HTLV-1 Versus HIV: 40 Years of Challenges from Discovery to Treatment for Human Retroviruses and Neurological Implications

The human T cell lymphotropic virus type 1 (HTLV-1) was the first retrovirus isolated in humans (1980) contributing to the rapid identification of human immunodeficiency virus (HIV) in 1983. ^1,2 Both retroviruses share the same transmission routes (sexual contact, contaminated blood breastfeeding, and perinatal). Screening of pregnant women and blood/tissue donors have been shown to be an efficient strategy to prevent the transmission of HTLV-1 and HIV.^2–,4 However, although HIV testing is routinely performed, HTLV-1 testing is not mandatory worldwide. ^{2 –4}

HTLV-1 infection is endemic in many regions of the world predominantly in southwestern Japan, the Caribbean islands, Central and South America, sub-Saharan Africa, Melanesia, and the Middle East. ⁴ It is estimated that ∼5–10 million people are infected with HTLV-1 worldwide. However, there are many areas of the world, where HTLV-1 prevalence was never studied. ⁴ In most countries, notification of HTLV-1 infection is not compulsory, although this is not true for HIV infection. ^2,4 HIV infection has a well-known worldwide distribution with higher prevalence in Africa, Caribbean, Eastern Europe, Central Asia, and Latin America. According to worldwide data from UNAIDS, 37.9 million people are living with HIV. ³

HTLV-1 infection can lead to the development of severe neoplasia (adult T cell leukemia—lymphoma) or chronic inflammatory diseases (neurological disorders, uveitis, conjunctivitis, sicca syndrome, interstitial keratitis, Sjogren's syndrome, Hashimoto's thyroiditis and Graves' disease, pulmonary disease, infective dermatitis, myositis and arthritis, among others) in about 5%–10% of infected individuals. ^1,4 HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a progressive neurological disorder characterized by weakness and spasticity generally associated with low back pain and neurogenic bladder. Ten years after disease onset, 30%–50% of patients progress to a wheelchair and 45% are unable to walk without crutches. ⁴ In our cohort of 47 patients with HAM/TSP in Rio de Janeiro city, 40% had rapid progression, 51% slow progression, 9% very slow progression. The average time between the onset of symptoms and the neurological diagnosis was 6.09 ± 6.13 years. ⁵ The majority (66%) of patients were restricted to a wheelchair. ⁵ In a different context from HIV, many health care professionals remain unaware of an HTLV-1 infection, as well as the long period of viral incubation or the development of associated diseases.

AIDS is the advanced clinical manifestation resulting from an immunodeficiency condition caused by HIV. ² Its main feature is the severe suppression of T cell-mediated immunity, which makes the individual susceptible to opportunistic infections and secondary neoplasms. ² In addition, neurological complications of HIV infection include conditions such as HIV-associated neurocognitive disorder, HIV-associated vacuolar myelopathy, and polyneuropathy. ² Since the 1980s, numerous antiretroviral medications have been tested and introduced for HIV treatment, leading to longer survival of HIV-infected individuals. ² Unfortunately, there is no specific treatment or vaccine for HTLV-1 infection. ⁴ Successful public health policies applied to HIV infection have not been implemented to prevent HTLV-1 infection. Overall, HTLV-1 infection and associated diseases should be considered neglected as they are endemic in low-income populations, receive reduced investment in research, in drug production and control.