MYXOMATOSIS : STILL ONE OF THE MAJOR THREATS FOR RABBIT BREEDING

The three most important viruses of rabbits include : Myxoma virus (MV), the poxvirus that causes Myxomatosis, the calicivirus ( genus Lagovirus) of Rabbit Haemorrhagic Disease (RHDV), and Lapine Rotavirus (LRV), which is an enteric agent. There a r some other viral agents in rabbits (parvovirus, coronavirus, adenovirus, calicivirus ( genus Vesivirus), enterovirus-like, reovirus, herpesvirus and coronavirus) but both their occurrence and their pa thological value are negligible. Myxomatosis was firstly introduced in Europe more t han 50 years ago and still represents a current and real problem. Recent studies have been focused to determine the role and function of the over 100 genes of MV encoding structural and essential prote ins; in particular the immunomodulatory MV (imMV) proteins (virokines and viroreceptors, immune m odulators and anti-apoptotic factor), involved in contrasting the host immune system response towards the MV infection. One of the main tools to control myxomatosis in endemic areas is use of the live attenuated vaccines that are able to induce traceable humoral immunity for a variable time even if the protection of rabbits from the infection is not fully guarantee. Therefore, thanks also to the knowledge on the im-MV proteins, a new family of biotechnology deleted vaccines will soon be produce and made available in a few years. These should be more safe and able to induce a wider immu nity and permit to apply the DIVA strategy i.e. to use serology to ascertain if the anti-MV antibodies d tected in a rabbits originate by an infection or a vaccination. RHDV is a non-cultivable calicivirus that infects r abbits and causes an acute fatal hepatitis, firstly described in China in 1984. The first consistent an tige ic variant called RHDVa, was identified in Italy and Germany in 1997. Nowadays it is present i n most parts of Italy and its field prevalence has reached value over 50%. In Europe it has been repor ted between 1997 and 2004 in Germany, France, Malta and Hungary. Outside Europe, it was reported in Reunion Islands, USA and South America and, taking account of the RHDV genetic sequences deposi ted at the NCBI databank, its presence in China is also evident from 1985. More recently some other RHDV isolates presenting peculiar characteristics were identified. Based on their reactivity with MAb s these strains could be considered as further and separate steps of variation of the RHDVa, possibly classified as sub-variants. The detection of seropositivity for RHDV in the ser a of healthy farmed and laboratory rabbits taken between 1975 and 1985, which is approximately ten y ars before the occurrence of the disease in Europe, has suggested the hypothesis of the existen ce of one or more “non-pathogenic” viral strains antigenically related to pathogenic RHDV. Rabbit Ca licivirus (RCV), the first of these non-pathogenic RHDV-like viruses identified in healthy rabbits, wa s detected in Italy in 1996. It is avirulent, repli cates in the intestine at a low titre and presents a 92% genomic identity with RHDV. The diffusion of RCV in different areas of Italy has been evaluated in f ive consecutive serological surveys during the peri od 1999-2008, throughout the determination at slaughte ring of anti-RHD antibodies in non-vaccinated meat rabbits from RHDV-free commercial rabbitries. The results clearly show that antibodies reactive with RHDV are present in several rabbit populations : almost 30% of controlled farms and over 80% of animals. The definitive proofs that an active infec tion had occurred came from the detection of IgA and IgM as well as the identification of viral stra ins by using PCR on faeces. In addition, the existe nce of other non-pathogenic caliciviruses in wild rabbi ts was suggested by the serological surveys of rabb it populations in European countries (UK and France), Australia and New Zealand. Either the identification of RNA particles related to RHDV in rabbit sera collected since 1955 in Britain and the 9 World Rabbit Congress – June 10-13, 2008 – Verona – It ly 880 very recent isolation and identification of one of these viruses in Australia finally confirmed such hypothesis. Enteric diseases have an important role in the rabb it industry since they produce severe economic losses due to mortality, growth depression and wors ening of conversion index. Among the different pathogens that could be found in rabbits suffering from enteropathy, viruses seem to have an important but not definitive role. Viruses and among others L apine Rotavirus (LRV) particularly, should not be able to induce primary episodes of high gravity but , acting as mild pathogens, they have the capacity of became endemic. The role and importance of virus es as primary aetiological agent of rabbit enteriti s are here discussed, by both reviewing the available literature and presenting the results of studies o f prevalence of the viruses identified in rabbits wit h enteritis. That is to recognise the main features and pathogenic abilities of different viral agents and to try to attribute them an etiological role in ent ric syndromes, relating their presence with pathologic lesions.