When discovered in humans, the TT-virus caused a scare. Now that it is seen as harmless, the medical community's interest has waned. But this could be a mistake, Steve Farrar hears
It is the disease that never was. Two years after its discovery, the mysterious TT-
virus is losing the attentions of the medical community as it emerges that millions of people worldwide are infected yet suffer no obvious medical problems as a result.
In fact, although the virus is very persistent and seems capable of evading the attentions of the human immune system, it appears completely benign.
This is a very different picture from that of two years ago, when a Japanese team researching hepatitis bugs announced that it had found a new type of virus infecting a man suffering liver disease, whose initials gave the virus its name.
To the horror of scientists who began to look for the microbe, it popped up around the globe and was rapidly identified in samples of donated blood. The name TT-virus was soon misdenoted as "transfusion transmitted" and there were fears that a new global health threat had been uncovered. Now it seems not.
Research has revealed that the TT-virus has a curious respect for its human host. This makes it an intriguing medical curiosity but not one that will attract much funding for further research.
Among those who have been investigating the TT-virus is a team from Edinburgh University's department of medical microbiology.
Lisa Jarvis, one of the team, said it was initially detected in just 2 per cent of volunteer blood donors from southeastern Scotland. But this soon changed. "Once more variants were found, those percentages just went up and up, and ultimately we found that virtually everybody is infected," Dr Jarvis said.
This high prevalence, evidence of the virus's startling success as an organism, has proved its undoing in terms of scientific research as it became increasingly clear that it was not harmful. "It may not have any clinical association, but it is still a very interesting virus as it has managed to evade the human immune system that would normally destroy it," Dr Jarvis said.
The TT-virus is thought to consist of a loop of DNA inside a protein shell, which lives and is believed to replicate itself inside human cells without damaging them.
Although more than 200 viruses have been identified that cause human diseases such as the common cold, influenza and HIV, fears that the TT- virus could be linked to liver damage now look increasingly unfounded.
Although not the first virus found to be benign to humans - known as orphan viruses - this one is of interest because of its remarkable persistence and prevalence.
Arie Zuckerman, professor of medical microbiology at the Royal Free and University College Medical School and director of the World Health Organisation reference centre on viral diseases in London, thinks funding to study the virus may become increasingly hard to come by but that this could be a mistake.
He believes the properties of the TT-
virus make it not just intrinsically interesting but also of potential medical use. "There are other examples of viruses searching for a disease but not quite as glaring as this one - it remains a very interesting virus, remarkably efficient, and if it is, as we suspect, harmless, it could be used as a vector for gene therapy. However, while one hopes that there will be no loss of interest in supporting research into it now that we know it is not transmitted by transfusion, I suspect money is going to be pretty tight."
It is possible that some strains of the virus may yet prove dangerous, though experts doubt it, or that the virus could have a harmful effect on certain animals, though again there is little evidence to support this.
The experts believe it is related to a group of common viruses found in farm animals and plants and is probably transmitted to humans in undercooked or raw foods.
And it may not be alone. Both Professor Zuckerman and Dr Jarvis have suggested that the discovery may prove to be the tip of the iceberg and that new, sensitive molecular techniques could soon reveal a great number of similarly benign and widespread viruses that have simply not been looked for yet.