Tar is very toxic to cats and can cause something called methemoglobinemia. This is a sometimes fatal change to the red blood cells. It can cause the red blood cells to lose the ability to carry oxygen. Cats with this problem develop a chocolate brown tint to their gums.
Also, if the cat licks and chews at this tar, he can ingest it, then vomit and aspirate it which causes a severe pneumonia.
Furthermore, the tar can cause dermatitis and inflammation at the site, particularly if it was hot when it went on (as it likely was in order for it to stick).
So, this is quite serious!
To get this off at home, you should start with an oily product like olive oil or butter. Put that on a facecloth or small flannel and rub it into the tar. The tar will mix with it. After rubbing you can then wash the oily residue off with Dawn dish detergent.
Here is more:
You can also use dioctyl sulfosuccinate which you can buy at a pharmacy. It is an ingredient used in pediatric enemas. It is gentle and mixes with the tar well.
Another option to try if the butter or oil is not working is Contractors Solvent.
Here is information about it:
"CONTRACTORS SOLVENT is a safer way to remove adhesives, tar, asphalt, road and roofing tar, oil-based paint and more from virtually any surface, including skin and hair. Professional Strength De-Solv-it® CONTRACTORS’ SOLVENT is non-hazardous, non-corrosive, and a highly safer alternative to acetone, paint thinners, and other traditional solvents."
More on safety of De-Solv-it:
I would strongly recommend that your kitty see a vet as the skin under the tar could be very burned and infected. He may well need some antibiotics. Please monitor him closely for brown gums, difficulty breathing, loss of appetite and lethargy. These would all be reasons to take him for immediate veterinary care!
I do hope that this helps you to help your cat!
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The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.