Pathogenesis and phylogenetic analyses of canine distemper virus strain 007Lm, a new isolate in dogs

Approaches to identify canine distemper virus with neurological symptoms on the basis of molecular characterization of hemagglutinin and fusion genes

Canine distemper virus (CDV), which causes severe infections in all domestic and wild carnivores, is transmitted by all secretions and excretions of infected animals. Despite the regular vaccination against it, CDV still manages to circulate in nature and is a worldwide problem in dogs. For many years in the world, the virus managed to circulate in nature. The current investigation aims to identify and characterize CDV in dogs with neurological symptoms and to determine whether CNS symptoms and phylogenetic data might be used to differentiate between CDV strains. The medical records of 35 dogs with central nervous system (CNS) symptoms were examined. An ELISA kit was used to identify CDV-specific IgG antibodies in all of the dogs' serum samples. RT-PCR confirmed the presence of CDV nucleic acid in 30 of these dogs. Of the RT-PCR-positive samples, 6 were randomly chosen for further sequencing, sequence comparisons, and phylogenetic reconstructions. Genes encoding the Hemagglutinin (H) and Fusion (F) proteins were partly sequenced and compared to other CDVs from throughout the world, including vaccine strains. The maximum likelihood method was used to build a phylogenetic tree using CDV H and F gene nucleotide sequences. According to phylogenetic analysis of partial H and F gene nucleotide sequences, the field CDVs in this investigation were unique and different from the vaccine strain. The phylogenetic analysis indicated that all Turkish CDV strains that induced CNS symptoms belonged to the European CDV clade. While the intricacy of the CNS and the complexities of glycosylation pathways may provide significant challenges to infections, future research will bring significant benefits by identifying evolutionarily conserved activities of N-glycosylation in CDV-infected dogs.

Canine Distemper Virus in Autochtonous and Imported Dogs, Southern Italy (2014–2021)

Simple Summary

In the period 2014–2021, the circulation of CDV in dogs of Southern Italy was investigated. In this time span a reduction in the circulation of CDV was observed, with a higher frequency of detection of the pathogen in imported dogs (18.4%) compared to stray (7.4%) and household (3.9%) animals. These results underline the effectiveness of the prophylaxis strategy on autochthonous dogs as well as the importance of continuous surveillance of CDV, especially in imported dogs.

Abstract

This study aims to investigate the presence of canine distemper virus (CDV) infection in 949 autochthonous or illegally imported dogs from Southern Italy, over a period of eight years (2014–2021). CDV RNA was detected in 6.8% (65/949) of the animals tested, with no detection of CDV in dogs sampled in 2020–2021. The frequency of CDV detection was higher in imported dogs (19/103, 18.3%) with respect to stray (27/365, 7.4%) and household dogs (19/481, 3.9%). On sequence and phylogenetic analyses of selected strains, the analyzed viruses belonged to the Arctic clade, which has already been reported in Italy and in Europe. The results of our study may suggest a reduction of CDV circulation in Southern Italy, while at the same time highlighting the need for strict controls on dog importation, in order to prevent the introduction of viruses from endemic countries.

Molecular and pathological screening of canine distemper virus in Asiatic lions, tigers, leopards, snow leopards, clouded leopards, leopard cats, jungle cats, civet cats, fishing cat, and jaguar of different states, India

The present investigation was conducted to rule out canine distemper (CD) diseases in Indian wild felids (Asiatic lions, tigers, leopards, snow leopards, clouded leopards, leopard cats, jungle cats, civet cats, fishing cat, and jaguar). The collected samples were screened for CD virus (CDV) by histopathology (HP), immunohistochemistry (IHC) and reverse transcriptase-polymerase chain reaction (RT-PCR) targeting H gene and N gene. The HP and IHC of suspected samples portrayed that 22 [11 leopards, 6 lions, 3 tigers, 1 snow leopard and 1 civet cat] out of 129 (17.05%) wild felids were positive for CD. The major pathological consequences were observed in spleen, lung, kidney and brain. The syncytia and intranuclear as well as intracytoplasmic eosinophilic inclusion bodies were seen in CDV infected cells. Although the histopathological lesions in spleen were more specific and consistent, however, the severe demyelinated leukoencephalitis (usually expected in CD infected dog) was not observed in the brain of any Indian wild felids. Conversely, the CDV antigen has been portrayed via IHC in pancreatic islets of Langerhans of tiger species for the first time in this study. Moreover, the concurrent CD and babesiosis has also been observed in a lioness without a usual coffee-coloured urine. The N gene and H gene of CDV isolates were amplified, sequenced and subsequently constructed the phylogenetic tree. The phylogenetic analysis of H gene revealed that the CDV isolates from Indian lion formed separate clade with CDV isolates from Indian dog and Indian palm civet cat. Furthermore, two CDV isolates from Indian tigers formed clade with Onderstepoort vaccine strain and CDV isolates from dogs of Uttar Pradesh, USA and UK. Evidently, CDV is circulating in Indian wild felids and causing diseases in them.

