You're speaking with Dr. Michael Salkin. Welcome to JustAnswer.
Barbara, there's rarely just one thing going on in a 15 year old. Pilchard's extreme cachexia (wasting) is often due to malignancy or major organ insufficiencies (renal, hepatic failure) at her age. Her vet will want to thoroughly examine Pilchard including diagnostics in the form of blood and urine tests. I must admit, however, that her presentation always indicates advanced disease that I might be able to manage in a palliative manner but cure shouldn't be considered.
Please check Pilchard's vital signs for me.
1) Check her gum and tongue color. They should be nicely pink - not whitish (anemia) or bluish/greyish (cyanosis/hypoxia/lack of oxygen to her tissues).
2) Check her respiratory rate at rest. She should be taking less than 30 breaths/minute while asleep or at rest.
3) Take her rectal temperature. Any body thermometer will do when placed 1.5" into her rectum for 1 minute. Normal is 38-39.2C. This is a two person job!
Please let me know what you find.
Thank you for your kind accept. I appreciate it. I can't set a follow-up in this venue so please return to our conversation - even after rating - with an update at your convenience.