Plant-based vaccine shows promise against deadly African horse sickness

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Tests suggest a plant-based vaccine against African horse sickness is at least as effective as the current live vaccine in protecting against one serotype of the deadly disease.

Highly infectious African horse sickness (AHS) is a scourge of domestic equines in sub-Saharan Africa.

There are well-established commercial live vaccines produced in South Africa in which the virulence has been attenuated (reduced).

While these live vaccines have been the primary control strategy to date, recent research has shown that these kinds of vaccines carry the risk of reverting to virulence, as well as the possibility of gene re-assortment between outbreak and vaccine strains.

The risks associated with these vaccines have encouraged attempts to develop new and safer alternatives.

Researchers have updated the science community on their progress with a plant-based vaccine in a short report published in the open-access journal Veterinary Reports.

University of Cape Town researcher Susan Dennis and her colleagues had previously reported on the immune response provoked in guinea pigs to their vaccine, which targets AHS serotype 5.

Their virus-like particle (VLP) vaccine had safely stimulated production of AHS serotype 5-specific neutralizing antibodies in the rodents.

They now report a similar response to the vaccine in horses.

“This,” they said, “is the first report demonstrating the safety and immunogenicity of plant-produced AHS VLPs in horses.”

Virus-like particles are non-replicating protein assemblies identical in size and shape to the intact virus, with which they share certain key features.

They are inherently safe when compared to live attenuated vaccines as they contain no infectious genetic material nor any of the non-structural proteins.

Nicoatiana benthamiana.
Nicoatiana benthamiana. (via Wikipedia)

The vaccine used in the study uses the plant Nicotiana benthamiana. The researchers used a genetic process in which four surface proteins from the virus were ultimately incorporated in the resulting VLPs in order to provoke an immune response.

The results showed that all four horses vaccinated with the plant-produced product seroconverted after two doses of the vaccine.

“Overall, the virus neutralization titres obtained indicate that the plant-produced AHS 5 VLP vaccine may be at least as effective as the current LAV in protecting against AHSV 5, but without any of the associated risks of a live vaccine such as reversion to virulence or genetic re-assortment with field or vaccine strains,” they reported.

“Furthermore, a similarly strong protective response against AHSV 8 was also elicited by all four horses, confirming the cross-protection between these two AHSV serotypes.”

Two horses that received the filtered plant extract had a temporary stiff neck. Otherwise, the vaccination did not cause any adverse reactions, and all horses have since remained healthy.

The study team said it was looking at running a challenge study using horses, as well as investigating the production of VLP vaccines against other AHS serotypes, and/or the production of VLPs that can target multiple serotypes in a bid to meet the demand for a new, safer and economically viable AHS VLP vaccine.

They said development of a VLP vaccine would address concerns regarding the use of a live vaccine and would also serve as a potentially acceptable preventative or rapid-response antidote in the wider international context.

The full study team comprised Susan Dennis, Martha O’Kennedy, Daria Rutkowska, Tsepo Tsekoa, Carina Lourens, Inga Hitzeroth, Ann Meyers and Edward Rybicki, variously affiliated with the University of Cape Town, the Council for Scientific and Industrial Research in South Africa, and the University of Pretoria.

Safety and immunogenicity of plant-produced African horse sickness virus-like particles in horses
Susan J. Dennis, Martha M. O’Kennedy, Daria Rutkowska, Tsepo Tsekoa, Carina W. Lourens, Inga I. Hitzeroth, Ann E. Meyers and Edward P. Rybicki
Veterinary Research 2018 49:105 https://doi.org/10.1186/s13567-018-0600-4

The study, published under a Creative Commons License, can be read here

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