Animal morbilliviruses and their cross-species transmission potential

Like measles virus (MV), whose primary hosts are humans, non-human animal morbilliviruses use SLAM (signaling lymphocytic activation molecule) and PVRL4 (nectin-4) expressed on immune and epithelial cells, respectively, as receptors. PVRL4's amino acid sequence is highly conserved across species, while that of SLAM varies significantly. However, non-host animal SLAMs often function as receptors for different morbilliviruses. Uniquely, human SLAM is somewhat specific for MV, but canine distemper virus, which shows the widest host range among morbilliviruses, readily gains the ability to use human SLAM. The host range for morbilliviruses is also modulated by their ability to counteract the host's innate immunity, but the risk of cross-species transmission of non-human animal morbilliviruses to humans could occur if MV is successfully eradicated.

Canine and Phocine Distemper Viruses: Global Spread and Genetic Basis of Jumping Species Barriers

Canine distemper virus (CDV) and phocine distemper (PDV) are closely-related members of the Paramyxoviridae family, genus morbillivirus, in the order Mononegavirales. CDV has a broad host range among carnivores. PDV is thought to be derived from CDV through contact between terrestrial carnivores and seals. PDV has caused extensive mortality in Atlantic seals and other marine mammals, and more recently has spread to the North Pacific Ocean. CDV also infects marine carnivores, and there is evidence of morbillivirus infection of seals and other species in Antarctica. Recently, CDV has spread to felines and other wildlife species in the Serengeti and South Africa. Some CDV vaccines may also have caused wildlife disease. Changes in the virus haemagglutinin (H) protein, particularly the signaling lymphocyte activation molecule (SLAM) receptor binding site, correlate with adaptation to non-canine hosts. Differences in the phosphoprotein (P) gene sequences between disease and non-disease causing CDV strains may relate to pathogenicity in domestic dogs and wildlife. Of most concern are reports of CDV infection and disease in non-human primates raising the possibility of zoonosis. In this article we review the global occurrence of CDV and PDV, and present both historical and genetic information relating to these viruses crossing species barriers.

EVIDENCE OF TWO COCIRCULATING CANINE DISTEMPER VIRUS STRAINS IN MESOCARNIVORES FROM NORTHERN COLORADO, USA

Canine distemper virus (CDV) is a highly contagious pathogen that principally infects wildlife and domestic carnivores. Peridomestic species such as raccoons ( Procyon lotor) experience outbreaks with high mortality. Clinical signs of infection include anorexia, fever, respiratory infection, and neurologic complications. Although not zoonotic, CDV poses a high risk to unvaccinated domestic animals and the conservation of endangered species. During 2013-16, we opportunistically collected wild and domestic carnivore specimens through a rabies surveillance program in northern Colorado, US. Brainstem and cerebellar tissue samples were independently tested for rabies and CDV by fluorescent antibody test. We tested a total of 478 animals for CDV, comprised of 10 wild and domestic carnivore species. A total of 15% (72/478) of all animals sampled tested positive for CDV, consisting of 24% (71/300) of raccoons and 4% (1/26) of coyotes ( Canis latrans), but coinfection with rabies virus was not observed among CDV-positive animals. We extracted RNA from positive tissues, and a reverse-transcription PCR was used to create complementary DNA. We amplified and sequenced the hemagglutinin gene from 60 CDV-positive tissues, and a median joining network and maximum likelihood phylogenetic tree revealed two major lineages among samples. Phylogenetic analysis indicated that our sequences were most similar to the America-2 ( n=55) and the America-3 ( n=5) CDV lineages circulating in North America. Our results indicated two distinct and distantly related clades of CDV overlapping geographically and temporally among raccoon populations in northern Colorado.

