The first virus to target Apple's OS X operating system has been identified in the wild.

Leap-A (also known as Oompa-A) spreads via the iChat instant messaging system, forwarding itself as a file called 'latestpics.tgz' to contacts on the infected user's buddy list.

When the file is opened on a computer it disguises itself with a JPEG graphics icon in an attempt to fool people into thinking it is harmless.

"Some owners of Mac computers have held the belief that Mac OS X is incapable of harbouring computer viruses, but Leap-A will leave them shell-shocked as it shows that the malware threat on Mac OS X is real," said Graham Cluley, senior technology consultant at Sophos.

"Mac users should not think it's OK to lie back and not worry about viruses. "

Cluley said that his company released a signature file for the virus at midday. Other manufacturers will follow suit, as antivirus researchers from rival companies cooperate on new threats.

Mac users on online forums have denied that the online pest is a worm and instead qualify it as a Trojan. It requires the user to manually download and open the file before a system is infected. Users who aren't running on administrator accounts furthermore are prompted to enter an administrator password.

None of those facts however precludes OSX/Leap.A from qualifying as a worm, Sophos countered. The main difference between a Trojan and a worm is the pest's ability to spread itself.

"OSX/Leap-A is programmed to use the iChat instant messaging system to spread itself to other users. As such, it is comparable to an email or instant messaging worm on the Windows platform," the security provider stated. "Worms are a sub category of the group of malware known as viruses."

• This story was updated at 18:33 to add information about the worm vs. Trojan debate. Tom Sanders contributed to this story from California.