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“HTLV, if you don’t know it exists, is really hard to find. Compared to HIV, it’s a nightmare.” 
Dr Robert Gallo in conversation with Dr Norman Swan.

HTLV is a virus few people have heard of, despite being endemic in a large swathe of Central Australia, Japan, Brazil, sub-Saharan Africa, the Middle East and elsewhere, affecting up to 10 million people worldwide. 

In 1980 Dr Robert Gallo and colleagues upset conventional wisdom that humans do not get retroviruses, when they discovered HTLV-1 and HTLV-2. Four years later, he co-discovered a third human retrovirus, HTLV-3 (later to be renamed HIV), as the cause of AIDS. But unlike HIV, HTLV has remained in the shadows.

Just how obscure it is was highlighted by a video account from a Northern Territory health worker presented at September’s Global Virus Network meeting at the Doherty Institute, by Dr Lloyd Einsiedel, an Alice Springs-based clinician for the Baker Institute.

The man had checked into hospital because he could no longer walk. He was transferred to a hospital in another state and was there for 10 days before a lab test confirmed HTLV-1.

“Neither myself nor the team that was looking after me had ever heard of it,” said the man, who wishes to remain anonymous.

“What surprised me is that I have been working in an area where there is a high rate of population that carries this virus, yet I had no idea. And what surprised me even more was that the specialists looking after me had no idea about this virus.

“So, we had to google it.”

HTLV-1 – Human T cell Leukaemia Virus –  is a blood-borne, sexually transmitted virus, that can be passed on unknowingly because people can be asymptomatic for many years. It can be transmitted through breastfeeding, unprotected sex, sharing needles, blood transfusions and organ donations.

Worldwide, HTLV-1 is known to cause a serious blood cancer, adult T cell leukaemia, and lymphoma​ (ATLL). Depending on the severity of disease, patients suffering from ATLL may die within a year of their diagnosis.​

The other commonly observed disease is HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP). This is a progressing, disabling and painful inflammation of the spinal cord which leads to walking disability and wheelchair usage over a period of 15-20 years – the very condition that immobilised the NT health worker who bravely shared his experience at the GVN. 

Dr Einsiedel spoke about the impact of HTLV-1 on central Australian Indigenous communities, where prevalence rates are up to 50 percent (in contrast to 36 percent for diabetes). 

Researchers now have evidence that HTLV-1 is a major contributing factor for a host of other diseases, such as sepsis, infective dermatitis and chronic lung disease, which are significantly more common in patients from indigenous communities with a high proportion of HTLV-1 infected T-cells compared to other community members. These associated conditions have a detrimental effect on life-span, quality of life and earning capacity.

HIV and HTLV expert Dr Fabiola Martin (University of York and University of Queensland) presented on the many intervention strategies that could be implemented to prevent further spread of HTLV-1.

Nearly 40 years since its discovery, HTLV-1 remains invisible. HAM/TSP does not have an International Classification of Diseases (ICD) code, which means there are no agreed standards for diagnosis, treatment or health system resourcing. “This is an insult to all people affected by HAM/TSP,” Dr Martin said. 

Dr Martin also raised the discrepancy in the international response to HTLV-1 in contrast to other viruses transmitted by sexual and mother-to-child contact.

“In Brazil, antenatal care and testing is only available in some cities, and in the UK antennal care testing is not available at all, despite it being known to be cost-effective. 

In Japan, HTLV-1 positive organs may be transplanted,” she said.

“I don’t know any country where HTLV is part of routine STI screening. The general population and healthcare providers are unaware of HTLV and how it is transmitted.”

This invisibility prevents the development of a locally and internationally agreed prevention strategy. It also leads to a lack of successful funding opportunities for scientists,  who are keen to develop a vaccine and a cure.

“In Brazil, HTLV antenatal care and testing is only available in some cities, and in the UK testing is not available at all. In Japan, HTLV-1 organs may be transplanted,” said Dr Martin, who runs a website on HTLV for patients and clinicians.

“I don’t know any country where HTLV is part of routine STI screening. The general population and healthcare providers are unaware of HTLV and how it is transmitted.”

Although HTLV-1 was the first human retrovirus discovered, HIV-1 displays much greater pathogenicity because of the way it spreads through the body. Because of this difference, HIV research has been prioritised. 

One of the most distinctive features of HTLV-1 is that this virus can be transmitted only through cell-to-cell contact. In contrast, says Doherty Institute HTLV researcher Damian Purcell, HIV readily sheds virions (or virus particles) that float freely in plasma (cell-free). 

Cell-to-cell viral transmission is more efficient than infection with a cell-free virus – this route circumvents the antibodies that are produced in response to free-ranging virions paying a house call. 

The result is that, unlike HIV, HTLV-1 is rarely detectable in the plasma of HTLV-1-infected subjects. This feature was resolved by Dr Gallo, and led him to describe HTLV as a very hard virus to study compared with HIV.

Dr Martin said ​HTLV-1 is not harmless. “We need to see it for what it is: a preventable virus, which can kill humans.”