Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry that your fellow Tie has been dealing with chronic diarrhea for the past two months, despite seeming to feel fine otherwise.
I understand that you are hesitant to dive into diagnostics because he feels so well but given the length of time he has been affected, that this has led to weight loss and that he has not responded to simple things like worming, a dietary change and antibiotic therapy I think diagnostic testing is warranted.
Chronic diarrhea can be secondary to heavy parasites infestation, but that is less likely in an adult dog. It can also be due to bacterial overgrowth, an inability due to digest and absorb food due to metabolic organ disease, (especially liver disease), pancreatic disease or endocrine disease (hyperthyroid, Addison's disease), or primary intestinal disease.
It is important to describe what sort of loose stools he has to try and localize the problem. Loose, small stools with mucous or bright red blood and straining or urgency to pass stools more frequently point more towards large bowel diarrhea or colitis, whereas watery stools with no mucous, no increased urgency or frequency to go, along with weight loss point more toward small bowel disease.
In his case it sounds like he has some straining and increased frequency, but no blood or mucous and the stools have been watery, so it may be that his entire intestinal tract is affected.
Chronic diarrhea does cause changes in motility of the gut and can lead to bacterial overgrowth in the gut. Metronidazole should have helped with that but perhaps he would benefit from probiotics such as Fortiflora, Proviable or Benebac. These can help replace appropriate bacteria.
I see that he has had a stool sample checked to look for parasites that may be part of his problem. In some cases we need to run several to find parasites as the eggs/cysts aren't shed with every stool.
Other diagnostic tests on his stools may be worthwhile too, such as fecal cultures or smears to look for bacteria or unusual parasites that may be hard to pick up of a regular fecal.
I recommend a fecal culture and direct smear to check for abnormal bacteria such as clostridia ect. be done. Rather than having them checked in the clinic I would also submit a few fresh stool samples to the laboratory to look for parasites. Laboratory facilities run more comprehensive testing and may pick up unusual parasites.
I would also check a complete blood count and biochemistry profile given his weight loss, we want to look for organ diseases and diabetes which would affect his stools and interfere with his ability to digest and absorb food.
The fact that he improved with a cortisone injection for a short time may point toward intestinal inflammation and an inability to digest or absorb nutrients from food or loss of proteins through a damaged intestinal wall. That would make sense with his symptoms and would explain why his veterinarian wants to perform an ultrasound on his abdomen. It may be useful to get intestinal biopsies too.
I understand that you are feeding a grain free, single protein source food but it may not be restrictive enough. It is quite possible that he has a food allergy/sensitivity or inflammatory bowel disease and that he needs a prescription low residue, easy to digest food or a hypoallergenic food to be able to properly digest and absorb his food and not have loose stools. I highly recommend a trial of either Hills i/d or Purina Veterinary Diets EN. No treats, table food or edible chewies while he is on his food trial. If he does well he can eat these foods for life as they are balanced. Having had 2 dogs with inflammatory bowel disease I have a personal preference for Purina Veterinary Diets EN. Dogs with food allergies can benefit from Hills z/d or Purina Veterinary Diets HA.
Dogs with inflammatory bowel disease will worsen with stressful situations. There may be times when he will need medications too, such as metronidazole or even steroids if that is his problem, but I have found that a consistent, easy to digest diet is very helpful for long term control.
There are other possibilities too as I mentioned.
Addison's disease, which is a poorly functioning adrenal gland, can lead to chronic diarrhea and/or vomiting. These dogs cannot handle stress at all because their adrenal gland doesn't produce cortisone when stressed and their electrolytes can be off too if their adrenal gland isn't controlling that normally either. We see vomiting, diarrhea and sometimes physical collapse in severely affected dogs. Testing is an ACTH response test to check adrenal gland function and checking electrolyte levels. Treatment is steroid replacement therapy and electrolyte replacement. Given that he seems to feel well otherwise this is less likely, as these dogs are generally very, very sick dogs.
Pancreatic insufficiency is another possibility. These dogs have a pancreas that produces a decreased amount of digestive enzymes, and the amount produced can wax and wane in some cases, especially early in the disease process. Testing is by running a blood test called a TLI which checks for digestive enzymes. Treatment is replacement of digestive enzymes at each meal. An easier to digest food would be expected to create less problems with digestion and as such less diarrhea.
In short more diagnostics need to be done. They can be as simple as more fecal checks and cultures, as well as checking pancreatic and/or adrenal gland function. An abdominal ultrasound could be very helpful. Or more invasive testing such as biopsies of his gastrointestinal tract to look for inflammatory bowel disease or infiltrative cancers such as lymphoma an be done.
In the meantime a diet change may help, and so may probiotics.
Please let me know if you have any further questions.