These little creatures are the worst infestation you can imagine, I had a tenant with them, I turned up to do a mid term inspection and one of the little blighters must have got on me the next thing my home was infested too. If you call the local council they will come round and spray but they need to do it a couple of times at least, they dont normally charge because they dont want them to spread, we had the spray man in three times we threw out the entire contents of the flat left it empty for three months and still found that they came back, they can live anywhere not eat for months on end and breed faster than you can ever imagine.
It would be very hard to actually pinpoint the blame, the main thing is to rid yourselves of them and try not to pass the blame because it will be very hard to prove, whoever walks into that flat is a potential carrier, speak to your insurer you may be covered, failing that let the tenants go as soon as and get the flat sprayed as many times as you can, we ended up selling both flats because there seemed no end to them, I wish I can be more helpful but we could find no escape from them, maybe if we had used a company like Rentokil the restults would have been differnt.
Your tenants must really be suffering too and would probably appreciate you terminating their contract as these things not only eat anything they drink blood too and leave very itchy painful marks..
I wish you the very best of luck, the thought of those things made the hair stand up on the back of my neck.