There is a very usefull patch for DansGuardian, called DgVirus. It adds additional protection level to DansGuardian with scanning downloadable content with F-prot and Clamav.
Steps to Reproduce:
1. download the patch from http://www.pcxperience.org/dgvirus/
2. apply to DansGuardian sources
3. build DansGuardian
Now DansGuardian (neither Squid) doesn't have anti-virus protection
Expected to have anti-virus web-content scanning
dgvirus is pretty inactive.
their patches are now obsolete; current version of dansguardian is 184.108.40.206