UC Davis

Canine distemper virus (CDV) is an enveloped, single-stranded negative-sense RNA virus belonging to the genus Morbillivirus, family Paramyxoviridae. Viral particles are composed of six structural proteins encoded by the six genes that compose the genome of CDV. The hemagglutinin (H) and fusion (F) proteins are the surface glycoproteins responsible for attachment and cell membrane fusion, respectively. The H protein drives host species range and cell tropism. It has a high mutation rate, and it is used to classify CDV isolates into distinct lineages, which are defined by sequence differences of 5% or more. These lineages are commonly restricted to specific regions and are named after the geographical locations where they are described.The disease resulting from CDV infection, distemper, is one of the most lethal diseases in dogs and a wide range of wild carnivores. Many species, including some endangered species, have had their numbers dramatically reduced due to distemper outbreaks. Distemper is associated with varied symptoms and lesions because of the wide range of systems that CDV can infect. Immunosuppression occurs as a result of CDV replicating and causing damage in the immune tissues. It also causes lesions in the respiratory, digestive, integumentary, and nervous systems. An extensive literature review of the virology, pathogenesis, and epidemiology of CDV is included in this thesis (Chapter 1). The high impact of distemper on canine health prompted the early development of a vaccine that is included in the typical vaccination series for dogs. It is regarded as highly successful. However, existing vaccines are based on CDV lineages that are no longer found in the wild, and they are significantly different from the currently circulating lineages. The H protein is the main target of the infected host’s immune system. As a result, it is subjected to high evolutionary pressure to avoid immune detection, resulting in the emergence of genetic variations. Moreover, reports of distemper in vaccinated dogs have been increasing during the last decade. All together this information has raised concerns about the current effectiveness of the CDV vaccine. In Chapter 2, we addressed this concern by performing a multi-institutional retrospective case-control study in which we compared cases of vaccine breaks with a control vaccinated population that did not develop distemper. The only clear predictor of vaccine break was the time between vaccination and onset of disease, indicating that animals that do not develop a protective response are quickly infected with the virus. The impact of the variability of CDV H protein is still not well characterized. To address this gap in knowledge, in Chapter 3 we studied the impact of H variability on the pathogenesis of distemper. We used in vitro and ex vivo approaches to study how H protein variations can affect CDV attachment and replication. We isolated CDV from tissues of infected animals originating from distinct geographical locations in the USA and sequenced their genomes. Several of the innovative approaches proved to be challenging, but we were able to demonstrate clear differences in tropism among the lineages included in the study, suggesting that H protein can have an important role at least in viral replication efficiency in specific cell types. Even though distemper is one of the longest-known diseases in dogs, the only available treatment is supportive care. There is a clear need for a specific and effective antiviral treatment for distemper. In chapter 4 we investigated the anti-CDV potential of several compounds with proven effectiveness for other viral diseases. We used multiple circulating lineages of CDV to investigate viral RNA replication and cytopathic effect (CPE) inhibition of the antiviral candidates. One candidate, the nucleoside analog GS-441524, had a strong antiviral effect and reduced both vRNA and CPE, while some other candidates with a similar mechanism of action were able to reduce vRNA production but not prevent cell death. Some compounds with previously reported anti-CDV effects in vaccine lineages, were not effective in our experiment, proving the importance of using a circulating virus for the development of antiviral compounds.