Cytomegalovirus (from the Greek cyto-, "cell", and megalo-, "large") is a viral genus of the viral family known as Herpesviridae or herpesviruses. It is typically abbreviated as CMV.
The species that infects humans is commonly known as human CMV (HCMV) or human herpesvirus-5 (HHV-5), and is the most studied of all cytomegaloviruses. Within Herpesviridae, CMV belongs to the Betaherpesvirinae subfamily, which also includes the genera Muromegalovirus and Roseolovirus (HHV-6 and HHV-7). It is related to other herpesviruses within the subfamilies of Alphaherpesvirinae that includes herpes simplex viruses (HSV)-1 and -2 and varicella-zoster virus (VZV), and the Gammaherpesvirinae subfamily that includes Epsteinâ€"Barr virus.
All herpesviruses share a characteristic ability to remain latent within the body over long periods. Although they may be found throughout the body, CMV infections are frequently associated with the salivary glands in humans and other mammals. Other CMV viruses are found in several mammal species, but species isolated from animals differ from HCMV in terms of genomic structure, and have not been reported to cause human disease.
Species
Several species of Cytomegalovirus have been identified and classified for different mammals. The most studied is Human cytomegalovirus (HCMV), which is also known as Human herpesvirus 5 (HHV-5). Other primate CMV species include Chimpanzee cytomegalovirus (CCMV) that infects chimpanzees and orangutans, and Simian cytomegalovirus (SCCMV) and Rhesus cytomegalovirus (RhCMV) that infect macaques; CCMV is known as both Panine herpesvirus 2 (PaHV-2) and Pongine herpesvirus-4 (PoHV-4). SCCMV is called Cercopithecine herpesvirus-5 (CeHV-5) and RhCMV, Cercopithecine herpesvirus 8 (CeHV-8). A further two viruses found in the night monkey are tentatively placed in the Cytomegalovirus genus, and are called Herpesvirus aotus 1 and Herpesvirus aotus 3. Rodents also have viruses previously called cytomegaloviruses that are now reclassified under the genus Muromegalovirus; this genus contains Mouse cytomegalovirus (MCMV) is also known as Murid herpesvirus 1 (MuHV-1) and the closely related Murid herpesvirus 2 (MuHV-2) that is found in rats. In addition, there many other viral species with the name Cytomegalovirus identified in distinct mammals that are as yet not completely classified; these were predominantly isolated from primates and rodents.
HCMV
Human cytomegalovirus is a species of virus that belongs to the viral family known as Herpesviridae or herpesviruses. It is typically abbreviated as HCMV and is alternatively known as Human herpesvirus 5 (HHV-5). Within Herpesviridae, HCMV belongs to the Betaherpesvirinae subfamily, which also includes cytomegaloviruses from other mammals.
Although they may be found throughout the body, HCMV infections are frequently associated with the salivary glands. HCMV infection is typically unnoticed in healthy people, but can be life-threatening for the immunocompromised, such as HIV-infected persons, organ transplant recipients, or new born infants. It can cause hydrops fetalis in infants. After infection, HCMV has an ability to remain latent within the body over long periods. CMV persists in the host because the viral genome encodes multiple proteins that interfere with MHC class I presentation of viral antigens. One viral protein blocks translocation of peptides into the lumen of the endoplasmic reticulum, while two other viral proteins cause degradation of MHC class I proteins before they reach the cell surface. Hence, diagnosis is done histologically by looking for inclusion bodies in salivary glands. A prevention by hygienic measures is included in information given to pregnant women.
HCMV is found throughout all geographic locations and socioeconomic groups, and infects between 50% and 80% of adults in the United States (>90% worldwide) as indicated by the presence of antibodies in a majority of the general population. Seroprevalence is age-dependent: 58.9% of individuals aged 6 and older are infected with CMV while 90.8% of individuals aged 80 and older are positive for HCMV. HCMV infection during pregnancy results in transmission to a developing fetus. Between 0.2% and 2% of newborns are infected in studies that have been carried out worldwide. HCMV infection occurs earlier in life and is more widespread in developing countries and, in developed countries, in communities with lower socioeconomic status. HCMV represents the most significant infectious cause of birth defects in industrialized countries. Congenital HCMV "infection is the leading infectious cause of deafness, learning disabilities, and mental retardation in children." CMV infection may also "have a large impact on immune parameters in later life and may contribute to increased morbidity and eventual mortality."
Disease
Need for vaccine to protect pregnant women
A vaccine against HCMV is currently under investigation. As a member of the TORCH complex, cytomegalovirus can cause congenital infection and disease, primarily sensorineural hearing loss. HCMV is the primary infectious cause of hearing loss recognized at birth, which has focused development of a vaccine to protect pregnant women. Development of such a vaccine has been emphasized as a priority by the National Vaccine Program Office in the United States. A phase 2 study of a CMV vaccine published in 2009 indicated an efficacy of 50%, with limited durability. Thus the protection provided was limited and a number of subjects contracted CMV infection despite the vaccination. In one case also congenital CMV was encountered, although there were fewer transplacental transmissions in the vaccinated groups than in the control group.
References
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