Skinny aggressive cat

Question
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Followup To
Question -
Hi,
 I started with 2 cats simular in age, Missie(f) and Munchkin(m) (both now going on 3 years old..) They have been good friends and still are. I then got married, and we got a 3rd kitten, Tillie(m), who is going on 2 years old. Everything was fine, until just before Christmas we bought a Siamease Mix kitten Sigwa(m). He is just turing 5 months and will be getting fixed in a few days.
 The problem is with Tillie and the new kitten Sigwa. Tillie is constantly swatting, growling and hissing at the baby. Tillie has lost lots of weight, and has become very bony dispite a normal appitite. (he does have dry skin, but has a healthy shiny fur. Tillie does go out quite a bit more often, and still gets along with the other 2 cats as usual (most of the time). Tilly seems a little more overall edgy/cautious towards me as well.. almost as though he is sulking. (except at night when I lock the kitten and the oldest in their craits, Tillie will be friendly and sleeps with me, and purrs and seems better.) The kitten started to take over the household straight off, attacking my oldest, though the oldest just ignors him for the most part. My girl, missie, just ignors all the males and does her own thing.
 Is Tilly just stressed out because of the new addition. We do have a small house, and all our cats have their shots and are wormed. I dont see any other reasons for his condition. They all eat, and I stay in the room to make sure they all get the same amount of food and will let them have their own corners if they are having an off day with the other cats to eat. They are all going into the vets later this week for their update shots, so I will ask them about it as well..
 I will be thankful for any help or advice you are able to give us. I want all my cats to be happy and healthy again!
Denise :)
Answer -
Hi Denise.  Sorry to hear you're having problems!  Although it has been a while, and I know it's frustrating, Sigwa is still very new to your other kitties.  I think, given time, Tillie will warm up to the kitten.  Just try to keep them separated when you aren't home to closely supervise, to prevent nasty fights.

I believe strongly that Tilly's weight loss is the result of a medical problem.  Were his appetite suffering, I'd say the stress of the new kitty could be a part of the problem.  But a cat who loses weight despite a normal appetite is suffering from a physical illness.

There are numerous possibilities - cancer, feline leukemia, feline AIDS, diabetes, hyperthyroidism, kidney failure, Inflammatory Bowel Disease, parasitic infections...that's just the tip of the iceberg.  Fortunately, many of the serious ones are uncommon in cats as young as Tillie, but they aren't unheard of.  

Feline AIDS and feline leukemia are most common in young cats and are becoming more and more common.  In fact, leukemia is now the number one killer disease in domestic cats.  It's a contagious virus that has no cure, and it is terminal.  Since Tillie goes out, I'd highly recommend having him tested for these diseases.  Ideally, make Tillie an indoor cat.  That way he can't bring the diseases back home and infect the other cats.  There are vaccines for these diseases if he will continue to go outdoors.  But they carry heavy risks and are nowhere near 100% effective.

Follow up Question...
All our cats were vaccionated for feline Lucumia, since it was known to be around a few towns from ours... I was wondering though, if a bad worm problem can cause weight loss? A few weeks ago he threw up occasionaly both tape AND round worms, and all cats were dewormed only a week ago. He has since looked happier, but has not had time to gain weight... This is what I was hopeing was wrong.. He is not sickly looking skinny, but has lost significant weight the past 3 months.. he also has that long slim body, long legs neck and super long tail of a Siamease.. so I know he is petite by nature as well..
 Anyways, I thank you for your first response. They will all be seeing a vet in 3 days, and I will find out more.
Denise B :)

Answer
Hello again.  Yes, a bad case of worms can certainly cause weightloss.  Hopefully, this was the problem, and he'll start putting weight back on now that he's been treated.  But I'm glad he'll be seeing the vet, anyway.

Just in case - roundworms and tapeworms can't be cured by the same medication unless given by prescription (such as Drontal).  If you use an over-the-counter medication, you'll need to give two separate meds to cover both roundworms and hookworms.  Praziquantel kills tapeworms.  Pyrantel pamoate or piperazine (I don't prefer this one, myself) will kill roundworms.