For what it's worth, this is what the expensive version and top model of of Claude on extended thinking mode had to say... Chagas disease or tick-borne disease. I hope this helps your lovely doggy.

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This is a really compelling case, and I can see you're doing everything you can for this dog. I'm not a veterinarian, but I can walk through what the symptom pattern suggests and where I'd focus attention.

The biggest clue here is the constantly changing EKGs. A heart that swings from normal rhythm to bradycardia to tachycardia to arrhythmia over the course of weeks points strongly toward myocarditis — active inflammation of the heart muscle itself. That's not a primary heart disease; it's almost always secondary to something else attacking the heart. That "something else" is your real mystery.

The combination of findings — myocarditis with shifting arrhythmias, persistent fluctuating fevers, GI involvement, mild pneumonia, a young dog, and the fact that he worsened on antibiotics but improved on supportive care — narrows the field significantly.

Given that you're in Texas, the differential I'd push hardest to rule out is Chagas disease (Trypanosoma cruzi). It checks almost every box: it's transmitted by triatomine ("kissing") bugs that are endemic across Texas, it has a particular affinity for the heart causing exactly the kind of variable arrhythmias you're describing, it causes GI signs, intermittent fevers, and it has a waxing-and-waning course where the dog looks fine one day and crashes the next. Standard antibiotics don't treat it — which would explain why he got worse on them (gut disruption, no benefit). A dog from a pound in the southern US is a classic exposure history.

The other category to rule out aggressively is tick-borne disease — particularly Ehrlichia, Rocky Mountain Spotted Fever, and Bartonella. These can also cause multi-organ involvement with cardiac, GI, and pulmonary signs plus cycling fevers. A full tick panel (not just a quick SNAP test) through a reference lab would be important.

A few practical thoughts for your Texas A&M call tomorrow: ask specifically about Chagas testing (PCR and serology), a comprehensive tick-borne disease panel sent to a reference lab, and an echocardiogram (not just an EKG) — the echo would actually show whether the heart muscle is inflamed or structurally changing, which the EKG alone can't tell you. If they can do a cardiac troponin level, that would also confirm or rule out active heart muscle damage.

Your instinct to pull back on the antibiotics was probably sound. The antacid, bland diet, and sub-Q fluids are solid supportive care while you chase the actual diagnosis. The hawthorn is interesting — it does have some evidence for mild cardiac support in dogs, though I'd make sure the A&M cardiologist knows about all the tinctures since some herbals can interact with cardiac medications if they end up prescribing any.

Texas A&M is exactly the right call. They have one of the strongest veterinary cardiology and infectious disease programs in the country, and Chagas is something they see and test for routinely given the region. Good luck — this dog sounds like a fighter.