Vaccines and Diagnostics for Transboundary Animal Diseases
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Vaccines and Diagnostics for Transboundary Animal Diseases

J. A. Roth, J. A. Richt, I. A. Morozov

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eBook - ePub

Vaccines and Diagnostics for Transboundary Animal Diseases

J. A. Roth, J. A. Richt, I. A. Morozov

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About This Book

Transboundary animal diseases (TADs) are a major threat to livestock. They are highly contagious or transmissible, and they have the potential to cause high morbidity and mortality in both susceptible animal populations and humans. In addition, not only are TADs detrimental to national economies, they are also a serious threat to world food security. This volume presents the proceedings of an international workshop on Vaccines and Diagnostics for Transboundary Animal Diseases that was held in Ames (Iowa, USA) in 2012. Experts and scientists from academia, industry and government reviewed the current status of vaccines and diagnostics for high priority TADs, decision-making and regulatory processes for veterinary biologics, and the roles and responsibilities of government agencies. The discussions also addressed achievements and gaps in vaccine and diagnostics development for 11 important TADs as well as the translation of research findings into licensed novel vaccines and diagnostics for high-priority TADs.

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Information

Publisher
S. Karger
Year
2013
ISBN
9783318023664
Roth JA, Richt JA, Morozov IA (eds): Vaccines and Diagnostics for Transboundary Animal Diseases. Dev Biol
(Basel). Basel, Karger, 2013, vol 135, p 59.
...............................
SESSION II

State of the art, progress and gaps in development of vaccines and diagnostics for high priority transboundary diseases for the NVS

Roth JA, Richt JA, Morozov IA (eds): Vaccines and Diagnostics for Transboundary Animal Diseases. Dev Biol
(Basel). Basel, Karger, 2013, vol 135, pp 61-72.
...............................

Vaccination for the Control of Rift Valley Fever in Enzootic and Epizootic Situations

B. Dungu1, M. Donadeu1, M. Bouloy2
1 Global Alliance for Livestock Veterinary Medicines (GALVmed), Edinburgh, Scotland, UK
2 Institut Pasteur, Paris, France
Key words: Rift Valley fever, vaccines, enzootic, epizootic
Abstract: Vaccination continues to be the most effective way to control Rift Valley fever (RVF), a zoonotic insect-borne viral disease of livestock. The irregular, cyclical and persistent nature of RVF in its occurrence in enzootic situations suggests that the vaccination strategy to be considered for these regions should be different from what is envisaged for free from risk regions. Currently available RVF vaccines have been extensively used for the control of the disease. However, these vaccines have shortcomings that have encouraged many research groups to develop new vaccine candidates that would address a large number of the current challenges, and be suitable for use both in disease-free regions and in different contingency and emergency preparedness strategies. The characteristics of different RVF vaccines and vaccination strategies are discussed in this report.

INTRODUCTION

Since the first description of a Rift Valley Fever (RVF)-like disease in Kenya in 1912-1913, and the first isolation of the RVF virus in 1931 in Kenya [1], RVF continues to be a major zoonotic and economically important disease of livestock in large regions of Africa and the Middle East, where it is endemic. The disease is also considered to be a big threat to other regions of the world. Countries at risk include those adjacent or close to affected regions (such as the Middle East, Europe and North Africa), and regions further away because the virus now features on most lists of potential biological warfare agents due to its severe zoonotic nature.
RVF is a multi-species insect-borne disease caused by the RVF virus, which belongs to the Phlebovirus genus of the Bunyaviridae family. The RVF virus is enveloped and spherical with a diameter of 80-120 nm. Like all members of the family, it possesses a single stranded tripartite RNA genome composed of three segments: the large (L), the medium (M), and the small (S) segment. The L segment codes for the RNA-dependent RNA polymerase L protein; the M segment codes for the precursor of the Gn and Gc glycoproteins, and the two non-structural NSm proteins. The S segment codes for both the N nucleoprotein and the non-structural NSs protein, by using an ambisense strategy [2].
A number of factors play a role in the occurrence of outbreaks: (i) the circulation of the RVF virus through mosquito vectors; (ii) the mosquito pressure (number of breeding sites and hatching frequency), which is highly dependent on environmental conditions, particularly rainfall events; and (iii) the distribution of domestic animal hosts, essentially ruminants (goats, sheep and cattle), vulnerable to increased vector/host contacts at night. However, there is extensive species susceptibility to RVF, as set out in Table 1.
Several outbreaks have occurred since the disease was first described in Kenya. Besides those countries linked to the Great Rift Valley formation, which stretch from the Red Sea through East Africa to Madagascar, RVF has occurred in West Africa, with serious outbreaks in Egypt, Mauritania and Senegal. Some of the major outbreaks include:
ā€¢ 1950-1951: Kenya; 100,000 mortality in sheep; 500,000 abortions [3];
ā€¢ 1977: Egypt; 200,000 human cases, some 600 reported fatalities [4];
ā€¢ 1987: Mauritania/Senegal; over 300 human deaths [5];
ā€¢ 1997-1998: large outbreak in Kenya/Somalia/Tanzania: over 300 human deaths [3];
ā€¢ 1998-1999: large outbreak in South Mauritania, also in Senegal;
ā€¢ 2000-2001: appearance of RVF beyond the African region, in Saudi Arabia [6];
ā€¢ 2007-2008: Sudan, 747 laboratory confirmed human cases, 230 deaths [7];
ā€¢ 2009-2010: South Africa: 242 lab-confirmed human cases with 26 deaths (unpublished data);
ā€¢ 2010: Mauritania: 63 human cases, 13 deaths [8];
Several other African countries have also reported small outbreaks or virus circulation as demonstrated by positive serology to RVF antibodies. Although several control measures have been suggested for RVF, vaccination is still the most effective tool, which should be accompanied by other measures such as effective surveillance, good diagnostic strategy and reliable emergency preparedness. This report focuses on the assessment of vaccination strategies for enzootic and epizootic situations.
Table 1: Species susceptibility to RVF [3]
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CURRENT VACCINATION STRATEGIES IN ENZOOTIC REGIONS

