Norton Security said that the rogue software's decline has not been as swift as expected, despite the efforts of Apple and third-party security firms to stamp it out.
"The statistics from our sinkhole are showing declining numbers on a daily basis. However, we had originally believed that we would have seen a greater decline in infections at this point in time, but this has proven not to be the case," said a Norton spokesperson.
"Currently, it appears that the number of infected computers has tapered off, but remains around the 140,000 mark."
The Flashback Trojan was reported to have infected some 600,000 Apple Macs earlier this month. Its creators exploited weaknesses in Java code to enable the malware to install itself on machines without permission. It then sends out a message to the hacker granting them access to the system's contents.
Apple issued a fix for the issue last week, several months after the Flashback Trojan was first discovered. Web security firm Kaspersky Lab criticised the company over its slow response time.
Apple recommends that all Mac users with Java installed should download the patch. The company is also said to be working with internet service providers to "disable" the networks which are sustaining the malware.
Earlier this week, Kaspersky Lab warned that it has discovered a new Mac Trojan, named Backdoor.OSX.SabPub.a, or "SabPub" for short, that could become more widespread than its predecessor.