Human T-cell leukaemia virus (HTLV) is a retrovirus that infects T-lymphocyte cells and can be spread in a number of ways including sexual intercourse, breastfeeding, and blood-to-blood contact. As HTLV infections are often symptomless, many people are unaware that they have the disease and act as silent carriers.

There are four main strains of HTLV and it is HTLV-I and HTLV-II that are most strongly associated with adverse clinical conditions.

Approximately 3–5% of people infected with HTLV-I go on to develop adult T-cell leukaemia/lymphoma (ATLL). ATLL is a rare, aggressive disease with a poor prognosis. In an open letter to the World Health Organization published in The Lancet in May of this year, doctors described HTLV-I as “the most potent carcinogenic oncovirus and potentially the most oncogenic risk factor including chemical carcinogens.”

Other conditions associated with HTLV-I infection are HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP), as well as uveitis in Japan and infective dermatitis in Jamaica, Brazil, and Africa. HAM/TSP is a slowly progressive disease that results in spastic paraparesis or weakness and stiffness in the legs. HAM/TSP can also result in impaired gait and bladder dysfunction.

HTLV-II has a limited association with haematological or myelopathy disorders. Instead, this virus has been linked to elevated lymphocyte and platelet counts and/or worse overall cancer mortality.

According to the most recent estimates, there are around 5–10 million people globally who are infected with HTLV-I. However, these estimates are based on studies from known endemic areas only and it is acknowledged that the actual number of HTLV-I-infected individuals is likely to be much higher.

While data regarding the epidemiology of HTLV are limited, it is known that the prevalence of HTLV infection varies greatly across different geographical regions. Areas known to be endemic for HTLV-I include southwestern Japan, parts of the Caribbean, Gabon, Colombia, French Guyana, the Mashhad region of Iran, Romania, and Brazil. Reports suggest that there are also rare isolated clusters of HTLV-I in Australia and Melanesia.

Screening for HTLV also exhibits high levels of geographical variation. In some countries, like France, Greece, Ireland, Netherland, Norway, Portugal, Spain, Sweden, the UK, and Australia, screening tests are routinely performed on blood donations as a part of blood safety measures. Antenatal screening of pregnant women also takes place in Japan.

Doctors are now calling for increasing funding, research, and preventative strategies for HTLV-I. A major concern cited in the call to eradicate this disease is that the increasing rate of human migration may accelerate the ability of this virus to spread and thus impact global health.

Strategies designed for stopping this disease will undoubtedly include improved disease awareness and greater screening measures and are thus likely to positively impact the HTLV tests in vitro diagnostics market.