Control of peste des petits ruminants: classical and new generation vaccines.

Peste des Petits Ruminants is a highly contagious disease of domestic and wild small ruminants. It seems to be the major constraint in the development of small ruminant production in areas where it is endemic. Included in the list A of the International Zoosanitary Code, it is also part of the FAO EMPRESS programme. Classically, it is characterised by fever, nasal and ocular discharges, diarrhoea, respiratory distress, mucosal erosive lesions and death in 40-80% of acute cases. All these clinical signs, apart from the respiratory symptoms, are very similar to those of rinderpest. The causal agents of both diseases are closely related and belong to the Morbillivirus genus. In the absence of homologous vaccine, and taking advantage of the close relationship between the two viruses, the attenuated tissue culture rinderpest vaccine has been used for a long time to protect small ruminants against PPR. At the end of 1980s, a PPRV strain was successfully attenuated by serial passages in Vero cells. Demonstrated to be very efficient in the protection of sheep and goats against a virulent challenge, this avirulent PPRV is now widely used in the control of PPR. Its thermostability has been dramatically improved by freeze drying. However, animals inoculated with this vaccine cannot be distinguished serologically from those infected with the wild type viruses. With the reverse genetics technique, it is now possible to engineer PPR marker vaccines which can allow such a differentiation. The genes of the PPRV immune protective proteins, the fusion and the haemagglutinin proteins, have been introduced into the genome of a capripox vaccine strain. The generated viruses have proved to be effective as a dual vaccine to protect against two major diseases of small ruminants: peste des petits ruminants and capripox infections.