Human T-lymphotropic viruses (HTLV) are retroviruses that primarily infect human T-cells, a type of white blood cell involved in the immune system. Of the four identified types, HTLV-1 and HTLV-2 are the most studied and clinically relevant. These viruses establish lifelong infections, yet most infected individuals remain asymptomatic. HTLV is distinct from Human Immunodeficiency Virus (HIV); while both infect T-cells, they cause different diseases.
Modes of Transmission and Prevention
HTLV-1 and HTLV-2 spread through three primary routes: from an infected mother to her child, through sexual contact, and via exposure to infected blood. Mother-to-child transmission occurs most frequently during breastfeeding, with approximately 25% of breastfed infants born to HTLV-1-seropositive mothers acquiring the infection. Transmission during pregnancy or birth is less common.
Sexual transmission is another route, with efficiency varying depending on the direction of transmission; it appears more efficient from males to females. Blood exposure, such as through transfusions or sharing contaminated needles among intravenous drug users, represents a highly efficient mode of transmission. HTLV is not transmitted through casual contact.
Prevention strategies address these transmission routes. To prevent mother-to-child transmission, formula feeding is often recommended for HTLV-1-infected mothers, especially in regions where access to safe formula and clean water is readily available. Using latex condoms can help prevent sexual transmission. Screening of donated blood products for HTLV-1 and HTLV-2 antibodies has significantly reduced transfusion-related transmission. Avoiding the sharing of needles for drug injection also prevents spread.
Associated Health Conditions
While most individuals infected with HTLV-1 or HTLV-2 remain asymptomatic, a small percentage may develop health conditions, often decades after initial infection. HTLV-1 is more frequently associated with disease than HTLV-2. The two most well-characterized conditions linked to HTLV-1 are Adult T-cell Leukemia/Lymphoma (ATL) and HTLV-1-Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP).
ATL is a rare and aggressive cancer of the T-cells, estimated to develop in about 2% to 5% of HTLV-1 infected individuals over a lifetime. The time from infection to cancer onset can vary, potentially taking 40 to 60 years. ATL presents in various forms, ranging from indolent to highly aggressive and often lethal.
HAM/TSP is a progressive neurological disorder characterized by inflammation of the spinal cord, leading to symptoms such as progressive weakness and spasticity in both legs, bladder dysfunction, and mild sensory changes. About 1% to 2% of HTLV-1 infected individuals develop HAM/TSP, with many eventually requiring walking aids or becoming wheelchair dependent. HTLV-1 has also been associated with other inflammatory conditions, including uveitis (eye inflammation), infective dermatitis, and certain joint inflammations.
HTLV-2’s link to specific diseases is less defined and generally milder than HTLV-1. Some associations with neurological conditions similar to HAM/TSP have been suggested, including stiffness and weakness in the legs, backache, and bladder issues. Some studies also indicate a possible increased risk of bacterial infections, particularly of the chest and bladder, and mild cognitive impairment in HTLV-2 infected individuals.
Diagnosis and Monitoring
Diagnosing HTLV infection involves blood tests that detect antibodies produced by the body in response to the virus. The initial screening test is an Enzyme-Linked Immunosorbent Assay (ELISA). If the ELISA result is reactive, a more specific confirmatory test is performed to verify the infection and determine the specific type (HTLV-1 or HTLV-2).
Confirmatory tests include Western blot or line immunoassay, which can distinguish between HTLV-1 and HTLV-2 antibodies. If serological tests are inconclusive, molecular testing, such as Polymerase Chain Reaction (PCR), may be used to detect the viral genetic material (proviral DNA). PCR can also quantify the proviral load, which represents the percentage of infected cells carrying the virus.
Following a diagnosis, long-term monitoring is recommended for individuals infected with HTLV. Regular check-ups are advised to watch for signs of associated diseases like ATL or HAM/TSP. While there is no specific trigger identified for disease progression, higher proviral loads in asymptomatic carriers may indicate an increased risk of developing disease, prompting more frequent clinical and laboratory monitoring.
Treatment and Management Strategies
Currently, there is no cure for HTLV infection itself. Medical management focuses on treating health conditions that may arise.
Treatment for Adult T-cell Leukemia/Lymphoma (ATL) involves chemotherapy and other therapies, often including antiviral agents like zidovudine combined with interferon-alpha for leukemic forms. The choice of therapy depends on the type and stage of ATL. Managing HTLV-1-Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP) focuses on symptomatic relief and slowing disease progression. This may include medications to reduce muscle spasticity and pain, such as baclofen or tizanidine.
Physical therapy plays a supportive role in HAM/TSP to help maintain mobility and muscle function. Immunomodulatory drugs like corticosteroids or interferon-alpha have been explored to manage the underlying inflammation, though results can vary. Counseling on preventing further transmission is an ongoing part of management for all infected individuals, reinforcing practices like safe sexual contact, avoiding needle sharing, and refraining from blood or organ donation.