The viruses

Human T-cell leukaemia viruses 1 and 2 are retroviruses (like HIV) and belong to the family Retroviridae. However, they belong to the genus oncovirinae (onco = oncogenic), whereas HIV belongs to a separate genus of lentivirus (lenti = slow). Like HIV they possess a reverse transcriptase enzyme, which converts the viral RNA into DNA in the first step of the replication cycle. This pro-viral DNA is capable of integrating in the cellular DNA.

Epidemiology

Human T-cell leukaemia virus 1 was first isolated accidentally in 1979 from a human T-cell line, during experiments to stimulate cells so they could be maintained in cell culture for a longer period of time. The virus was quickly associated as the cause of adult T-cell leukaemia (ATL), which had been described in 1977, and because of a clustering of cases in southern Japan it was suspected to have an infectious aetiology. It was the first human retrovirus to be isolated (pre-dating the isolation of HIV). A few years later the second human retrovirus HTLV 2 was also isolated in the human T-cell line.

Human T-cell leukaemia viruses 1 and 2 are closely related, with some serological cross-reactivity between the two.

Route of spread

Both HTLV 1 and 2 are blood-borne viruses with essentially similar routes of spread as HIV. See Table 13.1.