Herpesviruses of Types 6, 7, and 8 in Human Pathology
T-lymphotropic human herpesvirus type 6 (HHV-6) comprising two closely related viral species 6A and 6B is ubiquitous in human population. After the decline of protective maternal antibodies it rapidly infects infants – more than 90% of children above the age of 1 year and adults are seropositive for antiviral antibodies.
Despite the fact that the virus can infect a broad range of human cells, the main targets for HHV-6 infection are activated CD4+ T lymphocytes.
Primary acute infection of HHV-6 affects children of 6 months to 3 years of age. Viral transmission occurs predominantly via oral secretions (saliva).
Typical febrile disease is known as exanthema subitum (or roseola infantum) that is followed by fever and skin rashes. The disease is self-limited.
It is generally ascertained now that HHV-6 may account for at least 10-20% of all febrile illnesses at this age.
After primary infection the virus comes into latency and persists lifelong in macrophages, bone marrow progenitors or CNS cells. The unique feature of latent HHV-6 genome is to make covalent linkages with host chromosomes.
Reactivations of virus are almost totally asymptomatic. Severe recurrent HHV-6 infections may develop after allogeneic transplantation in graft recipients.
Similarly, T-lymphotropic human herpesvirus type 7 (HHV-7) also appears to be a ubiquitous viral agent, which most infections arise in childhood. Persistent infections are established in salivary glands; the virus can be isolated from saliva of infected individuals.
The distinct relations between HHV-7 and human disorders remain to be established. HHV-7 as well as HHV-6 may have concern to the development of human chronic fatigue syndrome.
The last human herpesvirus, called Kaposi’s sarcoma-associated herpesvirus (KSHV) or human herpesvirus type 8, was first detected in 1994 in Kaposi’s sarcoma biopsies of AIDS patients.
This virus is lymphotropic. Viral replication is very slow. It influences cellular genetic elements responsible for cell proliferation and host immune response (cytokine production, chemokine receptor expression, etc.) Human herpesvirus 8 seems to account for Kaposi’s sarcoma, vascular tumor of mixed cellular origin in AIDS patients.
KSHV is not as ubiquitous as other herpesviruses, affecting about 5-10% of human population. It might be sexually transmitted among homosexual men, e.g. following HIV infection. Also the virus can be transmitted via solid organ transplantations.