Figure 1. Simplified canine distemper epidemiology assuming a partially immunized dog population (B: grey segment, vaccinated and immune; white segment, susceptible). Virulent wild-type Canine distemper virus (CDV) is maintained endemically in various species of wildlife carnivores in which disease occurs at low to moderate incidence, primarily in young individuals (A). Wild-type CDV can be introduced from A into B (horizontal arrow) and cause small scale regional outbreaks (black dots) among susceptible dogs. C represents a potentially susceptible species/population which is immunologically naive (virgin population) because transmission of virulent CDV from either A or B has been prevented. This is due to spatial (geographic) separation or because of species-specific host factors which allow efficient viral replication only after adaptation. Once the virus penetrates the border, population C is at risk of a fulminant epizootic associated with high morbidity and mortality rates among all age classes.

Figure 2. Unrooted phylogenetic tree of Canine distemper virus (CDV). Evolutionary relationships are based on the analysis of the open reading frame encoding the H glycoprotein, the viral attachment factor. The construction of the tree is based on the maximum likelihood method. The genetic distances are drawn to scale (bar indicates 0.01 nucleotide replacements per site). Numbers indicate the percentage of bootstrap samples out of 1000 replications in which the respective cluster is supported. Values > 90% significantly signal a separate position of the cluster. Other phylogeny methods (parsimony, neighbour-joining), as well as the use of the deduced amino acid sequence of the H protein for comparisons, yielded very similar topologies. The insert displays the topology when isolates of the Phocid distemper virus (PDV) are added. PDV, the causative agent of the 1988 European seal mass mortality, is the morbillivirus species most closely related to, though distinct from, CDV. The natural host spectrum of these viruses is at least partially overlapping. No species abbreviation, dog; Bl, black leopard (zoo); Cl, Chinese leopard (zoo); F, ferret; Jv, javellina; M, mink; Rc, raccoon; Rd, raccoon dog; Den, Denmark; Ger, Germany; Con, Convac strain; Ond, Onderstepoorf strain. All sequences were accessed from the GenBank database.