Rift Valley fever (RVF) is a zoonotic viral disease caused by Rift Valley fever virus (RVFV), which is a virus within the genus Phlebovirus and family Bunyaviridae. RVF is enzootic and endemic in Africa, Saudi Arabia and Yemen and poses a significant threat to both human and animal health [1-3]. The disease is most severe in sheep, goats and cattle, causing abortions in pregnant females and high mortality in young animals [4-8].

Vaccination is considered to be the most effective way to prevent and control the expansion of the disease. However, the available attenuated vaccines for RVF cause abortions and teratogenic effects (Smithburn strain vaccine) or are thermolabile (CL13 strain vaccine) [6, 9, 10]. An evaluation of efficacy and safety of the CL13 vaccine in ewes at different stages of pregnancy indicated that the vaccine did not induce clinical manifestation of RVF, such as abortion in pregnant ewes, teratogeny in their offspring or pyrexia in vaccinated animals [11]. Vaccination with CL13 vaccine also prevented clinical RVF following virulent challenge. A recent study carried out by Daouam et al. [12] revealed that the CL13 vaccine strain was unstable in both lyophilized and liquid forms at 22-25 °C and at 37 °C, which highlight the importance of cold chain when using the CL13 vaccine in endemic tropical countries. In response to this need, the CL13 vaccine strain was made thermostable through three cycles of heating (56 °C) and selection of thermostable particles (12). The thermostable vaccine clone (CL13T) was found to be safe when tested on sheep, goats and cattle with no clinical signs or side effects observed. Neutralizing antibodies were detected in vaccinated animals for a minimum of one year in sheep and goats, and for at least 4 months in cattle indicating that the vaccine also provide a good protection [12].

Recent outbreaks of RVF in western Africa have resulted in high mortality and severe clinical signs among dromedary camel populations [13]. Serological surveys in northern and eastern Africa have revealed high percentages of seropositive camels after the occurrence of an outbreak [14, 15]. Camels are therefore susceptible to RVFV, and are likely to play a role in the epidemiology of the disease and should be included in any vaccination program. There have been no previous reports of camels being vaccinated against RVF, and no RVF vaccines including a thermostablized vaccine have been tested for safety and efficacy in this species. This study therefore was conducted to evaluate the safety and efficacy of the thermostable live attenuated CL13T vaccine in camels.

This study reveals that camels mounted a strong and long-lasting neutralizing antibody response when vaccinated with a single dose of the live CL13T RVF vaccine and that the vaccine is safe to use, producing no significant side-effects in the vaccinated animals. The neutralizing antibody response was similar to that observed in small ruminants and cattle after vaccination with the live RVF clone 13 vaccine [17, 18].

This is the first report that evaluates the safety and efficacy of a live attenuated thermostabilized RVF vaccine in camels. To be sure that this vaccine protects camels against RVF, challenge studies would need to be carried out. However in the case of RVF, neutralizing antibodies are known to be reliable predictor of protection after vaccination [17, 19]. The high levels of neutralizing antibodies observed in the vaccinated camels may indicate therefore that the CL13T vaccine is likely to be protective in camels for up to 12 months.

It is highly recommended to vaccinate livestock to prevent the occurrence of disease in susceptible animals and if possible virus amplifying hosts, in order to break the epidemiological transmission cycle. With the high densities of dromedary camels in many areas where RVF is present, it is extremely important to be able to protect camels, as well as other susceptible species.

This is the first study that has evaluated the safety and immunogenicity of a live attenuated RVF vaccine in camels. A recent study, reported camel vaccination with a recombinant adenovirus encoding RVFV envelope glycoprotein. A lower antibody neutralizing titre was observed in camels comparatively to other species (sheep, goats and cattle) with a dose of 10⁹ injected intramuscularly in the presence of an adjuvant [19]. No challenge was carried out for the protection testing.

In our study, the obtained antibody response in vaccinated camels was similar to that registered in sheep and better than cattle response with a dose of 10⁶ only, injected by S/C route. Although this is a preliminary study, the presence of neutralization antibodies is likely to correlate with protection. To be noted that RVF protection of camels has never been tested by experimental infection and no challenge model has been developed for camels.

In summary, the study showed that the CL13T live attenuated vaccine is safe to use in camels and that a single dose of the vaccine stimulated strong and long-lasting neutralizing antibody response for up to 12 months. In the absence of any tested live vaccine for camels, the CL13T vaccine could be considered for camel protection against RVF after a field trial that include sufficient number of animals under the normal living conditions.

CL13, clone 13; CL13T, clone 13 thermostable; PV, post-vaccination; RVF, Rift Valley fever; RVFV, Rift Valley fever virus; SC, subcutaneously; Vero, African Green monkey kidney cells; VN, virus-neutralization