In regions where outbreaks have occurred, the disease always seems to reoccur. Therefore, eradication or elimination in endemic regions is not a viable option at the moment. Control measures in endemic zones are aimed at early warning, emergency preparedness and minimising the impact of the disease. Measures for the prevention of the disease in regions at risk or free from RVF should be considered differently. In enzootic situations, vaccination is still the most effective method of protecting livestock. Although endemic in most of Sub-Saharan Africa, a very limited number of countries use vaccination for the control of RVF. In countries where vaccination is practiced, different approaches are used, as shown in Table 2.

Characteristics of current RVF vaccines

The advantages and disadvantages of different types of RVF vaccines are set out in Table 3. Two types of vaccines are generally utilised in countries that use vaccination for the control of RVF: live and attenuated vaccines and inactivated vaccines. The most commonly produced and used live RVF vaccines are based on the Smithburn virus derived from a strain isolated from mosquitoes in Western Uganda in 1944 and passaged 79-85 times by intracerebral inoculation of mice: this resulted in the loss of hepatotropism, the acquisition of neurotropism and the capacity to immunise sheep safely when administered parenterally [9]. The South African RVF Smithburn vaccine is based on the 103 mouse brain passage levels of the virus, while Kenya uses the 106 passage level to produce the vaccine, all in Baby Hamster Kidney anchored cell culture systems. The two major producers of the live RVF Smithburn vaccine, Onderstepoort Biological Products (OBP) in South Africa and the Kenya Veterinary Vaccines Production Institute (KEVEVAPI) have produced millions of doses since 1952 and 1960 respectively, with the vaccine having been widely used throughout Africa and the Middle East [10].
Table 2: Classification of countries based on RVF occurrence and approach to vaccination strategy
RVF Situation
Examples of countries
Current vaccination/Control strategy
Endemic with regular outbreaks
Kenya, Tanzania, Egypt, Madagascar, Sudan
Vaccination at sign of outbreak, Egypt, Sudan: continuous/regular vaccination, No vaccination (Madagascar)
Endemic with sporadic/reoccurring outbreaks
South Africa, Saudi Arabia, Mauritania
Continuous/yearly vaccination, No vaccination (Mauritania)
Endemic with no large scale outbreaks (or not reported), but with serological evidence of virus circulation
Senegal, Mali, DR Congo
No vaccination
Free high risk
Middle East, North Africa
(Active) surveillance
Free low risk
Europe, Americas
Surveillance, talks about vaccine banks
Table 3: Different types of RVF vaccines
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The live attenuated RVF Smithburn vaccines based on the Smithburn virus have several disadvantages: they may induce abortions, malformations in the foetuses of vaccinated animals, hydrops amnii, and prolonged gestation in a proportion of vaccinated dams. Their use during an outbreak is not advised as they are based on a live virus. Since RVF outbreaks occur in irregular cycles, a number of countries do not to implement vaccination between outbreaks. Another reported problem with the live attenuated RVF Smithburn vaccine is the poor antibody response in vaccinated cattle [11].
In an attempt to address problems associated with the residual virulence of the Smithburn vaccine, as well as the poor antibody response in vaccinated cattle, an inactivated vaccine was developed and has been used since the 1970s in South Africa. This vaccine can be used in all livestock species, at different physiological stages, including pregnancy, and during outbreaks. This inactivated RVF vaccine makes it possible to vaccinate cows that can then confer colostral immunity to their offspring. Given the poor immunogenicity of this vaccine in cattle, it requires a booster three to six months after initial vaccination, followed by annual inoculations [11]. This inactivated RVF vaccine is currently produced in Egypt and in South Africa.
Since 2008, a new vaccine called RVF Clone 13 has been registered and used in South Africa. It is based on an avirulent RVF virus isolated from a non-fatal case of RVF in the Central African Republic that had been passaged in mice and Vero cells, and then plaque purified in order to study the homogeneity of virus subpopulations. A clone designated 13 did not react with specific monoclonal antibodies against NSs and when further investigated was found to be avirulent in mice, yet immunogenic [12]. This vaccine has been evaluated for safety and efficacy in sheep [13] and cattle [14]. More than 10 million RVF Clone 13 vacci...

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