I found this Powerpoint presentation which helped me a lot to understand what happened with Tigris. As not everyone will have Powerpoint (I have it in the office but not at home I will quote what I found there:

UTD-Diseases

Of 143 cats with UTD (Urinary Tract Diseases) they found
1.4% with bacterial infections (what we call UTI)
22.4% with urethral plugs (Rutherford's case)
21 % with crystals (struvite or oxalate) (Boots' or Fister's case)
53.8% so called idiopathic (reason unknown, could be viruses, auto-immune and many others) (this is Tigris' case).

Now these urethral plugs consist mainly of tissue and cellular debris but can also contain minerals like struvite (which is also the stuff the most crystals are made from).

They say that in the case of obstruction, this obstruction of course must be relieved and this is done mostly via catheterization and flushing.

To help that it doesn't happen again, one should feed moist food, encourage drinking and in case struvite is involved, feed special diets.

That's the main thing I could find out about plugs and now I hope Rutherford is getting better