TRANSMISSION AND VIRULENCE OF RINDEPEST VIRUS.pdf

The project entitled 'Transmission and virulence of rinderpest virus' has been approved andfinanced by the European Economic Community, General Direction XII (EEC-GD XII — STD2).It comprises a total of four laboratories:- Institute for Animal Health, Pirbright Laboratory, Great Britain;- Laboratoire de virologie et d'immunologie, [Virology and immunology laboratory] of theFaculty of Veterinary Medicine of the Universite de Liege, Belgium;- Institut d'elevage et de medecine veterinaire (IEMVT), [Livestock and Veterinary MedicineInstitute], Maison-Alfort, France;- Laboratoire de recherches veterinaires et zootechniques de Farcha, [Laboratory for veterinaryand Zootechnical Research of Farcha], N'Djamena, Tchad.The purpose of this project is to study some rinderpest's viral strains of different pathogenecity,which will firstly give us the opportunity to understand the basics ofpoSe variations, to assess ifthis phenomenon can be associated to the virus's persistence and, finally, to bring to the forefactors, badly understood until then, which influence the virus's pathogenecity. Secondly, thefollowing aspects of those strains will be studied:- genes sequences of their F and H proteins;- incubation period;- duration of the virus's excretion in case of an infection;- minimum time required for an infecting contact;- number of animals infected, over a given period of time, by an animal excreting the virus.The results obtained in the course of those experiments will lead to the development of amathematical model of rinderpest infection.Rinderpest constitutes a disease which is acute, febrile, inoculable, very contagious and mainlytransmissible through direct contact. It affects almost all species belonging to the Artiodactylesfamily among which ruminants and swines are the most sensitive (PLOWRIGHT, 1968). Withrabies, it is the longest known affection and perhaps the most deadly for cattle. It is still affecting (ikAfrica, India and the Near East (PLOWRIGHT, 1985). The disease is known since the fifthcentury, but it is in 1902 that NICOLE and BEY demonstrated the filterability of the virus.As a result of a virus belonging to the Paramyxoviridae family, of the morbillivirus type, thisdisease is characterised by a more or less apparent typhic condition, an inflammation of mucousmembranes, that of the digestive tractus in particular, with erosion and necrosis.Although there is only one antigenic type of rinderpest virus, whatever its geographic origin,many isolated strains fundamentally differ over the world because of its level of virulence. As aresult, they were coarsely divided into three categories (TAYLOR, 1986). The first categorycomprises those called hypervirulent which cause 100% mortality in animals. Strains of thesecond category have an average virulence resulting in a mortality rate of 33%. The third oneencompasses hypovirulent strains leading to a mortality rate from 0 to 5% which result howeverin abortive forms, with truncated clinical expression.The duration of the incubation period, the infection and the mortality noticeably vary accordingto the virus's virulence, the receptivity or the natural resistance of animals, the intensity ofcontacts, the number of viral units excreted by an animal and finally, to a lesser extent, to theenvironment in which contact is made. Indeed, those factors play a crucial role in thetransmission speed of the virus from one animal to another and, consequently, influence thespread of the disease in a given population.The existence of atypical forms, indeed asymptomatic (subclinical) makes the possibledissemination of the virus in herds even more dreadful and, by way of consequence, renders theclinical diagnosis almost impossible. This complicates even more the epidemiological study ofthe disease (PROVOST et al., 1973).Several in vivo experiments in relation to the pathogenic power of isolated strains wereundertaken (COOPER, 1932; PROVOST, 1958; ROBSON et al., 1959; PLOVVRIGHT, 1963,1964; LIESS et al., 1964; PROVOST, 1972; TAYLOR et al., 1965, 1986). But those results arefragmentary and only provide information on the isolated virus.To date, there is no information in literature about the compared study of rinderpest virus/strains of different pathogenecity in the transmission through close contact over variable periodsof time.The part played by the laboratory of Farcha in this project is to achieve a compared study of threerinderpest virus strains of variable virulence in sensitive animals in order to address theabovementioned problems. Samplings and data are collected with a view to forward them toEuropean laboratories which are partners involved in the said project.