The Human Circoviruses 



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The human circoviruses, Torque Teno virus (TV) and Torque Teno-like Mini Virus (TLMV),  have been shown to have worldwide distribution.  As in their genomic organization and morphology, the two viruses are extremely similar in their epidemiological features.  They both have been found at extremely high seroprevalence rates (greater than 75%) in human populations around the world.  These high seroprevalence rates have 2 major implications: first, that many infected individuals appear to have persistent or chronic infections with human circoviruses; furthermore, these infections have not yet been clearly linked to human disease.  Human circoviruses can thus be considered orphan viruses.  

Scanning Electron Micrograph Image of Porcine Circovirus 




Human circoviruses have been isolated in and appear to preferentially infect leukocytes.  They have also been detected in the feces and saliva of infected individuals.  It has been proposed that TTV and TLMV are transmitted through aerosolized respiratory droplets.  Respiratory transmission of human circoviruses would help to explain the high seroprevalence rates of the viruses.   Parenteral transmission of TTV and TLMV is most likely also a route of  transmission, given the extremely high seroprevalence rates have been detected in hemodialysis patients, transplant recipients, and intravenous drug users.  Vertical transmission, both perinatally and through infected breast milk, as well as sexual transmission is also thought to occur.  At this point, however, further epidemiological studies are needed to confirm all viable routes of human circovirus transmission. 


An Opportunistic Infection?

High genetic diversity within viral isolates in a single infected individual is one of the salient qualities of the human circoviruses.  It also appears that infected with one viral strain does not protect an individual from co- or super-infection with another strain or quasi-species of the same virus.  Genetic diversity in isolates, as well as higher than typical seroprevalence rates, have been found in immunocompromised patients, leading to the hypothesis that TTV and TLMV are opportunistic infections in the human host.  In particular, high rates of the virus have been found in individuals infected with HIV, type 1, with intravenous drug use being the highest risk factor for TTV co-infection in individuals with HIV/AIDS.  Although a correlation has been found between TTV and HIV infection, no associations have been confirmed between clinical course or HIV disease progression as a result of human circovirus infection.