Hello, I am Dr. B, a licensed veterinarian and I would like to help you with this poor wee lad today.
Now when we see blood in their feces, this tells us that this cat is experiencing an issue based in the lower portion of the gut. This means that we do have a range of issues to consider for him. Now since you haven't noted that the blood is profuse or constantly pouring forth, then concerns for rectal tumors or rectal trauma would be less likely and something we can put at the bottom of our list of concerns for him to start. Furthermore, since you have not reported him being painful in his back end and no blood without the feces, anal gland disease would also be less of a concern here.
And that means we can focus on conditions where the blood is associated with passage of feces. Specifically, we can see this with parasitic infection (ie whipworms) and with inflammation/infection of the colon (colitis).
In regards XXXXX XXXXX approach here, first you may consider worming to remove the concern of parasitic causes. Since your kitten shares the tray, you may want to worm him too. Ideally, you want to use a good quality wormer that will cover against whipworms. Examples would be Panacur, Drontal (LINK) or Milbemax (LINK) as it will cover all the worms in question and help rule them out as the cause of this colonic bleeding. These are available over the counter at the vets, pet stores, and some pharmacies. You can often mix this into smelly food (Panacur granules tend to be particularly well tolerated) and administer it without the cat knowing.
As well, if we have an inflammatory colitis then this can sometimes be settled with a
light/easily digestible diet. (Though bacterial induced colitis will often require antibiotic treatment). Examples of bland diets would be boiled chicken, boiled white fish, scrambled eggs (made with water and not milk), or meat baby food (avoid those with garlic powder in the ingredients) There are also veterinary prescription diets that can be used in cases like this (ie Hill’s I/D or Royal Canin’s sensitivity). The easily digestible diet will be better tolerated and absorbed by the compromised gut. We usually will try them on the diet for a week until signs settle, and then slowly weaned back to their normal diet over a week.
If you try the above and this doesn't settle in the next 24-48 hours, or if you do see per-rectal swelling (thus indicating anal gland disease) or the volume of blood is more then a few drops mixed in then he'd need to see a vet for antibiotics against bacterial colitis. Since RSPCA cannot help you, I would advise that if he doesn't settle that you ring Cat's Protection (LINK). They have a large volume of branches that cover the country and often will come out to people's houses to help transport poorly strays if need be.
So, do rule out the parasitic differential by treating for worms today and consider putting this stray on a easily digestible diet for inflammatory colitis. But if that doesn't settle his signs then speak to your local branch of Cat's Protection and they will likely give you a had with this wee lad.
I hope this information is helpful.
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