What is HTLV-I?

HTLV-I is the short name of the "Human T-Lymphotropic Virus type I". It is called this because it infects a type of white blood cell called a T-lymphocyte. HTLV-I is a virus. A virus is a minute organism that usually can only be seen when magnified many thousand times by an electron microscope. Viruses do not have their own metabolism (life-support system) and can only make all their component parts (ie. genetic code, enzymes, sugars and proteins) by infecting living cell. Once formed viruses are released from the infected cell. To replicate again they must find a new cell to infect.

Worldwide distribution of HTLV-I infection

Worldwide distribution of HTLV-I infection Worldwide distribution of HTLV-I infection

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Who is likely to be infected by HTLV-I?

HTLV-I is a very old virus, which appears to have infected and moved with mankind for hundreds, perhaps thousands of years. It is thought to have migrated during ancient times with native American Indians in North and South America, with Australian aborigines and the Melanesian people of the South West Pacific, and to Japan. During the last few centuries it has migrated from Africa to the Caribbean and again to North and South America. In some areas more than 1% of the population carry the virus. The same rates of infection are seen in populations wherever they migrate. In Europe HTLV-I is mainly found among people who have originated from these endemic areas.

How would I know if I am infected

Most people who are infected with HTLV-I are completely unaware of the infection because they are perfectly well. As they have no symptoms or signs of the infection this is known as asymptomatic carriage. We have estimated that in the UK 22,000 people are infected with HTLV-I but most are asymptomatic carriers and less than 1000 are aware of the infection.

  • You are a blood donor and have been screened for HTLV-I and other infections.

  • A family member or your partner is infected with HTLV-I.

  • You have developed symptoms which resemble those the virus can cause.
The presence of HTLV-I infection can be detected by a blood test. Soon after infection with HTLV-I the body will respond by producing antibodies to fight the infection. HTLV-I antibodies, which are produced only in response to HTLV-I infection and not to any other infection, can easily be detected in the laboratory but only if the specific test is done. HTLV-I infection does not show up in routine blood tests such as those requested by your family doctor or in routine hospital tests.

Because HTLV-I causes a lifelong infection the presence of these antibodies is proof of infection. The absence of HTLV-I antibodies excludes HTLV-I infection unless the infection has only just occurred and the test was done before the body has had time to respond to the infection. If a recent infection is suspected the antibody test should be repeated three months later.

How does HTLV-I infection occur

HTLV-I can be transmitted from person to person in four ways:
  • From an infected mother to her baby. Up to 1:4 children born to mothers with HTLV-I infection will become infected. However most infections occur through breast-feeding and if this is avoided less than 1 in 20 babies will become infected. The risk of infection through breast-feeding increases with the duration of breast-feeding and may be low during the first three months.

  • HTLV-I infection is not routinely tested for in the UK ante-natal clinics so if you are pregnant and think you may be a carrier, you need to speak to your doctor or midwife.

  • Between sexual partners through unprotected (no condom) intercourse. The risk of transmission from an infected man is greater than from an infected woman. The best information indicates that in a steady relationship lasting 5 years there is a 7% chance of transmission. Since the use of condoms protects against many other, more infectious, infections we recommend the use of condoms, unless a couple are trying to have a baby.

  • Through transfusion of blood from an HTLV-I infected donor. The risk may be as high as 85% but depends on how the blood is handled and stored. Persons with HTLV-I infection should not donate blood, organs or sperm and should not carry an organ donor card. All blood donations in the UK are screened for HTLV-I infection.

  • Through the sharing or re-use of needles and syringes to inject drugs. The use of disposable equipment for injections prevents transmission.

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Does HTLV-I cause any disease?

The vast majority of persons infected with HTLV-I DO NOT DEVELOP ANY DISEASE due to HTLV-I. The virus appears to remain in the body throughout life without causing any harm at all. A small minority, about 1 person in 20, will develop disease due to HTLV-I but this usually occurs only after several decades of infection. There are two main types of disease caused by HTLV-I:

Adult T-cell Leukaemia/Lymphoma (ATLL). This is a rare form of cancer of the blood seen in less than 20 patients per year in the UK. It is usually treated with anti-cancer drugs but recent progress has shown that starting with anti-cancer treatment and then switching to anti-viral treatment is better. Bone marrow transplantation should be considered for patients in remission with ATLL.

HTLV-I-associated myelopathy (HAM). This is an inflammation of nerves in the spinal cord that causes stiffness and weakness of the legs, backache, a 'weak' bladder and constipation. Not all of these symptoms may be present, especially at the beginning. The disease often starts very slowly and the symptoms may be attributed to arthritis or getting old. Different medicines are available to treat the various symptoms: stiffness, pain, 'weak' bladder etc. The use of anti-inflammatory medications is now under investigation. In the UK about 10 persons are diagnosed with HAM each year.

Other HTLV-I-associated diseases. HTLV-I can also cause inflammation of the eye (uveitis), joints (arthritis), muscles (myositis), lung (alveolitis) and skin (dermatitis). These conditions are even less common than ATLL and HAM and the skin condition is usually only seen in tropical climates.

HTLV-I and other infections
Strongyloidiasis: A warm infection acquired in the tropics can, after lying dormant for years, cause a serious illness in HTLV-I carriers. Although rare in the UK, all HTLV-I carriers who have lived in the tropics should be screened for Strongyloidis.

Can HTLV-I infection be treated?

At present there is no treatment to cure (eradicate) the infection. Since 95% of all infected persons go through life without developing any HTLV-I-associated diseases any such treatment would have to be not only effective but also very safe.

Is everyone infected with HTLV-I at equal risk of developing an HTLV-I-associated disease?

ATLL is unlikely to develop following infection acquired in adult-life. This means that avoiding infection of babies by avoidance of breast-feeding is very important for the prevention of ATLL in the next generation.

HAM seems to be less common among Japanese with HTLV-I infection than among other populations and evidence is emerging that the immune system is important in controlling infection. HTLV-I asymptomatic carriers with less virus in the blood are less likely to develop HAM.

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Updated: July 2008