An ancient virus is rapidly spreading through the world's intravenous drug-user (IDU) population. The virus has taken a strong hold here, so much so that Ireland now has the fastest rate of new infections in Europe.
The virus has an ancient history. Human T-cell Lymphotropic Virus type II (HTLV-II) is a retrovirus and has been endemic in both North American Indian and South American pygmy tribes for tens of thousands of years, where it is spread by breast-feeding.
What is new is that it has moved into a new category of host, the IDU population. More dangerously, this move has also changed the way the virus mutates. It has started mutating up to 350 times faster among IDUs, according to Prof Bill Hall, UCD professor of medical microbiology and a consultant microbiologist at St Vincent's Hospital in Dublin.
Prof Hall and colleagues from the Rega Institute for Medical Research of Leuven, Belgium, published a major report last week on the advance and rate of mutation of HTLV-II in the US Proceedings of the National Academy of Sciences. He has been studying the virus for about 10 years.
"We have a virus that is evolving hundreds of times faster than normal," as seen in IDUs, he said. "We have to be concerned about the rate of mutation and the rate of infection. We are worrying a lot because so many of the drug-abusers are co-infected with HTLV-II and HIV."
He found HTLV-II infection rates of up to 60 per cent in current and former IDUs in Ho Chi Minh City in southern Vietnam. Italy and Spain have rates of 8 to 10 per cent, he said, but Ireland's rate at 16 per cent and rising is comparable with the US range from 15 to 25 per cent.
At the moment researchers are more concerned about HTLV-II's potential for disease than the level of disease it causes now.
It is like "a virus that has been looking for a disease," Prof Hall said.
The closely related HTLV-I virus can cause a form of aggressive leukaemia and also myelopathy in about 2 per cent of cases. The latter disease affects the spinal cord, leading to a progressive paralysis of the legs. HTLV-II can cause the same condition, and the research team estimates that incidence would also be about 2 per cent.
The virus has a relatively small genome, and its rapid dispersal through the IDU population has speeded up its opportunities for mutation and for "new emerging diseases", Prof Hall said. "The virus could become even more pathogenic" and bring about a much more serious disease or provoke a higher disease incidence.
"This dramatic acceleration of the evolutionary rate seems to be related with the mode of transmission," the authors state in the paper.
Its mutation rate in tribal populations was typical of that seen in other viruses, between 150 and 350 times slower than HTLV-II in IDUs.
The authors also describe the development of a novel new way to track the evolution of a virus. Computer models are used to examine the types of genomic changes that occur and can provide definitive timings for the arrival of a virus into a population.
"The virus is behaving like a clock," Prof Hall said. This system was used to determine that HTLVII reached non-tribal Western populations in 1955 and a related viral form, HTLV-IIa, appeared in 1975. It identified the endemic spread in tribal populations compared to the epidemic spread in the IDU population.
HTLV-II is an ancient virus that existed before humans reached the Americas. It was thought to have moved with Asian migrants as they populated North America when there was a land bridge linking the two continents across what is now the Bering Sea.
The computer models have allowed the researchers to create a highly detailed family tree for the virus based on the rate and type of mutation. This tree links related strains but also allows for dating of when divisions occurred and new strains evolved.
It is HTLV-II's latest manifestations that are making viral experts nervous. Quite simple single mutations could change the virus into a dangerous new entity. It is also unclear whether any synergy could develop between HTLV-II and HIV given the very high degree of co-infection among IDUs.
The report's authors are Drs Marco Salemi, Martha Lewis, John Fergal Egan, William Hall, Jan Desmyter and Anne-Mieke Van damme.