Outbreak and genotyping of canine distemper virus in captive Siberian tigers and red pandas

In this study, four canine distemper virus (CDV) strains were isolated from captive Siberian tigers (Panthera tigris altaica) and red pandas (Ailurus fulgens) during two separate CDV outbreaks in a zoo in Guangdong province, China. Sequence alignment and phylogenetic analyses based on the full-length hemagglutinin (H) and fusion (F) genes showed that they were closely identical to genotype Asia-1. Prior to confirmation of CDV in Siberian tigers, to control spread of the disease, a live attenuated combination CDV vaccine was used among almost all carnivore animals except for red pandas in which another recombinant combination CDV vaccine was used. However, about two months later, CDV re-emerged and caused the death among red pandas. Based on the vaccination records, the live combination vaccine could be considered an ideal weapon against CDV in zoo carnivore animals. Although the recombinant combination CDV vaccine was safe for red pandas, its protection effectiveness remains to be further investigated. Moreover, according to the outbreak interval time and sequence characterization, we suspected that stray cats circulating in the zoo were the intermediate host, which contributed to CDV spread from stray dogs to zoo animals. This study revealed the importance of vaccination and biosecurity for zoo animals.

Homologous recombination is a force in the evolution of canine distemper virus

Canine distemper virus (CDV) is the causative agent of canine distemper (CD) that is a highly contagious, lethal, multisystemic viral disease of receptive carnivores. The prevalence of CDV is a major concern in susceptible animals. Presently, it is unclear whether intragenic recombination can contribute to gene mutations and segment reassortment in the virus. In this study, 25 full-length CDV genome sequences were subjected to phylogenetic and recombinational analyses. The results of phylogenetic analysis, intragenic recombination, and nucleotide selection pressure indicated that mutation and recombination occurred in the six individual genes segment (H, F, P, N, L, M) of the CDV genome. The analysis also revealed pronounced genetic diversity in the CDV genome according to the geographically distinct lineages (genotypes), namely Asia-1, Asia-2, Asia-3, Europe, America-1, and America-2. The six recombination events were detected using SimPlot and RDP programs. The analysis of selection pressure demonstrated that a majority of the nucleotides in the CDV individual gene were under negative selection. Collectively, these data suggested that homologous recombination acts as a key force driving the genetic diversity and evolution of canine distemper virus.

Influence of vaccine strains on the evolution of canine distemper virus

Canine distemper virus (CDV) is a major dog pathogen belonging to the genus Morbillivirus of the family Paramyxoviridae. CDV causes disease and high mortality in dogs and wild carnivores. Although homologous recombination has been demonstrated in many members of Paramyxoviridae, these events have rarely been reported for CDV. To detect potential recombination events, the complete CDV genomes available in GenBank up to June 2015 were screened using distinct algorithms to detect genetic conversions and incongruent phylogenies. Eight putative recombinant viruses derived from different CDV genotypes and different hosts were detected. The breakpoints of the recombinant strains were primarily located on fusion and hemagglutinin glycoproteins. These results suggest that homologous recombination is a frequent phenomenon in morbillivirus populations under natural replication, and CDV vaccine strains might play an important role in shaping the evolution of this virus.

Preparation and Identification of a Single-chain Variable Fragment Antibody Against Canine Distemper Virus

The variable regions of the heavy chain (VH) and light chain (VL) were amplified by RT-PCR from the hybridoma 1N8, which secretes the monoclonal antibody against CDV N protein (aa 277-471). The VL and VH amplicons were combined using SOE-PCR by a 12 amino acid flexible linker (SSGGGGSGGGGS), which produced the scFv gene (named scFv/1N8). After sequence analysis, the scFv/1N8 gene was cloned into the prokaryotic expression vector PET32a with a His-tag. The recombinant scFv/1N8 protein was successfully expressed in recombinant Escherichia coli by IPTG induction. Moreover, the binding activity and specificity of the scFv were determined by indirect ELISA (His-tag) and competitive ELISA. The recombinant scFv/1N8 protein reported here will provide some basis for further antiviral drug research based on the scFv molecule.

