
As she is also very active is she othrwise wll. What is the condition of her coat, breath etc.
I have had an anorexic Elkhound (normally the reve3srse is the problem with this breed). she would only eat 4 days out of 7 and around ehr seasons she would not eat for a couple of days at a time. In the end I fed her on Arden Grange Prestige which is very energy and nutrient dense, and also has one of the highest manimal protein contens of the commercially available foods. this kept her very fit and muscular, albeit lean. It took until she was around 4 years old when I started feding ehr normal adult food, as hshe then ate most days, though still regularly leaves a few bitc in her bowl.
Funnily enough when she has ben in whelp and rearing her litters she has earten like a horse, even getting plump, but by her next season has rreturned to her normal wiry self and eating habits.
Raw Green Tripe is also something most canines find very attractive and can be used to good effect to put on weight.
Are you abso;utely sure your bitch is actually seriously underweight and not just slim. My slim girl would look fat if she weighed the average 20kg for a bitch of this breed, so just right at about 18kg and too slim at 17kg.
My 10 month old pup weighs about 18kg, and is just right for her developement, even though she is full height. she will gradually gain a few more pounds with maturity.