ClamAV integration in Messaging Server makes use of the clamd daemon that is provided as part of the ClamAV package. clamd is a multi-threaded process that listens on a socket for requests to process messages. After processing the message, it sends back a response and closes the connection. The client portion, clamdscan from the ClamAV installation, is not used. This function is done by a shared library called libclamav.so, which is part of Messaging Server.
libclamav.so is loaded the same way as the Brightmail SDK is loaded.