Phylodynamic analysis of the canine distemper virus hemagglutinin gene

Background

Canine distemper (CD) is one of the most contagious and lethal viral diseases in dogs. Despite the widespread use of vaccines, the prevalence of the CD virus (CDV) has increased at an alarming rate in recent years. In this phylodynamic study, we investigated the spatiotemporal modes of dispersal, viral demographic trends, and effectiveness of vaccines for CDV. A total of 188 full-length CDV hemagglutinin (H) gene sequences dataset were subjected to recombination analysis, including seven from modified live vaccine (MLV) strains and 12 from Taiwan specimens. After excluding the MLV strains and potential recombinant strains, alignments of 176 of 188 previous CDV strains were further used to analyze phylodynamic characteristics, and evidence of selection, and co-evolution.

Results

The CDV genotype consisted of MLV-associated genotypes such as America-1 and Rockborn-like strains, which were characterized by long terminal branches and no distinct geographical patterns among lineages. In contrast, wild-type isolates clustered into lineages with a spatiotemporal structure and short terminal branches. Co-circulation and extensive diversification were simultaneously observed. The sequence variation signature was shaped by both geographic diversity and host tropism. Codon 506 was identified as a multi-epistatic interacting in the H protein.

Conclusions

The topological signature revealed in this study suggests different epidemic scenarios. For example, a ladder-like backbone is a hallmark of directional selection, and cladogenesis at long terminal branches indicates the emergence of a surviving lineage. The stable effective viral population of CDV indicate the effectiveness of vaccines currently used to control the virus.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-015-0491-9) contains supplementary material, which is available to authorized users.

Use of quantitative real-time RT-PCR to investigate the correlation between viremia and viral shedding of canine distemper virus, and infection outcomes in experimentally infected dogs

We used real-time RT-PCR and virus titration to examine canine distemper virus (CDV) kinetics in peripheral blood and rectal and nasal secretions from 12 experimentally infected dogs. Real-time RT-PCR proved extremely sensitive, and the correlation between the two methods for rectal and nasal (r=0.78, 0.80) samples on the peak day of viral RNA was good. Although the dogs showed diverse symptoms, viral RNA kinetics were similar; the peak of viral RNA in the symptomatic dogs was consistent with the onset of symptoms. These results indicate that real-time RT-PCR is sufficiently sensitive to monitor CDV replication in experimentally infected dogs regardless of the degree of clinical manifestation and suggest that the peak of viral RNA reflects active CDV replication.

The V protein of canine distemper virus is required for virus replication in human epithelial cells

Canine distemper virus (CDV) becomes able to use human receptors through a single amino acid substitution in the H protein. In addition, CDV strains possessing an intact C protein replicate well in human epithelial H358 cells. The present study showed that CDV strain 007Lm, which was isolated from lymph node tissue of a dog with distemper, failed to replicate in H358 cells, although it possessed an intact C protein. Sequence analyses suggested that a cysteine-to-tyrosine substitution at position 267 of the V protein caused this growth defect. Analyses using H358 cells constitutively expressing the CDV V protein showed that the V protein with a cysteine, but not that with a tyrosine, at this position effectively blocked the interferon-stimulated signal transduction pathway, and supported virus replication of 007Lm in H358 cells. Thus, the V protein as well as the C protein appears to be functional and essential for CDV replication in human epithelial cells.

Phylogenetic analysis of the haemagglutinin gene of canine distemper virus strains detected from giant panda and raccoon dogs in China

BACKGROUND

Canine distemper virus (CDV) infects a variety of carnivores, including wild and domestic Canidae. In this study, we sequenced and phylogenetic analyses of the hemagglutinin (H) genes from eight canine distemper virus (CDV) isolates obtained from seven raccoon dogs (Nyctereutes procyonoides) and a giant panda (Ailuropoda melanoleuca) in China.

RESULTS

Phylogenetic analysis of the partial hemagglutinin gene sequences showed close clustering for geographic lineages, clearly distinct from vaccine strains and other wild-type foreign CDV strains, all the CDV strains were characterized as Asia-1 genotype and were highly similar to each other (91.5-99.8% nt and 94.4-99.8% aa). The giant panda and raccoon dogs all were 549Y on the HA protein in this study, irrespective of the host species.

CONCLUSIONS

These findings enhance our knowledge of the genetic characteristics of Chinese CDV isolates, and may facilitate the development of effective strategies for monitoring and controlling CDV for wild canids and non-canids in China.

