The isolation and genetic characterization of canine distemper viruses from domestic dogs (Canis familiaris) in South Africa

Abstract

Studies comparing canine distemper virus (CDV) strains from Africa with that of previously described lineages have been hampered due to a lack of field strains for in vitro experiments. There are no reports of CDV isolations in southern Africa, and although CDV is said to have geographically distinct lineages, molecular information of African strains has not yet been documented. Clinical specimens consisting of whole blood, spleen, lungs, brain and cerebrospinal fluid from dogs with clinical signs suggestive of distemper were obtained from private veterinary practices and diagnostic laboratories in Gauteng province, South Africa. One hundred and sixty two specimens from 124 dogs were inoculated into Vero cells expressing canine signalling lymphocyte activation molecule (Vero.DogSLAM). Cytopathic effects (CPE) in the form of syncytia formation and cell necrosis were observed in 33 (20.4%) specimens within 24 hours of inoculation and the presence of CDV was confirmed with the aid of the direct fluorescent antibody test and electronmicroscopy (EM). Viruses isolated in cell cultures were subjected to reverse transcription – polymerase chain reaction (RT-PCR), and the complete H gene was sequenced and phylogenetically analysed with other strains from GenBank. The animals all displayed the catarrhal, systemic, or nervous signs of the disease. Four of the dogs positive for CDV by viral isolation had a history of prior vaccinations to canine distemper. Seventy three percent of all positive dogs were less than 12 months old. There was no breed or sex predisposition to CDV infection in domestic dogs. Phylogenetic comparisons of the complete H gene of CDV isolates from different parts of the world (available in GenBank) with wild-type South African isolates revealed nine clades. All South African isolates form a separate African clade of their own and thus are clearly separated from the American, European, Asian, Arctic and vaccine virus clades. It is likely that only the ‘African lineage’ of CDV may be circulating in South Africa currently, and the viruses isolated from dogs vaccinated against CDV are not the result of reversion to virulence of vaccine strains, but infection with wild-type strains. Copyright