All rental units come with an "implied warranty of habitability," which is an obligation on the landlord (or his, her, or its agent) to keep the units in safely habitable condition. This warranty is held to include taking care of pest, vermin, etc. infestations, including bed bugs. If the property manager knew of the problem and did not take reasonable steps (the issue is not success; the issue is whether appropriate efforts were made), then the property manager and/or the building owner may be liable for costs  (such as to exterminate and clean), medical costs (if anyone needed treatment), and possibly for an abatement of rent during the condition; also they can be forced to take care of the situation. It would be worth consulting with a landlord-tenant attorney; good luck.