from Wolfram Brune writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:
Cytomegaloviruses (CMVs) are highly species-specific as they replicate almost exclusively in cells of their natural host species. However, the molecular basis of species specificity remains poorly understood. In cells of a foreign host a post-penetration block to viral gene expression and genome replication appears to restrict viral replication and spread. In some cases, infected cells of a foreign host undergo programmed cell death, indicating that apoptosis acts as a cellular antiviral defense mechanism to prevent viral replication. A few recent studies suggested that mediator and effector molecules of the interferon system and antiviral defenses operating at PML nuclear bodies (PML-NBs) might also be involved in restricting the host range of CMVs. Moreover, a recently isolated spontaneous mutant of murine CMV, which is capable of replicating to high titers in human cells, provided a new opportunity to study the mechanisms of CMV host species specificity. In this spontaneously adapted virus, mutations in the region encoding the viral Early1 (E1) proteins were found to be responsible for the extended host range phenotype. Further investigations of the CMV host species specificity should lead to a better understanding of the viral replication machinery, interfering host cell factors, and viral countermeasures.
Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention