What causes sudden weight loss and weakness in a 4-month kitten?

Sudden weight loss is always a very big warning light that should not be taken lightly, it's usually a sign of an internal disease that needs to be investigated, i strongly recommend taking Winter to the vet in order to get it checked, he will need to have a blood test done and possibly an xray as well in order to attempt to diagnose the reason for these symptoms.

Given her age there are several things that cross my mind, but a common issue in these young kittens is parasites. Sometimes their parasite burden can be so high that they cannot maintain adequate glucose levels, and also can become uninterested in food and sometimes will begin vomiting. I would recommend deworming her. Also, it would be important to rule out signs of infection and/or sepsis. I think she needs to be seen by a veterinarian right away. Such a young and small animal can deteriorate quickly even with easy to resolve issues like gastrointestinal parasites.

Unfortunately no, there is nothing you can do without taking her to a vet. She sounds quite ill, and she truly has to have veterinary care; in fact, I would consider her current symptoms a medical emergency. She sounds very weak and she's likely starving, and she needs veterinary care now if you want to help her. Please take her into a vet right away.

These are certainly concerning behaviors for your young kitten. I would be suspicious of intestinal parasites causing the problem, especially if he/she has not yet been wormed. I would recommend taking your cat in to the vet to be checked out and make sure nothing too serious is going on. Hopefully it is something simple like parasites that can be treated easily. Best of luck!

Poor Rusty! Cats can have worms even if you don't see them in the feces. Oftentimes, there are eggs in the feces, but those can only be seen with a microscope. Sudden weight loss in cats can also be caused by diabetes, kidney disease, systemic illness, infection, or injury among other causes. I would have Rusty seen by a vet asap, and bring along a sample of his feces for the vet to examine. In addition to looking at his feces, the vet can do a physical exam, blood tests, and a urinalysis to diagnose what is causing his weight loss. Once the cause of the weight loss is diagnosed, the vet can discuss treatment options with you.

A 3 pound weight loss is indeed significant and not normal. There could be many reasons for this, and there is just no way to pinpoint the cause without a hands-on exam and possibly blood work. I suggest you take your kitty to a veterinarian for a wellness exam.

It sounds like he is getting sick. Without additional clinical signs it is hard to say exactly what is causing his lethargy and decreased appetite but he could be starting with a respiratory virus or a gastrointestinal virus. He could also have parasites that need to be treated. If it continues than he should be examined by a vet to determine the cause. Your vet can run lab work to help diagnose and then determine the best way to treat. If he is unvaccinated, please discuss a vaccine plan with your vet once he has recovered from this illness.