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Cross-Species Transmission of Viruses and Pandemic Spread: The Case of Carnivore Protoparvovirus 1

Introduction: Emergence of Pathogenic Parvoviruses in Dogs

Carnivore protoparvovirus 1 is considered an iconic example of cross-species transmission leading to a global pandemic in the late 20th century.

During the 20th century, several novel parvoviruses were isolated, often in association with disease outbreaks. A particularly striking example is the emergence of pathogenic parvoviruses in dogs during the late 1970s. This emergence was notable for its sudden and global nature, with reports of the disease appearing almost simultaneously in different parts of the world, including the USA, Australia, and Europe.

Observations from the 1970s

In the 1970s, a new canine disease was reported in kennels across the globe. The sudden appearance of this disease in multiple locations simultaneously was a remarkable phenomenon, raising questions about the nature of its emergence. As noted by C. Lenghaus and M.J. Studdert in 1980, this rapid and widespread emergence was surprising, leading many to question whether the disease had existed undetected before 1978 or was truly a new phenomenon.

Evolutionary Insights and Challenges

As more genomic sequences became available, several studies focused on the evolution of parvoviruses. These studies suggested that parvoviruses could evolve at rates comparable to RNA viruses, a claim that has since become part of the accepted lore regarding these viruses. However, this assumption raises concerns about several aspects of the story:

  • Rapid Evolution: The notion that the emergence of these viruses involved rapid evolution is debated. Some evidence suggests that parvoviruses have been circulating in mammals for millions of years, which contradicts the idea of a sudden, rapid evolutionary event.

  • Iatrogenesis: The possibility that the disease could have been caused by medical or veterinary interventions has been largely excluded in the literature, but this exclusion warrants further examination.

  • Threat to Wild Carnivores: The potential impact of Carnivore protoparvovirus 1 on wild carnivores.

Shifting Perspectives on Parvovirus Ecology and Evolution

Recent years have seen a fundamental shift in how scientists view parvovirus ecology and evolution. It is now recognized that parvoviruses, including protoparvoviruses like CPV, have circulated in mammals for millions of years. These viruses are also known to be extremely resilient, capable of surviving in the environment for extended periods (e.g., years in soil).

Given this, it seems unlikely that Carnivore protoparvovirus 1 is an entirely new virus in dogs. Instead, the emergence of the disease associated with CPV infection might be new, but the virus itself could have been circulating in various carnivore populations for a long time.

Serological Evidence and Its Limitations

The serological evidence, which suggests that CPV was absent in dogs prior to 1970, is not without its limitations. The earliest tested samples date back to the early 1970s, which might not provide a complete picture of the virus's presence in earlier periods.

Even if the serological data are accurate, it remains difficult to conceive that dogs were never infected with viruses related to Carnivore protoparvovirus 1 throughout their evolutionary history. It is more plausible to consider that infection patterns may ebb and flow over time, influenced by environmental factors or changes in host immunity. These fluctuations could potentially explain why the virus seemed to emerge suddenly in the 1970s.

Conclusion

The emergence and pandemic spread of Carnivore protoparvovirus 1 in dogs during the late 20th century is a complex phenomenon that challenges our understanding of viral evolution and ecology. While the disease associated with the virus might be new, the virus itself is likely an ancient pathogen that has adapted to survive and spread under favorable conditions. Further research is needed to fully understand the dynamics of parvovirus transmission, evolution, and its impact on both domestic and wild animal populations.