To protect cattle against LSD, the Neethling strain is the most commonly used worldwide in endemic countries. The strain has been attenuated by 61 serial passages of a field isolate and then through 20 passages on the chorioallantoic membrane (CAM) and three passages on renal primary cells. The strain was then passed 10 times on Madin Darby Bovine Kidney (MDBK) cells, then five times in primary bovine testis cells by Weiss (1968)
[87,99,119][62][74][95]. In addition to the original Neethling strain, other derivative strains with similar sequences (99% homology) were used as commercial vaccines
[120][96] The Neethling strain has been shown to be effective in cattle; however, it can cause adverse inflammation at the injection site, accompanied by fever and decreased milk production
[121][97]. The strain can also cause symptoms similar to the disease with lower intensity, called ‘Neethling disease’. Several studies have reported the development of adverse reactions after the vaccination of cattle against LSDV. According to Ben-Gera et al. (2015)
[21], the LSD Neethling vaccine administered to cattle induced adverse effects with a very low incidence (0.38%). In Croatia, a free-disease country, the vaccine exhibited adverse effects in 0.09% of vaccinated cattle
[122][98]). As these effects have been reported by breeders, the data can be underreported
[123][99]. Agianniotaki et al. (2017)
[124][100] were able to identify the vaccine virus in cattle vaccinated with Neethling strain that showed mild clinical signs of LSDV. Collected nodules, mainly from the head and neck, were smaller and more superficial than those found in animals infected with virulent LSDV and were not accompanied by fever, discharge, and enlarged lymph nodes that characterise the disease. Similarly, in a study in Greece carried out by Katsoulos et al. (2018)
[121][97], skin lesions in the form of small nodules (<0.5 cm) were observed in 19 among 215 vaccinated animals. Bedekovic et al. (2017)
[125][101] were able to detect the vaccine virus in the nodules, blood, and milk of cattle vaccinated with Neethling vaccines.
Two strains—KSGP O 240 and KSGP O 80—have also been used to protect cattle against LSD in the horn of Africa and Israel. Poor protection was induced when used in cattle and widespread pathognomonic reactions to LSD were reported in dairy cattle vaccinated with KSGP strain O–240
[20,126][20][102]. Salib and Osman (2011)
[127][103] reported that an LSD epidemic occurred in Egypt although cattle were vaccinated with the KSGP O-240 vaccine. Similarly, in Ethiopia, a morbidity rate of 22.9% was reported in cattle vaccinated with the same strain
[20]. However, the vaccine failure could be linked to insufficient vaccination coverage and the quality of vaccines
[31][32]. The onset of the disease in cattle following vaccination with the KSGP strain could be explained by the insufficient attenuation of strains used. The KSGP O-180 strain was attenuated by 18 passages on muscle cells, while the KSGP O-240 strain was attenuated by only six passages
[3,110,128][3][85][104].
In general, homologous vaccines offer good protection and are able to control the diseases when vaccination coverage reaches 75%
[123][99]. Depending on the strain and the animal sensitivity, neutralising antibodies appear 14 days post-infection or vaccination and peaks between 28- and 35-days post-vaccination. Vaccinated animals can be protected even when antibodies are no longer detectable because of the dominance of cell-mediated immunity.
There is little information regarding the duration of immunity to capripox vaccines. Some authors report protection for at least two years after immunisation with live vaccines against SPV
[105,116,129][80][91][105]; others have been able to demonstrate protection against challenge with goatpox virus more than four years after vaccination
[130][106].
Few inactivated vaccines have been developed to protect areas at risk or in case of incursion in a free-disease country. The protection conferred with inactivated capripoxviruses has been described by Awad et al. (2003) as ‘non-solid and of short duration’, requiring a booster every six months
[131][107]. More recent studies, however, showed complete protection after the challenge and persistence of antibodies for a longer time period. In a sheep study, a group of 16 sheep was immunised with an inactivated and live attenuated vaccine. The animals were monitored for serological responses and challenged with a virulent strain. All vaccinated sheep were protected against experimental infection and developed antibodies that lasted up to nine months post-vaccination
[22]. In a cattle experiment, cattle were immunised using the inactivated Neethling vaccine. A high percentage of reactors had antibodies elicited and protected cattle against challenge. The vaccine was also tested in the field and showed at least 80% seroconversion
[23]. A second LSD inactivated vaccine was tested recently in cattle and showed complete protection against challenge
[24].
