What Is the Human T-Cell Lymphotropic Virus?

The Human T-Cell Lymphotropic Virus (HTLV) was the very first retrovirus ever discovered by scientists (a retrovirus is only composed of RNA but it uses a special enzyme to produce DNA before inserting itself into the host's broader genome). Previously it was believed that human retroviruses didn't exist.

Today there are four known Human T-Cell Lymphotropic Viruses, labeled chronologically by discovery simply as HTLV-1, HTLV-2, HTLV-3, and HTLV-4.

HTLV-1 was discovered in a patient who had been diagnosed with a cutaneous T-cell lymphoma in 1979. In 1981, HTLV-2 was discovered in a patient diagnosed with hairy cell leukemia.

In 1983, scientists believed they had discovered HTLV-3, but that discovery turned out to be a retrovirus from a different taxonomic genus and today is well-known as HIV.

HTLV-3 and HTLV-4 were only discovered recently, in 2005. Little is known or understood about either virus.

Human T-Cell Lymphotropic Virus-1

Thus far, the most important of the four HTL viruses remains the first one discovered, HTLV-1. This virus is highly concentrated among populations in parts of Japan, Africa, the Caribbean islands and South America.

The HTLV-1 virus is transmitted via three major routes:

• Breast milk from mother to child
• Sexual intercourse
• Blood transfusions

The virus has been proven via epidemiology to be the primary causative agent in at least four diseases: adult T-cell leukemia/lymphoma (ATL), HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP), HTLV-associated uveitis, and infective dermatitis.

Adult T-cell Leukemia/lymphoma (ATL)

Adult T-cell leukemia/lymphoma (ATL) is a very rare and highly aggressive lymphoid cancer that has been strongly linked to infection by the HTLV-1 virus. However, of the millions believed to be carriers of the virus, only a very small minority will develop this cancer. It is not currently known who among the carriers will develop the disease or why. While some therapeutic regimens have been tested against the disease, they have been met with limited success; rather, many researchers are focused on preventing viral transmission as a matter of public health policy among high-risk populations.

(Adult T-cell leukemia/lymphoma has also been classified into four subtypes, but that is beyond the scope of this entry. For more information on these subtypes, see Tsukasaki K. Adult T-cell leukemia-lymphoma. Hematology. 2012 Apr;17 Suppl 1:S32-5.)