Unable to load abstract class when trying to load in drupal queue.

In my module, I have implemented a queue to send out all digests as per the subscribed frequency. For this, I am using messsage_digest notifiers. Now when the cron job runs it aggregates all the messages and adds to the queue to be sent out. Now when the queue runs it tries to load the notifier class. Let's say digest_day notifier is being called and it tries to load the MessageDigestDay class. This class extends MessageDigest. Now when I am inside the queue worker function MessageDigest class is not found and it gives an error that MessagDigest class can not be found at (.....) location. This works everywhere else, just not inside the queue worker callback. If I explicitly include the abstract.inc file in all of my notifier class, it works fine.

Anyone having the same issue?

here is my code from worker callback:

$item = unserialize($item);

$uid = $item['uid'];

if(!isset($uid)) {
// log something here
return;
}

$notifier_name = $item['notifier'];
$message = $item['digest'];
$plugin = message_notify_get_notifier($notifier_name);
$class = ctools_plugin_load_class('message_notify', 'notifier', $notifier_name, 'class');
$notifier = new $class($plugin, new Message());