Canine distemper virus with the intact C protein has the potential to replicate in human epithelial cells by using human nectin4 as a receptor

Recent outbreaks in monkeys have proven that canine distemper virus (CDV) causes diseases in a wide range of mammals. CDV uses SLAM and nectin4 as receptors to replicate in susceptible animals. Here, we show that human nectin4, but not human SLAM, is fully functional as a CDV receptor. The CDV Ac96I strain hardly replicated in nectin4-expressing human epithelial NCI-H358 cells, but readily adapted to grow in them. Unsurprisingly, no amino acid change in the H protein was required for the adaptation. The original Ac96I strain possessed a truncated C protein, and a subpopulation possessing the intact C protein was selected after growth in NCI-H358 cells. Other CDV strains possessing the intact C protein showed significantly higher growth abilities in NCI-H358 cells than the Ac96I strain with the truncated C protein. These findings suggest that the C protein is functional in human epithelial cells and critical for CDV replication in them.

Canine distemper spillover in domestic dogs from urban wildlife

Canine distemper virus (CDV) causes a major disease of domestic dogs that develops as a serious systemic infection in unvaccinated or improperly vaccinated dogs. Domesticated dogs are the main reservoir of CDV, a multihost pathogen. This virus of the genus Morbillivirus in the family Paramyxoviridae occurs in other carnivorous species including all members of the Canidae and Mustelidae families and in some members of the Procyonidae, Hyaenidae, Ursidae, and Viverridae families. Canine distemper also has been reported in the Felidae family and marine mammals. The spread and incidences of CDV epidemics in dogs and wildlife here and worldwide are increasing.

Phylogenetic analysis of the haemagglutinin gene of canine distemper virus strains detected from breeding foxes, raccoon dogs and minks in China

Canine distemper virus (CDV) infects a variety of carnivores, including wild and domestic Canidae. Genetic/antigenic heterogeneity has been observed among the various CDV strains, notably in the haemagglutinin (H) gene, that appears as a good target to gather epidemiological information. Based on sequence analysis of the H gene, wild-type CDV strains cluster into distinct geographic lineages (genotypes), irrespective of the species of isolation. The sequence of the H gene of 28 CDV strains detected from both vaccinated and non-vaccinated breeding foxes, raccoon dogs and minks from different geographical areas of China during the years 2004-2008 was determined. All the CDV strains but two (strains HL and HLJ2) were characterized as Asia-1 genotype and were highly similar to each other (96.2-99.7% at the amino acid [aa] level) and to other Asia-1 strains (96.1-99.5% aa) previously detected in China. The CDV strains HL and HLJ2 were both collected from foxes in Heilongjiang province in 2005. Strain HL resembled CDVs of the Arctic genotype (GR88-like) and displayed high aa identity (98.0%) to the Chinese canine strain Liu. By converse, strain HLJ2 was barely related to CDVs of the Asia-2 genotype (88.7-90.3% aa identity), and could represent a novel CDV genotype, tentatively proposed as Asia-3. These results suggest that at least three different CDV genotypes, distantly related (81.8-91.6% aa identity) to the vaccine strains, Onderstepoort-like (America-1 genotype), are currently circulating in breeding foxes, raccoon dogs and minks in China, and that the genotype Asia-1 is predominant. Whether the diversity between wild-type CDVs and the vaccine strains may affect, to some extent, the efficacy of the vaccines deserves further investigations.

First isolation and characterization of canine distemper virus in Vietnam with the immunohistochemical examination of the dog

Canine distemper caused by canine distemper virus (CDV) is a contagious, incurable, often fatal, multisystemic viral disease that affects the respiratory gastrointestinal and central nervous system. Strains Vn86 and Vn99 of CDV were isolated, we believe for the first time, in Vietnam from two 4-month-old autopsied dogs pathologically showing non-suppurative encephalitis with pneumonia, lymphoid depletion and severe gastroenteritis. These strains caused syncytium cytopathic effect in Vero cells and Vero cells expressing canine signaling lymphocyte activation molecules. The titers of cell-associated viruses of both strains were higher than for released viruses. Molecular analysis showed that both new isolates of CDV joined to the group of classic type that is far from the Asia 1 and Asia 2 groups. These results indicated that first isolation and characterization of canine distemper virus in Vietnam with the immunohistochemical examination of the dog.

