zones, since the first introduction of the virus.

The level of herd immunity induced by Smithburn Strain Vaccine significantly declined with elapse of years. The percentage of IgG positive animals declined from 95% to 66.7% after one year and it would decline to zero after six years and eleven months. The vaccine neither was able to produce proper protective antibodies in all animal species particularly cows, nor was safe in immune compromised animals and pregnant ones during gestation period leading to high rate of abortion. Moreover, the isolation of the virus from aborted fetus has proved in utero transmission of the vaccine virus. The vaccine virus not only caused abortion and death of fetus at parturition, but also caused harmful changes in internal organs and propagated inside hepatic cells in a manner similar to natural infection.

Formalin-inactivated vaccine: Commercially produced from OBP and EVSVRI, the virus strain was adapted for growth in Baby Hamster Kidney (BHK) cell, with aluminum hydroxide gel adjuvant for immunization of cattle, sheep and goats, irrespective of the age and stage of pregnancy. The vaccine is neither licensed for use in human nor commercially available but has been used to protect personnel who either work in laboratories with RVFV or would be exposed to RVF infection, after receiving three doses on days 0, 7 and 28, to provide good long immunity with neutralizing antibody titers.

A comparative study conducted to assess the response in cattle to live and inactivated RVF vaccines revealed that a booster dose of inactivated vaccine after 3 months of the first vaccination was safe and able to evoke a good response sufficient to protect cattle against RVF for at least 1 year, not adversely affected by variation in temperature during transportation and that induced long term neutralizing antibodies may persist for 21 months after booster dose at any age and any stage of pregnancy.

Vector vaccines: RVFV vaccines based on viral vectors were developed of which several could be evaluated for human application. Experimental vaccines based on the poxvirus Modified Vaccinia Ankara (MVA) were previously communicated. laboratory developed a vector vaccine based on the paramyxovirus Newcastle Disease Virus (NDV). A single vaccination induced neutralizing antibodies and protected lambs from viremia and clinical signs. This vaccine, here referred to as NDV was developed for use in livestock, but several characteristics of the vaccine render it a particularly strong candidate for human application as well. The potential use of NDV as a vaccine vector for application in humans is well accepted.

Clone 13 vaccine: The commercial OBP vaccine named (RVF Clone 13) has recently been registered, marketed in a form of freeze dried live attenuated virus (Clone 13 strain), and extensively used in South Africa. Clone 13 is a naturally attenuated isolate of RVF virus with a large deletion in the S segment, it obtained during 1974 RVF outbreak in Central African Republic and proved to be highly immunogenic leading to long-term immunity as well.

Published efficacy and safety studies of clone 13 vaccine have shown that the vaccine protects animals properly without inducing undesirable clinical signs, such as abortion in pregnant ewes, pyrexia or fetal malformation in their offspring. In a more recent study, it was reported that the vaccine virus is able to cross the ovine placental barrier and spread to the fetus resulting in malformations and stillbirths.

The vaccine has the potential to be used as DIVA vaccine for RVF, but the accompanying diagnostic tests are not yet commercially available. Although the currently available commercial vaccines have made great contributions to RVF control over the past 80 years, they are associated with some safety and efficacy concerns, but not limited to risk of abortion, pyrexia, fetal malformation, teratogenic effects, viraemia, risk of reassortment, short shelf-life, revaccination and risk of incomplete inactivation in killed vaccines. The most prominent among these candidates is a recombinant Capripoxvirus (CPV) vaccine which was developed to protect against RVFV as well as against sheep poxvirus infection.

Rift Valley fever is an important zoonotic, mosquito-borne viral disease that causes severe human illness and death, as well as significant economic losses in the livestock industry, which may lead to heavy morbidity and mortality among the livestock especially sheep and goat, its needs insects and mosquito for its transmission. From animals, the virus is transmitted to humans through direct contact, such as contact with the raw products or secretions of infected animals It is considered as an occupational disease of livestock handlers, dairy farmers, abattoir workers and veterinarians. At any point of time the virus can emerged and spread to longer distance within a short span of time, it results due to import and export restriction is significant particularly in those countries which livestock contributes great share in their economy. Vaccination and vector control are the main strategies to reduce the incidence of RVF. Based on the above conclusions the following recommendations are forwarded:

• Restricted quarantine area should be established around infected country
• Vaccination of animals before outbreak of virus
• When importing from infected countries, veterinary certification
• Use personal insect protective measures to avoid insect bites
• Avoid contact with domestic animal’s cows, goats and sheep
• Avoid contact with their products, avoid handling raw meat
• Avoid ingestion raw milk and milk products
• Collaboration of veterinary and public health for prevention and control of RVF

I declare that this review has been composed solely by Elias Gezaw Anbu and Jemal Abrahim and that it has not been submitted, in whole or in part, in any previous application for a degree and all the information contained in this review is in accordance with the facts or truths to our knowledge.

Not applicable.

Data and information of this review will be made available up on request of the corresponding author.

There is no conflict of interest in preparation and submission of this manuscript.

They are permitted to publish this review without any interference from the authors.

Not applicable.

Elias Gezaw conceived the review, coordinated the overall activity and drafted review manuscript and participated in the collection of the information. Jemal Abrahim contributed in drafting and reviewing the manuscript. Both authors read and approved the final manuscript.

The authors’ heartfelt thanks Jigjiga University for proving us an internet access.

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