Hello, I'm Dr. Kara. I have over 20 years of experience as a veterinarian and I'd like to help with your concerns about Daisy's decreased appetite and lethargy.
In most cases a poor appetite is triggered by eating something they should not, too much table food, too many treats or something they find outdoors.
More serious causes of a decreased appetite include viral or bacterial infections, chronic pancreatitis, esophageal reflux, a dietary allergy or sensitivity, inflammatory bowel disease, internal organ failure (kidney or liver disease), a full or partial gastrointestinal obstruction or even infiltrative cancers such as lymphoma.
Gastrointestinal parasites rarely cause a decreased appetite in an adult dog, they can lead to loose stools though so continue to monitor her stools.
If your girlfriend's dogs had just moved in I would think that this may be a stress response but since they moved in two months ago and everyone seems to be getting along that is much less likely.
If the other dogs all seem fine then some sort of contagious disease (like a viral or bacterial infection) seems less likely, though it is possible that the others just aren't showing signs yet so I would keep an eye on them all.
Because this has persisted for several days ideally she would see her veterinarian. If that isn't possible for whatever reason there are some things we can try at home.
I would continue to keep her secluded so you can keep a close eye on her and watch for further symptoms.
To try and settle her stomach and improve her appetite you can give either:
1) Pepcid ac (famotidine) at one 10mg tablet per 20 to 40 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at one 10mg tablet per 20 to 40 pounds of body weight every 24 hours
These are acid reducers and should help her feel better and hopefully improve her appetite.
I would pick up all food for now and water for a couple hours to allow her stomach to settle after the acid reducers. In a couple hours when you give her water make sure it is in small amounts only. If she drinks too much too quickly that can lead to vomiting. To get some electrolytes in you can also offer her a 50:50 mix of pedialyte and water.
In 12 to 24 hours offer a bland diet of 1/3 boiled minced white skinless chicken or lean hamburger and 2/3 boiled white rice mixed with some low salt chicken or beef broth to make it easy to lap up and swallow. If she refuses that you can offer a little meat baby food. If she refuses both then don't push it. You know at that point that she needs a veterinary examination.
But if things go well and she does eat and doesn't vomit feed her the bland diet for 2 to 3 days then slowly start to mix back in her regular food, a little more at each meal. It should take about 5 to 7 days to slowly convert her back to her regular diet.
If he continues to not eat well, runs a fever (more than 103F rectally), has a tense painful belly or if she refuses to eat at all she should see her veterinarian for an examination, diagnostics, injectable anti-nausea drugs intravenous fluids and supportive care.
Please let me know if you have any further questions.