Development of a challenge-protective vaccine concept by modification of the viral RNA-dependent RNA polymerase of canine distemper virus

We demonstrate that insertion of the open reading frame of enhanced green fluorescent protein (EGFP) into the coding sequence for the second hinge region of the viral L (large) protein (RNA-dependent RNA polymerase) attenuates a wild-type canine distemper virus. Moreover, we show that single intranasal immunization with this recombinant virus provides significant protection against challenge with the virulent parental virus. Protection against wild-type challenge was gained either after recovery of cellular immunity postimmunization or after development of neutralizing antibodies. Insertion of EGFP seems to result in overattenuation of the virus, while our previous experiments demonstrated that the insertion of an epitope tag into a similar position did not affect L protein function. Thus, a desirable level of attenuation could be reached by manipulating the length of the insert (in the second hinge region of the L protein), providing additional tools for optimization of controlled attenuation. This strategy for controlled attenuation may be useful for a "quick response" in vaccine development against well-known and "new" viral infections and could be combined efficiently with other strategies of vaccine development and delivery systems.

Genetic diversity of Hungarian canine distemper virus strains

To achieve proper diagnosis of dogs based on acute clinical symptoms and poorly preserved field samples taken from animals that died due to canine distemper (CD), a new differential diagnostic test has been developed based on polymerase chain reaction (PCR). In this study, more than 150 samples collected from dogs showing respiratory, gastrointestinal and neurological signs suggesting canine distemper virus (CDV) infection were examined. The samples consisted of urine, blood and nasal swabs collected from clinically ill patients, sent to our laboratory by clinicians from various veterinary clinics throughout Hungary. Various organs collected during the necropsy of dogs with pathological changes that suggested CDV infection were also included. Three distinct PCRs were designed. For diagnostic purposes, a primer pair specific to a 409 bases-long segment within the conservative part of the large polymerase region (L) of the CDV genome was designed. Using this test, out of the 150 analyzed samples, 46 (30.66%) proved to be positive for CDV, indicating that CDV still represents a high risk to the canine population in Hungary. For the phylogenetical analysis, a primer pair that completely encompasses the hemagglutinin (H) gene of the CDV genome was designed. The amplicons of this region were sequenced in both directions using the appropriate primers. Our results indicate that several different CDV genotypes are currently present in Hungary. Nine of the analyzed Hungarian strains turned out to belong to the so-called Arctic group of CDVs, and were most closely related to non-European strains from North America, China and Greenland, as well as to the phocine distemper virus 2 (PDV-2) isolated from Baikal seals (Phoca sibirica). One of the Hungarian strains showed high similarity to other European isolates from Denmark, Germany, Italy and Turkey, as well as to other isolates from geographically more distant regions, such as the USA. Three Hungarian strains seem to join a new cluster that is formed by only a couple of strains, one isolated from a mink in Denmark, and another from a dog in North America. Using a third set of primers, a restriction fragment length polymorphism (RFLP) assay has also been designed for the fast and reliable differentiation of the wild-type CDVs from the vaccine strains.

Stability of canine distemper virus (CDV) after 20 passages in Vero-DST cells expressing the receptor protein for CDV

Isolates 007Lm, S124C and Ac96I and a Vero cell-adapted Onderstepoort strain of canine distemper viruses (CDV) were examined for stability after passages in Vero cells expressing the canine signaling lymphocyte activation molecule (dogSLAM, the intrinsic receptor to CDV). These viruses passage once in Vero cells expressing dogSLAM (Vero-DST) cells (original) and after 20 passages (20p) were compared by using sequence analyses and growth characteristics. All four strains of 20p grew well and were slightly better than their originals. The 20p viruses developed a cytopathic effect slightly lower than the original strains. A few changes in amino acids in the H gene were between the 20p and the original viruses, but the sites of changes were not specific. Fragments of P, M and L genes of all strains showed no nucleotide changes after the passages. These results showed that: (1) passages of CDVs in Vero-DST cells induced amino acid changes only in the H gene, not in the P, M and L genes, unlike in a previous study with Vero cells; (2) passages did not markedly affect the growth characteristics of every viral strain. These results indicate that Vero cells expressing canine SLAM allow the isolation and passaging of CDV without major changes in viral genes.