The low vaccination coverage is linked to poor infrastructure with limited access to flocks. It has been reported that the use of a bivalent vaccine that protects against two infections in one injection would minimise the cost of vaccination, overcome the constraints of multiple injections, and allow a large vaccination coverage. Among the combined vaccines that have been developed, a formalin-inactivated SPV vaccine with vaccination against anthrax and clostridium infections has been tested
[132][108]. In another assay, the SPV vaccine was combined with anthrax
[133][109]. Recently, many authors developed and tested the combination of SPV with Peste des Petits Ruminants (PPR) against both diseases. The SPV–PPR combined vaccines have been subsequently used at a large scale in Morocco and other African countries
[113,134,135,136][88][110][111][112].
The use of poxviruses as vaccine vectors in recombinant vaccines presents many advantages such as the large size of the viral genome (140–300 kbp) which can contain up to 25,000 bases of foreign DNA, their thermal stability, and their replication in the cytoplasm of infected cells without integration into the host genome. Several recombinant capripoxvirus-vectored vaccines have been generated using genes of Rift Valley fever
[137[113][114],
138], peste des petits ruminants
[139,140[115][116][117][118],
141,142], rinderpest
[143][119], bluetongue
[144,145][120][121]), foot-and-mouth disease virus Mp1-2A polyprotein
[146][122], EG95 antigens from Echinococcus granulosus
[147][155 |
] |
[ |
131 |
] |
SPV RM65 |
Safe and protective |
- |
Partially protective |
[21,156] | [21][132] |
SPV Perego |
Safe and protective |
- |
- |
[103, | [78 | 157] | ][133] |
SPV Rumania Fanar |
Safe and protective |
- |
- |
|
SPV Romania |
Safe and protective |
Safe and protective |
Partially protective |
[57,158] | [31][134] |
SPV Bakirkoy |
Safe and protective |
- |
Partially protective |
[102,115] | [77][90] |
LSD Neethling |
Partially protective |
- |
Causes Neethling disease, protective |
[23,57] | [23][31] |
LSD KSGP 0180 |
Safe and protective |
Safe and protective |
Safe and protective |
[110] | [85] |
LSD KSGP 0240 |
Safe and protective |
Safe and protective |
Residual virulence, partial protection |
[20,112,126, | [20][87][102] | 127 |
The lack of protection of SPV vaccines against goatpox has previously been reported by Prasad and Datt (1973) using SPV
[159][135]. In Saudi Arabia, Abuelzein et al. (2003)
[160][136] reported the appearance of the disease in goats vaccinated with SPV Romania vaccine locally produced; however, few data on vaccination coverage and vaccination conditions were presented. Although no challenge was performed, Abdelfatah et al. (2019) detected a satisfactory level of peripheral blood mononuclear cells (PBMCs) and lymphocytes in goats vaccinated with the Romania strain
[158][134]. In a recent paper, Hamdi et al. (2020) established full protection of goats against the Romania strain in an experimental goatpox infection
[57][31].
The use of SPV-based vaccines to protect cattle against LSV has been documented in Africa and the Middle East based on field observations. Studies carried out in Israel with RM65 strain demonstrated lower protection in cattle when vaccinated with the sheep dose
[156][132] or at 10-times the sheep dose
[21] when compared to the Neethling vaccine. In Jordan, a vaccination campaign was conducted in cattle using a vaccine based on the RM65 strain. The vaccine caused fever, reduced milk production, and the development of nodules in vaccinated cattle
[161][137]. In Turkey, Sevik and Dorgan (2016) reported LSD vaccination failure with SPV Bakirkoy strain, at 3–4 times the sheep dose
[115][90]. In Egypt, vaccination against LSDV in cattle was carried out with SPV Romania strain; however, cases of infection emerged in vaccinated herds as reported by Ali et al. (1990)
[162][138], Abdallah et al. (2018)
[163][139], and Zeedan et al. (2019)
[164][140].
