New retroviruses jump from monkeys to humans | Two new strains of HIV found.

Two new retroviruses - the type of virus which causes AIDS - have jumped from non-human primates to people, a new study reveals.

The study of blood samples from nearly a thousand bushmeat hunters or handlers in Cameroon showed that at least six viruses had crossed from monkeys to the people who were exposed to freshly caught bushmeat. And two of these viruses have never been seen before in humans.

The newly discovered human T-cell lymphotropic virus 3 (HTLV-3) and HTLV-4 are closely related to the known viruses, HTLV-1 and HTLV-2. These are implicated in cancers like leukaemia and can cause inflammatory or neurological diseases.

Retroviruses such as HTLV or HIV insert their genetic material into a host cell's DNA. The emergence of HIV is widely blamed on a primate retrovirus, SIV, jumping to humans. Previously, it was thought that the emergence of these viruses was limited by the rarity of successful cross-species transmission.

But the identification of two entirely new human retroviruses from one study, along with a previous discovery by the same group that simian foamy viruses can jump from monkeys to humans, may be ominous.
No fluke

"What's increasingly clear is that the hunting and butchering of non-human primates is associated with the transmission of retroviruses to humans," says Nathan Wolfe, at the Johns Hopkins Bloomberg School of Public Health in Baltimore, US, who led the study.

He says the new results suggest the team's previous find of simian foamy viruses in bushmeat hunters was "not just a fluke". In the blood-screening study, 13 out of 930 people were found to be infected with simian retroviruses, and two with the new human strains.

The new findings are no surprise, agrees Martine Peeters at the retrovirus laboratory of the Institute of Research for Development in Montpelier, France. She and her colleagues, along with other groups, had described simian TLV-3 in primates in Cameroon in 2004 so the discovery of a human equivalent is not unexpected. Now scientists will look for the simian equivalent of HTLV-4.

HTLV-1 and -2 are not very pathogenic says Peeters, but about 1% of those infected may go on to develop leukaemia.
Global distribution

Wolfe points out that HTLV-1 and 2 are now global viruses, infecting 22 million worldwide. "This finding is in a class of disease known to have global distribution and known to cause disease. It's not just a few obscure viruses crossing over and staying only in hunters," he told New Scientist.

He says the next issue to tackle is whether these new retroviruses reach a "dead end" in hunters, or are capable of human-to-human transmission.

While HTLV-3 and HTLV-4 are not a serious threat to global health, Peeters warns that "it shows that there is still retroviral transmission from primates to humans, so maybe one day another SIV could become another HIV".

HIV-2 is believed to have jumped from mangabey monkeys and HIV-1 from chimpanzees. A new HIV could potentially jump from another species, she cautions.

The work was presented at a retrovirus conference in Boston, US, on Friday.

HIV was born out of a union more than a million years ago between two viruses that infected different species of monkey, a study of the genetic history of HIV-like viruses suggests.

The union took place when chimpanzees preying on the monkeys contracted the viruses, which then combined to form a new type of simian immunodeficiency virus (SIV), the primate version of HIV. It wasn't until the 1930s, though, that this jumped to humans feeding off chimpanzee bush meat in what is now the Democratic Republic of Congo.

SIV in chimps is the precursor to HIV-1, the most widespread form of the human virus. HIV-2, which is restricted to west Africa, is thought to have transferred from sooty mangabeys in Guinea-Bissau in the 1960s.