Question of the Week

Question :
Posted On : 14 May 2023
What is the difference between infections caused by HIV1 and HIV2? Are HTLV1 and HTLV2 the same as HIV 1 and 2? If different, what is the difference in structure and clinical presentations of these 2 viruses? Which family do they belong to in the virological taxonomy?
Expert Answer :
I can understand the confusion because way back {about 25 years ago} some groups were calling the virus we now know as HIV1 by the term HTLV3. This was because HTLV1 and 2 had already been characterised, and a 3rd human retrovirus with a propensity for infecting T lymphocytes seemed a good candidate for this name. However, molecular sequencing showed it to have very little homology with HTLV viruses, and hence - eventually - it was internationally agreed to call it HIV1.

HIV1 and 2, and HTLV1 and 2 are all human retroviruses.

HIV1 and 2 are relatively closely related Lentiviruses, although HIV2 is much less immunosuppressive, and from a paediatric perspective, much less efficiently transmitted from infected mothers to their infants. HIV2 is inherently resistant to the 1st generation NNRTI drugs such as Nevirapine {so single dose NVP is of no use in preventing mother-to-child transmission}.

HTLV1 and 2 are classified as delta-retroviruses and are distinct from the Lentiviruses. HTLV1 causes a completely different spectrum of illness to HIV, and patients present with T-cell leukaemia or tropical spastic paraparesis {TSP} , HTLV1-associated myelopathy {HAM}. Geographically HTLV1 has a very different distribution to HIV - Japan, the Caribbean islands, etc. Infected children generally remain asymptomatic throughout childhood. Infection with HTLV1 is nothing like as destructive to the immune system, and opportunistic infections and wasting are not part of the typical presentation, although Strongyloides infection is more common and PCP has been described.

HTLV2 is associated with a similar spectrum of illness as HTLV1, but is less pathogenic and many individuals harbour HTLV2 without developing lymphoma. Both viruses are thought to be particularly efficiently transmitted via breastfeeding. These are `old world` viruses that have been co-existing with humans for hundreds of generations. Some really interesting studies using HTLV molecular sequencing of proviral DNA were published back in the late 1980`s, early 1990`s which showed it was possible to trace the path of populations that migrated across the Behring Straights when a land bridge still existing, from SE Asia into the Americas. The native American Indian populations had similar sequence homology, including communities in S. America. Today HTLV2 is found in iv drug-using populations as well as in Native American Indians.
Answer Discussion :
lusungu muchelenganga
HTLV and HIV all belong to the family Retroviridae but have different immunopathogenicity. Although HIV1 and 2 are both Retroviruses/Lentiviruses they have over 50% structural differences and more are yet to be established. Although HIV1 was discovered first,HIV2 has less immunopathogenicity compared to HIV1 and so disease progression to AIDS is slower in these patients.Some drugs used in HIV1 such as Efavirenz and other NonNucleoside Reverse Transcriptase Inhibitors(NNRTIs) do not work in HIV2.
Ravindra Kumar
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mohammad odeh
hiv 2 is more severe and its more common in africa
belong to retroviral

Angelo Fortune
Joy Nkomeni
lymphotropic virus

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