New African Horse Sickness vaccine shows promise

June 25, 2009

Research into a type of vaccine against African Horse Sickness considered more suitable to populations never before exposed to the fatal disease has shown promising results.

The findings are good news for countries in Europe considered under growing threat from the midge-borne disease, capable of killing up to 90 per cent of horses exposed to it.

The research, published in the open access journal PLoS One, focused on inducing an antibody response to the virus which causes the disease in ponies inoculated with a vaccine based on Recombinant Modified Vaccinia Ankara (MVA), a virus considered to have a higher safety profile.

The British and Australian researchers acknowledged that vaccines played a crucial role in control of the disease.

However, there are concerns over the use of polyvalent live attenuated vaccines, particularly in areas where the virus is not endemic. It was therefore important to consider alternative approaches for vaccine development.

The researchers carried out a pilot study to explore the ability of Recombinant Modified vaccinia Ankara vaccines carrying specific African Horse Sickness genes.

They created vaccines carrying one of three different genes, called VP2, VP7 and NS3, from the African Horse Sickness virus (AHSV).

Six Welsh mountain ponies were divided into pairs. Each pair was given one of the three vaccines and their antibody responses were analysed.

The VP2 vaccine induced a strong neutralising antibody response against the AHS virus. The VP7 also induced an antigen-specific response. However, the NS3 vaccine did not produce a detectable response.

The researchers said their study showed the potential of such vaccines in affording protection against the disease, particularly a version carrying the VP2 gene.

"Further work to investigate whether these vaccines would confer protection from lethal AHS Virus challenge in the horse is justifiable," they wrote.

African horse sickness is transmitted by the bite of certain Culicoides biting midge species.

It is endemic in tropical and sub-tropical areas of Africa, south of the Sahara, but outbreaks have occurred outside Africa, resulting in high mortality rates and severe economic loses.

Such outbreaks were reported in the Middle East in 1959, and in North Africa and Spain during 1969 and 1987. In the latter outbreaks, an extensive vaccination programme and movement-control measures led to complete eradication of the disease.

The researchers said the live polyvalent vaccines that are commercially available in South Africa have been developed by cell-culture attenuation (weakening) of the virus.

"However, concerns still exist over their use, particularly in those countries where the disease is not endemic because of potential gene segment reassortment between field and vaccine strains, potential reversion to virulence and inability to distinguish vaccinated from infected animals," they wrote.

In the past, inactivated vaccines have been shown to induce protective immunity, but are not readily available.

The researchers explained that the modified vaccinia Ankara strain used in the trial vaccines had lost the ability to productively infect the cells of mammals.

In backgrounding their research, they pointed to what they called the unprecedented emergence of the Culicoides-transmitted bluetongue virus in Europe and the Mediterranean - a virus closely related to African Horse Sickness.

"This has been attributed to the northward expansion of the major Bluetongue virus vector, Culicoides imicola, possibly influenced by climate change; as well as the involvement of indigenous European Culicoides species.

"Due to the similarities between Bluetongue and African Horse Sickness viruses and their vectors, it has been suggested that should AHSV incur into Europe, there is the potential for it to become as widespread as Bluetongue virus.

"As there are concerns over the use of modified live AHSV vaccines, the development of efficacious and safer AHSV vaccines, suitable for use in both endemic and non-endemic regions, is therefore an important focus of research."