Q: My 7-year-old cat is getting thin. When I lived in Colorado, I never let him outside. Now I am back in South Dakota and my cat goes outside. During the summer, he spent quite a lot of time outside. My cat has had all of his vaccinations done, including leukemia.
A: There are many possible reasons why your cat is getting thin. You didn’t say anything about your cat’s appetite and thirst, so it’s hard for me to comment.
Diabetes causes cats to lose weight, while maintaining a good appetite. Diabetics also drink more and urinate more. You didn’t say anything about his thirst in your letter. Hyperthyroidism and kidney disease are two other potential causes for weight loss in cats, although these tend to affect older cats. Because your cat goes outdoors and presumably encounters other cats, the feline leukemia virus (FeLV) and the feline immunodeficiency virus (FIV) must be considered as possible causes for your cat’s weight loss.
You said that you had your cat vaccinated against FeLV, and that’s great, because FeLV has a terrible prognosis, and the vaccine is highly effective. You didn’t mention if your cat was vaccinated against FIV. This vaccine was developed a few years ago. It provides good immunity to FIV, however, when your cat is vaccinated with the FIV vaccine, antibodies against the FIV virus will appear in the bloodstream. If your cat gets sick in the future and is tested for FIV, the antibodies will cause the test to appear positive. There is no way to distinguish whether the antibodies that are detected were induced by the vaccine, or induced by infection with FIV. So, if your cat was indeed vaccinated against FIV, the test result will be difficult to interpret. (Actually, a test to distinguish the two has recently been developed, but is not widely available.)
Assuming your cat was not vaccinated against FIV, I would have your cat tested for FeLV and FIV, just to make sure that your indoor-outdoor cat hasn’t contracted either of these harmful viruses. (Even though your cat was vaccinated against FeLV, no vaccine provides 100% immunity, so checking the FeLV status is prudent.) Your vet will likely want to perform some basic laboratory tests (for example, a complete blood count, serum chemistry panel, urinalysis and fecal evaluation) and perhaps some other tests, depending on the physical exam findings, to diagnose the cause of your cat’s weight loss.