Investigations on the response of cattle vaccinated with Romania strain showed proliferation of lymphocytes and stimulation of gamma interferon and interleukin 4; however, no challenge has been conducted to test the efficacy of the vaccine
[70,71,165][45][46][141]. Mikhael et al. (2016) observed, through serology and hypersensitivity testing, that the conferred protection by SPV Romania in cattle was insufficient
[166][142]. In another study, Mikhael et al. (2017) tested a bivalent SPV Romania and GPV vaccine in cattle and assessed the humoral response by seroneutralisation, ELISA, and the cellular responses by stimulation of lymphocytes
[167][143]. The authors concluded that the bivalent vaccine induced a better response than the monovalent Romania vaccine but recommended the use of a homologous strain. Similar results were reported by Aboul Soud et al. (2018)
[168][144], who demonstrated that no serological response was induced in cattle vaccinated with the Romania strain, while a trivalent capripox vaccine (composed of SP Romania, GPV Held, and KSGP 0180) induced antibodies in 66% of vaccinated animals. In a recent experiment, partial protection was obtained in cattle vaccinated with Romania strain, no serology detected and at challenge, three out of seven vaccinated cattle showed viraemia and clinical signs similar to control animals
[57][31].
The Gorgan goatpox strain was also tested in sheep, compared to the SPV vaccine at different doses, and challenged with a virulent SPV strain. Only sheep vaccinated with the SPV vaccine were protected and the authors, therefore, concluded that the Gorgan strain was unable to induce effective immunity in sheep against SPV infection
[154][130].
In cattle, several studies showed the effectiveness of GPV strains to protect cattle against LSD. The Kedong and Isiolo strains, isolated in sheep in 1950 and identified as goatpox viruses
[118][94], were able to protect vaccinated cattle against challenge
[109][84]). The Gorgan strain, commonly used in Iran for vaccination against LSD, was tested in comparison with the KSGP O-180 strain and the Neethling strain in cattle. The Gorgan strain was more immunogenic than the two other strains at challenge
[56][30]. However, the authors recommend undertaking large-scale studies to confirm their findings. Varshovi et al. (2017)
[71][46] investigated humoral and cellular response in cattle after vaccination with Gorgan strain and found it immunogenic. In a recent experiment, complete protection in cattle was obtained by Zhugunissov et al. (2020) against challenge after the use of Gorgan strain at 10 times goat dose
[169][145].
LSD vaccine based on Neethling has been used to protect sheep against SPV by Hamdi et al. (2020b); the authors report partial protection against virulent SPV challenge
[57][31].
It appears that the consensus on one universal vaccine against the three capripoxvirus infections is not realistic. In addition, the geographic distribution of diseases is different. Thus, in some countries, the presence of SPV/GPV may or may not be accompanied by that of LSDV
[2]. For instance, in North Africa, SPV is endemic, while no cases of LSD or GPV were reported. In southern Africa, LSD is endemic; however, SPV and GPV are absent. The European continent is free from GPV and SPV (except in Greece where outbreaks of SPV are still notified), while LSD has been reported in many countries of southeastern Europe. In the Middle East, central and eastern Asia, the three diseases are endemic. The use of a vaccine containing a virus not circulating in the country is not indicated
[27][146].
Capripoxvirus infections are emerging diseases that represent a threat to ruminant industry farming in large geographical areas. Mixed flocks of sheep, goats, and cattle living in proximity is a common occurrence in endemic countries, and vaccination should be carried out systematically using the homologous vaccine, live or inactivated according to the epidemiological situation.