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Bivalent Vaccine to Overcome Acquired Immunity in Pox and Peste des Petits Ruminants

10 April 2015

In Africa, the Middle East and Asia, pox and peste des petits ruminants (PPR) have devastating effects on goat and sheep farms.

A bivalent vaccine effective against both pox and PPR was recently developed by the agricultural research development organisation CIRAD.

However, as is the case with other vaccine vectors, there were fears that its efficacy might be limited by acquired immunity to one of the two diseases in regions where they are endemic.

To test this hypothesis, virologists from CIRAD and their partners conducted a series of experiments on goats already immunized against one virus or the other.

Pox and peste des petits ruminants have a major economic impact in Africa, the Middle East and Asia, where they are widespread. Vaccination is a key weapon against these highly contagious diseases of goats and sheep.

To cut the cost of vaccination campaigns, a bivalent vaccine effective against both pox and peste des petits ruminants has been developed.

However, in view of the distribution of these diseases, which overlap in many regions worldwide, acquired immunity against one of the two diseases could interfere with the vaccine's capacity to induce immunity against the other. Pre-existing immunity, or pre-immunity, would therefore limit the efficacy of the bivalent vaccine in the field. To shed light on this particularly critical, practical issue, virologists from CIRAD and their partners undertook a series of experiments on animals using the vaccine.

Highly Contagious Viral Diseases

Peste des petits ruminants (PPR) is a highly contagious viral disease that affects domestic and wild small ruminants. It is caused by a virus of the genus Morbillivirus : Peste des Petits Ruminants Virus (PPRV). It is endemic to many countries in Africa, the Middle East and Asia.
Sheep pox and capripox are also contagious viral diseases.

They are caused by viruses of the genus Capripoxvirus (CaPV). They affect small ruminants and are found in several regions worldwide, some of which overlap with the PPR distribution zone.

A Bivalent Vaccine Effective Against Pox and PPR

In their experiments, the researchers used a Capripoxvirus (CaPV) vaccine strain, Kenyan Sheep 1 (KS-1) that is effective against pox, which was genetically modified so as to express PPR virus antigens.

This recombining CaPV-PPR strain is an interesting candidate vaccine, capable of protecting against both pox and PPR.

Pre-immunity Limits Vaccine Efficacy

The researchers carried out infection tests (PPR then CaPV) on animals pre-immunized against one disease or the other and then vaccinated using recombinant CaPV-PPR vaccines.

They showed that while the recombinant vaccine protected animals previously infected with the PPR virus against the CaPV virus, the same could not be said for protection against PPR of animals pre-immunized against CaPV: the protection provided by the vaccine against the PPR virus was much lower in these animals.

As seen with the main viral vector families, pre-immunity against the CaPV viral vector thus results in a loss of efficacy when using recombinant CaPV in areas where these viruses are found and circulate endemically.

Towards New Multivalent Recombinant Vaccines

These results, which call for further studies, highlight the need to develop new generations of multivalent CaPV recombinant vaccines that perform better.

In particular, it is necessary to draft strategies to counter the problem of pre-immunity. CIRAD's virologists are now conducting new research to this end.

February 2015