By Zoe Cormier of Nature magazine

A class of virus has for the first time been shown to jump from animals to humans--and then to infect other humans.

The virus is described in PLoS Pathogens today. The team that discovered it might also have found the first human to be infected: the primary carer for a colony of titi monkeys (Callicebus cupreus) that suffered an outbreak.

The culprit is an adenovirus, one of a class of viruses that cause a range of illnesses in humans, including pneumonia. But this particular strain has never been seen before. It has been dubbed TMAdV, or titi-monkey adenovirus.

"It's always been thought that adenoviruses are not likely to be causes of outbreaks or pandemics because they have never been known to cross between animals and humans," says Charles Chiu, director of the UCSF-Abbott Viral Diagnostics and Discovery Center at the University of California, San Francisco (UCSF), who led the study. Now that assumption needs to be re-examined.

In May 2009, a deadly outbreak of respiratory illness spread through a colony of titi monkeys at the California National Primate Research Center in Davis. Of the 65 monkeys housed in one building, 23 developed symptoms, including pneumonia. As a result, 19 died or were put down.

Chiu and his colleagues analyzed tissue taken from the affected monkeys, and identified a previously unknown virus. Genetic sequencing revealed it to be an adenovirus, although its genome was substantially different to those of all known related viruses.

But what tipped the researchers off that there was something unusual about this virus, says Chiu, was what happened when they tried to culture it. "It was unusual to see it grow well in human cell lines, but not monkey" cells, he says. This suggested that the virus could infect humans as well as titi monkeys. "After we interviewed all of the staff, the only person who said they had been sick was one researcher--the one who had had the closest daily contact with the colony," says Chiu.

That researcher experienced flu-like upper-respiratory-tract symptoms for four weeks. More crucially, a family member who had never visited the primate centre also became ill--demonstrating that TMAdV can spread between humans.

Unknown origins

The titi monkeys might not have been the original hosts of the virus. Of those that developed symptoms, 83% died--a fatality rate that would prevent the virus from circling in the population without wiping out the monkeys. In human-specific strains of adenovirus, death rates usually only reach 18%.

The original host species could be humans, who passed the virus to the monkeys, only for it to jump back to humans--or it could be another animal, such as a rodent. The researchers are collecting blood samples from monkeys and humans from all over the United States, Brazil and Africa to help them to discover the virus' origins.

Chiu says that there is no reason to suspect that there will be a pandemic of TMAdV, as there has been with other viruses that spread to humans from animals. A survey of blood samples from 81 random, healthy blood donors from the western United States found that two people already had significant levels of antibody to TMAdV. "This virus then potentially crossed into the human population a long time ago and is now circulating at low levels," says Chiu.

But the more we know about this and other new viruses, the better, says Eric Delwart, a virologist at the Blood Systems Research Institute at UCSF. "Characterizing animal viromes facilitates the detection of related viruses, and may shave a few precious days from identifying a new virus in the event of a future severe outbreak," he says.

The discovery also raises the possibility of using adenoviruses as vectors in gene therapy, in which a virus is used to correct defects in a patient's genes, says Chiu. "The fact that TMAdV appears to infect two or more different species but is not common in the human population also suggests this might be a therapy that could have broader applicability," says Chiu, because it means the virus on a wider range of targets. Other labs are already investigating gene therapies using adenoviruses. "This could open up new and better treatment possibilities," says Chiu.

This article is reproduced with permission from the magazine Nature. The article was first published on July 14, 2011.